美国国家公共电台 NPR One Head, Two Brains: How The Brain's Hemispheres Shape The World We See（在线收听）

SHANKAR VEDANTAM, HOST:

This is HIDDEN BRAIN. I'm Shankar Vedantam. If you type in the words left brain versus right brain on YouTube, it's not long before you'll find yourself in a vortex of weird claims and outlandish hype.

(SOUNDBITE OF YOUTUBE VIDEO MONTAGE)

UNIDENTIFIED PERSON #1: With left brain imbalance, the overall function of the brain is stunted so that the oldest parts of the brain, the reptile brain, takes over on an instinctual level.

UNIDENTIFIED PERSON #2: Men - men need formulas. We need systems. That's the left brain, by the way, for men.

UNIDENTIFIED PERSON #3: Now, the problem is is that most people are either exclusively left-brained or right-brained. They're one or the other.

VEDANTAM: For decades, pop psychology books and plenty of YouTube videos have made dramatic claims about people who are left-brained and people who are right-brained. It got to the point that respectable scientists felt they had to steer clear of the study of hemispheric differences.

IAIN MCGILCHRIST: I was told, when I got involved in this area, don't touch it. It's toxic. Don't even go there.

VEDANTAM: This week on HIDDEN BRAIN, we follow the work of a researcher who went there. What he's found is much more nuanced and complex than the story on YouTube. His conclusions, though, might be even more dramatic. He argues that differences in the brain and Western society's preference for what one hemisphere has to offer have had enormous effects on our lives.

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VEDANTAM: Iain McGilchrist is a psychiatrist. He has spent years studying the human brain through case studies of his patients and a detailed examination of scientific research. He's found himself fascinated by a question that has intrigued philosophers and scientists for centuries. Why is the human brain divided in half? How does each hemisphere shape our perceptions? Iain's book on this topic has been on my radar for many years. It's called "The Master And His Emissary." Iain joined me for a chat in our studios in Washington, D.C. I asked him to start with a basic overview of what the two hemispheres do.

MCGILCHRIST: In motor terms, it's fairly straightforward that the left hemisphere controls the right side of the body and receives messages from it and vice versa. But in terms of psychological life, they have quite different kinds of roles. They have quite different dispositions. And I believe evolutionarily, they are - if you like - addressing different questions.

VEDANTAM: If you look at the last 20 or 30 years again, there's been a lot of work - or speculation, really - looking at how these two hemispheres might operate when it comes to perception, when it comes to behavior. You argue in the book that there has been many oversimplifications of how the two hemispheres work and what their different roles are. What does that look like? What does this world of oversimplification look like?

MCGILCHRIST: Well, the conventional model is something that sprang up probably in the '60s and '70s and had some life into the '80s and even into the '90s and is now, probably, mainly at home in middle-management programs and pop psychology books. And I was told when I got involved in this area - don't touch it. It's toxic. Don't even go there. And basically, that was that the left hemisphere is logical and verbal and the right hemisphere is kind of moody and possibly creative. But all of this turns out to be much more complicated, and some of it's plain wrong.

VEDANTAM: When we look at the evolution of the brain, not just among humans but other species, do we find a similar division in other species?

MCGILCHRIST: We certainly do. This is not something that was invented by human beings. It's there in all mammals, amphibians, reptiles, fish, insects, nematode worms - which have, you know, like - one of them has 302 neurons, but it's working asymmetrically. And in fact, the oldest creature that we know of that has a neural net of any kind is called nematostella vectensis. It's 700 million years old, and it's thought of as the origin of neural networks. Guess what. The neural network is asymmetrical.

VEDANTAM: And so in some ways, this does prompt a question. Why would you have...

MCGILCHRIST: (Laughter).

VEDANTAM: Why would you have the brain be divided, right? I mean, it does beg the question.

MCGILCHRIST: It certainly begs the question, particularly if you buy - which I emphatically don't - the idea that the brain is a computer because if it is, surely, it's a vast waste of computing power to have this brain divided into two bits.

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VEDANTAM: If you go to an antique store, you might find posters showing a human head with the brain divided like a map. Reason is in one quadrant, emotion in another. Memory is over here, imagination there. For a long time, the popular representations of hemispheric differences focused on what different parts of the brain do. Iain says what really distinguishes the hemispheres is not what they do but how they do the same things differently.

MCGILCHRIST: The reason that we got things wrong, in my view, is that we were looking at simply the functions as we saw them and divided them up in much the way that we would if we thought about a machine. This part of the machine does this; this part of the machine does that. And if you, instead of using the machine model, use the model of this is part of a human being, a person, you would ask a slightly different question, which is not just - what does it do? - but how does it do it? And that turns out to be a profoundly important question when looking at the hemispheres because they are both involved in doing everything. But there is a difference. It's that, quite consistently, each hemisphere does all these things in a totally different way - with a different kind of spirit towards a different end, if you'd like to put it that way.

