What is a thought? Is it something physical? How can you hear a voice in your head? And whose voice is it anyway? And what does this have to do with a small marine animal who eats its own brain. Welcome to Intercosmos with me, David Eagleman. I'm a neuroscientist and an author at Stanford and in these episodes we sail deeply into our three pound universe to understand some of the most surprising aspects of our lives. Today's episode is about thoughts. We have them constantly, even for the best meditators. It's difficult or impossible to stop the fire hose of words and images and ideas. We all talk about our thoughts. We sometimes act on our thoughts, we draw them with little thought bubbles and cartoons. But what in the world is a thought physically? So many years ago, a person at a party begged me to watch the movie called The Secret. So I watched it for fifteen minutes, and I regret to say that I will never get those fifteen minutes back. And I knew that this was the pinnacle of moronic when the first guy states quote, what most people don't understand is a thought has a frequency. Every thought has a frequency we can measure a thought, and so if you're thinking that thought over and over again, you're emitting that frequency on a consistent basis. And then the next Schremndrick says, thoughts are sending out that magnetic signal that is drawing the parallel back to you. That's the signal you're putting out into the universe. And then a third genius says, it has now been proven scientifically that an affirmative thought is hundreds of times more powerful than a negative thought. And every time someone thinks a thought in this movie, the graphics show the person having a thinking expression on their face, and then an energy wave bursts from their head, and then it cuts to a wide shot of the planet and you see this energy wave transmit across the universe. Now, the most striking thing about these statements in the movie is not simply that they are incorrect. To me, the shocking thing was how these guys looked right at the camera and asserted them as though these were not just completely made up, and then they used phrases like it has been scientifically proven. So in a few weeks, I'm going to do an episode about how we judge the value of any scientific idea and what it would mean to even say that something is scientifically proven. But I'm going to bite my tongue for now put that aside, because for today's episode, what I really care about is what really is a thought. So to set the table for this, I'm going to put thoughts aside for a second and tell a story that I originally wrote in my book Incognito, And this took place in nineteen forty nine. A guy named Arthur Alberts traveled from his home in New York to villages between the Gold Coast and Timbuktoo in West Africa. And he brought his wife, and he brought a camera and a jeep, and because of his love for music, he brought a tape recorder. Now he wanted to open the ears of the Western world. So what he did was record some of the most important music to ever come out of Africa. But Alberts ran into social troubles while he was using this tape recorder because none of the natives had ever seen anything even vaguely like this before. So one native heard his voice played back to him, and he accused Alberts of quote stealing his tongue and According to the biography, Albert's only narrowly averted being pummeled by taking out a mirror and convincing the man that his tongue was still intact. So it's not difficult to see why the natives found the tape recorder so strange and counterintuitive. Just imagine that you had never ever seen this technology before. The thing is that a voice seems ephemeral and ineffable, right. A voice has no weight. You can't hold voice in your hand. A voice just exists for a moment. It's totally untouchable, and then it's gone. There doesn't seem to be any physical trace. So it comes as a surprise that a voice is physical. If you build a little machine that's sensitive enough to detect tiny compressions of the molecule in the air, you can then capture those density changes and reproduce them later. We call these little machines microphones, and we reproduce the density changes on tape or with zeros and ones, And every one of the billions of podcasts and playlists on the planet is proudly serving up bags of feathers once thought irretrievable. When Alberts played the music back from the tape recorder. One tribesman described the feat as tremendous magic. And so it goes with thoughts. What exactly is a thought? It doesn't seem to weigh anything. It feels ephemeral and ineffable. You wouldn't think that a thought has any shape or smell or any sort of physical instantiation. Thoughts seem to be a kind of tremendous magic. So it comes as a surprise that a thought is physical, just like voice is. Thoughts are underpinned by biological stuff. We know this because alterations to the brain change the kinds of thoughts we can think. In a state of deep sleep, there are no thoughts. When the brain transitions into dream sleep, there are unbidden, bizarre thoughts. During the day, we enjoy our normal, well accepted thoughts, which people enthusiastically modulate by spiking the chemical cocktails of the brain with alcohol or narcotics, or exercise or coffee. So the state of the physical material determines the state of the thoughts, and issues like obsessive compulsive disorder or schizophrenia tell us that when certain networks in the brain become overactive or miscalibrated, the character of thinking changes. It's this kind of thing that tells us that the physical stuff is irreversible, tied to the thinking. And we know this also because when people get even small bits of brain damage, let's say, because of a stroke or a tumor, that can change their capacity to understand music, or name animals or see colors, or judge risk, or make decisions, or read signals from their bodies, or understand the concept of a mirror, or hundreds of other changes in thinking that we can witness in the clinics every day, and the consequences of injury is a big part of how brain science has mapped out the general blueprints of the brain. So the bottom line lesson which has emerged over centuries is that our internal thoughts about hopes or fears or desires, they all emerge from this strange three pound organ and when the brain changes, so do our thoughts. So although it's easy to intuit that thoughts don't have a physical basis, that there are something like feathers on the wind, they in fact depend directly on what's happening in this small, enigmatic mission control center locked in the silent vault of the skull. So what is a thought. It's the result of billions of neurons firing and coordinated patterns. As we've talked about in other episodes, the neurons communicate using electrical impulses and chemical signals, and they form vast networks that operate together, such that at any moment you have some millions of neurons doing something coordinated, and in the next moment it's a different collection of neurons that are working together. Now, it may not surprise you to know that a thought is not located in one place in the brain. It's an emergent property. It's a collaboration between these millions or billions of neurons. When you have a thought, it's like a symphony playing inside your head. Every neuron involved is an instrument in the orchestra, and no single part can create the music by itself. So we're gonna come back to the brain in a moment, but before we do, I want us to really zoom in on the subjective experience. How would you describe the experience of a thought. When you try to capture what a thought is, the most commonplace to go is your inner monologue. This is the voice in your head that you use to problem solve or criticize, or plan or reflect. So take just a second to pay attention to the voice inside your head, that constant stream of chatter. Now you might think, what voice inside my head? That's the voice? Now. Your internal voice can involve deliberate problem solving, like what should I do next here? Or it can involve involuntary commentary like oh so stupid to me, why did I do that? Or it can be whole imagined conversations like Okay, I say this, and then she says that, and then I respond like this. It can be helpful for things like working memory. So for example, you're trying to log into a website and you get texted some six digit pass code, and you need to remember that code while you switch windows, So you internally say the numbers over and over. So you're using the internal voice to keep the information in mind as part of your working memory. There's so many places where the internal voice comes in. We tend to talk to