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If Your Brain Were Cut in Half, Would You Still Be One Person?

Yes, with minor disabilities. Roger Sperry’s split-brain research convinced him that the mind and free will are real

In a recent podcast, “Splitting the Brain and Staying You”, Robert J. Marks asked Dr. Michael Egnor, “If you lose all four of your limbs, are you still you? Most people would say yes. What if your brain were cut into two pieces? Would you still be you?” What makes us unique, single human beings? Dr. Marks and Dr. Egnor look at the work of neuroscientist Roger Sperry (1913–1994) who was a Nobel Prize winner in 1981 for Physiology and Medicine. He studied people whose brains had been split:

A partial transcript follows:

01:45 | The research of Roger Sperry

Michael Egnor: Roger Sperry (below right) was a neuroscientist who worked mainly in California. He was very interested in the consequences of an operation called corpus callosotomy. A corpus callosotomy is an operation in which the neurosurgeon basically splits the brain in two. In that surgery, which I’ve performed and my colleagues perform, the corpus callosum, which is a large bundle of neurofibres that connects the two hemispheres of the brain is cut so that the two hemispheres of the brain are functionally disconnected. There’s no longer a material connection between them.

04:06 | Splitting the brain

Michael Egnor: The reasoning behind the surgery is that there are some people who have small seizures in one hemisphere of the brain who have those seizures travel across the corpus callosum into the other hemisphere and when the seizure does that, it becomes a major seizure instead of a small seizure. That can be very disabling and there are people who have twenty or thirty of those major seizures a day and medication doesn’t always work. So the point of the corpus callosotomy is to prevent the major seizure from happening. It’s reasonably effective. There are many ways of doing it. Sometimes the entire corpus callosum isn’t cut but only part of it. But sometimes the entire corpus callosum is cut and Sperry felt that these patients were very interesting from the standpoint of neuroscience.

Robert J. Marks: When you say they were cut, the right and left hemisphere were totally separated from each other (picture the brain in the image below cut in half).

Michael Egnor: For the most part, yes. There are small regions in the anterior part of the brain called the anterior commissure and the posterior commissure where there was still some potential for connection but 99% of the connections between the two hemispheres are cut by cutting the corpus callosum.

05:28 | What happens when you split the brain?

Michael Egnor: So Sperry asked a question. He said, “What happens to these people?” It was clear that, by cutting the corpus callosum, their seizures were made better but were they still one person? What did cutting the brain, basically in half, do to a person? So he studied these patients in great detail.

I’ve had patients with this as well and what he found is what I and other neurosurgeons who have dealt with this have found, that you cut the brain basically in half and—except for the fact that their seizures usually get better—they’re no different. They’re perfectly all right.

Robert J. Marks: Isn’t that incredible?

Michael Egnor: Right. If you were to meet these people, if they sat down in front of you and you had a conversation with them, you couldn’t tell the difference’ They’re perfectly normal people. And they can’t tell the difference. They don’t feel any different.

What Sperry did, though, was he studied them very, very carefully. And he found that there were subtle differences that—for example, it’s well known that, if you look straight ahead, everything to the left of the midline of where you’re looking is seen via the right hemisphere of your brain and everything to the right of where you’re looking is seen by the left hemisphere of your brain. So the visual fields kind of cross in the brain.

And Sperry showed that the left hemisphere is mainly the hemisphere that mediates speech and the right hemisphere tends to mediate geometrical and spatial understanding. If the corpus callosum is cut, the two hemisphere have perceptual abnormalities. If you sow the right hemisphere an apple, it’s capable of knowing that it is an apple but it is not capable of mediating speech in saying that it is an apple. Only the left hemisphere can do that. So he was able to understand the functioning of the hemispheres in a little more detail.

But all of the functional abnormalities that he found, number one, they were undetectable in everyday life. In fact, that’s why he won the Nobel Prize. You don’t win the Nobel Prize for finding out obvious things. So in everyday life, these people were perfectly normal. On very careful, subtle testing, you could find these perceptual abnormalities.

