A mind split in two – or Strange Case of Dr Left and Mr Right
One of the things I ask myself when deciding what to write about for this newsletter is “What are the things I know that I wish more people knew?” That’s how I decided to write about Bayes’ theorem for my last essay, and about the split-brain for this one. The split-brain is a weird consequence of a specific type of neurosurgery that deeply shakes our understanding of the mind, but for whatever reason, it’s much less known by the general public than other bizarre scientific discoveries like, say, quantum mechanical phenomena, despite being, in my opinion, equally as puzzling and mind-boggling.
As you might know, the cerebrum is divided into two hemispheres, the left and the right. Holding them together is a bundle of nerve fibers called the corpus callosum. If you are a healthy human being, this is all great. But if you suffer from epilepsy, these fibers provide a way for seizures starting in one side of the brain to spread to the other.
In the late 1930s, doctors devised a simple solution to this problem: you open a patient’s skull and cut their corpus callosum in the middle. The scientific understanding of the time was that the corpus callosum didn’t do much besides aiding in the spread of epileptic seizures, so the procedure shouldn’t have major unexpected consequences. And so they tried, and astonishingly, the surgery seemed to work just fine. Patients of callosotomy, as the procedure was called, recovered well, had fewer seizures, and could go on with their lives normally. Everything indicated that the corpus callosum was indeed of little use.
A few days after the procedure, however, some patients started noticing weird things. As they were buttoning up a shirt with one hand, the other would start trying to take the shirt off. As one hand reached for an object, the other would reach for something else entirely. In the supermarket, the two hands would start fighting over which items to pick up from the shelves. It was always the left hand, patients claimed, that was the insubordinate one, independently of which hand was the dominant one before surgery.
The first scientist to rigorously study what was happening was Roger W. Sperry. A zoologist by training, Sperry had spent many years studying what happened to the behavior of mice when you rewire their neurons, first at the University of Chicago and then at the California Institute of Technology. But his most remarkable discoveries had come from studying cats with severed corpora callosa. Startlingly, Sperry had noticed that you could teach a skill to one of the cat’s cerebral hemispheres and not to the other. To do so, he would cover one of the cat’s eyes, teach him a skill, and then switch the eye that was covered. (To be clear, Sperry had not only cut the cat’s corpus callosum but also his optic nerves in such a way that information shown to one eye could only go to the corresponding cerebral hemisphere.) The cat, who seconds earlier had appeared to dominate the trick, now seemed to have never learned the trick at all. Could it be that only one of the cat’s cerebral hemispheres had learned it, while the other remained completely ignorant? Could a similar thing be happening with the human patients whose hands seemed rebellious?
Sperry got the opportunity to test this hypothesis in 1960 after he was contacted by neurosurgeons Joseph Bogen and Phillip Vogel. They would perform a callosotomy on an epileptic war veteran and wanted Sperry to test the patient for cognitive and behavior changes.
No one expected much. After all, most of the callosotomy patients seemed to behave just fine afterward. The experiments with cats were fun, but it was absurd to imagine something similar could happen to humans. Michael S. Gazzaniga, then a graduate student who had recently joined Sperry’s lab and was asked to prepare the set of tests, recalls:
Riveting as these findings were, they seemed incoherent when considered in the context of human behavior. Could a left hand not know what the right hand is holding? Preposterous. … Looking back at those early days, it is hard to overstate the adventurous nature of our project. Nobody thought the patient would actually provide evidence the mind could be split. Weeks earlier, a case of callosal agenesis, a birth defect where there is a complete or partial absence of the corpus callosum, had come through the laboratory and nothing seemed out of the ordinary. From a larger view, even though one of the world’s greatest neurobiologists was involved, neither Sperry nor certainly I, a green-as-could-be new graduate student, had any significant experience examining patients. To others it might have seemed to be a fool’s game and a waste of time. However, it was not, because at Caltech, the attitude was always, “try it.”
In fact, Gazzaniga’s report of the experiments is so awesome that I think you should read it yourself, so I reproduced an excerpt below. But first, let me give you some context. The patient Gazzaniga was tasked with testing was called W.J.. Differently from the cats Sperry experimented on, W.J. did not have his optic nerves severed, so covering one eye would not suffice to isolate information to only one hemisphere. So the scientists had to take advantage of some weird facts about brain anatomy. First of all, they knew that all brains (with severed corpora callosa or otherwise) have this twisted organization that causes the hemispheres to control the movements of the opposite side of the body; this is, your left hemisphere controls your right hand, whereas your right hemisphere controls your left hand. For the eyes, however, things work a little bit differently. The left half of each eye sends information to the left hemisphere (same side), and likewise for the right half of each eye. However, due to the laws of optics, everything to the left of where a person is focusing on ends up projected on the right side of both retinas, while everything to the right ends up on the left. As a consequence, an image on the left visual field gets processed by the right hemisphere, and vice versa. I know that all this might sound very confusing, but it is rather quite simple: your left hemisphere controls and sees everything on the right, and your right hemisphere controls and sees everything on the left.