ourselves when we're planning something, when we're weighing options. The internal dialogue seems to be really helpful in simulating different scenarios. So you're running your internal voice all the time and subjectively, it seems to be a mental space that can seem almost as vivid as external speech. It follows all the rules of grammar and syntax, and it can be emotionally laiden with encouragement or criticism or anywhere in between. Now, how does this happen? How can you generate a voice internally and hear it well? When we eavesdrop on the brain, let's say, using brain imaging like fMRI, what we find is a network of areas involved in speech production, like Broca's area, which is a key region involved in talking out loud. And we find areas involved in language comprehension like Wernicke's area and the auditory cortex more generally, as though you're listening to spoken language from somebody else talking. And also what we see is a network that we summarize as the default mode network. And this is a network that becomes active when you're not focused on monitoring the outside world, but instead when you think about yourself, where you daydream or you reflect on your past, or you imagine your future. So all these networks are cranked up when you're talking to yourself. And one really important point, when you're generating your own speech is that you have something called corollary discharge, which just means that when your brain generates internal speech, it sends a copy of what it's doing to other parts of the brain to let them know this is coming. I'm the one who did this. That's how the rest of the brain knows that the inner voice is self generated instead of confusing it with external speech. As a side note, if you're a regular listener to this podcast, you know that I've been publishing on the topic of schizophrenia for many years, and my hypothesis is that auditory hallucinations in schizophrenia might have to do with a miscalibration of the timing of signals such that the internal voice gets misinterpreted as someone else's voice. In other words, when you're talking to yourself under normal conditions, you generate the voice, then you hear the voice, and the corollary discharge tells you that it was your own. But there's something wrong with the timing of the corollary discharge. Then it doesn't seem like you're the one who generated the voice, and you have to attribute the voice to someone else. The internal voice feels like it must be external. If you're interested in more on this hypothesis about schizophrenia, check out episodes thirty three and forty four. Okay, so back to the internal voice, which in normal circumstances we understand as our voice in our heads. Here's my question, is this inner monologue really the main way that we think? What if some of our thoughts don't come with words at all? So that's the question that psychologist Russell Hurlbert set out to answer with a set of experiments in the nineteen nineties. And here's how it works. Imagine that you are given a beeper that beeps off at random times during your day, and the instant a beeps, your job is to write down whatever was just in your mind. Beep, what are you thinking about right now? So when Hurlbert analyzed the data, he found that only about twenty six percent of people's thoughts were verbal, meaning that they involved actual words or inner speech. The other seventy four percent completely nonverbal. So what does nonverbal thought mean? Well, when your thoughts are suddenly probe, lots of people find that they were just thinking about, for example, vivid mental images like picturing a beach or remembering somebody's face, or thinking about some moment they just saw at the restaurant, and that's all their thought was at that moment. It was visual. There were no words involved, just pictures. So what's going on in the brain when you have these kind of vivid mental images. Well, if I were to ask you to think about a Siberian tiger, your prefrontal cortex right behind the forehead takes on the task and starts broadcasting to see who can fulfill the request. So your memory systems chug into place to determine what a Siberian tiger is, combining all the different examples of Siberian tigers that you've seen before, and your visual cortex is recruited to generate a picture. Your emotion centers might even contribute, shading this thought with feelings of alertness or awe or fear. So when you think about that Siberian tiger, hundreds of millions of neurons across different areas of your brain are lighting up, working together to create something that feels whole and immediate. And of course, if you're a musician and the pager beeps, you might find that you're thinking about something auditory. You're thinking about which notes would sound great. Right after this transition from this chord to this court. But there's no words involved in that, and in the brain we can measure activity in your auditory cortex while you're doing that, And if you're a perfumer, you might be doing smell imagery. You're thinking about the way that this other perfume smell than how you're trying to get a little closer to that. But as you are internally smelling, there are no words associated with that. It's just olfactory imagery. And this sort of sensory thinking can come in all kinds of flavors, like feeling the warmth from the sun on your face or the tightness of your chest during stress. So that's what Hurlbert found people were thinking about quite often when you probe them at random, sensory imagery without language. But that was just the beginning. The kind of thoughts that people were went beyond just sensory imagery. Lots of other times people described they were thinking about how to do something physical, like how to position their hands for reaching into the oven, or how to remove the cover off the printer, or how far to turn the steering wheel to get their car into the parking spot. But they were thinking about how to do these moves, and there were no words involved. It was a physical activity they were simulating. This is called motoric imagery. Their brains were thinking their way through something. And other thoughts are even more difficult to pin down than imagining senses or imagining movement because they're more abstract, like a vague sense of unease with no clear words or pictures attached. So thoughts come in many flavors, and I'll just make a thirty second side note here about whether it makes sense to call the activity in the unconscious brain think. For example, you're trying to remember the name of that song and it's on the tip of your tongue, but you just can't remember it. And then hours later, when you're not even thinking about it, the answer suddenly pops into your mind. Your unconscious mind has been working on it in the background, even though you had no awareness of it. So should we call that thinking even when it happens unconsciously. Just for the purposes of having a clear definition, it's probably going to make the most sense to call that something like processing, and will reserve the word thought for the conscious conclusion of that behind the scenes activity, So at least for the moment, I'm not going to call the unconscious activity thought. So this simple experiment of pinging people at random to ask them what they're thinking, what's in their minds at this exact moment, This experiment tells us something important, which is that thinking is not just talking to ourselves. It's broader than that. We have different kinds of thoughts in different formats. We have inner speech, but we also have mental imagery, like when you're visualizing what your kitchen red design could look like, and you have abstract thinking like when you're contemplating infinity or justice or love. And possibly there are still other things which could fall under the umbrella of thought. So thinking is a rich, multi dimensional experience. Okay, so here's where we are so far. We talked about the internal voice and visualization and imagining sound or smell or bodily feelings, and imagining motor movement, and even abstract sorts of thoughts. But if you've been listening to these episodes for a while, you know that I'm obsessed with the difference between people's internal experiences and thinking is no exception. When we look at the diversity of thought, some people seem to lean more towards verbal thinking. They talk things out in their heads. Others are visual things, seeing vivid mental images as their primary mode of thinking. And some people experience thoughts more like abstract concepts without the voice or the images. And this goes hand in hand with something I've talked about in many episodes, which is that, as far as we