The Divided Mind: A Conversation with Iain McGilchrist (Episode #234)
What this covers
In this episode of the podcast, Sam Harris speaks with Iain McGilchrist about the differences between the right and left hemispheres of the human brain.
Released: February 1, 2021
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McGilchrist argues that the right and left hemispheres of the brain have fundamentally different modes of engaging with reality—the right hemisphere as the 'master' providing broad, embodied, contextual understanding, and the left as the 'emissary' handling abstract, theoretical, symbolic processing—and that modern Western culture has become pathologically dominated by left-hemisphere modes of thinking, undermining our connection to the world.
- The hemispheres are not functionally identical but have evolved distinct and complementary roles across all vertebrates, with the left hemisphere specialized for symbolic/theoretical processing while the right attends broadly to embodied, contextual reality
- Split-brain research and stroke studies reveal that the left hemisphere is actually less reliable, more prone to confabulation and jumping to conclusions, and more emotionally reactive (especially anger), while the right hemisphere provides superior judgment, social understanding, and contact with reality
- The corpus callosum's function is primarily inhibitory—preventing interference between hemispheres—and modern culture's ascendance of left-hemisphere abstraction over right-hemisphere embodied wisdom has created a civilization 'in thrall to the emissary' and detached from genuine understanding
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The difference between the two hemispheres is not primarily in what they do (function), but in how they do it—the manner, style, and way in which they process information—and each hemisphere is involved in all functions including reason, language, emotion, and visuospatial processing, just in characteristically different ways
“the real difference between the two hemispheres was what they do which is the right answer or the right way right question perhaps to ask of a machine what does it do but it's not necessarily the right question to ask if a person of a person or may be more interested in the how in what way in what manner this is done”
Before split-brain surgery was ever performed, in the 19th century, neuropathologists and anatomists (such as Wigan) discovered through autopsies that people had lived entirely normal conscious lives with only a single hemisphere of the brain, meaning that consciousness does not require both hemispheres, and the right hemisphere alone is sufficient for consciousness
“but it's also worth saying that after usually about the first five or six months most split brain subjects started to lose this into manual conflict as it's called so it's something that the person sort of accommodated too but it's also not just true i mean on the other hand it's also not just split brain patients that must be thinking very differently and seeing the world very differently”
The study of hemispheric lateralization was heavily discouraged in academic neuroscience during the careers of most researchers trained after the 1980s, with senior scientists warning junior researchers that studying this topic would 'taint their career' and harm their academic prospects, specifically telling them 'don't allow your career to be tainted, you can do well, don't do this'
“when i got to my medical training and so on the topic of difference between the hemispheres was a really a non-non-subject it was considered entirely pop psychology it was tacky people plead with me don't don't allow your career to be tainted you can do well don't do this you know don't get involved in this issue it's all been rubbished a long time ago”
Roger Sperry, the Nobel Prize-winning neuroscientist who pioneered split-brain research, was also a philosopher who stated that the problem with modern Western civilization is that it has relegated and ignored the right hemisphere
“roger sperry who as you mentioned was one of the the most important neuroscientists of that era investigating this phenomenon for which she was given the nobel prize said and he was no mean philosopher actually as well as being a neuroscientist and he said that the problem with modern western civilization is that it has relegated the it ignores the right hemisphere anyway”
Alan Schneider in Sydney has found that suppressing left-frontal-cortex activity and enhancing right-frontal-cortex activity can facilitate problem-solving, including mathematical problem-solving, suggesting that the right hemisphere has superior capabilities for complex reasoning that are normally suppressed by left-hemisphere dominance
“the problem solving of a certain kind um alan schneider in sydney has experimented on this uh can be facilitated by suppressing the left frontal cortex and enhancing the right frontal cortex so that complex problems including mathematical problems can be more easily solved”
With evolution of language in humans, one hemisphere—the left—became specialized for dealing with symbols of experience (abstract, theoretical processing) rather than the gathering of experience itself, and the left hemisphere is systematically worse at all the ways in which we grasp hold of the world because it has been deliberately kept away from direct engagement with reality in order to do theoretical processing
“with the evolution of language we've decided to devote one part of the brain for dealing entirely in theory of the the symbols of experience rather than the gathering of experience itself”
Heublings Jackson, a great father of modern neurology in the 19th century, remarked that it is not common enough for us to wonder at the fact that the brain is divided in the way it is, suggesting that the evolutionary basis for hemispheric division deserves more scientific attention than it has received
“the chap called hewling's jackson who's a great one of the great fathers of modern neurology in the 19th century said it's not common enough for us to wonder at this fact that the brain is divided in this way”
The corpus callosum's primary function appears to be inhibitory rather than facilitative—it does not simply pass information between hemispheres but rather allows one hemisphere to suppress or prevent interference from the other hemisphere, and even in a perfectly functioning intact brain much of the inter-hemispheric communication is inhibition rather than positive exchange of information
“the connection between the hemispheres so much involved with inhibition rather than facilitation these were questions that haunted me”
The tip-of-the-tongue phenomenon reveals the structure of the boundary between consciousness and unconsciousness: when trying to remember a word, the word itself is absent from consciousness (there is a vacancy), yet this vacancy has structure because you can exclude candidate words even though you cannot consciously produce the target word
