with Spencer Greenberg
the podcast about ideas that matter

Episode 156: The range of human perceptual experiences (with Anil Seth)

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May 4, 2023

How does the world differ from our perception of it? Where is color located? Is the self constructed in the same way our concept of the world is constructed? Aside from being interesting bits of trivia, why does any of that really matter? In what ways does perception most often differ among humans? How different are art and science?

Anil Seth is a neuroscientist, author, and public speaker who has pioneered research into the brain basis of consciousness for more than twenty years. His mission is to advance the science of consciousness, and to use its insights for the benefits of society, technology, and medicine. He is Professor of Cognitive and Computational Neuroscience at the University of Sussex; Co-Director of the Canadian Institute for Advanced Research Program on Brain, Mind, and Consciousness; a European Research Council Advanced Investigator; and Editor-in-Chief of the academic journal Neuroscience of Consciousness. He has published more than 180 research papers and has been recognized by Web of Science, over several years, as being in the top 0.1% of researchers worldwide. A former Wellcome Trust Engagement Fellow, his two TED talks have been viewed more than thirteen million times, he has appeared in several films, and he has written for Aeon, The Guardian, Granta, New Scientist, and Scientific American, and he is lead scientist on the Dreamachine project. His new book Being You: A New Science of Consciousness was an instant Sunday Times Bestseller and a 2021 Book of the Year for The Economist, The New Statesman, Bloomberg Business, The Guardian, The Financial Times and elsewhere. Check out Dreamachine, take part in The Perception Census, visit Anil's website, or follow him on Twitter, Instagram, Facebook, or LinkedIn.

JOSH: Hello, and welcome to Clearer Thinking with Spencer Greenberg, the podcast about ideas that matter. I'm Josh Castle, the producer of the podcast and I'm so glad you've joined us today. In this episode, Spencer speaks with Anil Seth about the nature of reality, sense perception and hallucination, and the relationship between art and science.

SPENCER: Anil, welcome.

ANIL: Thank you, Spencer. It's a pleasure to be here.

SPENCER: So today we're going to talk about perception and how things may not be the way that we perceive them to be. And to get us started, can you tell us a little bit about why the world is not necessarily as we see it?

ANIL: The idea that the world that we experience is not the world as it actually is has been around for ages. And it makes a lot of sense, because the world is very rich. If you just think about vision, we have all this electromagnetic radiation that we swim in from radio waves to gamma waves. And we know that the eyes (the cells in our eyes) are only sensitive to a tiny slice of that reality. And out of that thin slice of reality, the brain creates this wonderful world of visual experience. And it can't be exactly the same as what we perceive because the world just doesn't show up out there with colors and the sorts of things that populate our experience. What we experience is always a construction that is related to what's there but it's not directly ever what's there. In philosophy, Kant made this point he called objective reality, this [inaudible], the unknown objective reality, always hidden behind a sensory veil.

SPENCER: Right now, if you lived 50,000 years ago, and you were walking around the world and you saw a red berry, it'd be very natural to think, “Oh, the redness is in the berry and I'm just seeing the redness that's out there in the world.” But we know today that redness is actually in your mind. There's no redness in the world. There's just electromagnetic waves that hit your eye, and then that turns into electrical signals. And eventually that turns into this experience of redness in the brain. Would you say that's accurate or anything you'd add to that?

ANIL: Almost. Firstly, I would say even when you walk around now, it still seems that the redness is in the berry. That's one of the things that makes perceptual experience work. It would be very strange if evolution had designed our brains so that we experienced colors as being constructions of the brain rather than as properties of the world. It is just helpful that, “Yeah. I see a berry and it seems to be red.” So it's no surprise that phenomenology, that conscious experience, works like this. Even when you know that's not the case, it still seems as though the berries are red. But what you are aware of is the redness, actually. I don't think it's right to say that it's in the mind, either. The artist, Cézanne, said that color is the place where the brain and the universe meet. It requires both. It's an interaction between both.

SPENCER: Do you unpack that a little bit? What do you mean by an interaction between both?

ANIL: Well, you're absolutely right that objectively out there, there's electromagnetic radiation of various wavelengths. And then the eyes (the cells in the eyes) respond to typically around three of these different wavelengths, and we call them red, green, and blue — but they're not actually red, green, and blue, they're just three different wavelengths — and then out of various mixtures of these wavelengths, the brain infers color. And what does inferring color mean? It means that the brain is picking up on the way in which light behaves. So, a particular color is really a way that the brain labels how surfaces reflect light. And the color that we experience is determined not only by the specific light that's reflected off a specific surface (like the red berry) but all the light that's around, all the rest of the light in the environment — which is why when you take a piece of white paper from indoors to outdoors, it still looks white, even though the light hitting your eyes on the paper has changed. Because the brain is taking into account the ambient lighting (which generally goes from yellowish indoors to bluish outdoors) to decide what is happening with the paper. It picks up the invariance of regularity and how the paper reflects light, and for the brain that is white. So whiteness is really an inference that the brain is making about how surfaces reflect light in the context of their environment. — And so of course, yes, in a sense, the mechanisms that underpin the experience of redness are in the brain. But for those to work, they depend on being shaped by a world out there. And there's still nothing that is actually red anywhere — not in the brain, neurons aren't read and there's no red paint to the new mind — the redness is part of a model that the brain is making off the world.

SPENCER: I feel that optical illusions help us gain insight into this. Because you can have these optical illusions where maybe the image looks like it's moving when in fact, it is not moving. Or it looks like one thing is longer than another when it's not. And it sort of illustrates that what's going on in the simulation within your mind is not always the same as what's happening in the actual world.

