On a recent episode of the Michael Shermer show, prominent neuroscientist Christof Koch was discussing mind–brain issues with neurosurgeon Michael Egnor:

Koch, who takes a materialist approach to the brain, hoped that the “shared brain” thought experiment would demonstrate his point:

Christof Koch: One way to test this, by the way, would be to use — and this will come, this will happen — is to use [57:44] something like Neuralink to try to connect your brain with my brain. Because I think what this will show, that if we essentially do a reverse commissurotomy, we add — let’s say for the sake of argument we add 200 million fibers from my brain to your brain — okay and I’ve written about it in my previous book.

What will happen? So first, I just see through your eyes — a little bit like augmented reality. I’m [58:09] still Christof but I see now what you’re seeing.

Michael Egnor: Right.

Koch: Just like VR or AR [58:14] (augmented reality). But then, at some point, there will be abrupt transition. If the number of fibers exceed a certain threshold, then your mind and my mind will merge. Mike will disappear. Christof will disappear. Instead there will be this new consciousness that has now four hemispheres, you know, that has two mouths and four arms and four legs. There will be a single conscious mind… experientially, it will be one mind.

Koch may have been unprepared for Egnor’s response.

But nature has already run this experiment…

Egnor: Reverse commissurotomies have been done. They’ve been done by nature.

There are conjoined twins who are conjoined at the brain and —

Koch: Yeah but, yeah, okay —

Egnor: And the most interesting pair of them is Krista and Tatiana Hogan —

Koch: Yeah, in Canada.

LIVE: Born conjoined at the head 10 years ago, meet Krista and Tatiana Hogan in a new documentary from @JudithPyke .https://t.co/v5H4AHmyop pic.twitter.com/bHtVNSZYNB

— CBC British Columbia (@cbcnewsbc) November 4, 2017

Egnor: And they share a thalamic bridge. So there’s a thalamic bridge and they’re absolutely fascinating. They share vision in each other’s eyes, they share sensation on each other’s skin, they share a lot of emotions. But they do not share personality at all. They’re very different people.

They don’t share, as far as I know, intellects. That is, it’s not as if one of them can study calculus, the other can study philosophy, and they both pass the test. So the hylomorphic framework fits so beautifully for this — that they’re sharing the material aspects of the brain, perceptions, but they don’t share intellect because they because it’s not material.

Koch: The bridge isn’t between their left, you know, it’s not like one has a right hemisphere and the other one — they have two separate, they have two sets of independent cortical hemispheres. They share a thalamic bridge but so in that sense, they’re separate now. I would say, if they join particular the prefrontal part, if it’s joined in both, then they would have a single intellect…

Michael Shermer: They would experience what the other one [1:00:29] is experiencing, that first person perspective.

Egnor: Correct. There’s no sign that they do that and —

Koch: Well [1:00:37], except when they go to sleep, well, if you read carefully — I mean they haven’t been studied because their parents keep them away from the prying eye of scientists — But when they go to sleep, they go to sleep at the same time, as you would expect. And the question is, once the ego defenses fall away during sleep… Do they both dream of the same thing or do they have independent dreams?

Egnor: Right. Fascinating stuff.

Shermer: They dream of electric sheep.

Koch: Do they dream of electric sheep? Probably not.

Egnor: Conjoined sheep.

From The Immortal Mind: (Worthy, June 3, 2025) on Krista and Tatiana Hogan:

Inseparability is a reality for the twins but, like “youth” or “career,” it is also an abstraction. The twins had to learn its meaning the hard way when they were quite young. As the CBC feature recounts:

“When they were little, they used to try to pull their heads apart. Their mother always told them they were stuck, so they would have to work things out. But as they’ve gotten older and the frustrations mount, they still fight. As they freely admit, some days they don’t like being together.

“She’s annoying,” says Tatiana, who promptly gives her twin a reassuring hug.”

So each twin had to separately realize the concept of inseparable for herself and internalize what it meant for her own future. (“Future” is another abstraction, of course.) (p. 78)

Egnor’s follow-up thoughts: “Split brain research and research on twins joined at the brain are in a sense mirror images of each other.”

Conjoined twins joined at the brain are rare but it does happen, and Tatiana and Krista Hogan are probably the best example. Despite the fact that the girls share some aspects of movement (the ability to move each other’s limbs), skin sensation and vision (they can feel touch on each other’s skin and see partially through each other’s eyes), some memories and some emotions, they have completely separate selves, with different and quite distinct personalities, different concepts, and different wills. They like different things, disagree often, and occasionally have fights.

This fits the Aristotelian-Thomistic hylemorphic framework very well — movement, sensation, memory and emotion are material powers caused by the brain, but intellect and will are immaterial and do not originate from the brain.

Split brain research and research on twins joined at the brain are in a sense mirror images of each other — i.e., the study of the study of brains split and brains fused. The science clearly is consistent with the hylemorphic understanding of the soul—i.e., that the intellect and will are immaterial powers of the human soul.

Christof Koch is a leading investigator at the Allen Institute for Brain Science. His most recent book is Then I Am Myself the World: What Consciousness Is and How to Expand It (Basic Books 2024). Michael Egnor is the author, with me, of The Immortal Mind: (Worthy 2025).

You may also wish to read:

Neuroscientist: Brain surgery can create “two conscious entities.” Dr. Egnor, who has done split-brain surgery himself, dismisses neuroscientist Christof Koch’s claim. Egnor: Consciousness is not the kind of thing that can be split, and there’s no evidence that one person can ever become two people.

and

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?