“one example that james gave is that if you think about what it's like to suffer the tip of the tongue phenomenon you're trying to remember a word you're trying to remember somebody's name and you just can't get anything there's so on on the one hand we're talking about what is absent from consciousness like the word is not there the name is not there there is a vacancy uh which you're struggling to fill but this vacancy has structure because someone can say to you is the name jim and you instantly no no this is not jim you can exclude jim because jim is not the name you're trying to think of and yet you don't know what the name is that you're trying to think of”
The possibility exists that in intact brains, parts of what is outside immediate conscious experience may themselves be conscious, creating a kind of 'Freudian spooky picture' where there are parts of your mind that have experiences and knowledge unknown to the conscious subject, and this possibility applies even if one believes that ultimately there is unified consciousness
“there's something that it's like to be part of your mind that you the so-called conscious subject isn't aware of in any given moment right there's some there's something that it's like to be part of your mind that you the conscious person in this moment doesn't directly experience and that's again even if you're convinced that that is a a possibility”
The popular conception that the right brain is emotional while the left brain is rational and reliable is largely inverted from the truth: the left hemisphere is actually less in touch with reality, more prone to jumping to conclusions, less reliable in judgment, and more prone to anger, while the right hemisphere is more intelligent in terms of IQ, attends more broadly to the world, perceives more, makes better judgments, and is less prone to quick conclusions
“the story was well the right brain is kind of emotional but the left brain is rational and it's dependable it may be a little bit boring but at least it's very dependable it tends to be our contact with reality whereas the right hemisphere is all very well if you want to paint pictures but you know and this is just so so terrible as a way of looking at them in many ways it's the inverse of the truth”
In split-brain patients, when information is presented to only one hemisphere, the other hemisphere—if it has no information about what was presented—will confabulate or make up a plausible explanation for why the person acted in a certain way, revealing that the left hemisphere is extraordinarily good at constructing false narratives and appears to believe its own fabrications
“when the left hemisphere knew really nothing at all because the information had all gone to the right hemisphere it would pretend that it knew all about what was going on so when it was asked why did you respond in a certain way about which it knew diddly squat because that had been the information the right hemisphere it had and that was why we had responded in that way it would make something up that was plausible and it is one way of looking at it is that the left hemisphere is extraordinarily good at making things up and it's a bullshitter in fact”
In split-brain patients, particularly in the early months after surgery, there are often conflicting behaviors where the two hemispheres attempt to perform incompatible actions simultaneously, such as a woman's right hand taking out one dress while the left hand puts it back and takes out a different one, or a person trying to pay money with one hand while the other hand takes it back and replaces it in the pocket
“sometimes people would show completely conflicting behavior so a woman would go to the wardrobe to take out a dress with her right hand and her left hand would take it and put it back and take out a different one or somebody would get out money to pay from the wallet and the other hand would take it away and put it back in this pocket”
The corpus callosum, which connects the two hemispheres with a band of fibers at the base of the brain, only directly connects about two percent of the brain's cortical fibers and is a mammalian invention—birds, reptiles, amphibians, and monotremes do not have a corpus callosum at all.
“the corpus callosum that connects the two hemispheres a band of fibers at the base of the brain that connects about two percent of the fibers of the brain directly is a mammalian invention up until mammals i.e in birds and reptiles amphibians monotremes there isn't a corpus callosum at all”
Transcranial magnetic stimulation (TMS) allows researchers to painlessly stimulate or suppress specific brain areas depending on the frequency of pulses, and when applied to knock out one hemisphere, it instantly releases full-fledged, ready-to-go decision-making patterns characteristic of that hemisphere, showing that these patterns were present and ready to manifest even in the intact brain
“you can produce this effect experimentally in normal subjects using transcranial magnetic stimulation which is a technique whereby you can painlessly stimulate or suppress uh depending on the frequency of the pulses areas of the brain and i don't know but you've probably talked about that at a in another podcast but in any case the point is this when you do that something full-fledged and ready to go is released so it's not like it was there you know when you when you knock out the left hand basically you knock out the right hemisphere you find instantaneously decisions being made which are characteristic of what we know to be the way of the right or the left hemisphere”
The reason hemispheric research was dismissed as 'pop psychology' was twofold: first, popular culture had appropriated split-brain findings in cartoonish ways (like advertising cars as 'for your right brain'), and second, some early researchers (1960s-1980s) drew conclusions too quickly from the initial split-brain data, leading serious neuroscience to overreact by dismissing the entire topic as pseudoscience
“i think there are two main reasons one is that as you say it had got into popular culture in a certain way so it was an ad you know the volvo a car for your right brain and this kind of thing and so people went oh please you know don't don't let's go near that so in order to remain aloof you know neurosciences oh no no no no it's not like that which indeed it isn't but the other reason is that there were some as i say some slightly too quick conclusions drawn in the early days in the 60s and 70s and on into the 80s”
The contralateral organization of the nervous system means that the right hemisphere sees only the left side of the world and the left hemisphere sees only the right side, organized by hemifield within both eyes rather than by which eye is used.
