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If reality is just hallucination, how come different people see same objects

If reality is just hallucination, how come different people see same objects

I wanted to ask this question for a long time but couldnt find any thing to support my argument. Today i found this https://www.ted.com/talk/anil_seth_how_your_brain_hallucinates_your_conscious_reality#t-694887, where the speaker Anil seth theoretize that our reality is just a hallucination. Exchange of information happens both ways inside and outside, i.e, we not only perceive the world around us but also actively generates it.

Perception is an active constructive process. To summarise,

Hallucination is uncontrolled perception, then Perception is controlled hallucination.

My question is, If thats the case then how two people would agree on the same state of world?


According toAnil Seth, in this talk, consciousness is an active construction process. In that view, for the subject to have an experience about world, physical signals from mind independent objects are processed by brain and a highly probabilistic mental content of the physical is experienced. Even though I mentioned objects, it could be material things and features.

But our sensory abilities are not inherently cable of perceiving everything. The range of vision is360-400 to 760-830nm. Similarly, for audition its20Hz to 20,000Hz. There are other phenomena like aftereffects and adaptations, which too work along with perception. The information thus processed by sensory faculty is further compared with prior data that is stored in the brain and brain tries to come up with a decent construction of mind-independent objects. This experience can be called a controlled hallucination as there are other possibilities for such construction.

The general organization and working pattern of typically developing humans brains are very similar, especially in the early processing of sensory signals. Similarly, the physical signals that it processes of mind-independent objects are also very similar. Given these two, it is highly likely that two people have similar experience, even though we cannot confirm it and can only say that we share a human like experience.

Reference:

  • https://plato.stanford.edu/entries/perception-problem/

  • https://en.m.wikipedia.org/wiki/Constructive_perception

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2888317/

  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4763133/


Additionally, active construction of reality doesn't mean there are basic objective reality. Usually our mind uses similar mechanisms to understand this basic reality. our byological makeup, psychological processes and broad social context provides us basic assumptions, and other tools to understand what is going on in the world. usually what we perceive is outcome of our interpertation of that basic reality by byological psychological, social tools. We might see world differently Because of our byological, psychological social differences. However, those differences are not so big that we can not understand each other.
Referenses: Burr, V. (2006). An introduction to social constructionism. Routledge. Raskin, J. D. (2002). Constructivism in psychology: Personal construct psychology, radical constructivism, and social constructionism. American communication journal, 5(3), 1-25.


Illusion came from the Latin word, “illusio” which means “to mock”. Rightly so, it occurs when something seems to be different from what it actually is. With the alteration of the stimulus, people experience “misperception”. This happens when the brain tries to fill in gaps in the organized sensory information. Though illusions are usually related with visual processes as it dominates the others, they are also associated with the other senses.

The following are the different kinds of illusions:

  • Optical illusion- makes use of visually deceptive illustrations like the Ebbinghaus and Hermann Grid illusions
  • Auditory illusion- characterized by sounds which are not actually present or improbable such as those in psychoacoustic tricks
  • Tactile illusion- this involves deceptions via touch such as the phantom limb wherein a patient still perceives pain in a leg which has already been amputated
  • Temporal illusion- concerned with the perception distortion of time like when minutes seem to significantly slow down to hours

Is the world real, or is it just an illusion or hallucination?

Is this real life? How do we know that we are not hallucinating it all? What if we're plugged into a Matrix-style virtual reality simulator? Isn't the universe a giant hologram anyway? Is reality really real? What is reality?

We asked renowned neuroscientists, physicists, psychologists, technology theorists and hallucinogen researchers if we can ever tell whether the "reality" we are experiencing is "real" or not. Don't worry. You're going to be ok.

Jessica L. Nielson, Ph.D.

Department of Neurosurgery, Postdoctoral Scholar, University of California, San Francisco (UCSF), Brain and Spinal Injury Center (BASIC)

What is our metric for determining what is real? That is probably different for each person. One could try and find a consensus state that most people would agree is "real" or a "hallucination" but from the recent literature using imaging techniques in people who are having a hallucinatory experience on psychedelics, it seems the brain is hyper-connected and perhaps just letting in more of the perceivable spectrum of reality.

When it comes to psychosis, things like auditory hallucinations can seem very real. Ultimately, our experiences are an interpretation of a set of electrical signals in our brains. We do the best to condense all those signals into what we perceive to be the world around us (and within us), but who is to say that the auditory hallucinations that schizophrenics experience, or the amazing visual landscapes seen on psychedelics are not some kind of bleed through between different forms of reality? I don't think there is enough data to either confirm or deny whether what those people are experiencing is "real" or not.

Sean Carroll

Cosmologist and Physics professor specializing in dark energy and general relativity, research professor in the Department of Physics at the California Institute of Technology

How do we know this is real life? The short answer is: we don't. We can never prove that we're not all hallucinating, or simply living in a computer simulation. But that doesn't mean that we believe that we are.

There are two aspects to the question. The first is, "How do we know that the stuff we see around us is the real stuff of which the universe is made?" That's the worry about the holographic principle, for example -- maybe the three-dimensional space we seem to live in is actually a projection of some underlying two-dimensional reality.

The answer to that is that the world we see with our senses is certainly not the "fundamental" world, whatever that is. In quantum mechanics, for example, we describe the world using wave functions, not objects and forces and spacetime. The world we see emerges out of some underlying description that might look completely different.

The good news is: that's okay. It doesn't mean that the world we see is an "illusion," any more than the air around us becomes an illusion when we first realize that it's made of atoms and molecules. Just because there is an underlying reality doesn't disqualify the immediate reality from being "real." In that sense, it just doesn't matter whether the world is, for example, a hologram our evident world is still just as real.

The other aspect is, "How do we know we're not being completely fooled?" In other words, forgetting about whether there is a deeper level of reality, how do we know whether the world we see represents reality at all? How do we know, for example, that our memories of the past are accurate? Maybe we are just brains living in vats, or maybe the whole universe was created last Thursday.

We can never rule out such scenarios on the basis of experimental science. They are conceivably true! But so what? Believing in them doesn't help us understand any features of our universe, and puts us in a position where we have no right to rely on anything that we did think is true. There is, in short, no actual evidence for any of these hyper-skeptical scenarios. In that case, there's not too much reason to worry about them.

The smart thing to do is to take reality as basically real, and work hard to develop the best scientific theories we can muster in order to describe it.

Holographic Principle

“If you asked anyone twenty years ago how many dimensions our world has, most of us would answer 'three spatial dimensions plus time.' The holographic principle would mean that this is actually a matter of perspective.” Craig Hogan to Motherboard

The theory suggests that the entire universe can be seen as a two dimensional information on the cosmological horizon, such that the three dimensions we observe are an effective description only at macroscopic scales and at low energies.

The Nature of Space and Time: Is Space Digital?

Space may not be smooth and continuous. Instead it may be digital, composed of tiny bits. Physicists have assumed that these bits are far too small to measure with current technology. Yet one scientist. Scientific American, 2014

Fredrick Barrett

Instructor in Psychiatry and Behavioral Sciences, Behavioral Pharmacology Research Unit, Johns Hopkins School of Medicine

With psychedelics or "classical (serotonergic) hallucinogens", individuals can often distinguish between perceptual disturbances, visualized experiences (it feels as if I was in another place, or I had traveled to another time, but I realized my physical body was still "here"), and whatever is happening "outside" in the "real" world. However, in psychosis (for instance, in the midst of a psychotic break in a person who has schizophrenia), hallucinations are quite clearly defined as something that an individual believes is real, persistent, and seemingly independent and autonomous in the world.

The "hallucinations" of schizophrenia and psychosis are accepted as real, and individuals with schizophrenia often do not have insight into the nature of their hallucinations as being "not real" to the rest of us. This highlights a bit of a misnomer in the name of the drug class "hallucinogens", in that the experiences with these compounds are not taken as consensual reality in the same way that psychotic hallucinations are taken as "real".

How or Why can we tell the difference between reality and what is perceived during the acute effects of psychedelics? I'm not sure science has definitively answered that question . but I think it may have to do with access to the insight that you've consumed a substance that can have these effects. It also may have to do with the transient effect of many perceptual disturbances and visualizations that can occur with hallucinogens. Maybe if the subjective effects of hallucinogens acted more like every-day perceptions (i.e. they weren't so extraordinary) or if they were more fixed or persistent (i.e. they didn't shift, warp, or morph so often) they would seem more real to the individual experiencing them.

George Musser Jr

The holographic principle doesn’t mean the universe isn't real. It just means that the universe around us, existing within spacetime, is ​constructed​ out of more fundamental building blocks. "Real" is sometimes taken to mean "fundamental", but that's a very limited sense of the term. Life isn't fundamental, since living things are made from particles, but that doesn’t make it any less real. It’s a higher-level phenomenon. So is spacetime, if the holographic principle is right. I talk about the holographic principle at length in my book, and I discuss the distinction between fundamental and higher-level phenomena in a recent blog post.

The closest we come in science to "real" or "objective" is intersubjective agreement. If a large number of people agree that something is real, we can assume that it is. In physics, we say that something is an objective feature of nature if all observers will agree on it - in other words, if that thing doesn’t depend on our arbitrary labels or the vagaries of a given vantage point ("frame-independent" or "gauge-invariant", in the jargon). For instance, I'm not entitled to say that my kitchen has a left side and a right side, since the labels "left" and "right" depend on my vantage point they are words that describe me more than the kitchen. This kind of reasoning is the heart of Einstein's theory of relativity and the theories it inspired.

Could we all be fooled? Yes, of course. But there's a practical argument for taking intersubjective agreement as the basis of reality. Even if everyone is being fooled, we still need to explain our impressions. An illusion, after all, is entirely real - it is the ​interpretation of the illusion that can lead us astray. If I see a smooth blue patch in the desert, I might misinterpret the blue patch as an oasis, but that doesn’t mean my impression isn't real. I'm seeing something real - not an oasis, but a refracted image of the sky. So, even if we're all just projections of a computer simulation, like The Matrix, the simulation itself has a structure that gives it a kind of reality, and it is ​our​ reality, the one we need to be able to navigate. (The philosopher Robert Nozick had a famous argument along these lines.)

Karl Friston

Institute of Neurology, University College London, Wellcome Principal Research Fellow and Scientific Director, Fellow of the Royal Society

First, you pose an extremely interesting question about how do we know we are hallucinating. Strictly speaking, one never has insight into a true hallucination, if one does, these are generally referred to as pseudo-hallucinations, which are not unrelated to illusions. The very distinction between illusions and hallucinations is itself fascinating. This is because it suggests we have the capacity to represent our own representations – or representational validity. This speaks to all sorts of deep philosophical issues for example, auto epistemic closure (in the sense of Thomas Metzinger), metacognition, self-awareness, lucid dreaming and so on.

The very fact that we can infer are perceptual influences are false speaks to a hierarchical composition of mind and perception in which not only do we have perceptual influences but also inferences about those inferences (CF metacognition). The implications for self awareness are clear. This is why people like Allan Hobson are so fascinated by lucid dreaming. This provides a wonderful test bed to compare situations in which dream reality is perceived as real and when one becomes aware of the fact that it is a dream. Neurobiologically, this seems to rest on frontal lobe activity, suggesting, again, a hierarchical aspect to our fantastic organ (i.e. the brain – that generates fantasies that are checked against reality).

The usual notion that perception is just hallucination grounded by sensations is somewhat subverted by the fact that we can, on occasions, know that our perceptual inference is false.

Metacognition is "cognition about cognition", "thinking about thinking", or "knowing about knowing". It comes from the root word "meta", meaning beyond.

Self-awareness is the capacity for introspection and the ability to recognize oneself as an individual separate from the environment and other individuals.

A lucid dream is any dream in which one is aware that one is dreaming.

Epistemic closure is a property of some belief systems. It is the principle that if a subject S knows p, and S knows that p entails q, then S can thereby come to know q. Most epistemological theories involve a closure principle and many skeptical arguments assume a closure principle.

Autoepistemic closure (Thomas Metzinge) is an epistemological, and not (at least not primarily) a phenomenological concept. It refers to an “inbuilt blind spot,” a structurally anchored deficit int he capacity to gain knowledge about oneself.

Rich Oglesby

There is a well known phrase: "We shape our tools and thereafter our tools shape us” (often associated with media theorist Marshall McLuhan, although it was actually a quote from Father John Culkin, a Professor of Communication at Fordham University in New York). This makes sense from an anthropological perspective - to put it crudely, whilst early humans evolved the ability to speak, the controlled sounds and utterances gained meaning to each other through localized consensus. Fast forward to the twentieth century and industrial nations, one can discover technologies that we can recognize their purpose yet have differences to our own, depending on our cultures and others - for example, the differences with electricial sockets or which side of the road you drive from one country to another. This was noted in William Gibson's book Pattern Recognition which he labelled 'mirror-world'. Technology alsocan become taken for granted and familiar over time unless we find ourselves taken out of our habituated situation - nothing so easily reminds ourselves of change as a power cut, taking us back a couple of centuries.

In the past twenty years or so in the industrial world, the biggest impact on our experiences has been from the field of computing. While many focus on the internet as the biggest game changer, it neglects developments and permutations which other computing tech has reached - how the computer monitor tech has crossed over into television displays, graphics cards have altered how we work with colours transforming Pantone, photography and printing, sound cards and music sequencers, mp3 and Flac. Personal computing technologies have radically changed the way we make, define and experience the world we exist in. To describe the last twenty years, the best term I can think of is the Recon-Naissance, combining the terms reconaissance (the practise of gathering, formulating or expressing information) and renaissance (both 'rebirth' and revival of interest), it is the widespread outcome of ideas and production of post-WW2 investment in computational and telecommunication technologies. The Renaissance Man polymath has been replaced with the Renaissance Machine - the personal computer. The same PC could be used by scientist or businessperson, coder or student, in the office or in the warehouse, in the studio or in the bedroom. This has been most advantageous to the modern creative.