What the scientists had to do, then, was to flash an image very fast on a patient’s left visual field for it to be processed by the right hemisphere, and to flash it on the right visual field for it to be processed by the left hemisphere. (It had to be fast so the patient didn’t have time to move his eyes to see it with both hemispheres.)
Finally, you should know that the left hemisphere is king when it comes to language processing. The right hemisphere can do a thing or two language-wise, but it’s the left who really does the talking. (Oh, and by the way, those popular claims about differences in the hemispheres, such as claiming one is rational and the other is creative, are totally false.) That's it, we can go to Gazzaniga now.
The big test came on a bright sunny Pasadena day. W.J. was rolled up to the entrance of the biology building on San Pasquale Avenue. Still recovering from surgery, W.J. used a wheelchair to get around. He still sported his helmet, which he had been wearing to protect himself from possible seizure-associated falls. Was this World War II veteran, who had been knocked out by a blow from the butt of a German rifle after a parachute jump behind enemy lines, going to reveal a deep secret? It did not seem likely. The morning began modestly enough. There was no drum roll as we entered the building. In fact, I was left alone to do the testing. Testing that proved to be mind-boggling. It remains so until this day. Here is how I recently described the moment:
M.S.G.: Fixate on the dot.
W.J.: Do you mean the little piece of paper stuck on the screen?
M.S.G.: Yes, that is a dot. . .. Look right at it.
W.J.: Okay.
I make sure he is looking straight at the dot and flash him a picture of a simple object, a square, which is placed to the right of the dot for exactly 100 milliseconds. By being placed there, the image is directed to his left half brain, his speaking brain.
M.S.G.: What did you see?
W.J.: A box.
M.S.G.: Good, let’s do it again. Fixate the dot.
W.J.: Do you mean the little piece of tape?
M.S.G.: Yes, I do. Now fixate.
Again I flash a picture of another square but this time to the left of his fixated point, and this image is transmitted exclusively to his right brain, a half brain that does not speak. Because of the special surgery W.J. had undergone, his right brain, with its connecting fibers to the left hemisphere severed, could no longer communicate with his left brain. This was the telling moment. Heart pounding, mouth dry, I asked,
M.S.G.: What did you see?
W.J.: Nothing.
M.S.G.: Nothing? You saw nothing?
W.J.: Nothing.
My heart races. I begin to sweat. Have I just seen two brains, that is to say, two minds working separately in one head? One could speak, one couldn’t. Was that what was happening?
W.J.: Anything else you want me to do?
M.S.G.: Yes, just a minute.
I quickly find some even more simple slides that only project single small circles onto the screen. Each slide projects one circle but in different places on each trial. What would happen if he were just asked to point to anything he saw?
M.S.G.: Bill, just point to what stuff you see.
W.J.: On the screen?
M.S.G.: Yes and use either hand that seems fit.
W.J.: Okay.
M.S.G.: Fixate the dot.
A circle is flashed to the right of fixation, allowing his left brain to see it. His right hand rises from the table and points to where the circle has been on the screen. We do this for a number of trials where the flashed circle appears on one side of the screen or the other. It doesn’t matter. When the circle is to the right of fixation, the right hand, controlled by the left hemisphere, points to it. When the circle is to the left of fixation, it is the left hand, controlled by the right hemisphere, that points to it. One hand or the other will point to the correct place on the screen. That means that each hemisphere does see a circle when it is in the opposite visual field, and each, separate from the other, could guide the arm/hand it controlled, to make a response. Only the left hemisphere, however, can talk about it. I can barely contain myself. Oh, the sweetness of discovery.
Thus begins a line of research that, twenty years later, almost to the day, will be awarded the Nobel Prize.
What Gazzaniga was seeing, in fact, were two hemispheres working independently. The left one could speak, so it could say when it saw something, but the right one could only point – using the left hand. Strikingly, one hemisphere seemed completely unaware of the experiences of the other. This explained why patients complained about the rebellion of the left hand: it was not that this hand was the insubordinate one, it was actually that both were working independently, but only one was able to speak.
These findings ignited a whole new research agenda in neuroscience. For one, what else besides language could only one of the sides of the brain do? But most importantly… What the hell was happening? Quoting historian of science Anne Harrington, “had the surgeon’s knife produced two independent minds within a single … skull?”
In a later experiment, Gazzaniga tested a teenage boy who had recently undergone callosotomy. He wondered whether the right hemisphere was completely incapable of language processing, as was then believed, or whether it was merely a matter of speaking. To test this, Gazzaniga offered the boy Scrabble tiles and started flashing questions to his right hemisphere. Journalist David Wolman describes a recording of the experiment:
In Gazzaniga's video, the boy is asked: who is your favourite girlfriend, with the word girlfriend flashed only to the right hemisphere. As predicted, the boy can't respond verbally. He shrugs and shakes his head, indicating that he doesn't see any word, as had been the case with W.J.. But then he giggles. It's one of those tell-tale teen giggles — a soundtrack to a blush. His right hemisphere has seen the message, but the verbal left-hemisphere remains unaware. Then, using his left hand, the boy slowly selects three Scrabble tiles from the assortment in front of him. He lines them up to spell L-I-Z: the name, we can safely assume, of the cute girl in his class.