“as you said the right hemisphere in a divided brain sees only the left side of the world and the left hemisphere sees the right side of the world it's not divided left and right eye it's the left hemifield within both eyes and and the right hemifield within both eyes”
Consciousness might not be generated solely or primarily by the brain but rather the brain might be receiving a stream of consciousness that is transmitted and transduced by neural structures, functioning more like an antenna or receiver than a generator or producer of consciousness
“i'm not convinced that the brain is merely a producer or secretor of consciousness so it becomes possible to think of consciousness that is a flow and that is transmitted transduced by the brain so you can see the brain as something that is receiving a stream of information to both hemispheres simultaneously”
The concept of hemispheric division is difficult for people to assimilate because it challenges their sense of being a unified conscious subject in the world; there is something existentially disconcerting about the implication that the mind is not unified.
“i think there may be an additional reason here which is that it's there's something impossible or or at least very difficult to assimilate about this finding into one's sense of one's own being in the world”
The left hemisphere has a very narrow beam of attention, highly clarified and precise, focused on about three degrees of the 360-degree attentional arc, whereas the right hemisphere has a very broad picture and is vigilant and on lookout all the time, surveying the full field of possible experience
“one distinction between the left and the right hemisphere which we must come onto at some point is that the left hemisphere has very narrow beam attention that is highly clarified and precise but it's only to like three three degrees of the 360 degree attentional arc whereas the right hemisphere sees a very broad picture and that is quite different it's on the lookout it's vigilant all the time”
The master and emissary parable describes a spiritual community where a wise master delegates daily business to a capable administrator, but the administrator comes to believe he is the master and loses the master's understanding, causing the whole community to fall to ruin—serving as a metaphor for the left hemisphere's usurpation of the right hemisphere's role.
“i see that as a parable very loosely based on a hint in nature to describe the relationship the advancing relationship between the right and left hemisphere and the way we have ended up in the world today enthrall to the emissary to the servant that doesn't really understand what the master would have known”
Consciousness is irreducible and should be defined in circular terms as synonymous with experience, following Thomas Nagel's framing that consciousness is simply 'something that it is like to be a system'—if you could trade places with a bat, there would be some qualitative character to that experience
“for me i'm again i think consciousness has a concept is is actually irreducible which is to say we define it in circular terms you know it's synonymous with experience you know i i like thomas nagel's framing that it's something that it's like to be a system so if a bat is conscious that's simply saying that there's something that it's like to be a bad if you could trade places with a bat you'd have some qualitative character to your being in the world”
Dreams provide evidence of partitioned consciousness: in non-lucid dreams, a dream character (the protagonist you are) can be surprised by a joke told by another dream character, implying that one part of the mind (the dream author) has written the joke to surprise another part of the mind (the protagonist), demonstrating that consciousness can generate novel content (a punchline) that surprises the conscious subject experiencing the dream
“in moments of you know dream you might experience a dream where there really seems like there's an author of the dream that has anticipated you as the protagonist of the dream not knowing what's going on i mean like having a dream where a dream character is telling you a joke that has a punch line that surprises you i mean like that's just an incredible experience they say you just you're the protagonist in your dream you meet a person who doesn't exist and you're obviously not aware of that because you're it's a dream it's not a lucid dream and this person tells you a joke and you're waiting to hear the punchline and then when the punch line is delivered it's actually funny and so how is it possible for part of your mind to have written on demand something that the other part of your mind will find funny”
The reason the two hemispheres are divided is explicable in terms of Darwinian evolutionary advantage—there is a fundamental adaptive problem that hemispheric separation solves.
“it's to do i believe with a problem which is entirely explicable in terms of darwinian evolutionary advantage”
Early in McGilchrist's career, he became troubled by the way literature was being analyzed and taught—scholars were taking embodied, implicit, unique works and making them disembodied and explicit, rendering them general rather than unique, and losing the meaning in the process.