With the development of the smartphone ten years ago, modern computing became pocketable. With it, computing components became smaller. Due to commercial popularity, upgrade cycles changed from a year and a half to just one. Information creation and reception became domesticated. It became mainstream and more conveniently portable. Music, photography and video could be captured and seen on the same device, replacing the personal media player and the portable camera. Life could be documented and experienced 'en plein Hertz'.

But the developments of the smartphone benefitted a once neglected but now up-and-coming field: Virtual Reality. With small displays and accelerometers now refined and cheaper, and gave the opportunity to start ups to produce a new experiences with a new computing medium. Initially produced to complement video games, other startups are producing other narratives, such as 360 documentaries, animations and first person tools for creativity and design. Whilst the consumer implementation of these ideas are not truly available yet, the technology is being used by scientists, architects, artists and gamers with current developer builds. It would appear how we engage and relate with information will change again - the Recon-Naissance is still going strong.


Perception As Controlled Hallucination

Perception itself is a kind of controlled hallucination. . . . [T]he sensory information here acts as feedback on your expectations. It allows you to often correct them and to refine them. But the heavy lifting seems to be being done by the expectations. Does that mean that perception is a controlled hallucination? I sometimes think it would be good to flip that and just think that hallucination is a kind of uncontrolled perception.

ANDY CLARK is professor of Cognitive Philosophy at the University of Sussex and author of Surfing Uncertainty: Prediction, Action, and the Embodied Mind. Andy Clark's Edge Bio Page

PERCEPTION AS CONTROLLED HALLUCINATION: PREDICTIVE PROCESSING AND THE NATURE OF CONSCIOUS EXPERIENCE

The big question that I keep asking myself at the moment is whether it's possible that predictive processing, the vision of the predictive mind I've been working on lately, is as good as it seems to be. It keeps me awake a little bit at night wondering whether anything could touch so many bases as this story seems to. It looks to me as if it provides a way of moving towards a third generation of artificial intelligence. I'll come back to that in a minute. It also looks to me as if it shows how the stuff that I've been interested in for so long, in terms of the extended mind and embodied cognition, can be both true and scientifically tractable, and how we can get something like a quantifiable grip on how neural processing weaves together with bodily processing weaves together with actions out there in the world. It also looks as if this might give us a grip on the nature of conscious experience. And if any theory were able to do all of those things, it would certainly be worth taking seriously. I lie awake wondering whether any theory could be so good as to be doing all these things at once, but that's what we'll be talking about.

A place to start that was fun to read and watch was the debate between Dan Dennett and Dave Chalmers about "Possible Minds" ("Is Superintelligence Impossible?" Edge, 4.10.19). That debate was structured around questions about superintelligence, the future of artificial intelligence, whether or not some of our devices or machines are going to outrun human intelligence and perhaps in either good or bad ways become alien intelligences that cohabit the earth with us. That debate hit on all kinds of important aspects of that space, but it seemed to leave out what looks to be the thing that predictive processing is most able to shed light on, which is the role of action in all of these unfoldings.

There's something rather passive about the kinds of artificial intelligence that Dan and Dave were both talking about. They were talking about intelligences or artificial intelligences that were trained on an objective function. The AI would try to do a particular thing for which they might be exposed to an awful lot of data in trying to come up with ways to do this thing. But at the same time, they didn't seem to inhabit bodies or inhabit worlds they were solutions to problems in a disembodied, disworlded space. The nature of intelligence looks very different when we think of it as a rolling process that is embedded in bodies or embedded in worlds. Processes like that give rise to real understandings of a structured world.

Something that I thought was perhaps missing from the debate was a full emphasis on the importance, first of all, of having a general-purpose objective function. Rather than setting out to be a good Go player or a good chess player, you might set out to do something like minimize expected prediction error in your embodied encounters with the world. That's my favorite general objective function. It turns out that an objective function like that can support perception and action and the kind of epistemic action in which we progressively try to get better training data, better information, to solve problems for the world that we inhabit.

Predictive processing starts off as a story about perception, and it's worth saying a few words about what it looks like in the perceptual domain before bringing it into the domain of action. In the perceptual domain, the idea, familiar I'm sure to everybody, is that our perceptual world is a construct that emerges at the intersection between sensory information and priors, which here act as top-down predictions about how the sensory information is likely to be. For example, I imagine that most people have experienced phantom phone vibrations, where you suddenly feel your phone is vibrating in your pocket. It turns out that it may not even be in your pocket. Even if it is in your pocket, maybe it's not vibrating. If you constantly carry the phone, and perhaps you're in a slightly anxious state, a heightened interoceptive state, then ordinary bodily noise can be interpreted as signifying the presence of a ringing phone.

It would work very much like, say, the hollow mask illusion: When people are shown a hollow face mask lit from behind, they see the concave side of the face as having a nose pointing outwards. Richard Gregory spoke about this many years ago. It's a standard story in this area. We human beings have very strong expectations about faces. We very much expect, given a certain bit of face information, that the rest of that information will specify a convex, outward-looking face.

The very same story gets to grips with phantom phone vibrations. It explains the White Christmas experiments, which is certainly one of my favorites in this area. People were told that they would hear the faint onset of Bing Crosby singing White Christmas in a sound file that they were going to be played. They would listen to the sound file and a substantial number of participants detected the faint onset of Bing Crosby singing White Christmas, but in fact there was no faint onset of White Christmas. There was no Bing Crosby signal there at all amongst what was simply white noise. In these cases, our expectations are carving out a signal that isn't there. But in other cases, perhaps someone speaks your name faintly and there's a noisy cocktail party going on, your expectations about what your name sounds like and the importance of anything that vaguely signals what your name sounds like conspire to up the weighting of the bits of the noisy signal that are there so that you hear your name fairly clearly.

Same thing if you're in the shower and a familiar song comes on the radio. Under those conditions, a familiar song sounds an awful lot clearer than an unfamiliar one. People might have thought that was a post-perceptual effect, as if you heard something fuzzy and then your memory filled in the details. But if the predictive processing stories are right, then that's the wrong way to think about it. This is just the same old story where top-down expectation meets incoming sensory signals with a balance that is determined by how confident you are in either the sensory signals or your top-down predictions.

The Bayesian brain, predictive processing, hierarchical predictive coding are all, roughly speaking, names for the same picture in which experience is constructed at the shifting borderline between sensory evidence and top-down prediction or expectation. There's been a big literature out there on the perceptual side of things. It's a fairly solid literature. What predictive processing did that I found particularly interesting—and this is mostly down to a move that was made by Karl Friston—was apply the same story to action. In action, what we're doing is making a certain set of predictions about the shape of the sensory information that would result if I were to perform the action. Then you get rid of prediction errors relative to that predicted flow by making the action.

There are two ways to get your predictions to be right in these stories. One is to have the right model of the world and the other is to change how the world is to fit the model that you have. Action is changing how the world is to fit the predictions, and perception is more like finding the predictions that make most sense of how the world is. But it turns out that they're operating using the same basic neural architecture. The wiring diagram for motor cortex and the wiring diagram for sensory cortex look surprisingly similar, and this story helps explain why. Indeed, the same basic canonical computations would be involved in both.

What's most interesting about predictive processing is the way it gives you a simultaneous handle on perception and action by showing they obey the same computational principles. It immediately invites you to think about having a model of the world that simultaneously drives how you experience and harvest information from the world. At that point, there's a standing invitation to stories like embodied cognition and the extended mind.

Once the predictive brain story is extended to the control of action in this very natural way, then there's a standing invitation to start thinking about how we weave worldly opportunities and bodily opportunities together with what brains are doing in a way that is going to make systematic sense of the extended mind story.

Before I go there, it's also worth saying a word or two about where the models that drive the predictions get to come from. Perceptual experience is the construct that lives on the border between sensory evidence and top-down prediction or expectation. That's what you're seeing in the White Christmas case and in the phantom phone vibration case. Just to see a structured world of objects around me means to know a lot about structured worlds of objects, and to bring those expectations to bear on the sensory signal. These are the stories that bring a structured world into view quite generally.

There are some rather nice cases that you can find online if you haven't already of so-called sine-wave speech cases, where speech gets stripped of some of its natural dynamics and what's left is a skeletal version of the speech. When you first hear it, it just sounds like a series of beeps and whistles, then when you hear the actual sound file and play that again, it sounds like a clear sentence being spoken because now you have the right top-down model, the right expectations. It's like hearing a familiar song when it's played in the shower on a bad radio receiver. It's a very striking effect and experience. It gives you a real sense of what is happening when a predictive brain gets to grips with the flow of sensory information.

Once you've played the real sentence, it might be something like, "The cat sat on the mat." So, you first hear beeps and whistles and you hear the sentence. Then you hear the beeps and whistles again, but this time through those beeps and whistles most people will clearly hear the sentence. After a while, you can become a native speaker of sine-wave speech so that you could be played a brand new one and you would hear the sentence through the noise. So maybe it will be useful to play some examples. Here we go.

[Audio samples. Begin listening at: 13:00]

I hope you've now had the experience of bringing a stream of somewhat unruly sensory information under an active predictive model and hearing how that can bring a structured world of words into view. The very same thing is happening in visual perception. It's the same effect that we were seeing in the White Christmas story, where your expectations are so strong that they make you think that there's a signal there when there isn't. But if predictive processing and stories of this kind are on track, then these are all exercises of the same constructive computational story. This is where human experience lives. As a philosopher, it sometimes interests me to wonder where this leaves the notion of veridical perception.

Perception itself is a kind of controlled hallucination. You experience a structured world because you expect a structured world, and the sensory information here acts as feedback on your expectations. It allows you to often correct them and to refine them. But the heavy lifting seems to be being done by the expectations. Does that mean that perception is a controlled hallucination? I sometimes think it would be good to flip that and just think that hallucination is a kind of uncontrolled perception.

The basic operating principle here is that you have a rich model of the world, a generative model, as it's known in this literature. What that means is a model that is not a discriminative model which just separates patterns out and says, "This is a cat and this is a dog," but rather a system that, using what it knows about the world, creates patterns that would be cat-like patterns or dog-like patterns in the sensoria. These systems learn to imagine how the sensory world would be, and in learning to imagine how the sensory world would be, they use that to do the classification and recognition work that otherwise would be done by an ordinary feed-forward discriminator. What that's doing is making perception and imagination and understanding come very close together. They're a cognitive package deal here, because if you perceive the world in this way, then you have the resources to create virtual sensory stuff like that from the top down.

Systems that can perceive the world like this can imagine the world, too, in a certain sense. That grip on the world seems to be very close to understanding the world. If I know how the sensory signal is going to behave at many different levels of abstraction and at many scales of space and time, so I can take the scene as it currently is and project it into the future and know what's going to happen if you hit the can and so on, that way of perceiving the world seems to me to be a way of understanding the world.

It will be very reasonable to ask where the knowledge comes from that drives the generative model in these cases. One of the cool things is that learning here proceeds in exactly the same way as perception itself. Moment by moment, a multilevel neural architecture is trying to predict the sensory flow. In order to do better at predicting the sensory flow, it needs to pull out regular structures within that flow at different time scales, so-called hidden causes or latent variables. Over time, with a powerful enough system, I might pull out things like tables and chairs and cats and dogs. You can learn to do that just by trying to predict the sensory flow itself.

A nice simple case of that will be something like learning the grammar of a language. If you knew the grammar of a language, that would be helpful in predicting what word is coming next. One way that you can learn the grammar of a language is to try again and again to predict what word is coming next. Pull out the latent variables and structure that is necessary to do that prediction task, and then you've acquired the model that you can use to do the prediction task in the future. These stories are a standing invitation to this bootstrapping where the prediction task that underlies perception and action itself installs the models that are used in the prediction task.

There's a pleasing symmetry there. Once you've got action on the table in these stories—the idea is that we bring action about by predicting sensory flows that are non actual and then getting rid of prediction errors relative to those sensory flows by bringing the action about—that means that epistemic action, as it's sometimes called, is right there on the table. Systems like that cannot just act in the world to fulfill their goals they can also act in the world so as to get better information to fulfill their goals. And that's something that active animals do all the time. The chicken, when it bobs its head around, is moving its sensors around to get information that allows it to do depth perception that it can't do unless it bobs its head around. When you go into a darkened room and you flip the light switch, you're performing a kind of epistemic action because your goal wasn't specifically to hit the light switch it was to do something in the room. But you perform this action that then improves your state of information so you can do the thing you need to do. Epistemic action, and practical action, and perception, and understanding are now all rolled together in this nice package.

It's interesting then to ask, if your models are playing such a big role in how you perceive and experience the world, what does it mean to perceive and experience the world as it is? Basically, what these stories do is ask you to think again about that question. Take the sine-wave speech example and ask yourself when you heard what was really there. Did you hear what was there when you heard it just as beeps and burps? Or did you hear what was there when you heard the sentence through the beeps and buzzes? I don't think there's a good answer to that question. If predictive processing is on track though, one thing we can say is that even to hear it as beeps buzzes is to bring some kind of model to bear, just one that didn't reach as deeply into the external causal structure as the one that actually does have words in it.

An upshot here is that there's no experience without the application of some model to try to sift what is worthwhile for a creature like you in the signal and what isn't worthwhile for a creature like you. And because that's what we're doing all the time, it's no wonder that certain things like placebo effects, medically unexplained symptoms, phantom phone vibrations, all begin to fall into place as expressions of the fundamental way that we're working when we construct perceptual experience. In the case of medically unexplained symptoms, for example, where people might have blindness or paralysis with no medically known cause, or more than that, very often the symptoms here will have a shape that in principle can't have a simple physiological cause.