ANIL: I think that's right to a point. I use them a lot to illustrate a lot of things that we're talking about. There are amazing, beautiful illusions of color where you can show people. For instance, a cube like a Rubik's cube, which has lots of little tiles that are different colors. And because of the shading, the Rubik's Cube appears with half of its surfaces in shade. And it seems that the two tiles have very different shades of brown. But then when you actually isolate them and just take them out of their context, you realize they're exactly the same. So illusions can be really, really effective at illustrating some of these points. But I think there's a danger because it sort of reinforces the false idea that when we're not being tricked by an illusion, we might be seeing things exactly as they are. And the illusions are distortions from a sort of underlying normative accuracy of conscious perception. Whereas, what I think is really happening is that everything that we experienced is an illusion in the sense that it's not directly reflecting what's there. It's always a brain-based construction. But sometimes, we can sort of see through the brain's constructive process and notice that it's a construction. That's what's happening in visual illusions.

SPENCER: Another analogy that I think could be helpful is that of VR (Virtual Reality), because you could put on a VR headset, and you can experience something in VR. And we know that what you experience in VR doesn't necessarily have any relationship to anything in the world. It's very clear, and then you kind of can make this analogy and say, “Well, our entire experience is sort of VR. And everything we're experiencing that is happening is sort of in our simulation in our brand, not in reality itself. The difference is that whereas in VR, there's no particular relationship between what you're experiencing in VR and what's happening in the world. Whereas in the simulation, in our minds (like our own internal VR), there is a fairly tight relationship. And that's because we wouldn't have survived if there wasn't. So yeah, I'm curious to hear how you feel about that analogy.

ANIL: Yeah. I think all analogies are useful up to a point. And I think you make the argument very well. There is a sense in which we inhabit biologically implemented virtual reality because our experience of the world is a construction. It's not the world just pouring itself directly into our minds to the transparent windows of our eyes. To that, as far as that goes, that's true. But then it is a bit limited as well. And I think one of the really important things to emphasize is that, even though the construction is very, very finely tuned, closely calibrated by objective reality — evolution has absolutely ensured that otherwise (as you say), we would not have survived very long. And I often use the term ‘perception is a kind of controlled hallucination.' And this often gets misunderstood as saying, “Well. Our brains just make everything up. It's all arbitrary. Why don't you just go and jump in front of a bus and see if that's a hallucination?” Of course, it isn't. Control is equally important — in normal perception, we see the world as it is useful for us to see it. And evolution, development and culture have all shaped that so our perception is very, very functional. Whereas virtual reality, as you say, can be completely divorced from what's out there. And the other limitation in that idea is that in VR situations, they're still themselves, they're still you that is separated from the world that you experienced. You strap your headset box on your face, and you can move your head around, maybe eye tracking is in there, whatever. But you are still where you are. You might see a virtual arm or have an avatar or something but it's really presenting a virtual world to you. And in reality, it's not only our experience of the world that is a construction, it's also the experience of the self, and the self from the inside, all our emotions, our moods, and our sense of body. All this is a construction and I think VR does attempt to get at this a little bit, but it doesn't get it very successfully in my perspective.

SPENCER: So that's very interesting. The idea that the self is also being constructed much the way that redness or your perception of what we're seeing is being constructed. Let's dig into that. So what do you really mean by the self as a construction?

ANIL: It's an old idea in philosophy. The Scottish philosopher, David Hume, called it a Bundle Theory of Self, the idea that the self is not one thing. It may seem to us again — especially if we have been brought up to thinking of some very literal religious tradition — that there is an essence of soul, an essence of you that might even persist after death, as that's what the self is. Whereas the idea to put in contrast with that is that the self is not the thing that does the perceiving this essence of you but the self is another kind of perception. The way that makes most sense is to really analyze what the experience of being a self is. It's not just one thing. We have experiences of the body, experiences of mood and emotions, experiences of this object in the world that is my body. I know there's some part of the universe that is me and other parts that aren't me. And then there are experiences of agency of intention to do things of what we might colloquially call free will. And then, at higher levels, we have experiences of being a continuous individual over time that we put a name to, my name, Anil Seth, and the experience of being part of a larger social network. Now, many instances in psychiatry and neurology and even in the lab show us that we can tease these different aspects of self apart, and people can lose one aspect of selfhood and retain all the others. And this is suggesting that the self is this collection of different experiences that tend to go together most of the time for most of us, but don't actually have to. And each of these different aspects of self — and this is my speculation rather than confirmed scientific fact — that each of these aspects of selfhood can be understood as a kind of perception as the brain interpreting sensory data of some sort or other and making its best guess about what's going on.

SPENCER: Would you say that applies to every one of these kinds of pieces of the self?Are all of them controlled hallucination? Or do you think there is some piece that sort of the actual center, the actual self in there?

ANIL: [Laughs] It is tempting, isn't it? It's tempting in the same way that however much you know that the redness of the berry is not actually in the berry. However much you can understand in the abstract, or see experiments about manipulations of self, it still seems as though there's an essence of me somewhere deep within, unless you are a really expert meditator, or on a very large dose of psychedelics or something where you get full ego dissolution and you might have a different experience of self. For most of us most of the time, it does seem as though there's an essence in that. But I do think that needs to be challenged. I don't think the fact that it seems that way means that it is that way. And for sure, this idea of controlled hallucination applies more readily to some aspects of self than others. The experience of the body as an object in the world is quite easy to accommodate in this framework. The brain takes sensory information from looking at the body, its feeling, its position, its space, and so on, and uses that information to update its predictions about where the body is. And we can show that's a sensible story for that aspect of self. For other aspects of self, it's a bit harder. It's a bit harder to explain what philosophers have called the narrative self. This is the self that has a name, has a history, and has a future. And the reason why it is more difficult is not because it's impossible, it's just further away from the act of perception, it's a bit more abstract. It's not really involved in interpreting sensory data. It's about the formation of memories, and so on. It's a more complicated story but the fundamental point where it applies equally is that, in fact, there is no place where the real self really resides.

SPENCER: I'm an amateur meditator, and I've experimented a lot of different types of meditation. And if there's one part of me that feels the most self-like, I would say that it's the kind of watching part. It feels like there's some part of me that's the place where things are being experienced. If I'm looking around, there's some part of me that's experiencing, “Oh, wait, that's a cookie. That's a car. Wait, that's a table.” But then, it could also experience other totally different things like it could experience a thought like, “Oh, wait, I'm having this thought now. Or wait, I'm feeling this emotion now.” It's almost like a spotlight that's watching and it's what the experiences are happening to. So I'm curious what you think about that?

