When the Feed Your Head podcast interview with neurosurgeon Michael Egnor went live last month, one topic that the host, Rabbi Adam Jacobs, asked about seems quite technical. But it is also quite significant:
Adam Jacobs: There is one experiment on a split-brain patient that I'd like to ask you about ... a split-brain patient who "was presented with conflicting messages in each of the two isolated visual fields: "The patient was able to respond to the conflicting information with perfect accuracy with either hand, even though each hand was controlled by a cerebral hemisphere that saw only one of the messages. She [the researcher] concluded that the patient could integrate and resolve the conflicting information, even though neither separated part of the brain actually saw at all." And then the conclusion "the patient's mind was more than the brain."
Michael Egnor: Yes.
Adam Jacobs: Can you please give me a couple of sentences explaining what was going on there and why this conclusion was reached?
A treatment for otherwise uncontrollable epilepsy has been to split the brain in half at the corpus callosum, the two-hundred-million-fiber bundle down the middle, to prevent seizures from becoming brainwide. So, the question is, how does the split affect the individual whose brain is now two separate halves?
Two Separate Minds?
Does that person end up with two separate minds? Prominent neuroscientist Christof Koch, who debated Dr. Egnor on Michael Shermer's podcast, is one expert who thinks so: "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."
In The Immortal Mind (Worthy, June 3, 2025), Dr. Egnor and I devoted some space to research on this question: Is it one mind or two?
Two neuroscientists, the Montreal Neurological Institute's Justine Sergent and the University of Amsterdam's Yair Pinto, noticed another interesting thing about split- brain patients: The disabilities that Sperry had identified are only perceptual. That is, they involve vision but not thought.
In 1983, Sergent reported her research on a split-brain patient who was presented with conflicting messages in each of the two isolated visual fields. The patient was able to respond to the conflicting information with perfect accuracy with either hand, even though each hand was controlled by a cerebral hemisphere that saw only one of the messages. She concluded that the patient could integrate and resolve the conflicting information even though neither separated part of the brain actually saw it all. The patient's mind was more than the brain.
In 1986, Sergent reported on a split-brain patient who could integrate visual information, such as whether two lines were aligned or would meet at an angle greater to or less than 90 degrees. showed an accuracy that was well above chance even though no part of the brain "saw" both images.
Again, in 1987, Sergent also reported research on two split-brain patients to whom she presented partial information in each of the split visual fields. Neither field was given enough information to make a final decision. Again the patients made an appropriate decision in most of the trials. She called their unity of consciousness "perceptual disunity and behavioral unity." Thirty years later, in 2017, neuroscientist Yair Pinto and his colleagues concluded, after reviewing decades of split-brain research, that the most accurate summary of the research is this: Split-brain patients have split perception but unified consciousness.
And There Is Other Research Too
Michael Egnor: The best example I can think of this and the way it's been done, I think in the most sophisticated way, was with Alice Cronin at MIT, who looked at split-brain patients. And what she did is she would present to one hemisphere using the visual fields. ...
She would put one picture, and in the other hemisphere, she would put three pictures, and one of the three pictures would conceptually match the picture in the other hemisphere. The other two would be unrelated.
One example of what she did is that she put a picture of a violin in one hemisphere and a picture of a violin in the other hemisphere. She put a picture of an artist's palette, a toilet plunger, and a light bulb.
And she said, "Tell me what concept links one of the pictures of the three to the other picture in the other hemisphere." And split-brain patients would always get it right. For example, in that instance, they would say, well, art is what links the violin and the artist's palate, the toilet plunger and the light bulb have nothing to do with the violin, really.
And she did it with hundreds of different trials of hundreds of different mixtures of pictures. And people with split-brain surgery were virtually always able to easily identify the concept and pick out the related images, even though no part of their brain saw both images.
From the paper:
All subjects performed the non-verbal matching task at well above chance level, with scores comparable to those attained when the task was performed completely within one or the other hemisphere. Transfer was equally successful in the two directions, though the pathway originating in the right and terminating in the left hemisphere may be more sensitive to affective components of the stimulus.
But the Question Still Arises
Is there a small, secret back door by which messages could get through to the other hemisphere? The split may not be altogether complete. Dr. Egnor commented,
Michael Egnor: ... when you cut the corpus callosum, you virtually completely separate the two hemispheres physically. So they're not in physical contact. There are roundabout ways that the hemispheres can talk to one another, at least theoretically, but they're very small ...
The problem with that theory was summed up by Yair Pinto, one of the main researchers in this area, a couple of years ago when he pointed out that when people count the axons in the workaround pathways, there are only about 1500 axons total in the brain in those pathways as compared to the 200 million in the corpus callosum.
So the degree of cutting is 99.999% there. And people have calculated how much information can be passed in those axons, the rate of information transfer. And Pento said that it's estimated it's about one bit per second, which is very slow, very, very slow.
And people [test subjects] conceptually will answer almost instantly. You'll show them these pictures and say, "Oh, art, it's the violin."
So I don't think the workaround theory works, and Pinto had a lot of questions about it. Also, Pinto summed it up as saying that in split-brain, patients, perception is split, but consciousness is unified.
And because there's no part of the brain that is conscious of these two regions, it would imply that the answer to Penfield's question, "Does the brain explain the mind completely?", is no. That there's a part of the mind that is aware of both, even though no part of the brain is aware of both.
Research continues. Dr. Egnor noted that there are roughly 1,500 papers in the neuroscience literature on the topic already. But current research suggests that a non-materialist approach to the neuroscience of the human mind is quite viable.
Cross-posted at Mind Matters News.