A nice example is you might get someone with a blind spot in their field of vision. If you ask them what the width of that blind spot is when it is mapped close to the eye and when it's mapped far from the eye, some people will have what's called tubular visual field defect, which means they say it's the same wherever it's mapped. This is optically, physiologically impossible. It's pretty clear in cases like that that what's doing the work is something like belief expectation prediction. It's their model of what it would be like to have a visual field defect that is doing the work.

In this broad sense of beliefs, it doesn't mean beliefs that you necessarily hold as a person, but somehow they got in there somehow. These multilevel systems harbor all kinds of predictions and beliefs which the agent themselves might even disavow. Honest placebos do work. For example, if someone is told that this pill is an inert substance, you can nonetheless get symptomatic relief from those substances as long as they're presented by people in white coats with the right packaging—mid levels of expectation are engaged regardless of what you, the person sitting at the top, thinks. In the case of medically unexplained symptoms, it looks like they're the physiological version of the White Christmas effect. There are bodily signals there, and if your expectations about the shape of those signals are strong enough, then you can bring about the experiences that those expectations describe, just like White Christmas only done here in this somatosensory domain.

There's interesting work emerging not just on medically unexplained symptoms, but even medically explained symptoms. If people live with a medically explained problem for long enough, they can build up all kinds of expectations about the shape of their own symptomology, which share a lot in common with the medically unexplained cases. The same person with a chronic condition on different days and in different contexts will have different experiences even if the physiological state, the bedrock state, seems to be exactly the same.

There's a nice paper that came out recently by Van den Bergh and colleagues which was arguing that in the case of chronic effects, chronic pain, for example, an awful lot of ordinary symptomology has very much the character of the symptomology in the medically unexplained cases. So, it puts neuro-typical and less typical cases on a continuum and on par, which is quite interesting.

Acute cases are somewhat different because there you haven't built up those regimes of expectation, and there's a very straight signal being dealt with. Although, even there it seems as if your long-term model of the world makes a big difference as to how that signal plays out. There's a large area here where work on placebo effects, medically unexplained symptoms, autism, the effects of psychedelics, schizophrenia, all of these things are being thought about under this general framework. Maybe this'll be one of the test cases for whether we make progress using these tools with understanding the nature of human consciousness.

We had a visit from Robin Carhart-Harris, who works on psychedelics and is now working on predictive coding. There are some very interesting ideas coming out there, I thought. In particular, the idea that what serotonergic psychedelics do is relax the influence of top-down beliefs and top-down expectations so that sensory information can find new channels. If we think about this in the context of people with depression, maybe part of what goes on there is that we hold this structured world in view, in part by our expectations—and they're not just about the world, they're also about ourselves—and if you can relax some of those expectations and experience a way of encountering the world where you don't model yourself as a depressive person, for example, even a brief experience like that can apparently have long-term, lasting effects.

Some of the Bayesian brain and predictive processing folks are doing some pretty cool things, looking at the action of psychedelics and the effects of sensory deprivation. For any of these things, you can ask how would those different balances—held in place by this prediction meets sensory information construct—play out under different regimes of neurotransmitters, for example, or under different environmental regimes where you might have a stroboscopic light being flashed at you very rapidly. The University of Sussex has one of these, and it creates surprisingly intense sensations. If you were to sit in it for a couple of hours, you might get full dissociation. Even for a few minutes, you get experiences of colors of an intensity that I've never experienced before.

If you begin to ask what these stories have to say, if anything, about the nature of human consciousness, there are several things to say. The first is that the basic construction of experience is already illuminated just by thinking in terms of this mixture of top-down expectations and bottom-up sensory evidence and the way that mixture gets varied in different contexts and by different interventions. At the same time, there's a strong intuition some people have that consciousness is special and that whatever tools I was using to make progress with the White Christmas experiments and phantom phone vibrations are not getting to grips yet with what matters most about consciousness, which is how it feels, the redness of the sunset, the taste of the Tequila, and so on.

There's quite a lot to say about how that should pan out. In some ways, my view is an illusionist view. A large part of this debate over consciousness is misguided because there's nothing there. There's a multidimensional matrix of real things, and among those real things, there's a tendency to think there's another thing and that other thing isn't real. That's one way of thinking about it.

Among the real dimensions are the perceptual dimension that we've spoken about, the dimension of acting to engage our world. There's a lot of super interesting work on the role of interoceptive signals in all of this. Apart from the exteroceptive signals that we take in from vision, sound, and so on, and apart from the proprioceptive signals from our body that are what we predict in order to move our body around, there's also all of the interoceptive signals that are coming from the heart and from the viscera, et cetera.

One of the effects of the general predictive processing story is that all of this is just sensory evidence thrown in a big pot. How I perceive the external world to be can be constantly inflected by how I'm perceiving my internal world to be. You see this, for example, in experiments where people are given false cardiac feedback. They're made to think that their hearts are beating faster than they are. And under conditions like that, if they're exposed to a neutral face, they're more likely to judge that the face is anxious or fearful or angry. It looks as if what's going on is that our constant intouchness with signals from our own body, our brains are taking as just more information about how things are.

In that sense, there's a Jamesian flavor to some of the work on experience that comes out of predictive processing where the idea is that emotion, for example, is very much tied up with the role that interoception plays in giving us a grip on how things are in the world. William James famously said that the fear we feel when we see the bear has a lot to do with the experience of our own heart beating and our preparations to flee, all of that bodily stuff. If you took all that away, perhaps the feeling of fear would be bereft of its real substance.

There is something genuine in there that being subtly inflected by interoception information is part of what makes our conscious experience of the world the kind of experience that it is. So, artificial systems without interoception could perceive their world in an exteroceptive way, they could act in their world, but they would be lacking what seems to me to be one important dimension of what it is to be a conscious human being in the world.

We've got a number of real dimensions to consciousness. One of them is bringing a structured world into view in perception in part by structured expectations. The other one is an inflection of all of that by interoception. You can then ask questions about the temporal depth of the model that you're bringing to bear, and that seems like an important dimension, too. If your model has enough depth and temporal depth, then you can turn up in your own model of the world. Technically here I can reduce prediction error by projecting myself into the future and asking what certain things a creature like me—the way I can see myself to be—might do, would serve to reduce prediction error in the future. In that way, I turn up as a latent variable in my own model of the world. That seems important in human consciousness, at least. That's part of what makes us distinguishable selves with goals and projects that we can reflect on. That matrix is real. The thing that I don't think is real is qualia.

To understand that, we need to take a more illusionist stance. To do that would be to ask some version of what Dave Chalmers has lately called the meta hard puzzle or the meta hard question. That would be, what is it about systems like us that explains why we think that there are hard puzzles of consciousness, why we think that the conscious mind might be something very distinct from the rest of the physical order, why we think there are genuine questions to be asked about zombies. What Chalmers thinks is that any solution to the meta hard question, the question of why we think there's a hard question, why we say and do the things that express apparent puzzlement of this kind—those are easy questions in Dave's sense.

You can say something about how you would build a robot that might get puzzled or appear to be puzzled about its own experience in those ways.

You might think, well there's something very solid about all this perceptual stuff. I can be highly confident of it, and yet how the world really is could be very varied. If you're the sort of robot that can start to do those acrobatics, you're the sort of robot that might invent a hard problem, and might begin to think that there's more than a grain of truth in dualism.

One thing that we might like to do is try to take an illusionist stance to just that particular bit of the hard problem while being realist about all the other stuff, thinking that there's something to say about the role of the body, something to say about what it takes to bring a structured world into view. Do all of that stuff and then also solve the meta hard puzzle, and you've solved all there is to solve. Whereas Dave Chalmers, I'm sure, will say, at that point, you showing us how to build a robot that will fool us into thinking that it's conscious, in certain sense it might even fool itself into thinking that it's conscious, but it wouldn't really because maybe it wouldn't have any experiences at all when it's doing all that stuff.

Dan Dennett's take on consciousness is a perfect fit with a predictive processing take on consciousness. For many years, Dan has argued that there's something illusory here, some self-spun narrative illusion. Predictive processing perhaps gives us a little bit more of the mechanism that might support the emergence of an illusion like that. Dan himself has written some interesting stuff on the way that predicting our own embodied responses to things might lead us down the track of thinking that qualia are fundamental special goings on inside us. I might predict some of my own ooing and awing responses to the cute baby, and when I find myself in the presence of the cute baby, I make those responses and I think that cuteness is a real genuine property of some things in the world.

What Dan has argued there is that maybe we get puzzled because we're fooled by our own Bayesianism here. This model of how things are gets to grips with how we're going to respond, and we then reify something within that nexus as these intervening qualia. But you don't need the weird intervening qualia you just have responses that come about in certain circumstances. There's a rather natural fit between Dan's approach and these approaches, and they're both a kind of illusionism where we're both saying whatever consciousness really is, it can't be what Dave Chalmers thinks it is.


Childhood hallucinations are surprisingly common – but why?

C hildhood has long been championed as a time for make-believe, but recent research has found that another form of unreality – hallucinations – is more common in children than we previously imagined. For years, kids’ accounts of seeing, hearing and experiencing things that weren’t really there were considered to be part of the same invented world – an “overactive imagination” a “fantasy world”. The Alice in Wonderland approach, perhaps. But as it was recognised that hallucinations can be reliably identified in children, science has begun to look at why these illusory experiences are many times more common during our early years.

Hallucinations often reflect a bizarre, blurry version of our realities and because play is an everyday reality for children, the content can seem similar. Both can contain quirky characters, strange scenarios and inspire curious behaviour. One child described how he saw a wolf in the house, another that he had “Yahoos” living inside him that ate all his medicine. On the surface, these could just as easily be a child’s whimsy, but genuine hallucinations have a very different flavour. “In play and make-believe, children are imagining,” says Elena Garralda, a professor of child and adolescent psychiatry at Imperial College London. “They do not have the actual perceptual experience of seeing and hearing.” Another key difference, notes Garralda, is that “hallucinations feel imposed and children cannot exercise a direct control over them”.

Recent studies have thrown up some surprising statistics about how common they are. One UK study found that almost two-thirds of children reported having at least one “psychotic-like experience” in their lives, a category that also includes unshiftable and unrealistic beliefs and fears. When focusing purely on hallucinations, a review of research found that 17% of 9-12-year-olds have these experiences at any one time. The number roughly halves in teenagers and drops again in adults. Since this type of research tends to focus on experiences that are selected because they can appear in mental health problems in adults, such as hearing voices, which are only a small part of the possible range of hallucinations, these figures are likely to be a low estimate.

It is interesting that hallucinations become less common as we move towards adulthood. Because very young children are more difficult to test and haven’t been studied as widely, it’s not clear whether we start out in a more hallucinatory world, which becomes increasingly stable as we age, or whether middle childhood is a peak time for unreal experiences. For all its reputation for causing emotional mayhem, puberty might be a stabilising force on our perceptions.

At this point, let’s just take a breather – a sanity check if you will – because a lot of people get worried when they think about the possibility of their child hallucinating. These figures don’t mean that if a child is having a hallucination that they are ill or unwell. In the majority of cases, children’s hallucinations disappear within a few days or weeks and are not a cause for concern. Childhood hallucinations are often sparked by life stresses, poor sleep and periods of low mood that fade when the difficult situations do. If the hallucinations are upsetting or persistent, however, it may be time to ask for a professional assessment.

Renaud Jardri has seen many children with hallucinations in his clinical practice and also researches the area as part of his role as a professor of child and adolescent psychiatry at the Lille University school of medicine. The criteria, he says, for judging whether a child needs professional support are whether the hallucinations are “frequent, complex, distressing and cause impairment”. For Jardri, hallucinations that are associated with positive emotions and don’t interfere with the child’s friendships and family life are usually benign.

In rare cases, medical problems can be the cause. Epilepsy can cause hallucinations, as can sleep disorders that affect consciousness and lead to the dream world invading the waking hours. For reasons that aren’t entirely clear, severe psychosis, represented by diagnoses such as schizophrenia, is extremely rare in young children. But when such conditions occur, the hallucinations can be both striking and terrifying. The six-year-old who described having Yahoos living inside him was one of these rare cases he heard them constantly speak to him, feared being poisoned, believed he could cast spells, smelt “bugs” in the tap water and saw nonexistent trails in the snow. This is a far from the common fear of “monsters under the bed” or isolated hallucinations that fade over time.

Then there are imaginary friends that are not hallucinations but vivid fantasies, which have been the subject of much adult hand-wringing over the years. Because of this, they have been surprisingly well researched and I am delighted to live in a world where there are genuine imaginary-friend scientists, as if Roald Dahl were alive and funding a research institute. It turns out that children with make-believe companions tend to have better social skills and more developed language abilities than kids who lack imaginary buddies. And neither, the research shows, are these illusory companions a compensation for a lack of real friends. They seem to reflect the child’s brain running in overdrive, expending excess energy, delighting in the limits of imagination and playing with the possibilities of the social world.


10 ways to respond when someone is experiencing dementia hallucinations

1. Determine if a response is needed
The first step is to determine whether the hallucination is bothering your older adult.

If it’s pleasant, you might not want to respond or call attention to it.

Just know and accept that it’s a dementia symptom and thankfully isn’t causing distress.

If the hallucination is upsetting them or causing them to do something unsafe, then it’s time to quickly step in to provide comfort or redirect to a safe activity.

2. Stay calm and don’t argue or try to convince using logic
When someone is having a dementia hallucination, it’s important to stay calm and avoid contradicting them.

What they’re seeing is a dementia symptom and is very real to them.

Trying to explain that it isn’t real simply won’t work because of the damage that dementia has caused in their brain.

In fact, knowing that you don’t believe them might make them even more upset and agitated.