“somebody in the past took great pains to create something that is unique embodied and largely speaking implicit in other words if you try and unpack it like explaining a joke or trying to say well this is what this poem means you know you really are losing a lot of the value and the meaning and people came along you know in seminar rooms and took the disembodied and made it thoroughly sorry took the embodied and made it thoroughly disembodied took the implicit and made it explicit and in the process rendered this entirely unique thing this completely unique experience something that was utterly general in nature”
The process of early psychological development involves both receiving information from both sides of the world through embodied and endocrine systems as well as neurological systems, and this information is processed as a whole, which is why people do not notice the dual-consciousness phenomenon that could theoretically arise from incomplete interhemispheric connectivity
“we all grow up with information coming to us from both parts of the world and it is communicated through the body and into the brain using both endocrine transmitters as well as the neurological system that we're describing right and the the normal person is receiving a picture all round of the world and this information is being taken as a whole so on the whole we don't find ourselves noticing this in fact if we noticed it it would be very damaging for us because we would find ourselves constantly torn like the person who's trying to pay and putting the money back in the pocket”
McGilchrist came to the study of neuroscience through his reading of Oliver Sacks' 'Awakenings,' which demonstrated that attending to individual patients while drawing philosophical conclusions about personhood, mind, and consciousness could illuminate what changes in the brain do to human experience.
“i thought i'd read oliver sack's book awakenings had just come out around that time i'm that old and uh i thought this is really fascinating here's someone who's attended to the individuality of his patients but made completely amazing philosophical conclude drawn philosophical conclusions they're very important about what happens when something changes your brain or your body and what that does to your your personhood to your to your mind and to to your whole humanity and i thought this is what i want to do”
Medical school training treated hemispheric division as obvious and mundane without questioning its purpose, despite the neurological principle that brain value depends on interconnection density, making hemispheric division paradoxical.
“when i was in medical school i mean obviously we we saw that it was there it is on the slab and it's divided and it was just taken for granted and nobody really said why what on earth is the point of having a mass of neuronal interconnections whose value we seem to believe is predicated on the sheer number of interconnections it can make why divide it right down the middle in this way”
The human brain is divided across the longitudinal fissure into left and right hemispheres, and this division exists not only in humans but in all mammalian brains, birds, reptiles, and even in ancient sea creatures and nematode worms, showing an asymmetrical neural network from the earliest organisms
“the human brain and you know not just human is this is true of the avian brain and all mammalian brains but for our purposes and most interestingly our own brains are divided across the the longitudinal fissure into left and right hemispheres”
Split-brain surgery, which involved dividing the corpus callosum to prevent epileptic seizures from overwhelming the entire brain, was successful in preventing seizures and provided a unique window into understanding how the two hemispheres operate independently, revealing that they can hold conflicting information and make different decisions
“the first split brain operations which was a procedure invented to aid patients who had constant epileptic seizures and somebody had the idea that it would be a good idea to divide the connection between the two hemispheres so that if a seizure started in one hemisphere it wouldn't automatically overwhelm the whole brain the other half would be able to carry on functioning and indeed it was a great success in achieving that but it gave people a window into the difference between these two worlds because you could actually by clever experimentation address problems and questions and test out each hemisphere on its own”
McGilchrist has spent decades—approximately 30 years researching the brain's lateralization and 10 more years writing new work—really studying the hemispheric literature in depth, which means his understanding is based on extensive immersion in the research rather than superficial overview, and very few neuroscientists have invested the career cost to do this level of deep study
“it took me 30 years basically to come up with the what i was able to write in the master in his emissary and another 10 years for what i've just written and i'm hoping will be published in the next 12 months so yes i mean it didn't start from a very auspicious place but i was completely convinced that something of great interest was being neglected and you asked why had people not sort of gone further with it i think the answer is that to make sense of it would have required 30 years and in doing so they would have basically forfeited their career because their when they were juniors their bosses wouldn't have wanted them to do research on lateralization they said no forget it that's all passe and as they got further on they wouldn't have got grants and they would have got promotion and so on so actually very few people have taken the trouble to really look at this in any great depth”
Most of what is happening in the brain—estimated at between 99.44 percent and over 99 percent of brain activity—is not within the field of consciousness, and what we call 'consciousness' is the narrow spotlight of attention that illuminates only a small portion of the brain's activity at any given moment
“consciousness means many different things and in one sense we think that consciousness is what is in my mind that i'm aware of right now and i'm focusing on but that is variously estimated to be between half a percent and five percent of what's going on in one's brain in fact i read a paper in which the the authors said that 99.44 of brain activity was not within the field of consciousness which is alarmingly precise”