ANIL: I think that's really interesting. And there might well be some data on this, I don't know. But I get the feeling that different people might give different answers to that kind of question. So I think the aspects of cognitive architecture that you're picking up, there is something about the control of attention. Our brains control our attention. They pay attention to particular things, whether it's something in the environment or a thought that emerges. And the experience of controlling that attention might be very important in our sense of self and our sense of consciousness. The scientist, Michael Graziano, makes this case very eloquently in his book about the social brain. He thinks that consciousness, in general, is closely associated with, or identical to, the brain having a model of how attention is controlled. But that's one answer. I think if you ask different people, you really get different things. For some people, self is fundamentally in the body. This really deeply embodied the feeling of being alive. For other people, it may be in the set of memories they have. For other people, it's really in the experience of freewill, the experience of ‘I'm the cause of this action. I'm the author of this thought.' And for other people, it's a bit more reflective and metacognitive. A little bit along the lines, what she was saying was that there's not only an experience, there's the experience of being the observer, the experience of having an experience, and it may be that one of these aspects is more common. But I don't think it'd be universal. And even if it is, that in itself doesn't mean that that's where the self really is. That just means that's one of the more dominant aspects of this collection of perceptions that collectively make up the experience of human selfhood.

SPENCER: Yeah, I agree. I don't think others would necessarily give the same answer. And without doing a bunch of meditation, I'm not sure a lot of people would even think about that part of themselves as being a distinct part. I think through meditations is where we start to feel that way.

ANIL: Yeah. That's great. I'm also an amateur meditator. I've no claim to be an expert like one of these people who've spent 10,000 hours really training their minds. But even with a small amount of meditation, you get a sense of what's at stake here. You get a sense of noticing how your own mental processes unfold, what happens to your attention, thoughts come and go. It gives you a little bit of a distance from what's happening in your mind. And that distance makes it a bit easier to see that all there really is, is just one experience after another. There's just experience happening. It's not happening to anybody, it's not happening for anything. It's just a continuous flow of conscious experiences. And the experience of selfhood is just part of that flow.

SPENCER: So I'm attempting to do a controlled hallucination of our listeners and simulate what their minds are doing right now. And what comes up for me there is, they might wonder, “Well, so what?” Right? Like, “Okay, maybe it's interesting and novel to think about, maybe our reality is controlled hallucination, but how does that affect me? Or why does that matter?”

ANIL: I think it matters in all sorts of ways. Fundamentally, of course, as a scientist, I'm just interested in how the thing works. And opening up the space between how things seem and how things are, that is what science and philosophy has been about for centuries, open our eyes. It seems as though we're at the center of the universe. It seems as though we're different from all other animals. But when we understand more about what's actually going on, the world becomes a much richer, more interesting, more fulfilling place. But it also makes a practical difference in everyday life, too. And this can be for a number of reasons. One is it provides us with a very useful handle on things like mental health, mental illness, psychiatric disease, brain injury conditions, and so on, where people have experiences that become very disconnected from what's going on, where we would usually use the word hallucinations, when people frankly hallucinate stuff. We can begin to understand what's going on and why it's happening. And that is very helpful. But even for healthy people with normal everyday life, I think it's really liberating to walk around and to understand a bit more about what's actually going on. It gives us a bit of humility, about our perceptual experience. If we realize that what we experience is indirectly related to what's there, and dependent to a large part on the particularities of our own minds and our own brains, then we can understand that actually, other people might see different things. They might have different experiences, and therefore might even believe different things in different cases. This is actually something we're studying quite a lot at the moment in this new project called the Perception Census, where we're trying to get a sense of how different our inner worlds actually are because not a lot is known about that. But that's a strong implication of all this. And in fact, people have known this for ages but they've only studied it in relatively small studies or on single aspects of perception before and we're really fascinated by painting a more comprehensive portrait of how we all differ on the inside just as how we differ on the outside. And of course, that has all, not only scientific implications, but social implications, policy implications, health implications, and so on.

SPENCER: As you point out with certain mental health conditions, the gap between the simulation inside and what's out there in the universe can grow. Someone can have visual hallucinations and things like that where there's really no connection between what's going on in their mind and what's out there in the world. I wonder for someone who is not experiencing these mental health challenges, how close do you think what they're experiencing is to what's going on?

ANIL: That's such a good question. And it's good, because it's almost the wrong question. But it's good, because it reveals itself to be the wrong question. Let me try and explain what I mean. When we say how close our perceptual experience is to what's really going on. I think that implies that ideally, it would be identical. That if everything was working correctly, if we had some sort of optimally designed brain in mind, we would see things exactly as they are. And that would be great. And I think both of those ideas are untrue. I don't think it's possible to ever see things exactly as they are. And I don't think that even if we could, that would be the best thing. That just takes us back to the start, this takes us back to Kant and the idea that there's this inaccessible reality. It's much more useful for us to see colors to construct colors than it would be to perceive the actual light that's coming into our eyes. Because it's changing all the time, and the light waves that come in change all the time. But what is useful for us is to understand how objects out there behave. And so, it's much more helpful for our perception to be systematically inaccurate, or systematically different from what's actually there. I think that's the key point here, we could never see things exactly as they are anyway. It doesn't really even make any sense because what we see is always an interpretation. It has to be. It's experiencing light waves, we're experiencing pressure waves in the years doesn't really mean anything. And we don't have any direct contact with external reality apart from through our senses. So it has to always be a construction. And then that construction is designed by evolution, not by criteria of accuracy, but by utility. We see things as they are useful for us, not as they actually are. But having said that, clearly these relationships are very close, they're very tightly regulated. So back to color, again. Color in one sense is very, very different from what's actually there. Because colors are not properties of the world without minds, and different species will experience very different colors, even different people experience different colors, as we're looking at with the Perception Census. So in one sense, it is totally inaccurate to experience colors. But in another sense, it's extremely useful to experience colors, too. Whether you tell an evolutionary story about enabling our ancestors to identify the right fruit in trees or whatever reason, there's very clear adaptive functional, adaptive utility, in creating or abstracting out this property of color from the messiness of lightwaves that allow us to behave more effectively in the world in which we are.