VEDANTAM: Iain believes the brain is divided into two hemispheres so that it can produce two different views of reality. One of the hemispheres, the right, focuses on the big picture. The left focuses on details. Both are essential. If you can't see the big picture, you don't understand what you're doing. If you can't home in on the details, you can't accomplish the simplest tasks. This fundamental difference in orientation turns out to have profound consequences for everything the two hemispheres do. Iain uses the example of learning a piece of music to explain how this works.

MCGILCHRIST: Imagine you are attracted to a piece of music. And you try playing it as a whole, and you love it.

(SOUNDBITE OF MELODIC PIANO PLAYING)

MCGILCHRIST: But you realize that there are bits that you are not getting right.

(SOUNDBITE OF DISCORDANT PIANO PLAYING)

MCGILCHRIST: So you know you need to practice your fingering at Bar 18.

(SOUNDBITE OF PIANO PLAYING)

MCGILCHRIST: And when you're taking it apart, you realize - oh, here we move into the subdominant. At this point, we're moving back to the dominant. You understand a whole lot of details, which are fine. You need to do the work, and you need to do the analysis. But when you come to play the piece, you must put all that out of your mind. Otherwise, you won't be able to play at all.

(SOUNDBITE OF MELODIC PIANO PLAYING)

MCGILCHRIST: The right hemisphere takes in the whole at the start. The left hemisphere unpacks that and enriches it. But then that work being done, it needs to be taken back into the whole picture, which only the right hemisphere can do.

(SOUNDBITE OF MELODIC PIANO PLAYING)

VEDANTAM: And it seems to me that this is not just, of course, when you're learning a piece of music. You have examples in the book of numerous cases, including in the animal world. When a bird is trying to pick up a grain of corn, it actually needs to be doing two different things at once.

MCGILCHRIST: All living creatures need to be able to attend to the world in two different ways, which require quite different attention at the same time. And this is simply not possible unless they can work relatively independently. On the one hand, in order to manipulate the world - to get food, to pick up a twig to build a nest - you need a very precise, targeted attention on a detail in order to be able to achieve that and be ahead of your competition. But if you're only doing that - if you're a bird just concentrating on the little seed, you'll become somebody else's lunch while you're getting your own because you need, at the same time, to be paying the precise opposite kind of attention - not piecemeal, fragmented and entirely detailed but sustained, broad and vigilant for predators and for other members of your species.

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VEDANTAM: Decades ago, scientists discovered that the two hemispheres are connected by a bundle of nerve fibers. They named it the corpus callosum. The corpus callosum is often described as a bridge, that it passes information back and forth between the hemispheres. But it turns out, it is much more like a traffic signal.

MCGILCHRIST: In order to collaborate, you need to both work together and work separately. If a surgeon collaborates with the scrub nurse, they don't both try and do the same job. They have distinct roles. And the corpus callosum enables both separation - mainly, I believe, separation - but also connection.

VEDANTAM: I mean, that's extraordinary, isn't it? When you think about this, it really looks like we actually have two different brains inside our head.

MCGILCHRIST: Certainly, the first people to look at split-brain patients thought that there were two distinct people there and they would talk to one another. I mean, these very famous names in the history of neurology, neuropsychology, they would talk about the two hemispheres as two people.

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VEDANTAM: Iain's book has a lot of science in it, but it's also a commentary on modern industrial societies. The title of the book comes from a parable, a myth about a wise spiritual master who rules over a land. The master appoints an emissary. He's a smart messenger. His job is to carry the master's instructions to the far corners of the land.

MCGILCHRIST: And this emissary was bright enough but not quite bright enough to know what it was he didn't know. And he thought, I know everything. And he thought, what does the master know, sitting back there seraphically smiling, while I do all the hard work? And so he adopted the master's cloak, pretended to be the master. And because he didn't know what he didn't know, the result was that the community fell apart, essentially.

VEDANTAM: Iain argues that the right hemisphere of the brain is supposed to play the role of the wise master of our mental kingdom. The left hemisphere is supposed to be the emissary. Iain says we have grown infatuated with the skills of the emissary. We prize the details but scorn the big picture. He makes an analogy about the relationship between the hemispheres.

MCGILCHRIST: I want to emphasize that I resist very strongly the idea that the brain is a computer. It's just nothing like a computer, actually. But in this one, limited sense, the left hemisphere is a little bit like a very, very smart computer. So you know what the data you've collected mean, but you haven't yet been able to analyze them. You put them into a machine that is just very clever at carrying out a routine. It doesn't understand. And then it spews out a result, which it also doesn't understand. But you then take back into the world where the data come from and go, I see.

So that is the relationship. Your left hemisphere is busy processing things to make sure they're consistent and unpacked, but your right hemisphere's seeing everything. I am suggesting that we have arrived at a place, not for the first time in the West, where we have slipped into listening only to what it is that the left hemisphere can tell us and discounting what the right hemisphere could have told us.

VEDANTAM: And in your analogy here, the right hemisphere is the master, the left hemisphere is the emissary.

MCGILCHRIST: In my interpretation of that myth, that's right. Yup.