can tell, subjective experiences exist on a spectrum. So let's zoom in on the inner voice. Some people report having constant chatter in their heads, while other people have little or no inner verbalization. That's called an endophasia no internal voice. As an example, when it comes to the inner monologue, my wife's internal radio is very loud, she says, she's always hearing it at full volume. For me, it happens to be pretty quiet most of the time. I'm going to link some papers in the show notes about studies on the variation of the internal voice. And when it comes to visualization, I've talked in other episodes about this from a fantasia to hyperfantasia. In other words, the spread from not really picturing anything visually in your mind to having very rich, colorful, movie like visualizations. And everyone is somewhere along that spectrum. Now, we haven't really measured this yet, but when it comes to more subtle issues of abstract thinking, like contemplating infinity or justice or love, it may be that people are having very different experiences of how strongly or intensely they're feeling that. Now, it's a little difficult to design an experiment to probe this, because it's purely an issue of somebody's subjective report, and it's not always easy to know if people are reporting accurately. But if this subjective experience of abstract thought is like everything else we've measured so far, it is surely going to differ from person to person. So it seems to me there's probably massive of divergence in what we mean from person to person when we talk about the experience of thought. If we can only know what the experience is inside another head, we might suddenly understand why Susan immediately sees the solution to the math problem, and why Amy keeps in mind so well what everyone else's emotion is, and why Steve is so interested in fixing broken radios, and why Tim spends all his intellectual efforts figuring out how to get other people to do work for him. And so on with the differences between every person you know. And by the way, this diversity in the inner experience this has real world implications. So just think about how people learn or solve problems. Schools and workplaces often prioritize verbal reasoning. But what if somebody's thoughts arrive more like pictures or more like physical sensations. What if their best ideas can't be put in words immediately? I think as we bring the individual differences in thinking into focus, we'll be able to increasingly build education to take advantage of the full spectrum of human cognition and understanding this diversity of thought. This also has implications for mental health. Verbal thoughts, for example, are strongly linked to rumination, which is the endless loops of self talk that fuel anxiety or depression. And meanwhile, nonverbal sensations like a tight chest or a racing heart, these are the things that dominate panic attacks. So really, understanding how differently people think and how to measure that could help us to manage these mental states more effectively. Okay, so we've been talking about our private internal experience of thoughts, but we still haven't nailed down with a thought is exactly and what could this possibly have to do with a c squirt finding its home? So let's start there. The sea squirt is a small marine creature that begins life as a free swimming larva. It has a little brain and a nervous system that helps it navigate and search for a suitable place to settle. Now, once it finds its permanent spot, it attaches itself to a surface like a barnacle, and then it undergoes a dramatic transformation because in this phase, once it's docked, it no longer needs its brain for movement or navigation, so it eats its brain for nutrition. It digests its own brain, and it uses that as nutrients for other bodily functions. So this illustrates two things. First, how incredible it is that some organisms can radically adapt their anatomy to fit their new role. But more importantly, for today's purpose, the main lesson from the sea squirt is that you only we need a brain for one purpose, and that is to move. If you've stopped moving, a brain serves only as a little snack for nutrition. And that's an idea that's been floating around in neuroscience for well over a century. The reason for the brain's evolution is movement control. The need to move and interact with the environment is the driving force behind the development of the nervous system. In other words, brains exist for one purpose, and that is to get around. So now let's return to thinking. The big idea for today is that thinking is like a physical movement. You're moving stuff around on the inside, but nothing on the outside. You are moving concepts instead of limbs. In other words, thinking is simply an outgrowth of the same brain mechanisms that govern moving. This idea reached way back into the scientific literature, but the most complete version of the argument that I know comes from the neuroscientist Rudolfo Ginas in his book called Eye of the Vortex. The key is that to get good at movements, your brain works to predict the outcome of possible actions. So as brains grew more sophisticated, they could run simulations that didn't necessarily result in overt behavior. So the brain generates predictions about the environment, and then it tries things out and it adjusts things based on the feedback, and that's how it refines future predictions. The key idea is that this predictive function of the brain eventually extended into the realm of cognition. So a thought is like an internalized movement simulations of possible actions or scenarios that don't necessarily result in overt behavior. When we think, the brain is doing a kind of mental rehearsal, like motor planning. It generates and navigates through thoughts by simulating potential outcomes, but all without the body physically moving, just the way that athletes mentally practice movements before performing them. In other words, thought can be viewed as the brain's way of moving through abstract mental landscapes, just as it would move through physical space. And what this means is that the mind is inseparable from the body's motor control systems. And I just want to note that this framework has far reaching implications for how we understand brains and brain disorders, because conditions that affect movement, like Parkinson's disease or motor neuron disease, might also give us insights into disorders of thought and consciousness and things that we lump into cognitive disorders like schizophrenia or obsessive compulsive disorder. These could perhaps be viewed through the lens of disrupted internal movements. The way to understand all of this is that the brain is loopy. Very primitive brains have inputs that lead to outputs, but our brains became more sophisticated such that you find all kinds of internal loops. One piece of brain anatomy worth mentioning here is a structure deep inside the brain called the thalamus. All the inputs and outputs of the brain stop in the thalamus like a trainway station, and what you get are these very sophisticated loops called the lamo cortical loops, which allows information to move around internally as though things are moving in the world, but without actually moving them. In this way, instead of the brain generating a movement and it happens right away, instead the brain can run a simulation internally to see what would be predicted to happen if the movement were to be made, and then eventually the simulations can be not just about pushing this button or lifting this coffee cup, but more abstract, like what would it be like if I got that job promotion, or how should I break this news to my friend? Or what is the optimal path to build a society for peace and justice. Now with new data about the brain, we can go even deeper to see how this would work. For example, in a recent episode, I talked with neuroscientist Jeff Hawkins on his theory about cortical columns. Think of cortical columns like little rice grain sized units that are all packed together in the cortex, and you have hundreds of thousands of them, and each one takes care of little overlapping aspects the world, and together they communicate and collaborate to build a larger internal model of the world. So here's a clip from that interview which I didn't include in the original cut, on the topic of what the columns are coding for. And Jeff points out that the cells that you find in mammals called grid cells, seem to be coding for two dimensional space, but they can also code for three dimensional space. And once you have these mechanisms for coding for movement in space, maybe those cells can do something more. Here's Jeff.