SPENCER: If we think about our perceptions as being inaccurate with respect to what's actually out there, I think there are different ways that can be inaccurate. One is compression. It's not that useful to see every single molecule, you kind of want to compress it all and say that's a tiger. That's much more useful. And it is inaccurate in a certain sense because you're losing a lot of information, but it's probably sort of more accurate to refer to it as compression than an accuracy. Because you're like taking a lot of information that's unnecessary. It's not helpful and kind of throwing it away and just focusing on the helpful. Another way you can be inaccurate is you could just be wrong about what's out there. You could see a tiger when there's no tiger (for example, having a visual hallucination). And then a third way you can kind of be inaccurate to what's out there is by having an experience that sort of doesn't have an objective truth value. So an example might be, maybe something happens, and then you feel like someone did something morally wrong. Well, it's not like your perception of a moral wrong refers to sort of a certain set of atoms being a certain configuration. It doesn't exactly correspond to something out there, but it's not really meant to in some sense. It also has sort of a social meaning and cultural meaning and so on. So anyway, I just want to point out that the idea of inaccuracy in sort of different definitions of inaccuracy that we could have going on here.

ANIL: Yeah, I think that's right. And of course, when you start talking about things like morality, and socially constructed phenomena, then accuracy means something very different. Accuracy becomes something that is totally relative to the culture that you're in. But it's hard to make sense of it, even for these lower levels of perception, too. I think the more useful way to think of it is whether our perceptual experience tracks things in the world in useful ways, or whether it doesn't. And our perceptual experiences (our perceptual apparatus), when it's working well, it tracks the external world in ways that are useful for us. And when something goes awry, it doesn't. It no longer tracks objective reality. And so then we start frankly hallucinating. I keep going back to color but there's something where color is perhaps not the best example, because color is what the philosopher John Locke would have called a secondary quality. It is something that is not mind-independent. We need a mind for color to exist. But then other things in the world, the objective external reality are mind-independent looks, primary qualities. Things like solidity is a primary quality, which is why even if you're not looking at a bus, and you step in front of it, it is going to hurt. There's a solid object there and that's true. And for our perceptual systems to be useful, they need to track that feature of the environment. But in our perceptual experience, the fact that something is a primary quality that it has a mind-independent existence, doesn't necessarily mean that we perceive it any more accurately. When we look at a bus, it is the way we experience it. It is still a construction. It's still not directly reflecting what's actually out there. So it's not a question of just compression and filtering; that's going on, too, for sure. So the brain is always trying to simplify because simplification is just useful for all sorts of reasons. But it's more than that, because it's simplification, bias, and construction. So color is not just a compressed description of the light waves that's out there. We only sense three wavelengths of light, but we construct millions and millions of different colors. That's an expansion. That's not a compression, but it's a very useful thing. So yeah, just just relying on accuracy and compression, I think only tells us part of the story.

SPENCER: All this talk about our internal simulations of reality raises a question of, how close are internal simulations to each other's? Because you could imagine that for any external world, there could be lots of different ways to simulate it internally, that might be roughly equally good for your survival or for figuring out what to do in the world. So an analogy for this would be like, imagine there's a VR world, different people put on VR headsets to experience the same world. But in one world, the creatures that are trying to kill you are represented as tigers, and in another it is represented by ghosts. But they're equally useful because the ghosts and tigers have the same properties, and you still gonna run away from them. So the fact that there's a difference in experience is sort of irrelevant. And so, I think about this with regard to our simulations, and we're out there trying to survive. And we might be representing things quite differently, but in an equally useful way. So I'm curious what your thoughts are on that.

ANIL: Yeah. I've had a lot of thoughts about this, something I've been thinking about quite a lot recently, actually. It gets back to this idea of perceptual diversity, that it is very likely that indeed, we all experience things quite differently. Now, this is something that's really uncontroversial. When we think about non-human animals (other animals) it is very well known that other animals inhabit entirely different sensory environments, even if they're in the same objective place. Bats have echolocation, bees see ultraviolet light, all these sorts of things. And we can never experience what it's like to be a bat, as the philosopher Thomas Nagel said, nor a bee. But we know that whatever the bat, or the bee, or the horse or something is experiencing, it is going to be different from what we're experiencing, even if we're sharing the same environment. What's slightly harder to really bear in is that it also applies to differences between people. We have this concept that's been around for a long time of neurodiversity. This idea that everybody is different, and that differences are not deficits. And this is a very important concept. A very important idea. But it tends to become associated with particular neurodivergent conditions, things like autism or ADHD. And ironically, it reinforces the idea that if you don't identify with one of these neurodivergent conditions, that you are a neurotypical; you see the world more accurately, more as it is and like everybody else. And that's an easy conclusion to come to you because, of course, it seems as though we see the world as it is. It doesn't seem as though it's a construction. We open our eyes, and there it is. But we will probably all experience things differently. And it's so hard to see, because as you just said, in many cases, it might not actually make any difference. We'll use the same words. We will behave the same way. And so these inner differences remain hidden behind underneath this common language, and hidden by our shared behavior, similar behavior. So you have to do some quite fine grained experiments to try and get at these individual differences, get people to make fine judgments between different colors, and rate how long something lasted, look at differences in time perception, and so on. And that's exactly what we're doing with this Perception Census project, where it's an online study. So I really hope your listeners would be interested in taking part because we're really trying to get as much data as possible. And we've got 20,000 people already in 100 countries, but we want to make this a massive survey of how different our inner worlds are. And we're looking at color, visual experience in general, the effect of expectations, time, music, emotion, all these sorts of things, to try and just reveal some of the richness that we have on the inside just as we have on the outside.