VEDANTAM: Coming up, we look more closely at Iain's view that we're living in a left-hemisphere world, and we dive into the ways the left and right hemispheres produce different accounts of reality.

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VEDANTAM: This is HIDDEN BRAIN. I'm Shankar Vedantam. In 1997, PBS aired a documentary about a patient named Joe. He had epilepsy.

(SOUNDBITE OF UNIDENTIFIED DOCUMENTARY)

JOE: I was having seizures, like, every day or so, or sometimes two or three a day.

VEDANTAM: Joe's doctors devised a treatment that sounded more sci-fi than like a real medical procedure. A surgeon literally split his brain in two.

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VEDANTAM: The surgeon sliced the corpus callosum, the nerve fibers that connect the left and right hemispheres. In Joe's case, the surgery accomplished what the doctors hoped. His seizures stopped.

(SOUNDBITE OF UNIDENTIFIED DOCUMENTARY)

JOE: I know the left hemisphere and right hemisphere now are working independent of each other, but you don't notice it. Now, you just kind of adapt to it. It doesn't you have any feeling - doesn't feel any different than it did before.

VEDANTAM: Psychiatrist Iain McGilchrist says many other patients with epilepsy have been helped by this procedure.

MCGILCHRIST: See, by simply separating the two hemispheres so that an electrical storm, if you like, in one hemisphere couldn't cross over and invade the other. They were able, actually, to carry on a remarkably normal life. So this was a life-saving procedure, but it also had the consequence that, by clever experimentation, you could deliver information to one hemisphere at a time and find out what that hemisphere knew and had to say about it.

VEDANTAM: These experiments showed the left and right hemisphere approach everyday tasks very differently. The left focuses on narrow details. The right, on general vigilance. To use a basketball analogy, the left hemisphere is focused on the mechanics of dunking the basketball.

(SOUNDBITE OF BASKETBALL BEING DUNKED)

VEDANTAM: The right, on where all the players are, the current situation in the game.

(SOUNDBITE OF BASKETBALL GAME AMBIENCE)

VEDANTAM: One sees the small picture.

(SOUNDBITE OF BASKETBALL BEING DRIBBLED)

VEDANTAM: The other, the big picture.

(SOUNDBITE OF BASKETBALL GAME AMBIENCE)

VEDANTAM: You see something similar when it comes to language.

MCGILCHRIST: Language has many components. One of them is attending to the tone of voice in which I say something. For example, I can say yes, or I can say yes. I can intone that in probably a dozen different ways with quite different meanings. So for example, I say, it's a bit hot in here. You, using your right hemisphere, know that what I mean is, could we have the door open? Could we put on the air conditioning? But your left hemisphere is wondering, meanwhile, why I'm supplying this quite unnecessary meteorological information.

VEDANTAM: (Laughter). So it's really focused on sort of the granular detail.

MCGILCHRIST: Yes.

VEDANTAM: Yeah.

MCGILCHRIST: And because of this, all kinds of things happen. Because of its narrow focus, it doesn't see anything that isn't explicit. It only sees what's right in the center of the focus of attention. And it doesn't understand things that are not said. Often, that's as important as what is said. The way in which it is said, my facial expression, my body language - all of this is lost, as well as the interpretation in the whole picture.

VEDANTAM: One of the important differences you point out is sort of understanding the role of metaphor in language. For example, which is that the left hemisphere really is incapable of understanding what metaphor is or how it works.

MCGILCHRIST: Yes. And that's no small thing because as some philosophers have pointed out, metaphor is how we understand everything. And they point out that, actually, particularly scientific and philosophical understanding is mediated by metaphors. In other words, the only way we can understand something is in terms of something else that we think we already understand. And it's making the analogy, which is what a metaphor does, that enables us to go, I see, I get it.

Now, if you think that metaphor is just one of those dispensable decorations that you could add to meaning - it's kind of nice but probably a distraction from the real meaning - you've got it upside down. Because if you don't understand the metaphor, you haven't understood the meaning. Literal meaning, however, is a peripheral, diminished version of the richness of metaphorical understanding. And what we know is the right hemisphere understands those implicit meanings, those connections of meanings, what we call connotations, as well as just denotations. It understands imagery. It understands humor. It understands all of that.

VEDANTAM: Do the hemispheres differ in how they think about time - their orientation towards the future and their orientation toward the past?

MCGILCHRIST: Well, that's an interesting question and, not in a clear-cut way, although the left hemisphere is tending to look for the next opportunity. So it's very goal-driven but very short-term goal driven. It wants to grasp things that are within reach. Remember, the left hemisphere is what controls our right hand with which we grasp things that are within reach. So it has a very direct, linear idea of a target and let's go and get it.

VEDANTAM: Now, is some of this played out in sort of the relationship they have with tradition, with history, as you say? In some ways, if I'm primarily focused on what's right in front of me, I don't really need to know what the last 2,000 years of history have taught me.