If they’re calm enough to explain, it may also help to understand what they’re seeing. Listen carefully and try to pick up clues to what they’re seeing.

But keep in mind that dementia damage in the brain may affect their ability to use the correct words. For example, they could unintentionally say cabbages when they mean green cushions.

3. Validate their feelings and provide reassurance
Be careful not to dismiss your older adult’s experience.

Brushing off what they’re seeing by saying something like, “Don’t be silly, there’s nothing there,” is likely to upset them.

It helps to allow them to talk about what they’re seeing. Having you take them seriously and provide reassurance increases their feeling of safety and security.

Focus on being kind and responding to their feelings rather than to the hallucination itself.

If they’re scared, you could say “That sounds scary, I can see how upset you are.”

Or if they’re happy, you might say, “How wonderful, I’m glad that makes you so happy.”

Other possible responses could be, “It sounds like you’re worried,” or “I know this is scary for you.”

You don’t need to pretend that you can see or hear what they can, just be supportive and do what you can to relieve any fear or anxiety as if it was a real threat.

For example, you could say ”I don’t hear or see anyone outside the window, but you seem worried. What can I do to help you feel safe?“

4. Check the environment and remove possible triggers
Oftentimes, dementia hallucinations can be triggered by things going on around your older adult.

Their dementia brain can interpret sights and sounds differently, causing hallucinations.

To remove possible triggers, check their environment for background noise or visual stimulation that could cause a problem.

For example, things like a TV or radio could make them believe that strangers are in the house, what’s happening on TV is real, or that they’re hearing voices.

Dim lighting could make shadowy corners a source of fear.

Reflections in shiny floors or windows when it’s dark outside and bright inside could make it seem like there are people in the house.

Similarly, mirrors can be another source of fear or confusion.

5. Offer simple answers and reassurances
When someone is having a dementia hallucination, don’t give long explanations about what’s happening. Trying to process what you’re saying may add to their distress.

Instead, respond in a calm, supportive way.

You could say something like, “Don’t worry. I’m here to protect you. I’ll make sure you’re safe.”

Gently hugging them or patting their arm or shoulder may also provide the comfort and reassurance they need if they’re scared or stressed.

Connecting with you may also be a welcome distraction from the hallucination.

6. Look for patterns
If hallucinations happen frequently, there could be a trigger that’s not obvious.

One way to figure out what could be causing the behavior is to track activities and try to find a pattern .

Taking notes or keeping a dementia journal may help you discover that certain hallucinations happen at a certain time of day, before or after meals, or is related to a physical need like using the bathroom or being in pain.

Or, it could be something as simple as a change in daily routine that’s making them feel confused or disoriented and causing hallucinations.

Keeping a log or taking notes helps you look for solutions and ways to avoid the situations that may be triggering hallucinations.

7. Distract and redirect
Another effective technique is to distract your older adult from their hallucination.

Try to switch their focus to an activity they enjoy.

You could ask them to help you with a chore that makes them feel successful , look at favorite family photos, sing their favorite song , do a fun puzzle , eat a tasty snack, or take a pleasant stroll to look at the view – even an indoor stroll would work.

Another way to distract is to direct their attention to you instead of the hallucination.

If they’re hearing voices, try chatting with them. It’s harder to hear those voices if you’re now having a conversation with them.

Or if they’re seeing someone or something, get to eye level and try to make eye contact with them. If they’re occupied with looking at you, it could make the hallucination less intense or even fade away.

8. Get support to help you cope
Caring for someone with dementia hallucinations is stressful. So it can be a big help to know that you’re not alone in dealing with issues like this.

That’s why caregiver support groups highly recommended.

Sharing your experience and getting advice and tips from others can make life easier.

There are also many great online groups that are free and private, here are 11 that we recommend .

9. Talk with the doctor to find out if there are medical causes
You may want to speak with your older adult’s doctor to find out if there could be a medical reason behind their hallucination.

This wouldn’t change the way you respond, but may help you find ways to to reduce or eliminate the behavior.

For example, some medical issues that can cause hallucinations include dehydration , urinary tract infections , kidney or bladder infections, head injuries from a fall , or pain .

Or if your older adult recently started a new medication, it could be a negative side effect of the drug or an interaction with another medication. Immediately report any changes in their behavior to the doctor.

And if your older adult is having trouble with hearing or vision, that could easily explain them hearing or seeing things that aren’t there.

10. Contact the doctor immediately if their safety or yours is at risk
If your older adult is severely distressed by hallucinations or if hallucinations cause them to hurt themselves or others, contact their doctor immediately to get help.

For example, they may be hitting out to try to defend themselves against a perceived attacker, run away from something that scares them, or something else dangerous.

These types of actions can easily lead to injury to them and you.

When you speak with their doctor, describe the symptoms, how often they happen, and if they’ve changed in intensity or frequency over time.

It helps if you’ve kept a log or notes that could help the doctor get a clearer picture of what’s happening.

If non-drug approaches aren’t working and there isn’t a medical condition that’s causing hallucinations, careful use of behavioral medication could improve the quality of life by reducing the intensity and frequency of hallucinations.


Paranoia

In a person with Alzheimer’s disease, paranoia often is linked to memory loss. It can become worse as memory loss gets worse. For example, the person may become paranoid if he or she forgets:

  • Where he or she put something. The person may believe that someone is taking his or her things.
  • That you are the person’s caregiver. Someone with Alzheimer’s might not trust you if he or she thinks you are a stranger.
  • People to whom the person has been introduced. He or she may believe that strangers will be harmful.
  • Directions you just gave. The person may think you are trying to trick him or her.

Paranoia may be the person’s way of expressing loss. The person may blame or accuse others because no other explanation seems to make sense.

Here are some tips for coping with paranoia:

  • Try not to react if the person blames you for something.
  • Don’t argue with the person.
  • Let the person know that he or she is safe.
  • Use gentle touching or hugging to show you care.
  • Explain to others that the person is acting this way because he or she has Alzheimer’s disease.
  • Search for things to distract the person, then talk about what you found. For example, talk about a photograph or keepsake.

Also, keep in mind that someone with Alzheimer’s disease may have a good reason for acting a certain way. He or she may not be paranoid. There are people who take advantage of weak and elderly people. Find out if someone is trying to abuse or steal from the person with Alzheimer’s. For more information, visit Elder Abuse.


Hallucinations in children with a fever

Hallucinations can sometimes occur in children who are ill with a fever. Call your GP if your child is unwell with a body temperature of 38C or above and you think they're hallucinating.

In the meantime, stay calm, keep your child cool and reassure them. Encourage them to drink plenty of fluids and give them paracetamol or ibuprofen (always read the patient information leaflet to find out the correct dose and frequency for your child's age, and check they're not allergic to medicines you give). The hallucinations should pass after a few minutes.


My nocturnal flying geometric manifestations

As a child, multicolored geometric shapes would regularly swoop across my bedroom, just as I was drifting off to sleep. I remember simultaneously marveling at them and wishing them away so I could sleep in peace.

It doesn’t happen so often nowadays, but once in a while, I’m still prone to my mathematical manifestations.

Perhaps I should be grateful for my geeky hallucinations – especially compared to the disturbing experiences some people have.


Hallucinations: What explains these tricks of the mind?

Hallucinations are sensory perceptions that appear in the absence of stimuli. Although they are often associated with illnesses such as schizophrenia, these phenomena can occur in the absence of mental ill health. But what explains these uncanny occurrences?

Share on Pinterest Not all hallucinations occur due to psychotic disorders or psychedelic drugs.

There are many types of hallucinations. They can be visual (sight hallucinations), auditory (sound hallucinations), olfactory (smell hallucinations), gustatory (taste hallucinations), or tactile (touch hallucinations).

For every sense, a form of hallucination is possible. The reason for this is that these phenomena are “ghost sensations” — perceptions of stimuli in the absence of actual external stimuli.

More often than not, when people think of hallucinations, they associate them with illnesses that can involve psychosis, such as schizophrenia, or neurocognitive disorders, such as forms of dementia.

Hallucinations are also associated with the use of mind-altering drugs, including LSD and DMT.

But hallucinations are not always due to psychotic disorders or psychedelics. Sometimes, they occur in the absence of these factors.

So what explains hallucinations, how common are they, and what role might they teach us about ourselves? In this Special Feature, we investigate.

A study that appeared in the British Journal of Psychiatry in 2017 suggests that hallucinations are far more common among people without psychotic disorders than scientists had previously thought.

The study authors — Dr. Ian Kelleher, from the Royal College of Surgeons in Ireland, and Jordan DeVylder, Ph.D., from the University of Maryland in Baltimore — analyzed data that they had obtained through the 2007 Adult Psychiatric Morbidity Survey, a nationally representative study of mental health in England.

These data included information on the mental health of 7,403 people aged 16 years and older throughout 1 year.

Dr. Kelleher and DeVylder found that visual and auditory hallucinations were almost equally prevalent among participants with borderline personality disorder and those with a nonpsychotic mental illness.

They also found that more than 4% of all the survey respondents — including those who had no diagnosed mental health issues — reported experiencing visual or auditory hallucinations.

“Hallucinations are more common than people realize. They can be frightening experiences, and few people openly talk about it,” Dr. Kelleher observed in an interview for International Business Times.

“Our research is valuable because it can show them they are not alone and that having these symptoms is not necessarily associated with having a mental health disorder. It breaks the taboo,” he added.

Research published in the Scandinavian Journal of Psychology in 2015 also showed that auditory hallucinations were surprisingly common in a representative sample of the general population in Norway.

In a cohort of 2,533 individuals, the “current lifetime prevalence of [auditory verbal hallucinations] was 7.3%.”

Phantom smells also seem to be a more common occurrence than people might think. In 2018, a study published in JAMA Otolaryngology — Head & Neck Surgery showed that 6.5% of people aged 40 years and over had experienced phantom odor perception.

This percentage is based on data from a cohort of 7,417 participants with a mean age of 58 years. However, when it came to explaining this phenomenon, the researchers were at a loss.

“The causes of phantom odor perception are not understood. The condition could be related to overactive odor-sensing cells in the nasal cavity or perhaps a malfunction in the part of the brain that understands odor signals,” says first author Kathleen Bainbridge, Ph.D., from the National Institute on Deafness and Other Communication Disorders at the National Institutes of Health (NIH) in Bethesda, MD.

Researchers have been trying to build a better understanding of the biological mechanisms behind the different types of hallucinations.

In 2019, investigators from the University of Oregon in Eugene conducted a study in mice to try to find out how hallucinations manifest in the brain.

Their study paper — published in Cell Reports — revealed some surprising findings. As the researchers induced visual hallucinations in the rodents by injecting them with a hallucinogenic substance, they saw that this did not “map out” how they expected it to in the brain.

The researchers observed that the hallucinating mice experienced fewer signals firing between the neurons of the visual cortex, the brain region associated with the interpretation of visual information.

“You might expect visual hallucinations would result from neurons in the brain firing like crazy or by mismatched signals. We were surprised to find that a hallucinogenic drug instead led to a reduction of activity in the visual cortex.”

– Senior author Prof. Cris Niell

Despite the initial surprise, the study’s senior author notes that it makes sense that visual hallucinations should appear in the brain in this way.

“Understanding what’s happening in the world is a balance of taking in information and your interpretation of that information. If you’re putting less weight on what’s going on around you but then overinterpreting it, that could lead to hallucinations,” he explains.

There is also a large amount of research into the causes and mechanisms associated with auditory hallucinations — some of the most fascinating kinds of ghost sensations.

For instance, a 2017 study featuring in eLife goes some way toward explaining how and why some people may think they hear voices that are actually not there.

According to the authors, this may be the effect of an error or “short-circuit” in processing internal speech and differentiating it from speaking out loud.

When people speak out loud, the researchers explain, the brain does two things. First, it sends instructions to the vocal cords, tongue, and lips — our vocal apparatus — to prompt them to move in the right way so as to vocalize the correct sounds.

At the same time, the brain also makes an internal copy of these instructions, which scientists call the “efference copy.” This duplicate allows the brain regions associated with hearing to predict the sounds that the vocal apparatus is about to make correctly.

This is part of how we recognize our own voice and speech.

“The efference copy dampens the brain’s response to self-generated vocalizations, giving less mental resources to these sounds because they are so predictable,” explains first author Prof. Thomas Whitford.

Through their study, Prof. Whitford and his colleagues found that the brain makes an efference copy not just of the thoughts intended for vocalization, but also of the inner talk — the internal chatter that people process in the background without expressing out loud.

It is likely, the scientists hypothesize, that auditory hallucinations arise when something goes wrong with the efference copy of people’s internal monologues.

“We all hear voices in our heads. Perhaps the problem arises when our brain is unable to tell that we are the ones producing them.”

– Prof. Thomas Whitford

Many of the hallucinations that people might experience are, in fact, of the run-of-the-mill variety. For example, a vague sensation that you have heard the phone ringing when no one actually called or catching the shadow of a silhouette from the corner of the eye when there is no one there.

Dr. Philip Corlett and Dr. Albert Powers, two scientists from Yale University and the Connecticut Mental Health Center in New Haven, CT, have conducted many experiments into the nature of hallucinations.

They argue that there is a spectrum of phantom sensations and that many hallucinations might actually reflect how our brains work — namely, by making predictions about stimuli and the environments that we navigate.

In a paper published in World Psychiatry in 2018, they write that hallucinations might emerge from a “mismatch” between the predictions that our brains make about our surrounding reality and the actual reality.

In another study paper, published in 2017 in the journal Science, the two researchers and Dr. Christoph Mathys, an associate professor at Aarhus University in Denmark, present the evidence they collected through a series of experiments that appear to support this notion.

“When we go about the world, we’re not just passively perceiving sensory inputs through our eyes and ears. We actually build a model in our minds of what we expect to be present,” Dr. Corlett told The Atlantic, noting that when those expectations do not come to pass, this may sometimes translate as a hallucination.