SPENCER: And where will people go if they want to participate?

ANIL: If they would like to participate and again, please do consider it, it would be wonderful. The easiest place to go is on my own web page, which is and follow the links from there. Or if you just type in Perception Census to Google, you'll probably find it as well.

SPENCER: Awesome. And what are some questions you hope to answer with the project?

ANIL: A lot of it is very exploratory. There's been a lot of studies of individual differences in perception but they tend to just focus on one single aspect or one or two aspects. So we are widening the lens. We want to look at many aspects of perception at the same time. And that will allow us to understand how they relate to each other. And that's what I'm really interested in here is how different aspects of variability relate and what might explain that variation. Are there underlying traits, underlying variables, underlying tendencies, that explain why we might experience color in a particular way, and therefore time in a particular way, or therefore emotion in a particular way? That's what I want to know. It's just a pretty blue sky. People don't really know at this scale and across the world. So we can also ask, “How does perception differ in different countries? How does it differ depending on where people live, what language they speak?” Lots of interesting questions there, too. And how does this variability and perception relate to its sort of more extreme ends where people might actually hallucinate? Or how does it relate to other not uncommon ways of perceiving — like synesthesia, where people experience a kind of mixing of the senses, where letters might have colors or shapes might have tastes — we tend to study these conditions in isolation. But I'm interested in finding out how they fit into a more continuous view of how we all differ and how we're all unique.

SPENCER: I think synesthesia is such a great example. Because a lot of times people who have synesthesia don't realize there's anything different about themselves for many years, and often until they're in their 20s or 30s. And then they mentioned how the word tiger is always blue. And people are like, “What are you talking about?” And people be like, “Wait, what?” This exists because they're able to function in the world, just like everybody else. And so there's no reason they have to think that they're perceiving things any differently than anyone else.

ANIL: Yeah, that's absolutely right. I think that just really underlines the point that we have a sort of naive realistic view on our own perceptions most of the time. We just assume that, “Okay, that's the way I see things. That's the way they are, and therefore everybody else sees things the same way.” And I really remember this. When I was an undergraduate student in the early 90s. I was studying psychology and I learned about synesthesia then. I remember describing it to a friend of mine at college — I think we were about 19 years old, something like that. — And I was describing to her and she said, “What do you mean? Isn't that what everybody experiences?” And it was exactly that. She had synesthesia, and she didn't realize it. And she didn't realize it was anything distinctive. And I always remember that. And I think it happens a bit less. Now, I think the idea of synesthesia is a bit better now. But I'm sure it still happens quite a lot. Because not everybody knows about synesthesia at all.

SPENCER: I have a friend who has a rare type of synesthesia, which is touch color synesthesia. So when she touches things, she experiences colors. And so having sex for her is just this overwhelming experience of colors everywhere.

ANIL: Wow. There are lots of interesting variations. The taste or shape is one that's always fascinated me. We tend to study one of the more common forms of synesthesia, which is grapheme-color synesthesia, which is when you see a letter, it might be a letter printed in black typeface that evokes a very specific color for these people. And this is a really useful and a really appropriate or sensible type of synesthesia to study because you can actually test it. And the way you test it is you get people to pick out of a big color palette, you get them to pick the color that they think that goes with the letter that they see. And then you ask them to do it again, five minutes later, or half an hour later. And if a nonsynesthete is basically trying to fake it, or thinks they might have it but are not sure, they won't be able to get exactly the same color, they might get close, they'll pick another shade of red. But if you do this for lots of letters, lots of times, they won't be that accurate. If you are actually synesthetic, and you really do have these experiences, then you'll get very close, because you'll just pick the color until it really reflects what's going on in your mind. And that's difficult or impossible to fake. So there's this really nice objective test for at least one kind of synesthesia.

SPENCER: What are some other kind of surprising differences that people have that they may not have considered where they sort of assume everyone else has things the same way that they do in their mind?

ANIL: Oh, that's a very good question. This is one of the things the Perception Census is hoping to discover. And a lot of this is not really known. One thing that is known a bit about — and we asked about this, too, in the Census — is hearing voices. So it's not that uncommon for people to hear voices in their head when nobody is speaking. And it's often underreported because people, sometimes quite rightly, are worried about being labeled as suffering from psychosis or having some form of mental illness. Because often that's the association made with things like psychosis and schizophrenia, that you start hearing voices in your head. A colleague of mine in Durham University in the UK, Charles Fernyhough, has this project called Hearing Voices. And it's been very successful in normalizing this experience. It turns out that quite a lot of people do hear voices. They're just not particularly distressing voices, they're just there and that's fine. And so they just go about their daily lives, and you wouldn't know any difference and so it doesn't surface, but it's still there on the inside. And this is another benefit of actually mapping out the diversity of our experience, because it just shows that we are all different. And that there's just a continuum between normal perception and the kinds of perceptual experiences that become challenging. And of course, sometimes that happens, if you are hearing a voice in your head that is constantly criticizing you or telling you to do bad things. Well, that really is a problem. And that needs to be treated. But many instances of voices in the head just aren't like that.

SPENCER: There's a TED Talk by a woman named Eleanor Longden, who had this really interesting experience where she had the voices that would narrate her life. And it didn't really bother her — she just sort of said, “Oh, yeah, that's just normal.” — And then one day, she tells a friend about it, and the friend becomes very distressed, and tells her she should go seek counseling. And then she goes to a psychologist, and the psychologist not only diagnosed her with schizophrenia, but made her afraid of the voices, because she started to view them now as something scary and bad. And that completely changed her perception of the voices. Whereas before they were innocuous, now they're scary and it sends her life into a tailspin. So I just think that's really, really interesting about how you can have this perception. It's very different from what most people experienced, but sort of view it in very different ways. You can view it as sort of neutral or bad, or you could even imagine viewing it in a positive way.