07:59 | Perceptual vs. intellectual abnormalities

Michael Egnor: But the other thing that he found was that all of these abnormalities were perceptual, none of them were intellectual… it wasn’t like you disconnected addition from subtraction or justice from mercy or integral calculus from differential calculus. There were no intellectual changes. These were all just perceptual. And what I believe this research shows, and what I believe is the most interesting thing about Sperry’s research was not what’s usually cited. Most people say that what’s interesting about Sperry was the abnormalities that he showed. What I think is interesting is the lack of abnormality that he found. That is, that these were basically normal people, with brains cut in half. And that is what ought to make us say Wow!

Robert J. Marks: And so there were no abnormalities in abstract reasoning and such…

Michael Egnor: Correct. It would be the same thing as if you were sitting at your computer and one of your mischievous kids took a chain saw and cut your computer in half while you are typing away and nothing changes. It works just fine. There’s something about this computer that I didn’t understand before. And it’s still working… Nobel laureate scientists studied these patients and found very little that is wrong with them. And specifically they found that the only differences were perceptual things, not intellectual things. So, in my view, what Sperry showed was that the intellectual aspect of the human mind is what philosophers call metaphysically simple. By that, I mean that it can’t be split, like matter can be. Any material thing, if you think about it, can be split, your cell phone or a piece of paper or a brain—you can cut it, cut it in half. If you think about it, what defines a material thing is that it has extension in space. So it’s got parts to it. Cut it down the middle, you’ve got a right part and a left part.

Does the mind have parts? While one might make a case that the perceptual powers can be sort of divided into parts, the intellectual powers cannot be. You can’t cut the intellect in half. If you did, you’d have two people. And you don’t get two people.

So what the split brain research showed, is that intellect—and will, which follows our intellect—are what’s called metaphysically simple. That is, they’re not composed of parts. And that is typical of a spiritual thing. Spiritual things are not splittable. You can’t split a spirit. It doesn’t make any sense to talk about splitting spirits. So I think that Sperry’s research strongly confirms that the intellect and will are immaterial powers of the mind.

11:05 | Possibilities of other brain connections

Robert J. Marks: I’m sitting here thinking about ways that the two hemispheres can still communicate. I wonder if these things have been considered. One of them would be a quantum connection where you have entangled states on both sides, maybe an electromagnetic connection, or maybe the idea that the brain might act like a hologram. It’s like looking through a window. If you take away half a window, you can still look through the window and see what’s on the other side. Do you know of anybody who has tried to explain away the split brain surgery using an argument of that sort?

Michael Egnor: I’m not aware of anyone who has brought up the point that the split brain surgery strongly supports the viewpoint that the intellect is metaphysically simple and is an immaterial power of the mind, even though the research obviously supports that view. The notion that there must be some other way for the hemispheres to connect, be it quantum or electromagnetism or something, may be sort of a category error. I think that the most logically rigorous way to look at it is that the intellect isn’t a material power of the mind so that talking about connection of the hemispheres related to the mind doesn’t really make any sense. If something is immaterial, then it doesn’t have material connections, by definition. You could chop the brain into a thousand pieces and it wouldn’t have an effect on the immaterial powers of the mind. [See Note below on the mixed ways in which the significance of Sperry’s research has been presented to the public.]

Show Notes

00:30 | Introducing Dr. Michael Egnor, Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook

01:45 | The research of Roger Sperry
02:56 | Corpus callosotomy vs. frontal lobotomy
04:06 | Splitting the brain
05:28 | What happens when you split the brain?
07:59 | Perceptual vs. intellectual abnormalities
11:05 | Possibilities of other brain connections
13:06 | How do the mind and the brain work together?
14:05 | The materialist approach to Sperry’s results

Note: Sources generally don’t emphasize the significance of the fact that split-brain patients showed normal behavior in everyday life. One source explains that Sperry did a great deal of work with cats and monkeys, where he severed the corpus callosum for the purposes of the experiment. Here’s what he found:

His experiments started with split-brain cats. He closed one of their eyes and presented them with two different blocks, one of which had food under it. After that, he switched the eye patch to the other eye of the cat and put the food under the other block. The cat memorized those events separately and could not distinguish between the blocks with both eyes open. Next, Sperry performed a similar experiment in monkeys, but made them use both eyes at the same time, which was possible due to special projectors and light filters. The split-brain monkeys memorized two mutually exclusive scenarios in the same time a normal monkey memorized one. Sperry concluded that with a severed corpus callosum, the hemispheres cannot communicate and each one acts as the only brain.

Sperry moved on to human volunteers who had a severed corpus callosum. He showed a word to one of the eyes and found that split-brain people could only remember the word they saw with their right eye. Next, Sperry showed the participants two different objects, one to their left eye only and one to their right eye only and then asked them to draw what they saw. All participants drew what they saw with their left eye and described what they saw with their right eye. Sperry concluded that the left hemisphere of the brain could recognize and analyze speech, while the right hemisphere could not.

Dina A. Lienhard, “Roger Sperry’s Split Brain Experiments (1959–1968)” at The Embryo Project Encyclopedia

It sounds as though the humans were much less affected than the cats and monkeys. Some sources emphasize the abnormality of the humans with split brains: “ At first the patient seemed quite normal, but experimentation showed certain activities such as naming objects or putting blocks together in a prescribed way could only be done when using one side of the brain or the other.” (PBS). Others acknowledge the relatively minor nature of the disabilities:

Yet, when Sperry started testing patients with split brains, he and other scientists were surprised. He found that not only could these patients continue to carry on most everyday functions after the two hemispheres were disconnected, but that the right brain wasn’t as word-deaf and word-blind as once thought. It wasn’t as advanced in language skills as the left, but patients using only their right brains could recognize such sophisticated spoken phrases as “a measuring instrument,” and could spell three- and four-letter words. Also, in split brain patients, both sides of the brain were clearly conscious, even when they weren’t aware of what the other side was seeing, hearing or thinking. While the two sides of the brain obviously worked in tandem when they were connected, they could operate independently if necessary.

Michael Parrish, “Roger Sperry: The Brains Inside The Brain” at Brain Connection
This image has an empty alt attribute; its file name is Corpus-callosum-Grays-Anatomy-public-domain.png

Parrish at Brain Connection goes so far as to admit the fundamental significance of Sperry’s findings:

The implications of split-brain research have been widely debated. Scientists and philosophers have long argued over what is known as the mind-body quandary, the relationship between our mind and the physical brain. Some scientists saw the work of Sperry and others as supporting the notion that the brain operates almost entirely mechanically, and that consciousness, reasoning and free will have almost no effect. But Sperry strongly felt otherwise…

What this meant to Sperry was that free will, and responsibility, were no illusion. “It is possible to see today,” he believed, “an objective, explanatory model of brain function that neither contradicts nor degrades but rather affirms age-old humanist values, ideals, and meaning in human endeavor.”

Michael Parrish, “Roger Sperry: The Brains Inside The Brain” at Brain Connection

It’s fair to say that the true significance of the split-brain experiments goes far beyond the significance of the lateralization of the brain; it also points to the immaterial nature of the mind.

Further reading on what happens to people whose brains are split or who live with half a brain:

Yes, split brains are weird, but not the way you think. Scientists who dismiss consciousness and free will ignore the fact that the higher faculties of the mind cannot be split even by splitting the brain in half. (Michael Egnor)

Some people think and speak with only half a brain. A new study sheds light on how they do it.

We will never “solve” the brain. A science historian offers a look at some of the difficulties we face in understanding the brain.


Four researchers whose work sheds light on the reality of the mind The brain can be cut in half, but the intellect and will cannot, says Michael Egnor. The intellect and will are metaphysically simple.

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If Your Brain Were Cut in Half, Would You Still Be One Person?