What is a thought? Is it something physical? How can you hear a voice in your head? And whose voice is it anyway? And what does this have to do with a small marine animal who eats its own brain. Welcome to Intercosmos with me, David Eagleman. I'm a neuroscientist and an author at Stanford and in these episodes we sail deeply into our three pound universe to understand some of the most surprising aspects of our lives. Today's episode is about thoughts. We have them constantly, even for the best meditators. It's difficult or impossible to stop the fire hose of words and images and ideas. We all talk about our thoughts. We sometimes act on our thoughts, we draw them with little thought bubbles and cartoons. But what in the world is a thought physically? So many years ago, a person at a party begged me to watch the movie called The Secret. So I watched it for fifteen minutes, and I regret to say that I will never get those fifteen minutes back. And I knew that this was the pinnacle of moronic when the first guy states quote, what most people don't understand is a thought has a frequency. Every thought has a frequency we can measure a thought, and so if you're thinking that thought over and over again, you're emitting that frequency on a consistent basis. And then the next Schremndrick says, thoughts are sending out that magnetic signal that is drawing the parallel back to you. That's the signal you're putting out into the universe. And then a third genius says, it has now been proven scientifically that an affirmative thought is hundreds of times more powerful than a negative thought. And every time someone thinks a thought in this movie, the graphics show the person having a thinking expression on their face, and then an energy wave bursts from their head, and then it cuts to a wide shot of the planet and you see this energy wave transmit across the universe. Now, the most striking thing about these statements in the movie is not simply that they are incorrect. To me, the shocking thing was how these guys looked right at the camera and asserted them as though these were not just completely made up, and then they used phrases like it has been scientifically proven. So in a few weeks, I'm going to do an episode about how we judge the value of any scientific idea and what it would mean to even say that something is scientifically proven. But I'm going to bite my tongue for now put that aside, because for today's episode, what I really care about is what really is a thought. So to set the table for this, I'm going to put thoughts aside for a second and tell a story that I originally wrote in my book Incognito, And this took place in nineteen forty nine. A guy named Arthur Alberts traveled from his home in New York to villages between the Gold Coast and Timbuktoo in West Africa. And he brought his wife, and he brought a camera and a jeep, and because of his love for music, he brought a tape recorder. Now he wanted to open the ears of the Western world. So what he did was record some of the most important music to ever come out of Africa. But Alberts ran into social troubles while he was using this tape recorder because none of the natives had ever seen anything even vaguely like this before. So one native heard his voice played back to him, and he accused Alberts of quote stealing his tongue and According to the biography, Albert's only narrowly averted being pummeled by taking out a mirror and convincing the man that his tongue was still intact. So it's not difficult to see why the natives found the tape recorder so strange and counterintuitive. Just imagine that you had never ever seen this technology before. The thing is that a voice seems ephemeral and ineffable, right. A voice has no weight. You can't hold voice in your hand. A voice just exists for a moment. It's totally untouchable, and then it's gone. There doesn't seem to be any physical trace. So it comes as a surprise that a voice is physical. If you build a little machine that's sensitive enough to detect tiny compressions of the molecule in the air, you can then capture those density changes and reproduce them later. We call these little machines microphones, and we reproduce the density changes on tape or with zeros and ones, And every one of the billions of podcasts and playlists on the planet is proudly serving up bags of feathers once thought irretrievable. When Alberts played the music back from the tape recorder. One tribesman described the feat as tremendous magic. And so it goes with thoughts. What exactly is a thought? It doesn't seem to weigh anything. It feels ephemeral and ineffable. You wouldn't think that a thought has any shape or smell or any sort of physical instantiation. Thoughts seem to be a kind of tremendous magic. So it comes as a surprise that a thought is physical, just like voice is. Thoughts are underpinned by biological stuff. We know this because alterations to the brain change the kinds of thoughts we can think. In a state of deep sleep, there are no thoughts. When the brain transitions into dream sleep, there are unbidden, bizarre thoughts. During the day, we enjoy our normal, well accepted thoughts, which people enthusiastically modulate by spiking the chemical cocktails of the brain with alcohol or narcotics, or exercise or coffee. So the state of the physical material determines the state of the thoughts, and issues like obsessive compulsive disorder or schizophrenia tell us that when certain networks in the brain become overactive or miscalibrated, the character of thinking changes. It's this kind of thing that tells us that the physical stuff is irreversible, tied to the thinking. And we know this also because when people get even small bits of brain damage, let's say, because of a stroke or a tumor, that can change their capacity to understand music, or name animals or see colors, or judge risk, or make decisions, or read signals from their bodies, or understand the concept of a mirror, or hundreds of other changes in thinking that we can witness in the clinics every day, and the consequences of injury is a big part of how brain science has mapped out the general blueprints of the brain. So the bottom line lesson which has emerged over centuries is that our internal thoughts about hopes or fears or desires, they all emerge from this strange three pound organ and when the brain changes, so do our thoughts. So although it's easy to intuit that thoughts don't have a physical basis, that there are something like feathers on the wind, they in fact depend directly on what's happening in this small, enigmatic mission control center locked in the silent vault of the skull. So what is a thought. It's the result of billions of neurons firing and coordinated patterns. As we've talked about in other episodes, the neurons communicate using electrical impulses and chemical signals, and they form vast networks that operate together, such that at any moment you have some millions of neurons doing something coordinated, and in the next moment it's a different collection of neurons that are working together. Now, it may not surprise you to know that a thought is not located in one place in the brain. It's an emergent property. It's a collaboration between these millions or billions of neurons. When you have a thought, it's like a symphony playing inside your head. Every neuron involved is an instrument in the orchestra, and no single part can create the music by itself. So we're gonna come back to the brain in a moment, but before we do, I want us to really zoom in on the subjective experience. How would you describe the experience of a thought. When you try to capture what a thought is, the most commonplace to go is your inner monologue. This is the voice in your head that you use to problem solve or criticize, or plan or reflect. So take just a second to pay attention to the voice inside your head, that constant stream of chatter. Now you might think, what voice inside my head? That's the voice? Now. Your internal voice can involve deliberate problem solving, like what should I do next here? Or it can involve involuntary commentary like oh so stupid to me, why did I do that? Or it can be whole imagined conversations like Okay, I say this, and then she says that, and then I respond like this. It can be helpful for things like working memory. So for example, you're trying to log into a website and you get texted some six digit pass code, and you need to remember that code while you switch windows, So you internally say the numbers over and over. So you're using the internal voice to keep the information in mind as part of your working memory. There's so many places where the internal voice comes in. We tend to talk to