ANIL: Yeah. It's a terrible story. That's really the opposite of what should happen. And this is one of the benefits of understanding a bit more about how we all do differ because it may make people less quick to do that, less quick to diagnose differences as deficit and set people down the wrong path, which, as you say, you make people afraid of their own experience. Well, that's not going to end well.

SPENCER: Another interesting difference that I think a lot of people don't realize exists is that some people can't imagine things in their mind, like they do visualizing, but it doesn't make any sense to them. And I'm curious how much that difference has been studied?

ANIL: It's increasingly studied. Again, I've sounded like a broken record. But this is one of the other things that we do look at in the Perception Census. It's relatively recent that this phenomenon of what's called ‘aphantasia' has been described as to be the complete absence of internal mental imagery. Now, I've always found it a bit harder to get a handle on because it's sort of the absence of something rather than the presence of something. And maybe it's because my own mental imagery seems to be quite weak, but I don't really know because it's very hard to know how to compare it. But there are some people who claim to have no internal mental imagery at all. If they close their eyes — just close your eyes and just try to bring into our minds the face of someone, maybe that we were talking to earlier today — and for me, something happens. It's not it's not that I see the face, in my mind. But I get some sort of fleeting, semi-visual impression. It's not just like a word or a name. So there's some sort of mental imagery going on. And when you ask people in a systematic way about how vivid that imagery is, and how similar it is to normal perception, you find a large range of answers. So some people will say that their internal mental imagery is really quite similar to their actual perceptual experience. For me, that's almost unimaginable. That sounds like, “Oh, wow, that would be like a movie in the head when you close your eyes like dreaming or something.” To me, it's not like that. There's no sort of saturated color, or perception or anything of that sort. But something is going on at the other end of the extreme, you have aphantasia and that seems to be the complete absence of imagery at all. It's important really to say here that wherever you lie on scales like this, it often has very little or no implications for what you can do. You just end up maybe doing things in slightly different ways. You might solve problems in different ways. If you have strong mental imagery, you might solve problems by forming an image in your head and interrogating and rotating it. If you're not strong with mental imagery, you might solve problems in different ways by thinking through things more logically, just some other form of intuition. And there's experiments to back this up that there's really not many differences in people's functional capacity in general. It is just that there are different ways of doing things.

SPENCER: So you don't think it makes some people better or worse at things based on what internal experience they have?

ANIL: Well, I think it does depend on what these things are. So if something is really testing a capacity for mental imagery, then sure, of course. But very few real world things just rely on that one single way of doing things internally. But synesthetes, there's actually one of my colleagues, Nicholas Rothman has led research that is looking at memory in synesthetes. And there's some evidence that synesthetes might have better memory than nonsynesthetes, perhaps because they have these other perceptual associations that help them ground their memories. And there's also people that argue that something like synesthesia really enhances creativity. But that's quite difficult to measure not because of synesthesia, but it is just because it's very hard to measure creativity in the real world.

SPENCER: I would guess that something like lack of internal imagery would make it very hard to be a sculptor, for example. If the object was in front of you, you could sculpt it. But if you had to sort of imagine what kind of sculpture to make, it seems like that'd be really difficult without internal mental imagery.

ANIL: You see, I don't know. So this is now just sort of [inaudible] isn't it? Because there may be a study on imagery and sculpture, but if there is, I don't know about it. But it strikes me that you might just have very different styles of sculpture. Those people who don't have strong mental imagery when they're performing sculpture, they might actually be better in some sense because they might instead just sculpting whatever comes into their mind, they pay more attention to what's actually out there, and may sort of reflect changes in texture and shape in a more interesting way because they're not just overly simplifying things in their minds. But on the other hand, of course, if you have strong mental imagery, then you might have more ability to change things around, rearrange things, make different forms of sculpture. So I'm still thinking about my point. I think you can have equally good sculpture, but it might well be different sculpture for images versus non-images. And the same for painting as well. Painting is super interesting. If you are really good at bringing an image into your mind, that might make you paint in a different way from if you don't and you're really paying attention to what you see. So again, I think it's just interesting diversity. Like biodiversity in an ecosystem, we're very familiar and comfortable with the idea that a flourishing ecosystem requires many different kinds of plant and animal and so on. And by the same token, I think a healthy society and a healthy culture of art requires many different styles of painting and sculpture. What I think is overlooked is how these different styles are not just styles of executing actions or styles of thought in terms of particular artists in the history of art, but styles that might be grounded in different ways of perceiving, and I'm super interested in that and how that might shed a really different window on the history of art.

SPENCER: Now, in your Perception Census, are you going to have some performance measures so that we can actually see if some of these internal differences map onto people being able to do some things that are worse?

ANIL: Sort of, but not really. And part of this is because we want to be very careful here, because the last thing we want to do is to have people come away from the Census with the impression that they can't do something or that they're lacking in some way. And we typically avoid phrasing things in terms of you did better than 90% of the people or you did worse than 90% of the people. And we try to give people feedback, firstly, we ask them to do the thing they did, why this particular illusion is interesting, and what we as scientists can discover about it, and teach a bit about perception in that way? So one objective is people will learn about the mind and the brain through this way. And the feedback that we can give is more about, “You formed this way, which is like 10% of people 90% of people may perform in a different way.” And we can say something about what that means. But it's not usually that it means that they're better or worse, it really honestly just means that they do things differently, and we can tell them how likely they are to fall in one way of doing things rather than another way of doing things. Part of it is also that we're not really measuring that. We're not giving them performance tasks of that sort. Now, we're really picking our tasks that are more tuned to measure difference rather than things being better or worse.


SPENCER: Before we wrap up, there's one more topic I wanted to touch on with you, which is the question of ‘How related or how close art and science are?' So do you want to start us up on that?