MCGILCHRIST: I think that's certainly right. Time can be seen rather like the flow of a river, which isn't made up of slices or chunks of river that are then put together. We, as personalities in time or cultures in time, are like this flow. The left hemisphere can't deal with anything that is moving. It fixes things. It likes things to be fixed because then you can grab them. You can't grasp your prey, you can't pick up something unless you can at least immobilize it for that second while you're interacting with it.

So it doesn't like flow and motion, which are, in my view, basic to not just life but actually to the cosmos. So instead, it sees lots of little punctuate moments, little slices of time. And things have to be put together by adding them up.

VEDANTAM: I mean, it's almost like a form of calculus, you know, of taking slices and then trying to integrate them together.

MCGILCHRIST: You're absolutely right. And calculus is an attempt, actually, to achieve something which is indivisible by dividing it in slices.

(SOUNDBITE OF MUSIC)

VEDANTAM: The two hemispheres even appear to have different value systems. The left hemisphere prefers to reduce moral questions to arithmetic.

MCGILCHRIST: For example, if you - and this experiment has been done - if you disable, temporarily, the right temporoparietal junction - which you can do with a painless procedure - and ask people to solve moral problems, they give quite bizarre answers to them based on entirely utilitarian understanding of them. An example is, a woman is having coffee with her friend. She puts what she thinks is sugar in her friend's coffee but it's in fact poison, and the friend dies. Scenario two, a woman is having coffee with her friend who she hates. (Laughter). She wants to poison her. And she puts what she thinks is poison in the coffee, but it's sugar, and the friend lives. Which was the morally worse scenario?

Now, all of us using our intact brains say, well, the one in which she intended to kill her friend. But no. If you disable the right hemisphere, the good old left hemisphere says, well, obviously, the one in which she died. The consequence is what matters. So values are not well-appreciated, I think, by the left hemisphere.

(SOUNDBITE OF MUSIC)

VEDANTAM: One of the most striking differences between the hemispheres is how they relate to each other. With its big-picture view of the world, the right hemisphere can see what the left hemisphere is doing, see the value that it produces. But the left hemisphere, with its narrow view of reality, doesn't recognize the value of the right. In other words, the left hemisphere not only sees a narrow view of the world, it believes that the narrow view that it sees is all there is to see.

Iain explains this through one exchange between a physician and a patient who experienced right hemisphere brain damage. Her left hemisphere is still intact. The patient has a strange belief about her own arm. We asked a couple of producers to read the exchange.

UNIDENTIFIED PRODUCER #1, BYLINE: (Reading, as physician) Whose arm is this?

UNIDENTIFIED PRODUCER #2, BYLINE: (Reading, as patient) It's not mine.

UNIDENTIFIED PRODUCER #1: (Reading, as physician) Whose is it?

UNIDENTIFIED PRODUCER #2: (Reading, as patient) It's my mother's.

UNIDENTIFIED PRODUCER #1: (Reading, as physician) How on earth does it happen to be here?

UNIDENTIFIED PRODUCER #2: (Reading, as patient) I don't know. I found it in my bed.

UNIDENTIFIED PRODUCER #1: (Reading, as physician) How long has it been there?

UNIDENTIFIED PRODUCER #2: (Reading, as patient) Since the first day. Feel. It's warmer than mine. The other day, too, when the weather was colder, it was warmer than mine.

VEDANTAM: I asked Iain how the patient could be unaware of her own arm and why she came up with this odd way to explain the presence of the arm attached to her.

MCGILCHRIST: Well, what we're seeing is a phenomenon called denial, which is a feature of the way the left hemisphere works. So if you have a left hemisphere stroke, so your right hemisphere still functioning, you're very aware of what deficits you have. If you have a right hemisphere stroke, you are completely unaware of there being anything wrong. So if you have a paralyzed left arm, which is often a consequence of right hemisphere stroke, more often than not you will deny that there's any problem with it. If asked to move it, you will say there, but it didn't move.

If, on the other hand, I bring it in front of you and say, whose arm is this, can you move it, they say, oh, that's not mine. That belongs to you, doctor, or to the patient in the next bed or, as in this cut, my mother. It's extraordinary because these are not people who in any way mad. They don't have a psychosis. But they're simply incapable of understanding that there is something wrong here that involves them.

VEDANTAM: When we look at patients who have damage not now to the right hemisphere but to the left hemisphere, Iain, do we see a pattern in terms of how they behave, what deficits they have, what they're able to do?

MCGILCHRIST: Yes. It's really fascinating because the consequences are so obvious. You can't speak. And sometimes you can't appreciate the structure of a sentence that's being said to you. The other thing that happens is you can't use your right hand, which is a bit of a bummer if that's your important hand. But effectively, the structure of reality is not changed. That's why it is easier to rehabilitate somebody after a left hemisphere stroke than after a right. The left hemisphere is the one that sees body parts whereas the right hemisphere is the one that sees the body as a whole. It has something called a body image, which is not just a visual image but an integrated image from all senses of the body.

