Discussing split brain surgery with neurosurgeon Michael Egnor on the Michael Shermer show (June 24, 2025), Christof Koch made the claim that — when the two halves of the brain are severed (to treat epilepsy) — two different consciousnesses arise in the same person.

Dr. Egnor, who has done the surgery himself, began to question materialism, in part because he found that split brain patients have only one consciousness.

We’ve transcribed the dialogue and Egnor offers some additional comments below:

Background: Neuroscientist Christof Koch is a leading investigator at the Allen Institute for Brain Science, as well as the author of a number of books about consciousness, most recently, Then I Am Myself the World: What Consciousness Is and How to Expand It (Basic Books 2024).

Michael Egnor, author, with me, of The Immortal Mind: A neurosurgeon’s case for the existence of the soul (Worthy 2025), is a professor of neurosurgery at Stony Brook University.

Michael Shermer: Let’s look [50:55] at some specific examples from your book that Christof can address. You talked about split brain patients who seem, they seem to know things they shouldn’t know because the halves are separated. Yeah, you want to address that?

A knife can’t cut the intellect

Michael Egnor: Sure. The split brain [51:13] surgery — I’ve done it — and what struck me as utterly remarkable was how normal these people are. That is, you cut the corpus callosum and after the surgery you cannot tell the difference. I mean, if you put Christof and me in a room with two people — one had split brain surgery; one didn’t — you make them wear a hat so you can’t see the scar, I don’t think we could tell the difference. You really need specialized instruments and techniques to find them.

That’s why Roger Sperry (1913–1994) won the Nobel Prize because he did all these experiments. And what has been shown, I think, has been summarized very nicely by Yair Pinto in the Netherlands, is that these patients have split perception and unified consciousness. That is, that their consciousness really shows all the hallmarks of being a unified whole. That certainly is their everyday experience.

And even in detailed testing, there’s Alice Cronin-Golomb, who is an MIT neuroscientist who did some beautiful work back in the 1980s where she presented images to both visual fields. What she found was that there was certainly a perceptual split in our ability to transmit verbal information across and geometrical information across, but there was no conceptual split. That is, concepts could be shared between both hemispheres without any sign of splitting.

And I think this corresponds beautifully to the hylomorphic Aristotelian understanding of the soul. That is, a knife can cut the material stuff which is perception. But a knife can’t cut the intellect because it’s not a material thing. It can’t cut something that’s not material. Split brain surgery, I think, very strongly supports the hylomorphic view.

hylomorphism … view according to which every natural body consists of two intrinsic principles, one potential, namely, primary matter, and one actual, namely, substantial form. – Britannica

Shermer: Christof?

Two conscious entities emerge?

Christof Koch: I would [53:06] beg to differ. So it is true, and in fact it was when the operation was first performed by someone in Chicago in 1938, he already commented on the remarkable unremarkableness of these patients — that you cut 200 million fibers, roughly which are the number of axons that connect the cortical hemispheres, and you find relatively little differences until you do the testing. And then it’s it becomes very apparent, of course, in many patients.

I don’t know the patient you’ve operated on, Mike. In many patients, they don’t have a complete commissurotomy. You know, you sometimes leave the posterior or the anterior commissure and of course you don’t touch some of the deeper areas, the deeper fibers in the deeper parts of the brain below the thalamus, like at the level of the superior caliculus, the tectum that connects the left and the right brain.

The Pinto study shows that, yes, unless you’re really extremely careful, it’s very easy for the right side, you see … so the standard interpretation is the following: The standard interpretation of these split brains is that there are two conscious entities. There is the left hemisphere, typically the speaking one, and that tends to dominate.

Certainly, a few days after the operation, if you silence that by injecting sodium amytal here, you can briefly anesthetize the left hemisphere. Then the right hemisphere emerges. It can now command the opposite side, the left side.

It typically doesn’t speak; it has sort of non-linguistic component of consciousness. So the idea is that there are two consciousnesses in there but it’s the right, if you talk to the patient, it’s always the right hemisphere. And unless you’re really very careful doing the experiment — and I don’t think Pinto was (we can go into the technical details) — the left hemisphere will take cues from the right hemisphere, you know. Because it’s like a clever Hans phenomenon except now it’s my — you know — for example, if I, you know, if I do this with my right hand, that seems to indicate, you know, that’s a cue for my left hand that something is on the other side. But it doesn’t quite know what [ it is. But that cue can be picked up by the right hemisphere, the — sorry — by the left arm, i.e, the right hemisphere. So it’s very difficult to do these tests because they’re not completely split. It’s not like you’re taking a knife and you cut down all the way down to the brain stem.