Other scientists note that even more pronounced hallucinations could sometimes be of benefit to the person experiencing them.

In a 2017 paper in Psychology and Psychotherapy: Theory, Research and Practice , Drs. Filippo Varese, Warren Mansell, and Sara Tai — from the University of Manchester in the United Kingdom — note that not all auditory hallucinations are distressing.

Their study, which enlisted people with mental health issues who also experienced auditory hallucinations, found that how people reacted to the voices that they thought they heard made an important difference to whether those voices hindered or encouraged them in their daily pursuits.

“Most voice-hearers with mental health difficulties in our study experienced their voices as a hindrance to achieving their goals and viewed their voices as distressing and problematic. But other voice-hearers find that voices facilitate their valued goals and are, therefore, a pleasant and constructive part of their lives,” Dr. Varese points out.

Going forward, the researchers say, such insights might help mental health professionals assist their clients in turning the tables on potentially distressing hallucinations.

“[…] we should seek to help clients explore how their voices relate to goals that are important to them and empower them to progress toward those goals. That would be a more meaningful and acceptable way of supporting them,” he suggests.


My nocturnal flying geometric manifestations

As a child, multicolored geometric shapes would regularly swoop across my bedroom, just as I was drifting off to sleep. I remember simultaneously marveling at them and wishing them away so I could sleep in peace.

It doesn’t happen so often nowadays, but once in a while, I’m still prone to my mathematical manifestations.

Perhaps I should be grateful for my geeky hallucinations – especially compared to the disturbing experiences some people have.


Paranoia

In a person with Alzheimer’s disease, paranoia often is linked to memory loss. It can become worse as memory loss gets worse. For example, the person may become paranoid if he or she forgets:

  • Where he or she put something. The person may believe that someone is taking his or her things.
  • That you are the person’s caregiver. Someone with Alzheimer’s might not trust you if he or she thinks you are a stranger.
  • People to whom the person has been introduced. He or she may believe that strangers will be harmful.
  • Directions you just gave. The person may think you are trying to trick him or her.

Paranoia may be the person’s way of expressing loss. The person may blame or accuse others because no other explanation seems to make sense.

Here are some tips for coping with paranoia:

  • Try not to react if the person blames you for something.
  • Don’t argue with the person.
  • Let the person know that he or she is safe.
  • Use gentle touching or hugging to show you care.
  • Explain to others that the person is acting this way because he or she has Alzheimer’s disease.
  • Search for things to distract the person, then talk about what you found. For example, talk about a photograph or keepsake.

Also, keep in mind that someone with Alzheimer’s disease may have a good reason for acting a certain way. He or she may not be paranoid. There are people who take advantage of weak and elderly people. Find out if someone is trying to abuse or steal from the person with Alzheimer’s. For more information, visit Elder Abuse.


10 ways to respond when someone is experiencing dementia hallucinations

1. Determine if a response is needed
The first step is to determine whether the hallucination is bothering your older adult.

If it’s pleasant, you might not want to respond or call attention to it.

Just know and accept that it’s a dementia symptom and thankfully isn’t causing distress.

If the hallucination is upsetting them or causing them to do something unsafe, then it’s time to quickly step in to provide comfort or redirect to a safe activity.

2. Stay calm and don’t argue or try to convince using logic
When someone is having a dementia hallucination, it’s important to stay calm and avoid contradicting them.

What they’re seeing is a dementia symptom and is very real to them.

Trying to explain that it isn’t real simply won’t work because of the damage that dementia has caused in their brain.

In fact, knowing that you don’t believe them might make them even more upset and agitated.

If they’re calm enough to explain, it may also help to understand what they’re seeing. Listen carefully and try to pick up clues to what they’re seeing.

But keep in mind that dementia damage in the brain may affect their ability to use the correct words. For example, they could unintentionally say cabbages when they mean green cushions.

3. Validate their feelings and provide reassurance
Be careful not to dismiss your older adult’s experience.

Brushing off what they’re seeing by saying something like, “Don’t be silly, there’s nothing there,” is likely to upset them.

It helps to allow them to talk about what they’re seeing. Having you take them seriously and provide reassurance increases their feeling of safety and security.

Focus on being kind and responding to their feelings rather than to the hallucination itself.

If they’re scared, you could say “That sounds scary, I can see how upset you are.”

Or if they’re happy, you might say, “How wonderful, I’m glad that makes you so happy.”

Other possible responses could be, “It sounds like you’re worried,” or “I know this is scary for you.”

You don’t need to pretend that you can see or hear what they can, just be supportive and do what you can to relieve any fear or anxiety as if it was a real threat.

For example, you could say ”I don’t hear or see anyone outside the window, but you seem worried. What can I do to help you feel safe?“

4. Check the environment and remove possible triggers
Oftentimes, dementia hallucinations can be triggered by things going on around your older adult.

Their dementia brain can interpret sights and sounds differently, causing hallucinations.

To remove possible triggers, check their environment for background noise or visual stimulation that could cause a problem.

For example, things like a TV or radio could make them believe that strangers are in the house, what’s happening on TV is real, or that they’re hearing voices.

Dim lighting could make shadowy corners a source of fear.

Reflections in shiny floors or windows when it’s dark outside and bright inside could make it seem like there are people in the house.

Similarly, mirrors can be another source of fear or confusion.

5. Offer simple answers and reassurances
When someone is having a dementia hallucination, don’t give long explanations about what’s happening. Trying to process what you’re saying may add to their distress.

Instead, respond in a calm, supportive way.

You could say something like, “Don’t worry. I’m here to protect you. I’ll make sure you’re safe.”

Gently hugging them or patting their arm or shoulder may also provide the comfort and reassurance they need if they’re scared or stressed.

Connecting with you may also be a welcome distraction from the hallucination.

6. Look for patterns
If hallucinations happen frequently, there could be a trigger that’s not obvious.

One way to figure out what could be causing the behavior is to track activities and try to find a pattern .

Taking notes or keeping a dementia journal may help you discover that certain hallucinations happen at a certain time of day, before or after meals, or is related to a physical need like using the bathroom or being in pain.

Or, it could be something as simple as a change in daily routine that’s making them feel confused or disoriented and causing hallucinations.

Keeping a log or taking notes helps you look for solutions and ways to avoid the situations that may be triggering hallucinations.

7. Distract and redirect
Another effective technique is to distract your older adult from their hallucination.

Try to switch their focus to an activity they enjoy.

You could ask them to help you with a chore that makes them feel successful , look at favorite family photos, sing their favorite song , do a fun puzzle , eat a tasty snack, or take a pleasant stroll to look at the view – even an indoor stroll would work.

Another way to distract is to direct their attention to you instead of the hallucination.

If they’re hearing voices, try chatting with them. It’s harder to hear those voices if you’re now having a conversation with them.

Or if they’re seeing someone or something, get to eye level and try to make eye contact with them. If they’re occupied with looking at you, it could make the hallucination less intense or even fade away.

8. Get support to help you cope
Caring for someone with dementia hallucinations is stressful. So it can be a big help to know that you’re not alone in dealing with issues like this.

That’s why caregiver support groups highly recommended.

Sharing your experience and getting advice and tips from others can make life easier.

There are also many great online groups that are free and private, here are 11 that we recommend .

9. Talk with the doctor to find out if there are medical causes
You may want to speak with your older adult’s doctor to find out if there could be a medical reason behind their hallucination.

This wouldn’t change the way you respond, but may help you find ways to to reduce or eliminate the behavior.

For example, some medical issues that can cause hallucinations include dehydration , urinary tract infections , kidney or bladder infections, head injuries from a fall , or pain .

Or if your older adult recently started a new medication, it could be a negative side effect of the drug or an interaction with another medication. Immediately report any changes in their behavior to the doctor.

And if your older adult is having trouble with hearing or vision, that could easily explain them hearing or seeing things that aren’t there.

10. Contact the doctor immediately if their safety or yours is at risk
If your older adult is severely distressed by hallucinations or if hallucinations cause them to hurt themselves or others, contact their doctor immediately to get help.

For example, they may be hitting out to try to defend themselves against a perceived attacker, run away from something that scares them, or something else dangerous.

These types of actions can easily lead to injury to them and you.

When you speak with their doctor, describe the symptoms, how often they happen, and if they’ve changed in intensity or frequency over time.

It helps if you’ve kept a log or notes that could help the doctor get a clearer picture of what’s happening.

If non-drug approaches aren’t working and there isn’t a medical condition that’s causing hallucinations, careful use of behavioral medication could improve the quality of life by reducing the intensity and frequency of hallucinations.


Hallucinations in children with a fever

Hallucinations can sometimes occur in children who are ill with a fever. Call your GP if your child is unwell with a body temperature of 38C or above and you think they're hallucinating.

In the meantime, stay calm, keep your child cool and reassure them. Encourage them to drink plenty of fluids and give them paracetamol or ibuprofen (always read the patient information leaflet to find out the correct dose and frequency for your child's age, and check they're not allergic to medicines you give). The hallucinations should pass after a few minutes.


Is the world real, or is it just an illusion or hallucination?

Is this real life? How do we know that we are not hallucinating it all? What if we're plugged into a Matrix-style virtual reality simulator? Isn't the universe a giant hologram anyway? Is reality really real? What is reality?

We asked renowned neuroscientists, physicists, psychologists, technology theorists and hallucinogen researchers if we can ever tell whether the "reality" we are experiencing is "real" or not. Don't worry. You're going to be ok.

Jessica L. Nielson, Ph.D.

Department of Neurosurgery, Postdoctoral Scholar, University of California, San Francisco (UCSF), Brain and Spinal Injury Center (BASIC)

What is our metric for determining what is real? That is probably different for each person. One could try and find a consensus state that most people would agree is "real" or a "hallucination" but from the recent literature using imaging techniques in people who are having a hallucinatory experience on psychedelics, it seems the brain is hyper-connected and perhaps just letting in more of the perceivable spectrum of reality.

When it comes to psychosis, things like auditory hallucinations can seem very real. Ultimately, our experiences are an interpretation of a set of electrical signals in our brains. We do the best to condense all those signals into what we perceive to be the world around us (and within us), but who is to say that the auditory hallucinations that schizophrenics experience, or the amazing visual landscapes seen on psychedelics are not some kind of bleed through between different forms of reality? I don't think there is enough data to either confirm or deny whether what those people are experiencing is "real" or not.

Sean Carroll

Cosmologist and Physics professor specializing in dark energy and general relativity, research professor in the Department of Physics at the California Institute of Technology

How do we know this is real life? The short answer is: we don't. We can never prove that we're not all hallucinating, or simply living in a computer simulation. But that doesn't mean that we believe that we are.

There are two aspects to the question. The first is, "How do we know that the stuff we see around us is the real stuff of which the universe is made?" That's the worry about the holographic principle, for example -- maybe the three-dimensional space we seem to live in is actually a projection of some underlying two-dimensional reality.

The answer to that is that the world we see with our senses is certainly not the "fundamental" world, whatever that is. In quantum mechanics, for example, we describe the world using wave functions, not objects and forces and spacetime. The world we see emerges out of some underlying description that might look completely different.

The good news is: that's okay. It doesn't mean that the world we see is an "illusion," any more than the air around us becomes an illusion when we first realize that it's made of atoms and molecules. Just because there is an underlying reality doesn't disqualify the immediate reality from being "real." In that sense, it just doesn't matter whether the world is, for example, a hologram our evident world is still just as real.

The other aspect is, "How do we know we're not being completely fooled?" In other words, forgetting about whether there is a deeper level of reality, how do we know whether the world we see represents reality at all? How do we know, for example, that our memories of the past are accurate? Maybe we are just brains living in vats, or maybe the whole universe was created last Thursday.

We can never rule out such scenarios on the basis of experimental science. They are conceivably true! But so what? Believing in them doesn't help us understand any features of our universe, and puts us in a position where we have no right to rely on anything that we did think is true. There is, in short, no actual evidence for any of these hyper-skeptical scenarios. In that case, there's not too much reason to worry about them.

The smart thing to do is to take reality as basically real, and work hard to develop the best scientific theories we can muster in order to describe it.

Holographic Principle

“If you asked anyone twenty years ago how many dimensions our world has, most of us would answer 'three spatial dimensions plus time.' The holographic principle would mean that this is actually a matter of perspective.” Craig Hogan to Motherboard

The theory suggests that the entire universe can be seen as a two dimensional information on the cosmological horizon, such that the three dimensions we observe are an effective description only at macroscopic scales and at low energies.

The Nature of Space and Time: Is Space Digital?

Space may not be smooth and continuous. Instead it may be digital, composed of tiny bits. Physicists have assumed that these bits are far too small to measure with current technology. Yet one scientist. Scientific American, 2014

Fredrick Barrett

Instructor in Psychiatry and Behavioral Sciences, Behavioral Pharmacology Research Unit, Johns Hopkins School of Medicine

With psychedelics or "classical (serotonergic) hallucinogens", individuals can often distinguish between perceptual disturbances, visualized experiences (it feels as if I was in another place, or I had traveled to another time, but I realized my physical body was still "here"), and whatever is happening "outside" in the "real" world. However, in psychosis (for instance, in the midst of a psychotic break in a person who has schizophrenia), hallucinations are quite clearly defined as something that an individual believes is real, persistent, and seemingly independent and autonomous in the world.

The "hallucinations" of schizophrenia and psychosis are accepted as real, and individuals with schizophrenia often do not have insight into the nature of their hallucinations as being "not real" to the rest of us. This highlights a bit of a misnomer in the name of the drug class "hallucinogens", in that the experiences with these compounds are not taken as consensual reality in the same way that psychotic hallucinations are taken as "real".