Take a moment to ponder about what had taken place. Gazzaniga had shown that the right hemisphere was also capable of communicating; if not by speaking, at least by arranging Scrabble tiles. But to me, this was a minor discovery relative to the puzzling phenomenon at hand: half of a boy was thinking about his crush, while the other half didn’t even know that a question was asked.
In the decades that followed, Gazzaniga performed a multitude of split-brain experiments, uncovering a new fascinating phenomenon each time. One in particular stands out because it makes us question how all of us reason about our actions.
In this experiment, Gazzaniga would present each hemisphere with a different image, each in turn related to one of four cards placed on a table. For instance, the right hemisphere would be shown a snow scene, while the left hemisphere would be shown a chicken claw. The patient would then be instructed to choose the cards that related to the images. The right hemisphere would correctly point to a snow shovel (because it had seen the snow scene) using the left hand, and the left hemisphere would correctly point to a chicken head (because it had seen a chicken claw) using the right hand.
But what would happen when Gazzaniga asked the patient why they had chosen the shovel? It was the right hemisphere that had done it, because it had seen a picture of snow, but it was incapable of speech. The speaking hemisphere, the left, was only aware of seeing a chicken foot, so it had no idea why it chose the shovel. Or so Gazzaniga expected. What he found, however, is that the left hemisphere had no problem in answering the question. It would immediately come up with an explanation relating the shovel to the chicken. “Oh, that’s simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” It was not just that the left hemisphere could think of a reason for choosing the shovel. It truly believed that it was for this reason that it did.
Could it be that all of us, with our intact (I’m assuming) corpora callosa, are also coming up with reasons for our actions that we deeply believe to be true when in fact they aren’t? I’m not even talking about complex Freudian behaviors such as yelling at your coworker because you didn’t receive enough maternal love as a child (I’m pretty sure that’s how psychoanalysis works). I’m talking about mundane decisions such as why you choose the chocolate pudding over the fruit salad. Could it be that it is something else in your brain that makes these decisions for you, while your conscious self’s only function is to come up with explanations for why you did it?
Michael Gazzaniga refers to this function of the brain as “the interpreter” in his book Tales from Both Sides of the Brain: A Life in Neuroscience:
The interpreter not only took note; it tried to make “sense” out of the behavior by keeping a running narrative going on about why a string of behaviors was occurring. It is a precious device and most likely uniquely human. It is working in us all the time as we try to explain why we like something or have a particular opinion, or rationalize something we have done. It is the interpreter device that takes the inputs from the massively modularized and automatic brain of ours and creates order from chaos. It comes up with the “makes sense” explanation that leads us to believe in a certain form of essentialism, that is, that we are a unified conscious agent. Nice try, interpreter!
But wait a minute, what does he mean when he suggests that the interpreter is trying to trick us into thinking we are a unified conscious agent? Are we not? Even in the context of callosotomy patients, what could it mean for two minds to be operating independently within a single head? Do the two hemispheres have different conscious experiences, two separate “selves” each thinking a different thing, as if they were two different persons? It’s important to note that this is very different from dissociative identity disorder (popularly known as “multiple personalities”). Gazzaniga’s patients were not merely changing the way they behave from one moment to the next, they were effectively displaying two different minds working at the same time, each, supposedly, with its own thoughts and perceptions of the world.
Consider the implications of this. For instance, what if the patient decides they are tired of the rebellious hand and wants to remove his right hemisphere entirely? It has been shown that patients that undergo hemispherectomy, this is, the complete removal of one hemisphere (which is also done to contain seizures), can mostly lead normal lives with only one hemisphere. Thus the left hemisphere could decide the right one is an annoying parasite and ask doctors to remove it. But if the split-brain produces two different minds, two different consciousnesses, removing a hemisphere would entail preventing a conscious entity from all of its future experiences. Wouldn’t this be ethically equivalent to murder?
We can go even further with these speculations. For instance, what if there already are parts of your brain that have their own separate conscious experiences? Take the visual cortex. It does all the visual processing that psychologists call “unconscious”. But how do we know? I mean, if you ask me I’ll tell you that I’m not conscious of any primary visual processing. But the me who is doing the answering is really just the part of my brain which has access to speech. Who speaks for the parts that do not?
In his book Waking Up: A Guide to Spirituality Without Religion, philosopher and neuroscientist Sam Harris writes:
All brains–and persons–may be split to one or another degree. Each of us may live, even now, in a fluid state of split and overlapping subjectivity. Whether or not this seems plausible to you may not be the point. Another part of your brain may see the matter differently.
As you might have already realized, I don’t have good answers for any of this. Nor does Sam Harris, Michael Gazzaniga, or W.J. for that matter. I think more people should know about the split-brain not because it provides us with definite answers but because it raises the right questions.
At this point, your brain is probably already hurting, with your right hemisphere begging for a callosotomy so it doesn’t have to think about all this. I think it’s enough for a day.
Thanks to Aline Zimerman, Gabriel Ferreira, João Mercadante, Paula Sampaio, and Victoria Oldemburgo de Mello for reading drafts of this.