Maybe the two halves are not really significantly severed?

Egnor: Right. Well the study by Cronin-Golomb at MIT involved three patients and [55:42] they had a commissurotomy, not just a callosotomy. So they had sectioning of the anterior commissure and of the hippocampal commissures, but not the posterior commissure.

I don’t believe — as far as I know, it is not routinely sectioned because it’s a very dangerous place to be; it’s by the aqueduct and you don’t want to be cutting anything there. So this was really as complete as it gets of separation of the hemispheres. And one explanation would be that there is queuing that goes on from side to side.

However, you can observe these people and they don’t appear to be queuing. The the other explanation would be subcortical pathways. There have been studies on the number of subcortical pathways, number of subcortical axons that are connecting the two hemispheres. And the number that I’ve that I’ve read is 1500 axons that connect the two hemispheres. As you said, there are 200 million axons in the corpus callosum. So that when you cut the corpus callosum you’re [56:46] cutting — I actually calculated this — it’s 99.99925% of the connections between the hemispheres So these disconnections are almost complete The subcortical pathways are very very small and have very few of them to mediate.

Koch: I don’t know about concepts but there certainly would be enough to mediate base emotions like surprise or [57:12] fear or the startle response.

Egnor: Yeah, but emotions also are hematological That [57:17] is, you make adrenaline and you can communicate with that. I believe that Pinto estimated (I’m not sure where he got this number from) he estimated that the rate of transfer in subcortical pathways in commissurotomy patients was no faster than one bit per second, was the number he used.

Egnor’s further reflections on his split brain debate with Koch

The remarkable thing about patients with split brains is that they do not have split consciousness, either in everyday life or in detailed neurological testing. Cronin at MIT showed that abstract concepts are shared by each split hemisphere without splitting of concepts. For example, a split brain patient can be shown a violin to one hemisphere and an artist’s palette to the other hemisphere, and can conceptually link them as forms of art, even though no part of the split brain patient’s brain has seen both images. This clearly implies that there is an aspect of the person’s mind that is not material and cannot be split with a knife.

Other investigators, including Justine Sergent and Yair Pinto, have observed similar unity of conceptual thought. Perception is split with a knife; conception is not split. This is remarkably consistent with the Aristotelian-Thomistic hylemorphic understanding of the human soul, which is that the intellect (which forms concepts) is spiritual/immaterial and does not originate from the brain, whereas the perceptual powers are material and do originate from the brain. Hylemorphic metaphysics explains the split brain phenomenon naturally and remarkably well.

Christof’s idea that split brain patients have split consciousness doesn’t really make sense and doesn’t correspond to the evidence. Consciousness is metaphysically simple — that is, my thoughts and sense of self can’t be split with a knife like the brain or a material thing can be split. What would it mean to say that I have “split” consciousness? Instead of Mike, there would be Mike and Joe, which wouldn’t be “split,” it would just be two people.

A category error

‘Split consciousness’ is an oxymoron, a category error. Consciousness is not the kind of thing that can be split, and there’s no evidence that one person can ever become two people. It’s science fiction, not science.

Adobe Stock Licensed split brain

Neuroscientists who refer to “split consciousness” refer to conflicting actions or motives in one person, but that happens in normal life. When I mistype the letter “a” by hitting “s” with my little finger, that’s just a mistake, not evidence of split consciousness in my brain (one “consciousness” didn’t want “a” and the other “s”). I just made a mistake.

Patients with split brains make more mistakes, because they have brain damage, but that’s not “split consciousness.” For example, people with cerebral palsy have a big problem with unintentional movements — they will try to move an arm, and a leg will move too, without their intending it.

This doesn’t mean that people with cerebral palsy have split consciousnesses — one consciousness that wants to move the arm, another than wants to move the leg. It simply means that they have disabilities with coordination because they have brain damage, the same kind of disability that split brain patients can have.

“Split consciousness” is a nonsensical idea and really just represents a disability — an incoordination — associated with brain damage. People with split brains have only one conscious self, and they have subtle and interesting disabilities associated with their brain damage.

Next: Neuroscientist Christof Koch proposes a bold experiment…

You may also wish to read: Mind: Neurosurgeon Michael Egnor vs. neuroscientist Christof Koch. Released yesterday evening, the debate, hosted by science writer and broadcaster Michael Shermer, was cordial, and quite relatable. At one point, the discussion turned on the case of a girl who had her sense of guilt removed surgically. What does that prove about the mind and the brain?