ourselves when we're planning something, when we're weighing options. The internal dialogue seems to be really helpful in simulating different scenarios. So you're running your internal voice all the time and subjectively, it seems to be a mental space that can seem almost as vivid as external speech. It follows all the rules of grammar and syntax, and it can be emotionally laiden with encouragement or criticism or anywhere in between. Now, how does this happen? How can you generate a voice internally and hear it well? When we eavesdrop on the brain, let's say, using brain imaging like fMRI, what we find is a network of areas involved in speech production, like Broca's area, which is a key region involved in talking out loud. And we find areas involved in language comprehension like Wernicke's area and the auditory cortex more generally, as though you're listening to spoken language from somebody else talking. And also what we see is a network that we summarize as the default mode network. And this is a network that becomes active when you're not focused on monitoring the outside world, but instead when you think about yourself, where you daydream or you reflect on your past, or you imagine your future. So all these networks are cranked up when you're talking to yourself. And one really important point, when you're generating your own speech is that you have something called corollary discharge, which just means that when your brain generates internal speech, it sends a copy of what it's doing to other parts of the brain to let them know this is coming. I'm the one who did this. That's how the rest of the brain knows that the inner voice is self generated instead of confusing it with external speech. As a side note, if you're a regular listener to this podcast, you know that I've been publishing on the topic of schizophrenia for many years, and my hypothesis is that auditory hallucinations in schizophrenia might have to do with a miscalibration of the timing of signals such that the internal voice gets misinterpreted as someone else's voice. In other words, when you're talking to yourself under normal conditions, you generate the voice, then you hear the voice, and the corollary discharge tells you that it was your own. But there's something wrong with the timing of the corollary discharge. Then it doesn't seem like you're the one who generated the voice, and you have to attribute the voice to someone else. The internal voice feels like it must be external. If you're interested in more on this hypothesis about schizophrenia, check out episodes thirty three and forty four. Okay, so back to the internal voice, which in normal circumstances we understand as our voice in our heads. Here's my question, is this inner monologue really the main way that we think? What if some of our thoughts don't come with words at all? So that's the question that psychologist Russell Hurlbert set out to answer with a set of experiments in the nineteen nineties. And here's how it works. Imagine that you are given a beeper that beeps off at random times during your day, and the instant a beeps, your job is to write down whatever was just in your mind. Beep, what are you thinking about right now? So when Hurlbert analyzed the data, he found that only about twenty six percent of people's thoughts were verbal, meaning that they involved actual words or inner speech. The other seventy four percent completely nonverbal. So what does nonverbal thought mean? Well, when your thoughts are suddenly probe, lots of people find that they were just thinking about, for example, vivid mental images like picturing a beach or remembering somebody's face, or thinking about some moment they just saw at the restaurant, and that's all their thought was at that moment. It was visual. There were no words involved, just pictures. So what's going on in the brain when you have these kind of vivid mental images. Well, if I were to ask you to think about a Siberian tiger, your prefrontal cortex right behind the forehead takes on the task and starts broadcasting to see who can fulfill the request. So your memory systems chug into place to determine what a Siberian tiger is, combining all the different examples of Siberian tigers that you've seen before, and your visual cortex is recruited to generate a picture. Your emotion centers might even contribute, shading this thought with feelings of alertness or awe or fear. So when you think about that Siberian tiger, hundreds of millions of neurons across different areas of your brain are lighting up, working together to create something that feels whole and immediate. And of course, if you're a musician and the pager beeps, you might find that you're thinking about something auditory. You're thinking about which notes would sound great. Right after this transition from this chord to this court. But there's no words involved in that, and in the brain we can measure activity in your auditory cortex while you're doing that, And if you're a perfumer, you might be doing smell imagery. You're thinking about the way that this other perfume smell than how you're trying to get a little closer to that. But as you are internally smelling, there are no words associated with that. It's just olfactory imagery. And this sort of sensory thinking can come in all kinds of flavors, like feeling the warmth from the sun on your face or the tightness of your chest during stress. So that's what Hurlbert found people were thinking about quite often when you probe them at random, sensory imagery without language. But that was just the beginning. The kind of thoughts that people were went beyond just sensory imagery. Lots of other times people described they were thinking about how to do something physical, like how to position their hands for reaching into the oven, or how to remove the cover off the printer, or how far to turn the steering wheel to get their car into the parking spot. But they were thinking about how to do these moves, and there were no words involved. It was a physical activity they were simulating. This is called motoric imagery. Their brains were thinking their way through something. And other thoughts are even more difficult to pin down than imagining senses or imagining movement because they're more abstract, like a vague sense of unease with no clear words or pictures attached. So thoughts come in many flavors, and I'll just make a thirty second side note here about whether it makes sense to call the activity in the unconscious brain think. For example, you're trying to remember the name of that song and it's on the tip of your tongue, but you just can't remember it. And then hours later, when you're not even thinking about it, the answer suddenly pops into your mind. Your unconscious mind has been working on it in the background, even though you had no awareness of it. So should we call that thinking even when it happens unconsciously. Just for the purposes of having a clear definition, it's probably going to make the most sense to call that something like processing, and will reserve the word thought for the conscious conclusion of that behind the scenes activity, So at least for the moment, I'm not going to call the unconscious activity thought. So this simple experiment of pinging people at random to ask them what they're thinking, what's in their minds at this exact moment, This experiment tells us something important, which is that thinking is not just talking to ourselves. It's broader than that. We have different kinds of thoughts in different formats. We have inner speech, but we also have mental imagery, like when you're visualizing what your kitchen red design could look like, and you have abstract thinking like when you're contemplating infinity or justice or love. And possibly there are still other things which could fall under the umbrella of thought. So thinking is a rich, multi dimensional experience. Okay, so here's where we are so far. We talked about the internal voice and visualization and imagining sound or smell or bodily feelings, and imagining motor movement, and even abstract sorts of thoughts. But if you've been listening to these episodes for a while, you know that I'm obsessed with the difference between people's internal experiences and thinking is no exception. When we look at the diversity of thought, some people seem to lean more towards verbal thinking. They talk things out in their heads. Others are visual things, seeing vivid mental images as their primary mode of thinking. And some people experience thoughts more like abstract concepts without the voice or the images. And this goes hand in hand with something I've talked about in many episodes, which is that, as far as we