But I've been looking at all the interesting neuropsychiatric syndromes, many of them described by Oliver Sacks, which follow brain damage. And all these quite extraordinary delusional hallucinating syndromes that most people can hardly believe can happen to a human being happen either only or very largely after damage to the right hemisphere, not after damage to the left. So the succinct answer is the left hemisphere is to do with functioning and utilizing - reading, writing and grasping - and it doesn't really deal with the structure of reality whereas the right hemisphere does.

VEDANTAM: What about emotion? Are there emotions that tend to lateralize more to one hemisphere than the other?

MCGILCHRIST: Broadly speaking, the right hemisphere is more emotionally literate. It reads emotional expression, and it gives emotional expressivity to a greater extent than the left. But it's not a simple matter. And some emotions to do with particularly understanding another person's point of view, what it feels like to be that person, are very profoundly connected with the right hemisphere. However, there are some emotions that are more particularly associated with the left hemisphere. Perhaps the most striking one is anger, which happens to be the most lateralized of all emotions. And it lateralizes to the left hemisphere.

So I think it's that the left hemisphere always has an immediate task because it wishes to accomplish. And if it encounters any opposition, it's dismissive, and it becomes enraged. I mean, that's a simplification, but I think it works. And after a right hemisphere stroke, the range of emotions open to somebody is limited. It's mainly irritability and anger.

VEDANTAM: You obviously have an interest in music. We've talked a little bit about learning a piece of music. I'm wondering if you can bring to mind a favorite piece of music that you return to every so often and describe to me what that would sound like if I only had my left hemisphere hearing it and I only had my right hemisphere hearing it.

MCGILCHRIST: OK. Well, part of the second movement of the Bach "Violin Concerto" would be a good place. It's a particularly beautiful piece.

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "VIOLIN CONCERTO IN A MINOR")

MCGILCHRIST: And it involves the intertwining of two violins accompanied by an orchestra.

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "VIOLIN CONCERTO IN A MINOR")

MCGILCHRIST: So there is definitely something going on at all levels. For most of us, understanding and appreciating a melody, understanding and appreciating harmony and understanding and appreciating complex rhythm is all served by the right hemisphere. And the bit that the left hemisphere gets is regular beat.

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "VIOLIN CONCERTO IN A MINOR")

MCGILCHRIST: One observation is that if you look at a score of a piece of music, if you read music at all, you see individual notes. It encourages you to think that there are separate notes. And so when we then listen to a piece of music, we think we're hearing separate notes. Actually, we're not. We're hearing an absolutely seamless flow in which we remember the notes that have just happened and we already anticipate where the notes are going to come next.

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "VIOLIN CONCERTO IN A MINOR")

MCGILCHRIST: So in the absence of a right hemisphere, we would be hearing much more of point-like, punctate sounds whereas in the right hemisphere, well, it's getting the flow, the melody, the overall picture. And to an extent, we need both. But for music, it really is mainly the right hemisphere. And there's a condition called amusia in which you either can't make sense of music or you cease to understand it. And that usually happens following a right hemisphere stroke.

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "VIOLIN CONCERTO IN A MINOR")

VEDANTAM: So it's really interesting. When you're analyzing something, you're trying to take it apart. And you can analyze a great symphony. And you can analyze it, you know, by looking at the different instruments that are playing. But you can also analyze it acoustically. You can sort of say, here are the different frequencies and the different wavelengths. And here's the tempo and the beat and so forth. But, really, what you're pointing out is that that is not music.

MCGILCHRIST: (Laughter) Right, not at all.

VEDANTAM: Music is actually the connections between them. It's what happens between the notes.

MCGILCHRIST: It does, indeed. And I sometimes give the analogy of - you know, a scientific approach to what is music? And a conventional scientific approach would be, well, let's drill down and see what it's made of. And after years of expensive electron microscopy, I can inform you that music is made up of notes. What is a note?

VEDANTAM: (Laughter).

MCGILCHRIST: A note is a simple tone. What does it mean? Actually, it means nothing. Well, let's take another one. That was an A flat. Let's take a B - doesn't mean anything. Now, if you put 35,000 of these together, you've got the Bach "B Minor Mass" (ph).

(SOUNDBITE OF SONG, "MASS IN B MINOR")

UNIDENTIFIED SINGERS: (Singing unintelligibly).

MCGILCHRIST: Where does it come from? Well, it can't come from the notes because we've established that they don't mean anything. So it must be something else, but all the rest is gaps. It's the gaps between the notes, the gaps that make melody, the gaps that make harmony and so on. But the gaps are just, as it were, silence. They don't mean anything either. So if you put lots of things together that don't mean anything with lots of gaps that don't mean anything, you get something...

VEDANTAM: (Laughter).

MCGILCHRIST: ...That means everything. How did that happen?

(SOUNDBITE OF JOHANN SEBASTIAN BACH'S "MASS IN B MINOR")

MCGILCHRIST: That's what I call betweenness (ph). So it's the notes and the gaps plus whatever happens when they're all edited together. Something new emerges. That is an idea from complexity theory. It's also an idea that is integral for those who are interested in the history of philosophy.