How or Why can we tell the difference between reality and what is perceived during the acute effects of psychedelics? I'm not sure science has definitively answered that question . but I think it may have to do with access to the insight that you've consumed a substance that can have these effects. It also may have to do with the transient effect of many perceptual disturbances and visualizations that can occur with hallucinogens. Maybe if the subjective effects of hallucinogens acted more like every-day perceptions (i.e. they weren't so extraordinary) or if they were more fixed or persistent (i.e. they didn't shift, warp, or morph so often) they would seem more real to the individual experiencing them.

George Musser Jr

The holographic principle doesn’t mean the universe isn't real. It just means that the universe around us, existing within spacetime, is ​constructed​ out of more fundamental building blocks. "Real" is sometimes taken to mean "fundamental", but that's a very limited sense of the term. Life isn't fundamental, since living things are made from particles, but that doesn’t make it any less real. It’s a higher-level phenomenon. So is spacetime, if the holographic principle is right. I talk about the holographic principle at length in my book, and I discuss the distinction between fundamental and higher-level phenomena in a recent blog post.

The closest we come in science to "real" or "objective" is intersubjective agreement. If a large number of people agree that something is real, we can assume that it is. In physics, we say that something is an objective feature of nature if all observers will agree on it - in other words, if that thing doesn’t depend on our arbitrary labels or the vagaries of a given vantage point ("frame-independent" or "gauge-invariant", in the jargon). For instance, I'm not entitled to say that my kitchen has a left side and a right side, since the labels "left" and "right" depend on my vantage point they are words that describe me more than the kitchen. This kind of reasoning is the heart of Einstein's theory of relativity and the theories it inspired.

Could we all be fooled? Yes, of course. But there's a practical argument for taking intersubjective agreement as the basis of reality. Even if everyone is being fooled, we still need to explain our impressions. An illusion, after all, is entirely real - it is the ​interpretation of the illusion that can lead us astray. If I see a smooth blue patch in the desert, I might misinterpret the blue patch as an oasis, but that doesn’t mean my impression isn't real. I'm seeing something real - not an oasis, but a refracted image of the sky. So, even if we're all just projections of a computer simulation, like The Matrix, the simulation itself has a structure that gives it a kind of reality, and it is ​our​ reality, the one we need to be able to navigate. (The philosopher Robert Nozick had a famous argument along these lines.)

Karl Friston

Institute of Neurology, University College London, Wellcome Principal Research Fellow and Scientific Director, Fellow of the Royal Society

First, you pose an extremely interesting question about how do we know we are hallucinating. Strictly speaking, one never has insight into a true hallucination, if one does, these are generally referred to as pseudo-hallucinations, which are not unrelated to illusions. The very distinction between illusions and hallucinations is itself fascinating. This is because it suggests we have the capacity to represent our own representations – or representational validity. This speaks to all sorts of deep philosophical issues for example, auto epistemic closure (in the sense of Thomas Metzinger), metacognition, self-awareness, lucid dreaming and so on.

The very fact that we can infer are perceptual influences are false speaks to a hierarchical composition of mind and perception in which not only do we have perceptual influences but also inferences about those inferences (CF metacognition). The implications for self awareness are clear. This is why people like Allan Hobson are so fascinated by lucid dreaming. This provides a wonderful test bed to compare situations in which dream reality is perceived as real and when one becomes aware of the fact that it is a dream. Neurobiologically, this seems to rest on frontal lobe activity, suggesting, again, a hierarchical aspect to our fantastic organ (i.e. the brain – that generates fantasies that are checked against reality).

The usual notion that perception is just hallucination grounded by sensations is somewhat subverted by the fact that we can, on occasions, know that our perceptual inference is false.

Metacognition is "cognition about cognition", "thinking about thinking", or "knowing about knowing". It comes from the root word "meta", meaning beyond.

Self-awareness is the capacity for introspection and the ability to recognize oneself as an individual separate from the environment and other individuals.

A lucid dream is any dream in which one is aware that one is dreaming.

Epistemic closure is a property of some belief systems. It is the principle that if a subject S knows p, and S knows that p entails q, then S can thereby come to know q. Most epistemological theories involve a closure principle and many skeptical arguments assume a closure principle.

Autoepistemic closure (Thomas Metzinge) is an epistemological, and not (at least not primarily) a phenomenological concept. It refers to an “inbuilt blind spot,” a structurally anchored deficit int he capacity to gain knowledge about oneself.

Rich Oglesby

There is a well known phrase: "We shape our tools and thereafter our tools shape us” (often associated with media theorist Marshall McLuhan, although it was actually a quote from Father John Culkin, a Professor of Communication at Fordham University in New York). This makes sense from an anthropological perspective - to put it crudely, whilst early humans evolved the ability to speak, the controlled sounds and utterances gained meaning to each other through localized consensus. Fast forward to the twentieth century and industrial nations, one can discover technologies that we can recognize their purpose yet have differences to our own, depending on our cultures and others - for example, the differences with electricial sockets or which side of the road you drive from one country to another. This was noted in William Gibson's book Pattern Recognition which he labelled 'mirror-world'. Technology alsocan become taken for granted and familiar over time unless we find ourselves taken out of our habituated situation - nothing so easily reminds ourselves of change as a power cut, taking us back a couple of centuries.

In the past twenty years or so in the industrial world, the biggest impact on our experiences has been from the field of computing. While many focus on the internet as the biggest game changer, it neglects developments and permutations which other computing tech has reached - how the computer monitor tech has crossed over into television displays, graphics cards have altered how we work with colours transforming Pantone, photography and printing, sound cards and music sequencers, mp3 and Flac. Personal computing technologies have radically changed the way we make, define and experience the world we exist in. To describe the last twenty years, the best term I can think of is the Recon-Naissance, combining the terms reconaissance (the practise of gathering, formulating or expressing information) and renaissance (both 'rebirth' and revival of interest), it is the widespread outcome of ideas and production of post-WW2 investment in computational and telecommunication technologies. The Renaissance Man polymath has been replaced with the Renaissance Machine - the personal computer. The same PC could be used by scientist or businessperson, coder or student, in the office or in the warehouse, in the studio or in the bedroom. This has been most advantageous to the modern creative.

With the development of the smartphone ten years ago, modern computing became pocketable. With it, computing components became smaller. Due to commercial popularity, upgrade cycles changed from a year and a half to just one. Information creation and reception became domesticated. It became mainstream and more conveniently portable. Music, photography and video could be captured and seen on the same device, replacing the personal media player and the portable camera. Life could be documented and experienced 'en plein Hertz'.

But the developments of the smartphone benefitted a once neglected but now up-and-coming field: Virtual Reality. With small displays and accelerometers now refined and cheaper, and gave the opportunity to start ups to produce a new experiences with a new computing medium. Initially produced to complement video games, other startups are producing other narratives, such as 360 documentaries, animations and first person tools for creativity and design. Whilst the consumer implementation of these ideas are not truly available yet, the technology is being used by scientists, architects, artists and gamers with current developer builds. It would appear how we engage and relate with information will change again - the Recon-Naissance is still going strong.


Perception As Controlled Hallucination

Perception itself is a kind of controlled hallucination. . . . [T]he sensory information here acts as feedback on your expectations. It allows you to often correct them and to refine them. But the heavy lifting seems to be being done by the expectations. Does that mean that perception is a controlled hallucination? I sometimes think it would be good to flip that and just think that hallucination is a kind of uncontrolled perception.

ANDY CLARK is professor of Cognitive Philosophy at the University of Sussex and author of Surfing Uncertainty: Prediction, Action, and the Embodied Mind. Andy Clark's Edge Bio Page

PERCEPTION AS CONTROLLED HALLUCINATION: PREDICTIVE PROCESSING AND THE NATURE OF CONSCIOUS EXPERIENCE

The big question that I keep asking myself at the moment is whether it's possible that predictive processing, the vision of the predictive mind I've been working on lately, is as good as it seems to be. It keeps me awake a little bit at night wondering whether anything could touch so many bases as this story seems to. It looks to me as if it provides a way of moving towards a third generation of artificial intelligence. I'll come back to that in a minute. It also looks to me as if it shows how the stuff that I've been interested in for so long, in terms of the extended mind and embodied cognition, can be both true and scientifically tractable, and how we can get something like a quantifiable grip on how neural processing weaves together with bodily processing weaves together with actions out there in the world. It also looks as if this might give us a grip on the nature of conscious experience. And if any theory were able to do all of those things, it would certainly be worth taking seriously. I lie awake wondering whether any theory could be so good as to be doing all these things at once, but that's what we'll be talking about.

A place to start that was fun to read and watch was the debate between Dan Dennett and Dave Chalmers about "Possible Minds" ("Is Superintelligence Impossible?" Edge, 4.10.19). That debate was structured around questions about superintelligence, the future of artificial intelligence, whether or not some of our devices or machines are going to outrun human intelligence and perhaps in either good or bad ways become alien intelligences that cohabit the earth with us. That debate hit on all kinds of important aspects of that space, but it seemed to leave out what looks to be the thing that predictive processing is most able to shed light on, which is the role of action in all of these unfoldings.

There's something rather passive about the kinds of artificial intelligence that Dan and Dave were both talking about. They were talking about intelligences or artificial intelligences that were trained on an objective function. The AI would try to do a particular thing for which they might be exposed to an awful lot of data in trying to come up with ways to do this thing. But at the same time, they didn't seem to inhabit bodies or inhabit worlds they were solutions to problems in a disembodied, disworlded space. The nature of intelligence looks very different when we think of it as a rolling process that is embedded in bodies or embedded in worlds. Processes like that give rise to real understandings of a structured world.

Something that I thought was perhaps missing from the debate was a full emphasis on the importance, first of all, of having a general-purpose objective function. Rather than setting out to be a good Go player or a good chess player, you might set out to do something like minimize expected prediction error in your embodied encounters with the world. That's my favorite general objective function. It turns out that an objective function like that can support perception and action and the kind of epistemic action in which we progressively try to get better training data, better information, to solve problems for the world that we inhabit.

Predictive processing starts off as a story about perception, and it's worth saying a few words about what it looks like in the perceptual domain before bringing it into the domain of action. In the perceptual domain, the idea, familiar I'm sure to everybody, is that our perceptual world is a construct that emerges at the intersection between sensory information and priors, which here act as top-down predictions about how the sensory information is likely to be. For example, I imagine that most people have experienced phantom phone vibrations, where you suddenly feel your phone is vibrating in your pocket. It turns out that it may not even be in your pocket. Even if it is in your pocket, maybe it's not vibrating. If you constantly carry the phone, and perhaps you're in a slightly anxious state, a heightened interoceptive state, then ordinary bodily noise can be interpreted as signifying the presence of a ringing phone.

It would work very much like, say, the hollow mask illusion: When people are shown a hollow face mask lit from behind, they see the concave side of the face as having a nose pointing outwards. Richard Gregory spoke about this many years ago. It's a standard story in this area. We human beings have very strong expectations about faces. We very much expect, given a certain bit of face information, that the rest of that information will specify a convex, outward-looking face.

The very same story gets to grips with phantom phone vibrations. It explains the White Christmas experiments, which is certainly one of my favorites in this area. People were told that they would hear the faint onset of Bing Crosby singing White Christmas in a sound file that they were going to be played. They would listen to the sound file and a substantial number of participants detected the faint onset of Bing Crosby singing White Christmas, but in fact there was no faint onset of White Christmas. There was no Bing Crosby signal there at all amongst what was simply white noise. In these cases, our expectations are carving out a signal that isn't there. But in other cases, perhaps someone speaks your name faintly and there's a noisy cocktail party going on, your expectations about what your name sounds like and the importance of anything that vaguely signals what your name sounds like conspire to up the weighting of the bits of the noisy signal that are there so that you hear your name fairly clearly.

Same thing if you're in the shower and a familiar song comes on the radio. Under those conditions, a familiar song sounds an awful lot clearer than an unfamiliar one. People might have thought that was a post-perceptual effect, as if you heard something fuzzy and then your memory filled in the details. But if the predictive processing stories are right, then that's the wrong way to think about it. This is just the same old story where top-down expectation meets incoming sensory signals with a balance that is determined by how confident you are in either the sensory signals or your top-down predictions.

The Bayesian brain, predictive processing, hierarchical predictive coding are all, roughly speaking, names for the same picture in which experience is constructed at the shifting borderline between sensory evidence and top-down prediction or expectation. There's been a big literature out there on the perceptual side of things. It's a fairly solid literature. What predictive processing did that I found particularly interesting—and this is mostly down to a move that was made by Karl Friston—was apply the same story to action. In action, what we're doing is making a certain set of predictions about the shape of the sensory information that would result if I were to perform the action. Then you get rid of prediction errors relative to that predicted flow by making the action.

There are two ways to get your predictions to be right in these stories. One is to have the right model of the world and the other is to change how the world is to fit the model that you have. Action is changing how the world is to fit the predictions, and perception is more like finding the predictions that make most sense of how the world is. But it turns out that they're operating using the same basic neural architecture. The wiring diagram for motor cortex and the wiring diagram for sensory cortex look surprisingly similar, and this story helps explain why. Indeed, the same basic canonical computations would be involved in both.

What's most interesting about predictive processing is the way it gives you a simultaneous handle on perception and action by showing they obey the same computational principles. It immediately invites you to think about having a model of the world that simultaneously drives how you experience and harvest information from the world. At that point, there's a standing invitation to stories like embodied cognition and the extended mind.

Once the predictive brain story is extended to the control of action in this very natural way, then there's a standing invitation to start thinking about how we weave worldly opportunities and bodily opportunities together with what brains are doing in a way that is going to make systematic sense of the extended mind story.