can tell, subjective experiences exist on a spectrum. So let's zoom in on the inner voice. Some people report having constant chatter in their heads, while other people have little or no inner verbalization. That's called an endophasia no internal voice. As an example, when it comes to the inner monologue, my wife's internal radio is very loud, she says, she's always hearing it at full volume. For me, it happens to be pretty quiet most of the time. I'm going to link some papers in the show notes about studies on the variation of the internal voice. And when it comes to visualization, I've talked in other episodes about this from a fantasia to hyperfantasia. In other words, the spread from not really picturing anything visually in your mind to having very rich, colorful, movie like visualizations. And everyone is somewhere along that spectrum. Now, we haven't really measured this yet, but when it comes to more subtle issues of abstract thinking, like contemplating infinity or justice or love, it may be that people are having very different experiences of how strongly or intensely they're feeling that. Now, it's a little difficult to design an experiment to probe this, because it's purely an issue of somebody's subjective report, and it's not always easy to know if people are reporting accurately. But if this subjective experience of abstract thought is like everything else we've measured so far, it is surely going to differ from person to person. So it seems to me there's probably massive of divergence in what we mean from person to person when we talk about the experience of thought. If we can only know what the experience is inside another head, we might suddenly understand why Susan immediately sees the solution to the math problem, and why Amy keeps in mind so well what everyone else's emotion is, and why Steve is so interested in fixing broken radios, and why Tim spends all his intellectual efforts figuring out how to get other people to do work for him. And so on with the differences between every person you know. And by the way, this diversity in the inner experience this has real world implications. So just think about how people learn or solve problems. Schools and workplaces often prioritize verbal reasoning. But what if somebody's thoughts arrive more like pictures or more like physical sensations. What if their best ideas can't be put in words immediately? I think as we bring the individual differences in thinking into focus, we'll be able to increasingly build education to take advantage of the full spectrum of human cognition and understanding this diversity of thought. This also has implications for mental health. Verbal thoughts, for example, are strongly linked to rumination, which is the endless loops of self talk that fuel anxiety or depression. And meanwhile, nonverbal sensations like a tight chest or a racing heart, these are the things that dominate panic attacks. So really, understanding how differently people think and how to measure that could help us to manage these mental states more effectively. Okay, so we've been talking about our private internal experience of thoughts, but we still haven't nailed down with a thought is exactly and what could this possibly have to do with a c squirt finding its home? So let's start there. The sea squirt is a small marine creature that begins life as a free swimming larva. It has a little brain and a nervous system that helps it navigate and search for a suitable place to settle. Now, once it finds its permanent spot, it attaches itself to a surface like a barnacle, and then it undergoes a dramatic transformation because in this phase, once it's docked, it no longer needs its brain for movement or navigation, so it eats its brain for nutrition. It digests its own brain, and it uses that as nutrients for other bodily functions. So this illustrates two things. First, how incredible it is that some organisms can radically adapt their anatomy to fit their new role. But more importantly, for today's purpose, the main lesson from the sea squirt is that you only we need a brain for one purpose, and that is to move. If you've stopped moving, a brain serves only as a little snack for nutrition. And that's an idea that's been floating around in neuroscience for well over a century. The reason for the brain's evolution is movement control. The need to move and interact with the environment is the driving force behind the development of the nervous system. In other words, brains exist for one purpose, and that is to get around. So now let's return to thinking. The big idea for today is that thinking is like a physical movement. You're moving stuff around on the inside, but nothing on the outside. You are moving concepts instead of limbs. In other words, thinking is simply an outgrowth of the same brain mechanisms that govern moving. This idea reached way back into the scientific literature, but the most complete version of the argument that I know comes from the neuroscientist Rudolfo Ginas in his book called Eye of the Vortex. The key is that to get good at movements, your brain works to predict the outcome of possible actions. So as brains grew more sophisticated, they could run simulations that didn't necessarily result in overt behavior. So the brain generates predictions about the environment, and then it tries things out and it adjusts things based on the feedback, and that's how it refines future predictions. The key idea is that this predictive function of the brain eventually extended into the realm of cognition. So a thought is like an internalized movement simulations of possible actions or scenarios that don't necessarily result in overt behavior. When we think, the brain is doing a kind of mental rehearsal, like motor planning. It generates and navigates through thoughts by simulating potential outcomes, but all without the body physically moving, just the way that athletes mentally practice movements before performing them. In other words, thought can be viewed as the brain's way of moving through abstract mental landscapes, just as it would move through physical space. And what this means is that the mind is inseparable from the body's motor control systems. And I just want to note that this framework has far reaching implications for how we understand brains and brain disorders, because conditions that affect movement, like Parkinson's disease or motor neuron disease, might also give us insights into disorders of thought and consciousness and things that we lump into cognitive disorders like schizophrenia or obsessive compulsive disorder. These could perhaps be viewed through the lens of disrupted internal movements. The way to understand all of this is that the brain is loopy. Very primitive brains have inputs that lead to outputs, but our brains became more sophisticated such that you find all kinds of internal loops. One piece of brain anatomy worth mentioning here is a structure deep inside the brain called the thalamus. All the inputs and outputs of the brain stop in the thalamus like a trainway station, and what you get are these very sophisticated loops called the lamo cortical loops, which allows information to move around internally as though things are moving in the world, but without actually moving them. In this way, instead of the brain generating a movement and it happens right away, instead the brain can run a simulation internally to see what would be predicted to happen if the movement were to be made, and then eventually the simulations can be not just about pushing this button or lifting this coffee cup, but more abstract, like what would it be like if I got that job promotion, or how should I break this news to my friend? Or what is the optimal path to build a society for peace and justice. Now with new data about the brain, we can go even deeper to see how this would work. For example, in a recent episode, I talked with neuroscientist Jeff Hawkins on his theory about cortical columns. Think of cortical columns like little rice grain sized units that are all packed together in the cortex, and you have hundreds of thousands of them, and each one takes care of little overlapping aspects the world, and together they communicate and collaborate to build a larger internal model of the world. So here's a clip from that interview which I didn't include in the original cut, on the topic of what the columns are coding for. And Jeff points out that the cells that you find in mammals called grid cells, seem to be coding for two dimensional space, but they can also code for three dimensional space. And once you have these mechanisms for coding for movement in space, maybe those cells can do something more. Here's Jeff.