ANIL: Oh, yes. I love this topic. And it was drilled into me at school that they're quite different and our whole system of education treats them as very different. But I think there's some beautiful similarities. Fundamentally, they're both hugely creative enterprises. Science isn't just about cranking a wheel and facts come out. Science is about imagination. It's about trying to conceive an explanation, trying to figure out what would be a way to test an explanation, trying to see patterns in things that other people haven't seen in a collaborative way, though it's incremental. So I think it is a different practice from art, of course. But fundamentally, this thread of creativity is there in both. And there's just so many fascinating overlaps. So I often think of this guy, Santiago Ramon y Cajal, who is the Father of neuroscience. He was really the first neuroscientist by many accounts. He is a Spanish guy. And he made his reputation with what's become called the neuronal doctrine in neuroscience. And this is the idea that neurons in the brain (brain cells) are separated from each other. And they communicate across a gap — the so-called synaptic cleft — by exchanging chemicals. Exactly how they did that wasn't known at the time but the idea that neurons were sort of separate cells, which was quite controversial. The alternative idea, proposed by an Italian neuroscientist called Camillo Golgi, was that there was just a single network. All neurons were connected in one seamless mesh inside the brain. And these were two competing ideas. And this was the very early 20th century, late 19th century, just when microscopes were becoming more available and techniques where you could apply little bits of chemical and stain just a few neurons so you could begin to see what's going on in these thickly entwined meshes of neurons and their fibers. And both of them were using microscopes — Golgi was more advanced on this, actually — and both of them would draw what they saw. They didn't have these sort of beautiful cameras that could go on top of microscopes; they would just look in the microscope and draw. And Golgi drew a mesh. So he drew what he thought was going on. He saw a mesh, so he drew a mesh. And Ramon y Cajal, was also an artist who had been trained as a visual artist, as a painter. And his pictures, firstly, were absolutely beautiful. His drawings of neurons — they've been exhibited in museums across the world; they're still stunning to look at — And because, of course, this is a bit of postdoc speculation, many others think because he was such a good artist, he drew much more than what was actually there. And going back to your conversation, he was much more sensitive to the visual signals that were coming into his eyes and ears. And he drew neurons that had gaps between other neurons. And he turned out to be absolutely right. So one of the major discoveries in neuroscience can be at least partly attributed to somebody having a strong artistic aptitude as well.

SPENCER: Why do you think people view these two systems, science and art, as being so different?

ANIL: This separation, I think, has got a pretty unedifying history. There was the cultural theorist C.P. Snow, who long ago wrote about the Two Cultures, the two cultures of art and science. I think it's probably felt on both sides historically, which is just rooted in stereotypical assumptions about what the other side does. So scientists might dismiss artists as being unconstrained by reality, just purely operating in the realm of imagination. And not really revealing anything interesting about nature, not conducting experiments, or collecting data, or anything like that. And of course, artists might have a stereotype of scientists as not being creative, just turning the handle of methods and facts. And both of these stereotypes are entirely wrong. And they get embedded in how education is delivered. Now, I remember at school having to choose between doing art subjects and science subjects and I wanted to do both and I couldn't. It was very disappointing. It is a great relief when they come back together again in later life. And they do come back together again, they're both different ways of trying to understand our place in nature. I often think about visual art in particular because in my scientific work, I focus quite a lot on vision. And if you look at the work of the Impressionists like Monet, Manet, and Bizarro, and Cezanne — well, I've been told recently that Cezanne is not a proper impressionist, so I'll bow to that knowledge. — But really these paintings, which were controversial at the time, because they seem quite fuzzy up close, were just famously dismissed as palette scrapings on a dirty canvas. But when you stand back from them, the impression of a coherent scene is so vibrant, so compelling, and is very different from a photograph. And the reason is that impressionist art is sort of reverse engineering the visual system to paint. Not the output, not like, “Here's a tree and on a river.” But it's painting the raw materials needed for the visual system to do its work. It's getting close to painting light. I think that's amazing. Artists like Rembrandt and his understanding of light and shade reveal and tell us so much about how the visual system works. And I have no idea whether they thought that's what they were doing. But that really is why I think their work can really be usefully seen that way. And later art history justifies that. There was an art historian called Ernst Gombrich, he wrote about the Beholder's Share. This is the idea that in viewing an artwork or experiencing an artwork, part of that experience is brought by the observer, by the beholder. It's in the act of the beholder, not in the artwork itself. And so when you look at an impressionist work, part of that aesthetic experience is because the brain is being engaged in making sense of these ambiguous scrapings on the canvas. And that's why it's so powerful. And to make that happen, the Beholder's Share is sort of an art historical version of the idea that perception is about making predictions and interpreting sensory data. So, there's a really, really nice confluence between art and science, certainly in the visual arts, but I think in all sorts of artwork as well. Music can be thought of as an investigation into how the auditory system works. So the more crosstalk we can get between art and science, I think the better.

SPENCER: When I think about the differences between art and science, I guess to oversimplify, I think of it as art is about creating an experience (either for the viewer of the art or for the artists themselves) where science is about understanding the way things work, understanding reality, understanding the universe, or understanding the mind. And I guess when I think about the overlap between the two, it's when those two goals kind of happen to coincide. Where, in order to create an experience, you have to understand something about the perceptual system, or when we are understanding the universe happens to line up with understanding how to create an effect in a viewer or an artist. So, I am curious to hear your thoughts on that.