VEDANTAM: So computer scientists and people who study complexity theory would call these emergent phenomena.

MCGILCHRIST: That's right. The only thing there is about emergent phenomena is that by labeling them emergent phenomena, we think we've somehow explained them. But we haven't. We've just kicked the can down the road because how the dickens do they emerge? That's the question that's interesting. That's not explicable.

VEDANTAM: There's another domain where what matters is not sort of the individual details but what happens in between. And the picture you're painting - one hemisphere seeing the big picture, the other seeing only what is literal, what is right in front of it - this has effects on things such as the world of humor. Understanding a joke requires some appreciation of the implicit - what is not being said as well as what is being said. Here's a clip from the TV show "The Big Bang Theory," which you might not be familiar with in Britain. The character Sheldon Cooper is a caricature of the left-brain scientist.

(SOUNDBITE OF TV SHOW, "THE BIG BANG THEORY")

JOHNNY GALECKI: (As Leonard) Hey, Penny. How was work?

KALEY CUOCO: (As Penny) Great. I hope I'm a waitress at The Cheesecake Factory for my whole life.

JIM PARSONS: (As Sheldon) Was that sarcasm?

CUOCO: (As Penny) No.

PARSONS: (As Sheldon) Was that sarcasm?

CUOCO: (As Penny) Yes.

PARSONS: (As Sheldon) Was that sarcasm?

GALECKI: (As Leonard) Stop it.

VEDANTAM: What do hemispheric differences tell us about humor here?

MCGILCHRIST: (Laughter) I love that. Well, the fact is that in order to understand humor, you have to be doing an awful lot of things. You have to be drawing on experience. You have to be understanding what's not said. And you have to be making connections that are not normally made. All of this is far better done by the right frontal cortex. And if you show cartoons to people with right hemisphere damage, they ask entirely inappropriate questions and make entirely inappropriate deductions about what they're seeing.

So humor is another example of something very human and very important that the left hemisphere doesn't get. Humor is an example of something else, which is the ability to understand the implicit in poetry. You can't really understand poetry by paraphrasing it any more than you can explain the joke and expect it still to be funny.

And that's very close to my heart because I used to work in the area of English literature. And in brief, I left it partly because I loved poetry too much. And it seemed to me that these internally implicit, unique, embodied creatures - the poems - were being turned into explicit, general and entirely abstract entities. So I thought this was a destructive process. I wrote a book called "Against Criticism" and went off to study medicine and become a psychiatrist (laughter).

VEDANTAM: So this would be, give me the 600-word summary of "Macbeth" and - assuming you basically have understood "Macbeth."

MCGILCHRIST: That's it. Yeah, yeah, yeah or - yes. There are six points you need to know about Jane Austen. And they are - you know, whatever it is, you know? - and being able to translate a poem, reduce it to some incredibly banal ideas that you could've found somewhere else. You know, you take a wonderful poem by Hardy that he wrote after his wife died.

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UNIDENTIFIED PERSON #4: (Reading) And to me, though Time's unflinching rigor in mindless rote has ruled from sight the substance now, one phantom figure remains on the slope, as when that night saw us alight. I look and see it there, shrinking, shrinking. I look back at it amid the rain for the very last time, for my sand is sinking. And I shall traverse old love's domain never again.

MCGILCHRIST: I mean, I can never read those, "Poems 1912-13," without tears. But if you ask me, what do they say? I'd say it's very sad when somebody you're close to dies, and you sometimes have regrets. That is not going to wash. (Laughter).

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VEDANTAM: Iain has a broad thesis about the brain but also a broad thesis about how these differences might affect our daily lives. Increasingly, he argues, we live in a world that prizes what the left hemisphere offers and has contempt for what the right hemisphere brings to the table. When we come back, I ask Iain about what happens when the emissary usurps the master.

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VEDANTAM: This is HIDDEN BRAIN. I'm Shankar Vedantam. Iain McGilchrist is the author of "The Master And His Emissary," a book about the divided brain. He uses research drawn from patients who have brain damage to one hemisphere or the other and patients who suffer from serious mental disorders. Iain argues that the left and right hemispheres of the brain have competing visions of reality and that increasingly we live in a world dominated by the left hemisphere. I asked Iain to imagine a world where all of us only had one hemisphere, the right.

What kind of a world would that produce?

MCGILCHRIST: The right hemisphere, if it were really without the left hemisphere, would see a lot of connections between things and would see a broad picture, but it might not be so good at focusing on details. Emotionally, the timbre might be somewhat melancholic and sad. Because I think it's one of the aspects, I'm afraid, of the right hemisphere's realism and sympathy, a capacity for empathy, that it does feel suffering. We would not be able to make calculations in the same way. Most arithmetic calculations are made by the left hemisphere.

So we would be good at coming up with ideas. We might not be good at actually sort of carrying out the nuts and bolts and getting it working as a machine.