It looks like the neurons. It's a speculative, but it looks like the neurons can learn whatever is the proper space for a particular problem. Yeah, so math may have a different sort of space than what you learn for cups, and it could be n dimensional. It's a little hard to think about.

It looks like the neurons. It's a speculative, but it looks like the neurons can learn whatever is the proper space for a particular problem. Yeah, so math may have a different sort of space than what you learn for cups, and it could be n dimensional. It's a little hard to think about.

This, but here's the way that we maybe can think about this is people have done this in VR, where they put people in let's say a four dimensional world, so it doesn't follow the normal three You know, if I go to the right and the right and the writing again, I'm not gonna end up in the same space. I end up in a different space. And people are are quite good at learning exactly this is not that hard for them.

This, but here's the way that we maybe can think about this is people have done this in VR, where they put people in let's say a four dimensional world, so it doesn't follow the normal three You know, if I go to the right and the right and the writing again, I'm not gonna end up in the same space. I end up in a different space. And people are are quite good at learning exactly this is not that hard for them.

Initially it's very disoriented exactly right exactly, but they are able to learn it, which is consistent with your idea. If I understanding that the neurons are flexible about which dimension.

Initially it's very disoriented exactly right exactly, but they are able to learn it, which is consistent with your idea. If I understanding that the neurons are flexible about which dimension.

The neurals don't know what they're representing. They don't know what where they're getting input from. They don't know what the input represents. It's just some pattern that's coming in from something that's moving, and in the movement could be can be expressed as a set of one dimensional vectors, and you're some intersection of those. So we could learn anything. It can learn fourth dimensional space. Now, I would think it would be hard to learn these things because you have to practice a lot. But this is why a mathematician might be really good and a non mathematician look at some math and say this is all gobbledygook to me, I can't and a mathematicians look at all these are like friends. These numbers are friends, these equations are friends. I know where they are, I know the relationship. I know how to move from here to here. I know what what action I have to take to get this equation to look like that equation. And so they've developed this sort of movement centro motor space for mathematics that if you haven't spent years doing it, it's like mystery, just gobblbook, right, So it takes time, and I certainly think we would be we would be really in trouble if all of our dimensional reference names and their brain changed over night, because nothing would make any sense. So but you could take some and movement of certain different directions. So a lot of practice you could become a mathematician potentially, or another lot of practice you might be good at whatever it is you know, understanding computer code, which is a totally different set of problems.