Before I go there, it's also worth saying a word or two about where the models that drive the predictions get to come from. Perceptual experience is the construct that lives on the border between sensory evidence and top-down prediction or expectation. That's what you're seeing in the White Christmas case and in the phantom phone vibration case. Just to see a structured world of objects around me means to know a lot about structured worlds of objects, and to bring those expectations to bear on the sensory signal. These are the stories that bring a structured world into view quite generally.

There are some rather nice cases that you can find online if you haven't already of so-called sine-wave speech cases, where speech gets stripped of some of its natural dynamics and what's left is a skeletal version of the speech. When you first hear it, it just sounds like a series of beeps and whistles, then when you hear the actual sound file and play that again, it sounds like a clear sentence being spoken because now you have the right top-down model, the right expectations. It's like hearing a familiar song when it's played in the shower on a bad radio receiver. It's a very striking effect and experience. It gives you a real sense of what is happening when a predictive brain gets to grips with the flow of sensory information.

Once you've played the real sentence, it might be something like, "The cat sat on the mat." So, you first hear beeps and whistles and you hear the sentence. Then you hear the beeps and whistles again, but this time through those beeps and whistles most people will clearly hear the sentence. After a while, you can become a native speaker of sine-wave speech so that you could be played a brand new one and you would hear the sentence through the noise. So maybe it will be useful to play some examples. Here we go.

[Audio samples. Begin listening at: 13:00]

I hope you've now had the experience of bringing a stream of somewhat unruly sensory information under an active predictive model and hearing how that can bring a structured world of words into view. The very same thing is happening in visual perception. It's the same effect that we were seeing in the White Christmas story, where your expectations are so strong that they make you think that there's a signal there when there isn't. But if predictive processing and stories of this kind are on track, then these are all exercises of the same constructive computational story. This is where human experience lives. As a philosopher, it sometimes interests me to wonder where this leaves the notion of veridical perception.

Perception itself is a kind of controlled hallucination. You experience a structured world because you expect a structured world, and the sensory information here acts as feedback on your expectations. It allows you to often correct them and to refine them. But the heavy lifting seems to be being done by the expectations. Does that mean that perception is a controlled hallucination? I sometimes think it would be good to flip that and just think that hallucination is a kind of uncontrolled perception.

The basic operating principle here is that you have a rich model of the world, a generative model, as it's known in this literature. What that means is a model that is not a discriminative model which just separates patterns out and says, "This is a cat and this is a dog," but rather a system that, using what it knows about the world, creates patterns that would be cat-like patterns or dog-like patterns in the sensoria. These systems learn to imagine how the sensory world would be, and in learning to imagine how the sensory world would be, they use that to do the classification and recognition work that otherwise would be done by an ordinary feed-forward discriminator. What that's doing is making perception and imagination and understanding come very close together. They're a cognitive package deal here, because if you perceive the world in this way, then you have the resources to create virtual sensory stuff like that from the top down.

Systems that can perceive the world like this can imagine the world, too, in a certain sense. That grip on the world seems to be very close to understanding the world. If I know how the sensory signal is going to behave at many different levels of abstraction and at many scales of space and time, so I can take the scene as it currently is and project it into the future and know what's going to happen if you hit the can and so on, that way of perceiving the world seems to me to be a way of understanding the world.

It will be very reasonable to ask where the knowledge comes from that drives the generative model in these cases. One of the cool things is that learning here proceeds in exactly the same way as perception itself. Moment by moment, a multilevel neural architecture is trying to predict the sensory flow. In order to do better at predicting the sensory flow, it needs to pull out regular structures within that flow at different time scales, so-called hidden causes or latent variables. Over time, with a powerful enough system, I might pull out things like tables and chairs and cats and dogs. You can learn to do that just by trying to predict the sensory flow itself.

A nice simple case of that will be something like learning the grammar of a language. If you knew the grammar of a language, that would be helpful in predicting what word is coming next. One way that you can learn the grammar of a language is to try again and again to predict what word is coming next. Pull out the latent variables and structure that is necessary to do that prediction task, and then you've acquired the model that you can use to do the prediction task in the future. These stories are a standing invitation to this bootstrapping where the prediction task that underlies perception and action itself installs the models that are used in the prediction task.

There's a pleasing symmetry there. Once you've got action on the table in these stories—the idea is that we bring action about by predicting sensory flows that are non actual and then getting rid of prediction errors relative to those sensory flows by bringing the action about—that means that epistemic action, as it's sometimes called, is right there on the table. Systems like that cannot just act in the world to fulfill their goals they can also act in the world so as to get better information to fulfill their goals. And that's something that active animals do all the time. The chicken, when it bobs its head around, is moving its sensors around to get information that allows it to do depth perception that it can't do unless it bobs its head around. When you go into a darkened room and you flip the light switch, you're performing a kind of epistemic action because your goal wasn't specifically to hit the light switch it was to do something in the room. But you perform this action that then improves your state of information so you can do the thing you need to do. Epistemic action, and practical action, and perception, and understanding are now all rolled together in this nice package.

It's interesting then to ask, if your models are playing such a big role in how you perceive and experience the world, what does it mean to perceive and experience the world as it is? Basically, what these stories do is ask you to think again about that question. Take the sine-wave speech example and ask yourself when you heard what was really there. Did you hear what was there when you heard it just as beeps and burps? Or did you hear what was there when you heard the sentence through the beeps and buzzes? I don't think there's a good answer to that question. If predictive processing is on track though, one thing we can say is that even to hear it as beeps buzzes is to bring some kind of model to bear, just one that didn't reach as deeply into the external causal structure as the one that actually does have words in it.

An upshot here is that there's no experience without the application of some model to try to sift what is worthwhile for a creature like you in the signal and what isn't worthwhile for a creature like you. And because that's what we're doing all the time, it's no wonder that certain things like placebo effects, medically unexplained symptoms, phantom phone vibrations, all begin to fall into place as expressions of the fundamental way that we're working when we construct perceptual experience. In the case of medically unexplained symptoms, for example, where people might have blindness or paralysis with no medically known cause, or more than that, very often the symptoms here will have a shape that in principle can't have a simple physiological cause.

A nice example is you might get someone with a blind spot in their field of vision. If you ask them what the width of that blind spot is when it is mapped close to the eye and when it's mapped far from the eye, some people will have what's called tubular visual field defect, which means they say it's the same wherever it's mapped. This is optically, physiologically impossible. It's pretty clear in cases like that that what's doing the work is something like belief expectation prediction. It's their model of what it would be like to have a visual field defect that is doing the work.

In this broad sense of beliefs, it doesn't mean beliefs that you necessarily hold as a person, but somehow they got in there somehow. These multilevel systems harbor all kinds of predictions and beliefs which the agent themselves might even disavow. Honest placebos do work. For example, if someone is told that this pill is an inert substance, you can nonetheless get symptomatic relief from those substances as long as they're presented by people in white coats with the right packaging—mid levels of expectation are engaged regardless of what you, the person sitting at the top, thinks. In the case of medically unexplained symptoms, it looks like they're the physiological version of the White Christmas effect. There are bodily signals there, and if your expectations about the shape of those signals are strong enough, then you can bring about the experiences that those expectations describe, just like White Christmas only done here in this somatosensory domain.

There's interesting work emerging not just on medically unexplained symptoms, but even medically explained symptoms. If people live with a medically explained problem for long enough, they can build up all kinds of expectations about the shape of their own symptomology, which share a lot in common with the medically unexplained cases. The same person with a chronic condition on different days and in different contexts will have different experiences even if the physiological state, the bedrock state, seems to be exactly the same.

There's a nice paper that came out recently by Van den Bergh and colleagues which was arguing that in the case of chronic effects, chronic pain, for example, an awful lot of ordinary symptomology has very much the character of the symptomology in the medically unexplained cases. So, it puts neuro-typical and less typical cases on a continuum and on par, which is quite interesting.

Acute cases are somewhat different because there you haven't built up those regimes of expectation, and there's a very straight signal being dealt with. Although, even there it seems as if your long-term model of the world makes a big difference as to how that signal plays out. There's a large area here where work on placebo effects, medically unexplained symptoms, autism, the effects of psychedelics, schizophrenia, all of these things are being thought about under this general framework. Maybe this'll be one of the test cases for whether we make progress using these tools with understanding the nature of human consciousness.

We had a visit from Robin Carhart-Harris, who works on psychedelics and is now working on predictive coding. There are some very interesting ideas coming out there, I thought. In particular, the idea that what serotonergic psychedelics do is relax the influence of top-down beliefs and top-down expectations so that sensory information can find new channels. If we think about this in the context of people with depression, maybe part of what goes on there is that we hold this structured world in view, in part by our expectations—and they're not just about the world, they're also about ourselves—and if you can relax some of those expectations and experience a way of encountering the world where you don't model yourself as a depressive person, for example, even a brief experience like that can apparently have long-term, lasting effects.

Some of the Bayesian brain and predictive processing folks are doing some pretty cool things, looking at the action of psychedelics and the effects of sensory deprivation. For any of these things, you can ask how would those different balances—held in place by this prediction meets sensory information construct—play out under different regimes of neurotransmitters, for example, or under different environmental regimes where you might have a stroboscopic light being flashed at you very rapidly. The University of Sussex has one of these, and it creates surprisingly intense sensations. If you were to sit in it for a couple of hours, you might get full dissociation. Even for a few minutes, you get experiences of colors of an intensity that I've never experienced before.

If you begin to ask what these stories have to say, if anything, about the nature of human consciousness, there are several things to say. The first is that the basic construction of experience is already illuminated just by thinking in terms of this mixture of top-down expectations and bottom-up sensory evidence and the way that mixture gets varied in different contexts and by different interventions. At the same time, there's a strong intuition some people have that consciousness is special and that whatever tools I was using to make progress with the White Christmas experiments and phantom phone vibrations are not getting to grips yet with what matters most about consciousness, which is how it feels, the redness of the sunset, the taste of the Tequila, and so on.

There's quite a lot to say about how that should pan out. In some ways, my view is an illusionist view. A large part of this debate over consciousness is misguided because there's nothing there. There's a multidimensional matrix of real things, and among those real things, there's a tendency to think there's another thing and that other thing isn't real. That's one way of thinking about it.

Among the real dimensions are the perceptual dimension that we've spoken about, the dimension of acting to engage our world. There's a lot of super interesting work on the role of interoceptive signals in all of this. Apart from the exteroceptive signals that we take in from vision, sound, and so on, and apart from the proprioceptive signals from our body that are what we predict in order to move our body around, there's also all of the interoceptive signals that are coming from the heart and from the viscera, et cetera.

One of the effects of the general predictive processing story is that all of this is just sensory evidence thrown in a big pot. How I perceive the external world to be can be constantly inflected by how I'm perceiving my internal world to be. You see this, for example, in experiments where people are given false cardiac feedback. They're made to think that their hearts are beating faster than they are. And under conditions like that, if they're exposed to a neutral face, they're more likely to judge that the face is anxious or fearful or angry. It looks as if what's going on is that our constant intouchness with signals from our own body, our brains are taking as just more information about how things are.

In that sense, there's a Jamesian flavor to some of the work on experience that comes out of predictive processing where the idea is that emotion, for example, is very much tied up with the role that interoception plays in giving us a grip on how things are in the world. William James famously said that the fear we feel when we see the bear has a lot to do with the experience of our own heart beating and our preparations to flee, all of that bodily stuff. If you took all that away, perhaps the feeling of fear would be bereft of its real substance.

There is something genuine in there that being subtly inflected by interoception information is part of what makes our conscious experience of the world the kind of experience that it is. So, artificial systems without interoception could perceive their world in an exteroceptive way, they could act in their world, but they would be lacking what seems to me to be one important dimension of what it is to be a conscious human being in the world.

We've got a number of real dimensions to consciousness. One of them is bringing a structured world into view in perception in part by structured expectations. The other one is an inflection of all of that by interoception. You can then ask questions about the temporal depth of the model that you're bringing to bear, and that seems like an important dimension, too. If your model has enough depth and temporal depth, then you can turn up in your own model of the world. Technically here I can reduce prediction error by projecting myself into the future and asking what certain things a creature like me—the way I can see myself to be—might do, would serve to reduce prediction error in the future. In that way, I turn up as a latent variable in my own model of the world. That seems important in human consciousness, at least. That's part of what makes us distinguishable selves with goals and projects that we can reflect on. That matrix is real. The thing that I don't think is real is qualia.

To understand that, we need to take a more illusionist stance. To do that would be to ask some version of what Dave Chalmers has lately called the meta hard puzzle or the meta hard question. That would be, what is it about systems like us that explains why we think that there are hard puzzles of consciousness, why we think that the conscious mind might be something very distinct from the rest of the physical order, why we think there are genuine questions to be asked about zombies. What Chalmers thinks is that any solution to the meta hard question, the question of why we think there's a hard question, why we say and do the things that express apparent puzzlement of this kind—those are easy questions in Dave's sense.

You can say something about how you would build a robot that might get puzzled or appear to be puzzled about its own experience in those ways.

You might think, well there's something very solid about all this perceptual stuff. I can be highly confident of it, and yet how the world really is could be very varied. If you're the sort of robot that can start to do those acrobatics, you're the sort of robot that might invent a hard problem, and might begin to think that there's more than a grain of truth in dualism.

One thing that we might like to do is try to take an illusionist stance to just that particular bit of the hard problem while being realist about all the other stuff, thinking that there's something to say about the role of the body, something to say about what it takes to bring a structured world into view. Do all of that stuff and then also solve the meta hard puzzle, and you've solved all there is to solve. Whereas Dave Chalmers, I'm sure, will say, at that point, you showing us how to build a robot that will fool us into thinking that it's conscious, in certain sense it might even fool itself into thinking that it's conscious, but it wouldn't really because maybe it wouldn't have any experiences at all when it's doing all that stuff.