VEDANTAM: Now let's run the opposite thought experiment. What if all of us just had only a left hemisphere? What would that world look like?

MCGILCHRIST: Well, obviously, we would lose sight of the big picture. That's the thing I've emphasized throughout. There'd be an emphasis on the details, instead. There would be a great emphasis on predictability, organizability, anonymity, categorization, loss of the unique and an ability to break things down into parts but not really see what the whole is like. There'd be a need for total control because the left hemisphere is somewhat paranoid. After right hemisphere damage, people often develop a paranoia, and that's because one can't understand quite what's going on and one needs, therefore, to control it. Anger would become the key note in public discourse. Everything would become black and white.

The left hemisphere needs to be decisive because, don't forget, it's the one that's catching the prey. It's no good at going, well, yeah, it could be a rabbit, but it might not be. It's going to go, I'm going to go for it. So it likes black and white. It doesn't like shades of meaning. So in this world, we would lose the capacity to see grades of difference. We would misunderstand everything that is implicit and metaphorical and have to make rules about how to achieve it.

VEDANTAM: And it's your contention, in some ways, that the world that we have come to live in is a world that increasingly looks like that latter picture?

MCGILCHRIST: I think what I observe is an overemphasis on predetermined systems of algorithms. The sense of social alienation. The way in which we live divorced from the natural world, which is a very new phenomenon. The insistence on extreme positions, which is what the left hemisphere understands, not a nuanced argument about the pros and cons of every single thing.

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MCGILCHRIST: Meaning comes out of living in a consistent culture where there is a sense of connection with one's past. And not just one's own past, but the past of the people who made you who you were, with the other people in the society to which you belong and to the world at large. The natural world and things that are just simply beyond our ken, the transcendental. These are very important things that the right hemisphere's much better equipped to understand, and I feel the loss of them in modern life is grievous.

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VEDANTAM: One of the unusual things about Iain's book is that it has detailed passages about neuroscience followed by discussions about thousands of years of art and music and literature. Iain argues that we have come to live in a world that prizes details over the big picture, what is literal over what is metaphorical. He then tries to show how these changes play out in culture. One criticism of his book by other scientists is that it steps off the ledge of science when it ventures into polemics about art and culture. I asked Iain what he made of that criticism.

MCGILCHRIST: Well, it depends how you look at it. If you look at it with your left hemisphere...

VEDANTAM: (Laughter).

MCGILCHRIST: It makes sense. If you look at it with your right hemisphere, it's nonsense. I mean, I think what it shows - and it's made, typically, by a certain kind of scientist who is not generally somebody who's interested in the big picture. Now, in the past, scientists couldn't leave school without having read quite a lot of history, literature and even doing a bit of philosophy. Nowadays, it's possible to be crammed with technical information and leave and go into an entirely technical scientific subject.

So in the past, scientists tend to take a humanistic view. They saw the import of their work in a broad context of human life. But if you spend all your life looking down a microscope at a tiny detail and hardly even know much about what's going on in the next cubicle to you, then somebody coming along and saying, this has something to do with human life, is very odd.

But if you actually think that researching on the brain has got to have human implications because - excuse me - the brain is something that makes it possible to be a human being, then, you know, it's bound to have implications. So I just think it's an - it's a feeling of discomfort with, as they would put it, straying off the field.

VEDANTAM: So let me push back on you just a little bit because I think there's an interesting rhetorical trick here. You're essentially saying that some of the people who criticize you are criticizing you because they're overly reliant on their left hemisphere. That does come a little close to basically saying that if you criticize my idea, there's something wrong with your brain. And that, essentially, seals you off from criticism, doesn't it?

MCGILCHRIST: Well, no, I don't think it does. I know it looks like that. And I do accept that there's no way around that because I - what I'm really pointing out is that I think that if you do take a narrow view, you will object to it. If you don't take a narrow view, you won't. But it's perfectly possible to criticize my thesis coherently. For example, you could take any one of the contentions in many areas across the board to do with, say, the appreciation of metaphor or creativity or facial recognition. And, you know, there are at least 20 of these.

And you could say, now I look at the research. I find all these things. And you don't take them into account at all. You're wrong, mate. And then I have to go OK. Or can I point you to this? And I think that I can make it cohere. This is a sensible, rational conversation. But just to go, if you talk about things I don't know about, I've got to tell you you don't - you're not welcome in the club.

VEDANTAM: (Laughter).

MCGILCHRIST: Well, that's not good enough.

VEDANTAM: So the book is, in some ways, a criticism of our increasing love and reliance on models. But as I was reading the book, I did notice that you, yourself, have built a very complex and compelling model here.

MCGILCHRIST: Oh, indeed.

VEDANTAM: Have you ever wondered whether you, yourself, might be captive to your left hemisphere and you, potentially, now can't see the problems with your own model?