The neurals don't know what they're representing. They don't know what where they're getting input from. They don't know what the input represents. It's just some pattern that's coming in from something that's moving, and in the movement could be can be expressed as a set of one dimensional vectors, and you're some intersection of those. So we could learn anything. It can learn fourth dimensional space. Now, I would think it would be hard to learn these things because you have to practice a lot. But this is why a mathematician might be really good and a non mathematician look at some math and say this is all gobbledygook to me, I can't and a mathematicians look at all these are like friends. These numbers are friends, these equations are friends. I know where they are, I know the relationship. I know how to move from here to here. I know what what action I have to take to get this equation to look like that equation. And so they've developed this sort of movement centro motor space for mathematics that if you haven't spent years doing it, it's like mystery, just gobblbook, right, So it takes time, and I certainly think we would be we would be really in trouble if all of our dimensional reference names and their brain changed over night, because nothing would make any sense. So but you could take some and movement of certain different directions. So a lot of practice you could become a mathematician potentially, or another lot of practice you might be good at whatever it is you know, understanding computer code, which is a totally different set of problems.

That's a really it's a really cool answer to that, because one would think we have a three dimensional reference frame, because that's the physical world we live in. But your point is these norms are actually flexible enough that you could get higher dimensionality when useful.

That's a really it's a really cool answer to that, because one would think we have a three dimensional reference frame, because that's the physical world we live in. But your point is these norms are actually flexible enough that you could get higher dimensionality when useful.

Right, Well, it looks like you look at evolutionary point of view, the first references you needed were two dimensional, right, if you're moving on the surface of the of the of the ground, or you're moving along the floor to the floor, right. And so these early reference systems, the grid cells and the play cells, and the antirhinal cortex and the hippocampus, they seem to be predominantly two dimensional. And one could argue back a little bit, argue that that makes sense because that's the older system, that's what animals had to start with. And maybe what happened was in the neocortex they generalized the system right, Right, So it's so that it looks like the grid cells and the ant hinod projects can represent three D structure because bats can do it. But they're truly wonky. There's all this research trying to figure out what's going on. They don't really inderstand it. But it's clearly two dimensional. But it's possible that in the cortext it says, okay, we can represent any dimensional. We've generalized this thing now beyond just navigating on the ocean floor on the Earth. And maybe first I had to do it for flying or climbing in trees, but now humans could do it for math and and history, and you know, we can build reference frames for everything.

Right, Well, it looks like you look at evolutionary point of view, the first references you needed were two dimensional, right, if you're moving on the surface of the of the of the ground, or you're moving along the floor to the floor, right. And so these early reference systems, the grid cells and the play cells, and the antirhinal cortex and the hippocampus, they seem to be predominantly two dimensional. And one could argue back a little bit, argue that that makes sense because that's the older system, that's what animals had to start with. And maybe what happened was in the neocortex they generalized the system right, Right, So it's so that it looks like the grid cells and the ant hinod projects can represent three D structure because bats can do it. But they're truly wonky. There's all this research trying to figure out what's going on. They don't really inderstand it. But it's clearly two dimensional. But it's possible that in the cortext it says, okay, we can represent any dimensional. We've generalized this thing now beyond just navigating on the ocean floor on the Earth. And maybe first I had to do it for flying or climbing in trees, but now humans could do it for math and and history, and you know, we can build reference frames for everything.

So that was my interview with Jeff Hawkins from a couple of months ago, and I include that to illustrate the degree to which different scientists are scratching at different versions of this idea that brains evolve to move through space. But once you've got that solved, the mechanisms can generalize to represent higher order concepts. So let's wrap today's episode. We saw two main lessons. The first is that thoughts are much more than words. Thinking comes in many formats, and the second is that to the brain, thinking may be just like moving, but it's internal moving through a cognitive landscape. In other words, the brain's capacity to control the body extends to its ability to simulate and navigate mental spaces. Through this lens, thinking is mental motion, and consciousness arises as the brain's continuous orchestration of movements, both real and imagined. Understanding how we think can enrich how we see ourselves. So here's a challenge. Spend today paying attention to your thoughts. Notice how often they're verbal, or visual, or emotional or something else entirely. If you get a chance, download an app that randomly beeps your phone throughout the day and jot down exactly what you were thinking. What were you thinking about right then? Was it words? Was it a feeling, a picture, something else. Our thoughts aren't always what we naively expect, and if most of our thoughts don't come in words, what does that say about who we are? How much of your identity is tied to the words in your head, and how much of it lies in the massive nonverbal undercurrents of your mind. Go to Eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcasts at eagleman dot com with questions or discussion, and check out and subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments Until next time. I'm David Eagleman, and this is inner Cosmos.

So that was my interview with Jeff Hawkins from a couple of months ago, and I include that to illustrate the degree to which different scientists are scratching at different versions of this idea that brains evolve to move through space. But once you've got that solved, the mechanisms can generalize to represent higher order concepts. So let's wrap today's episode. We saw two main lessons. The first is that thoughts are much more than words. Thinking comes in many formats, and the second is that to the brain, thinking may be just like moving, but it's internal moving through a cognitive landscape. In other words, the brain's capacity to control the body extends to its ability to simulate and navigate mental spaces. Through this lens, thinking is mental motion, and consciousness arises as the brain's continuous orchestration of movements, both real and imagined. Understanding how we think can enrich how we see ourselves. So here's a challenge. Spend today paying attention to your thoughts. Notice how often they're verbal, or visual, or emotional or something else entirely. If you get a chance, download an app that randomly beeps your phone throughout the day and jot down exactly what you were thinking. What were you thinking about right then? Was it words? Was it a feeling, a picture, something else. Our thoughts aren't always what we naively expect, and if most of our thoughts don't come in words, what does that say about who we are? How much of your identity is tied to the words in your head, and how much of it lies in the massive nonverbal undercurrents of your mind. Go to Eagleman dot com slash podcast for more information and to find further reading. Send me an email at podcasts at eagleman dot com with questions or discussion, and check out and subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments Until next time. I'm David Eagleman, and this is inner Cosmos.