ANIL: Well. It's probably dangerous to ascribe purposes to art and science, sort of that uni-dimensional. Of course, science has many different purposes and it depends partly on who's funding it. Science can be about control. It can be about making money. It doesn't always have to be the idea of discovering how things work. And of course, a criterion for a successful scientific theory is often aesthetic. People think a theory is beautiful. And that's treated as a good thing in scientific explanations. And the same with art — and unless well versed in what the various purposes art is made for — but it's certainly not just to create the experience. Again, there's lots of social purposes that art is put to. But I agree with you that when there is an overlap, that's when something really interesting can happen in interaction between the two things. I've been recently involved in this project. Actually, this is where the Perception Census came out of which is one of the deepest art-science collaborations I've certainly ever experienced, where we were using stroboscopic light to give people visual hallucinations, and take them on an extraordinary kind of journey into the power and potential of their own minds. And this project had roots both in science for 60 years ago, when the British neuroscientist William Gray Walter discovered this phenomena and was the first person to really systematically study what happens in the brain when you sit in front of a strobe light with your eyes closed. And you have these explosions of color and shape in your mind. But at the same time, an artist called Brian Grison had discovered this effect and he wanted to make it a public art object, which had a strong social message to get people to resist the idea that art was something you passively consume to try and give people the first person experience of being the author of their own experiences, not the consumer of them. That historical interaction itself was super interesting. And then just recently, last year, we had the opportunity with a group of collaborators and musicians and architects and so on, to regenerate this for the new century. It's called the Dream Machine. And this is exactly what we did. And it was partly scientific because we needed to know how it works. We wanted to collect some data. Everybody has a different experience and why and how different. But fundamentally, to your point, we couldn't present it as an experiment. We didn't want to present it as an experiment. We wanted to offer it as an experience as something that we invited people to participate in. And it would be theirs. So they would have ownership of their own experience. So it is a project that would never have happened through an arts motivation or a science motivation alone. It really needed both. And it's still a super interesting thing to be involved with.

SPENCER: So from the point of view of someone attending the Dream Machine, what is the experience? What happens?

ANIL: So you go in. It's usually quite a big space — so we were in four different cities in the UK last year in a big church, in a public market, and in an ice rink — and the whole space is architected so that it feels very comfortable, warm, and welcoming. You take your shoes off, you then go into the Dream Machine itself, which is a big, blue box-like structure that can fit about 30 people sitting in a circle (and very comfy seating), you lie down, there's a sort of breathing exercise and so on. And then you close your eyes and flashing lights start, and they're just white light, that's all that's going on: flashing white light and music. And for about half an hour — maybe a bit less or more — you have visual experiences. It's different for every person, but pretty much everyone has a visual journey. And they can experience all sorts of things, colors and shapes and patterns, geometries, movements, sometimes complex imagery with people, places, and so on. But often, colors have a depth and vividness that exceeds what they might experience in everyday life. And then the opposite of imagery, if imagery is always a slightly faded version of the normal perceptual world we experienced, in the Dream Machine, it's often the other way, it's often more intense. And surprisingly, people don't generally expect this. And then after half an hour, they come out of the Dream Machine, and they're given lots of opportunities to talk about it and to draw pictures — we have 20,000 drawings of people's experiences in the Dream Machine now — and fill out surveys to give us some more systematic data about what's going on. And what we found over and over again, is firstly, people have extraordinary positive emotional reactions to this for the most part. Some people find it a bit freaky, but it's a small minority. People are surprised this happens, they don't believe that it's just white light, because they experience all these colors. But if they've listened to this podcast, they won't be surprised because they know colors are generated in the brain. And they develop this curiosity about the mind and the brain. Because we open our eyes and the world is just there, there's often no particular reason to think about what's going on under the hood to make this happen. It's very easy to take for granted that we just experienced the world as it is and ourselves as we are. But when you have an experience like the Dream Machine, it drives home in the first person, that experience can come from your own mind and brain and that your mind and your brain are extremely powerful. So for me this is one of the great legacies of this; it leads people to cultivate real appreciation for the complexity, the wonder, the potential of their own minds and brains, the uniqueness and how they're different from but related to others.

SPENCER: Is it known why flashing white lights would create such unique experiences and different people?

ANIL: Not entirely. And this is one of the things that actually we've been working on as a side project for quite a number of years. And we're putting a lot more effort into it now. And there's some other groups working on it, too, though not that many. There are some clues. One thing that has been known since the 1950s is that when you close your eyes and the light is sufficiently strong, what happens is it still activates the visual system, because light will still get into your eyes, go through the eyelids, and activate yourself and your retina. And you can see this if you record from the visual cortex. You can see big oscillations, big waves that are at the same frequency as the strobe light. So the strobe light is in training the brain. So that's got something to do with it. I think intuitively for me what's going on is that because you have your eyes closed, your brain is not expecting anything visual to be happening. You've kind of told the brain like, “Don't worry about vision, stand down, just listen. Do whatever but vision is offline for a bit, don't worry.” So it's in that state, but then it's getting activated anyway because the strobe light is sufficiently powerful. And so now the visual brain is in a funny situation where it's saying, “All this stuff is going on. And I have to make sense of it. But I don't know how to make sense of it because I'm not expecting anything to be going on at all.” And the only thing giving structure to the input is the intrinsic wiring of the brain itself. So my hypothesis here — and it's just a hypothesis at the moment — is that the phenomena that people experience in the Dream Machine is the result of the brain looking at itself. It's the brain interpreting this activity in terms of its own intrinsic structure. And so the geometrical patterns that are very commonly reported, might relate to the repeated, geometrical patterns that we see if we look at how the visual cortex is wired up. But why there's any experience at all? That is still a big question and I don't yet have an answer for that.

SPENCER: Anil, thanks so much for coming on. This was really fun.

ANIL: Oh. Thank you very much, Spencer, I really enjoyed the chat. Thank you for inviting me.


JOSH: A listener asks: Do you feel more people are getting diagnosed with ADHD because of increased awareness? Or do you feel like the internet is making people develop ADHD-like behaviors?

SPENCER: Well, I haven't looked at the stats. I do think it's probably true that more people are being diagnosed with ADHD. Probably some of that is from increased awareness. I know tons of people that didn't realize their ADHD until their 30s or even later. So I do think increased awareness is part of that. And I do know a bunch of people who once they realized that they have ADHD and got treated for it, they were like, “Wow, this is so much better.” I do wonder whether social media sites, especially Tiktok, could give people certain ADHD-like qualities. It's not the same as giving someone ADHD. But watching, let's say 30-second videos for five hours (some people do daily). And having them change so fast and have them be so stimulating, it has a certain ADHD-like quality to it, I feel. And I wonder about the cumulative effects of doing that day-after-day-after-day. If that could give people a more ADHD-like mind and then that might actually mean that they seek treatment for ADHD, and maybe even get diagnosed with it more. So it's just a question mark in my mind.




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