MCGILCHRIST: That's a very good point. And it's not - I'm not critical of models, actually, in themselves. I'm critical of particular models because, in fact, we can't understand anything - this is one of my basic points - except by having a model with which we compare it. So that is always a limitation. We don't move from a world in which we have models to a better one in which we don't. We move from a bad model to a better one. So every model has its limitations, but some form, simply, a better fit. And that is what the progress of science is.

It's always what I would call a gestalt. Does this gestalt, which is an overall appreciation of a whole rather than nit-picking at little details - does this generally speaking answer better to the picture of reality I have? And does it answer some questions that were not answered under the old model? So that is a perfectly good question to ask.

I'm not blind to the problems of it. I'm open to and desirous of dialogue with people about its virtues and its vices. And if there are vices - and there undoubtedly have to be - then I appreciate being able to alter my model to incorporate this new information. So that is how science progresses.

VEDANTAM: We talked a little bit about some of the critics that you've had. But I was - also had a question about some of your admirers. I mean, your book has had a considerable amount of success. It's been very well-received. But I do wonder whether some of the people who like the book like it in some ways because it allows them to say, you know, the reason I'm not successful at work is because, you know, I'm a right-brain person stuck in this workplace with all these left-brain technocrats. And it's really a way to sort of bring back the left brain, right brain pop psychology, this time with fMRI studies. Do you worry about that?

MCGILCHRIST: I think what I'm saying could be, obviously, misused by people who want to deceive themselves. But I do also think it may open their eyes to something that is real. For example, I have treated many patients who come into the consultation and immediately put down a mobile phone, a pager and a third electronic appliance. And I can sense already what's wrong with them. And many of my patients, I say to them at the end, you know, it's not that there's pathology in you. It's that there's pathology in the workplace, in the corporation for which you work and perhaps in the society to which you belong. And so I don't want you to approximate yourself more and more to what is demanded of you because it is inhuman. It is dehumanizing - and no wonder you're feeling a bit depressed.

And to be fair, what most people say to me - and I get people writing me from all walks of life all the time - saying, you describe what I'm experiencing at work. What they say is not, you've suddenly explained, you know, what I don't like about my job. What they say is, you've given me a whole new view of my life. Some people say, you've improved my marriage. You've made me enjoy my work better. And you've confirmed to me - this is a very common thing - you have confirmed to me something that I knew at some level, but I just didn't have the language to express. You have helped me articulate it.

VEDANTAM: One of the things that's really important to mention, Iain, is that even though so much of your work is a corrective to our overreliance on the left hemisphere, you are cautious to separate yourself from those who would say the worlds of science and reason are expendable, that those worlds don't actually give us anything of value.

MCGILCHRIST: Absolutely the opposite, absolutely the opposite. I love science. Since a child, I was captivated by science. I depend on science in my work, and I depend on scientific discoveries for my life. The argument in my book, as people have pointed out, is sequential, analytical and rational. In fact, people say is quite a left-hemisphere book. And I say, good, I hope I used both my hemispheres in writing this book because if not, it wouldn't be a very good one. So we need both. And what I feel is that science and reason depend on a balance of these things. There is a distinction to be made between rationality - by which I mean the mindless following out of rationalistic procedures - and what I would call reason - which, since the Renaissance, has been exalted as the mark of a truly educated person, which is to make balanced, informed judgments - but not just informed by data but informed by an understanding in the whole context of a living being belonging to a vibrant society of what this actually means.

In other words, judgment - judgment has been taken out of our intellectual world and replaced by something a machine can do. And that may look good to a certain kind of way of thinking, but I think it's a disaster. The right hemisphere sees the need of the left. That's in the image of the master and the emissary - the master knowing the need for the emissary, the emissary not knowing the value of the master. And if I may use a quotation from Einstein, I think this gives us the full picture - he said that "the rational mind is a faithful servant. The intuitive mind is a precious gift." We live in a society that honors the servant but has forgotten the gift.

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VEDANTAM: Iain McGilchrist is the author of "The Master And His Emissary: The Divided Brain And The Making Of The Western World." A new documentary about his book has just been made. It's called "The Divided Brain." Iain, thank you for joining me today on HIDDEN BRAIN.

MCGILCHRIST: It's been a huge pleasure. Thank you.

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VEDANTAM: This episode was produced by Rhaina Cohen and edited by Tara Boyle and Jenny Schmidt. Our team includes Parth Shah, Thomas Lu and Laura Kwerel. Emefa Agawu performed the Debussy piece. Kevin Beesley read the Thomas Hardy poem. Alex Curley and Lauren Landau perform the scene of the doctor and patient. Our unsung hero this week is Jude Asfar (ph). Jude is a regular listener of HIDDEN BRAIN and a longtime fan of Iain's book. A few months ago, she sent me an email letting me know that Iain was going to be in Washington for an event. Because of Jude's note, we were able to schedule an in-person interview with Iain during his brief visit to Washington. We're always grateful when our listeners look out for us and point us in the direction of interesting guests. Thank you, Jude. If you learned something from this episode, please talk about it with a friend and encourage them to subscribe to our show. I'm Shankar Vedantam, and this is NPR.

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