Dan Dennett's take on consciousness is a perfect fit with a predictive processing take on consciousness. For many years, Dan has argued that there's something illusory here, some self-spun narrative illusion. Predictive processing perhaps gives us a little bit more of the mechanism that might support the emergence of an illusion like that. Dan himself has written some interesting stuff on the way that predicting our own embodied responses to things might lead us down the track of thinking that qualia are fundamental special goings on inside us. I might predict some of my own ooing and awing responses to the cute baby, and when I find myself in the presence of the cute baby, I make those responses and I think that cuteness is a real genuine property of some things in the world.

What Dan has argued there is that maybe we get puzzled because we're fooled by our own Bayesianism here. This model of how things are gets to grips with how we're going to respond, and we then reify something within that nexus as these intervening qualia. But you don't need the weird intervening qualia you just have responses that come about in certain circumstances. There's a rather natural fit between Dan's approach and these approaches, and they're both a kind of illusionism where we're both saying whatever consciousness really is, it can't be what Dave Chalmers thinks it is.


Hallucinations: What explains these tricks of the mind?

Hallucinations are sensory perceptions that appear in the absence of stimuli. Although they are often associated with illnesses such as schizophrenia, these phenomena can occur in the absence of mental ill health. But what explains these uncanny occurrences?

Share on Pinterest Not all hallucinations occur due to psychotic disorders or psychedelic drugs.

There are many types of hallucinations. They can be visual (sight hallucinations), auditory (sound hallucinations), olfactory (smell hallucinations), gustatory (taste hallucinations), or tactile (touch hallucinations).

For every sense, a form of hallucination is possible. The reason for this is that these phenomena are “ghost sensations” — perceptions of stimuli in the absence of actual external stimuli.

More often than not, when people think of hallucinations, they associate them with illnesses that can involve psychosis, such as schizophrenia, or neurocognitive disorders, such as forms of dementia.

Hallucinations are also associated with the use of mind-altering drugs, including LSD and DMT.

But hallucinations are not always due to psychotic disorders or psychedelics. Sometimes, they occur in the absence of these factors.

So what explains hallucinations, how common are they, and what role might they teach us about ourselves? In this Special Feature, we investigate.

A study that appeared in the British Journal of Psychiatry in 2017 suggests that hallucinations are far more common among people without psychotic disorders than scientists had previously thought.

The study authors — Dr. Ian Kelleher, from the Royal College of Surgeons in Ireland, and Jordan DeVylder, Ph.D., from the University of Maryland in Baltimore — analyzed data that they had obtained through the 2007 Adult Psychiatric Morbidity Survey, a nationally representative study of mental health in England.

These data included information on the mental health of 7,403 people aged 16 years and older throughout 1 year.

Dr. Kelleher and DeVylder found that visual and auditory hallucinations were almost equally prevalent among participants with borderline personality disorder and those with a nonpsychotic mental illness.

They also found that more than 4% of all the survey respondents — including those who had no diagnosed mental health issues — reported experiencing visual or auditory hallucinations.

“Hallucinations are more common than people realize. They can be frightening experiences, and few people openly talk about it,” Dr. Kelleher observed in an interview for International Business Times.

“Our research is valuable because it can show them they are not alone and that having these symptoms is not necessarily associated with having a mental health disorder. It breaks the taboo,” he added.

Research published in the Scandinavian Journal of Psychology in 2015 also showed that auditory hallucinations were surprisingly common in a representative sample of the general population in Norway.

In a cohort of 2,533 individuals, the “current lifetime prevalence of [auditory verbal hallucinations] was 7.3%.”

Phantom smells also seem to be a more common occurrence than people might think. In 2018, a study published in JAMA Otolaryngology — Head & Neck Surgery showed that 6.5% of people aged 40 years and over had experienced phantom odor perception.

This percentage is based on data from a cohort of 7,417 participants with a mean age of 58 years. However, when it came to explaining this phenomenon, the researchers were at a loss.

“The causes of phantom odor perception are not understood. The condition could be related to overactive odor-sensing cells in the nasal cavity or perhaps a malfunction in the part of the brain that understands odor signals,” says first author Kathleen Bainbridge, Ph.D., from the National Institute on Deafness and Other Communication Disorders at the National Institutes of Health (NIH) in Bethesda, MD.

Researchers have been trying to build a better understanding of the biological mechanisms behind the different types of hallucinations.

In 2019, investigators from the University of Oregon in Eugene conducted a study in mice to try to find out how hallucinations manifest in the brain.

Their study paper — published in Cell Reports — revealed some surprising findings. As the researchers induced visual hallucinations in the rodents by injecting them with a hallucinogenic substance, they saw that this did not “map out” how they expected it to in the brain.

The researchers observed that the hallucinating mice experienced fewer signals firing between the neurons of the visual cortex, the brain region associated with the interpretation of visual information.

“You might expect visual hallucinations would result from neurons in the brain firing like crazy or by mismatched signals. We were surprised to find that a hallucinogenic drug instead led to a reduction of activity in the visual cortex.”

– Senior author Prof. Cris Niell

Despite the initial surprise, the study’s senior author notes that it makes sense that visual hallucinations should appear in the brain in this way.

“Understanding what’s happening in the world is a balance of taking in information and your interpretation of that information. If you’re putting less weight on what’s going on around you but then overinterpreting it, that could lead to hallucinations,” he explains.

There is also a large amount of research into the causes and mechanisms associated with auditory hallucinations — some of the most fascinating kinds of ghost sensations.

For instance, a 2017 study featuring in eLife goes some way toward explaining how and why some people may think they hear voices that are actually not there.

According to the authors, this may be the effect of an error or “short-circuit” in processing internal speech and differentiating it from speaking out loud.

When people speak out loud, the researchers explain, the brain does two things. First, it sends instructions to the vocal cords, tongue, and lips — our vocal apparatus — to prompt them to move in the right way so as to vocalize the correct sounds.

At the same time, the brain also makes an internal copy of these instructions, which scientists call the “efference copy.” This duplicate allows the brain regions associated with hearing to predict the sounds that the vocal apparatus is about to make correctly.

This is part of how we recognize our own voice and speech.

“The efference copy dampens the brain’s response to self-generated vocalizations, giving less mental resources to these sounds because they are so predictable,” explains first author Prof. Thomas Whitford.

Through their study, Prof. Whitford and his colleagues found that the brain makes an efference copy not just of the thoughts intended for vocalization, but also of the inner talk — the internal chatter that people process in the background without expressing out loud.

It is likely, the scientists hypothesize, that auditory hallucinations arise when something goes wrong with the efference copy of people’s internal monologues.

“We all hear voices in our heads. Perhaps the problem arises when our brain is unable to tell that we are the ones producing them.”

– Prof. Thomas Whitford

Many of the hallucinations that people might experience are, in fact, of the run-of-the-mill variety. For example, a vague sensation that you have heard the phone ringing when no one actually called or catching the shadow of a silhouette from the corner of the eye when there is no one there.

Dr. Philip Corlett and Dr. Albert Powers, two scientists from Yale University and the Connecticut Mental Health Center in New Haven, CT, have conducted many experiments into the nature of hallucinations.

They argue that there is a spectrum of phantom sensations and that many hallucinations might actually reflect how our brains work — namely, by making predictions about stimuli and the environments that we navigate.

In a paper published in World Psychiatry in 2018, they write that hallucinations might emerge from a “mismatch” between the predictions that our brains make about our surrounding reality and the actual reality.

In another study paper, published in 2017 in the journal Science, the two researchers and Dr. Christoph Mathys, an associate professor at Aarhus University in Denmark, present the evidence they collected through a series of experiments that appear to support this notion.

“When we go about the world, we’re not just passively perceiving sensory inputs through our eyes and ears. We actually build a model in our minds of what we expect to be present,” Dr. Corlett told The Atlantic, noting that when those expectations do not come to pass, this may sometimes translate as a hallucination.

Other scientists note that even more pronounced hallucinations could sometimes be of benefit to the person experiencing them.

In a 2017 paper in Psychology and Psychotherapy: Theory, Research and Practice , Drs. Filippo Varese, Warren Mansell, and Sara Tai — from the University of Manchester in the United Kingdom — note that not all auditory hallucinations are distressing.

Their study, which enlisted people with mental health issues who also experienced auditory hallucinations, found that how people reacted to the voices that they thought they heard made an important difference to whether those voices hindered or encouraged them in their daily pursuits.

“Most voice-hearers with mental health difficulties in our study experienced their voices as a hindrance to achieving their goals and viewed their voices as distressing and problematic. But other voice-hearers find that voices facilitate their valued goals and are, therefore, a pleasant and constructive part of their lives,” Dr. Varese points out.

Going forward, the researchers say, such insights might help mental health professionals assist their clients in turning the tables on potentially distressing hallucinations.

“[…] we should seek to help clients explore how their voices relate to goals that are important to them and empower them to progress toward those goals. That would be a more meaningful and acceptable way of supporting them,” he suggests.


Childhood hallucinations are surprisingly common – but why?

C hildhood has long been championed as a time for make-believe, but recent research has found that another form of unreality – hallucinations – is more common in children than we previously imagined. For years, kids’ accounts of seeing, hearing and experiencing things that weren’t really there were considered to be part of the same invented world – an “overactive imagination” a “fantasy world”. The Alice in Wonderland approach, perhaps. But as it was recognised that hallucinations can be reliably identified in children, science has begun to look at why these illusory experiences are many times more common during our early years.

Hallucinations often reflect a bizarre, blurry version of our realities and because play is an everyday reality for children, the content can seem similar. Both can contain quirky characters, strange scenarios and inspire curious behaviour. One child described how he saw a wolf in the house, another that he had “Yahoos” living inside him that ate all his medicine. On the surface, these could just as easily be a child’s whimsy, but genuine hallucinations have a very different flavour. “In play and make-believe, children are imagining,” says Elena Garralda, a professor of child and adolescent psychiatry at Imperial College London. “They do not have the actual perceptual experience of seeing and hearing.” Another key difference, notes Garralda, is that “hallucinations feel imposed and children cannot exercise a direct control over them”.

Recent studies have thrown up some surprising statistics about how common they are. One UK study found that almost two-thirds of children reported having at least one “psychotic-like experience” in their lives, a category that also includes unshiftable and unrealistic beliefs and fears. When focusing purely on hallucinations, a review of research found that 17% of 9-12-year-olds have these experiences at any one time. The number roughly halves in teenagers and drops again in adults. Since this type of research tends to focus on experiences that are selected because they can appear in mental health problems in adults, such as hearing voices, which are only a small part of the possible range of hallucinations, these figures are likely to be a low estimate.

It is interesting that hallucinations become less common as we move towards adulthood. Because very young children are more difficult to test and haven’t been studied as widely, it’s not clear whether we start out in a more hallucinatory world, which becomes increasingly stable as we age, or whether middle childhood is a peak time for unreal experiences. For all its reputation for causing emotional mayhem, puberty might be a stabilising force on our perceptions.

At this point, let’s just take a breather – a sanity check if you will – because a lot of people get worried when they think about the possibility of their child hallucinating. These figures don’t mean that if a child is having a hallucination that they are ill or unwell. In the majority of cases, children’s hallucinations disappear within a few days or weeks and are not a cause for concern. Childhood hallucinations are often sparked by life stresses, poor sleep and periods of low mood that fade when the difficult situations do. If the hallucinations are upsetting or persistent, however, it may be time to ask for a professional assessment.

Renaud Jardri has seen many children with hallucinations in his clinical practice and also researches the area as part of his role as a professor of child and adolescent psychiatry at the Lille University school of medicine. The criteria, he says, for judging whether a child needs professional support are whether the hallucinations are “frequent, complex, distressing and cause impairment”. For Jardri, hallucinations that are associated with positive emotions and don’t interfere with the child’s friendships and family life are usually benign.

In rare cases, medical problems can be the cause. Epilepsy can cause hallucinations, as can sleep disorders that affect consciousness and lead to the dream world invading the waking hours. For reasons that aren’t entirely clear, severe psychosis, represented by diagnoses such as schizophrenia, is extremely rare in young children. But when such conditions occur, the hallucinations can be both striking and terrifying. The six-year-old who described having Yahoos living inside him was one of these rare cases he heard them constantly speak to him, feared being poisoned, believed he could cast spells, smelt “bugs” in the tap water and saw nonexistent trails in the snow. This is a far from the common fear of “monsters under the bed” or isolated hallucinations that fade over time.

Then there are imaginary friends that are not hallucinations but vivid fantasies, which have been the subject of much adult hand-wringing over the years. Because of this, they have been surprisingly well researched and I am delighted to live in a world where there are genuine imaginary-friend scientists, as if Roald Dahl were alive and funding a research institute. It turns out that children with make-believe companions tend to have better social skills and more developed language abilities than kids who lack imaginary buddies. And neither, the research shows, are these illusory companions a compensation for a lack of real friends. They seem to reflect the child’s brain running in overdrive, expending excess energy, delighting in the limits of imagination and playing with the possibilities of the social world.


Illusion came from the Latin word, “illusio” which means “to mock”. Rightly so, it occurs when something seems to be different from what it actually is. With the alteration of the stimulus, people experience “misperception”. This happens when the brain tries to fill in gaps in the organized sensory information. Though illusions are usually related with visual processes as it dominates the others, they are also associated with the other senses.

The following are the different kinds of illusions:

  • Optical illusion- makes use of visually deceptive illustrations like the Ebbinghaus and Hermann Grid illusions
  • Auditory illusion- characterized by sounds which are not actually present or improbable such as those in psychoacoustic tricks
  • Tactile illusion- this involves deceptions via touch such as the phantom limb wherein a patient still perceives pain in a leg which has already been amputated
  • Temporal illusion- concerned with the perception distortion of time like when minutes seem to significantly slow down to hours