Dr. Theodore Schwartz is a prominent Cornell University neurosurgeon. In addition to publishing many scholarly articles, he is the author of Gray Matters: a Biography of Brain Surgery (2024). He’s a very thoughtful guy and his recent essay at Psyche, “What removing large chunks of brain taught me about selfhood”, caught my attention.
Dr. Schwartz: “As a brain surgeon…I’ve severed the brain in two and watched in amazement as my patients wake up feeling like their complete and undivided selves.” (February 17, 2025)
I’ve had the same experience. After split brain surgery, patients wake up feeling completely unified, like just one person, despite the surgical disconnection of the two halves of their brain. A few patients have transient disorders like “alien hand syndrome” but this is rare. By and large, these people are normal in ordinary activities of life.
Dr. Schwartz: “When I first did this type of operation, I had fantasies that they might suddenly refer to themselves as ‘we’ rather than ‘I’. Thankfully, this never occurred…the patient’s sense of a unified self is the illusion.”
That’s not true. The split-brain patient’s sense of a unified self is real, not an illusion.
I say this for two reasons.
It makes no sense to say that two people have an illusion that they are one person. To have an illusion presupposes that the subject with the illusion is one person. Two people would have two illusions, or they would have similar illusions, or share illusions, or conspire to claim to have the same illusion, etc. But having an illusion— even an illusion that I am one person after having my brain split in two— presupposes that I am a single person that has the illusion. The claim that two people have one illusion— not just share similar illusions, in which case they are just two people with two similar illusions— makes no sense.
There is clear neuroscientific evidence for unified consciousness in patients with split-brains. Neuroscientist Justine Sergent studied split-brain patients and found that while some perceptual abilities are indeed split— for example, the right side of the visual field is seen via the left hemisphere, and vice versa— there remains a genuine unity to the human mind. Sergent showed images of different objects to each of the two split hemispheres, and found that patients could compare the objects reasonably accurately, even though no part of the brain perceived both objects:
From her paper: “[We found] the coexistence of perceptual disunity and behavioural unity, and they suggest that, even when the two disconnected hemispheres receive different information, the commissurotomized brain works as a single and unified organism.”
Across a wide variety of tasks, split-brain patients with a complete and radiologically confirmed transection of the corpus callosum showed full awareness of presence, and well above chance-level recognition of location, orientation and identity of stimuli throughout the entire visual field… These findings suggest that severing the cortical connections between hemispheres splits visual perception, but does not create two independent conscious perceivers within one brain.
Sergent and Pinto found that patients with split-brain surgery did have subtle perceptual disabilities associated with the split nature of their brains, but they nonetheless were capable of integrating the split information and remained one conscious individual.
In other words, the normal sense that split-brain patients have that they are one person with one center of consciousness is not an illusion. They are, in fact, one person with one mind, even after splitting the brain hemispheres. This means that there is an aspect of the mind— “soul” is perhaps a better word here— that is not split by the neurosurgeon’s scalpel.
Our conscious unity, even after split-brain surgery, is not an illusion. Each of us is a physical creature with a single spiritual soul, which is immaterial and cannot be split with a knife. This is not only the perennial teaching of the great religions, but the evidence of the best neuroscience.
Professor of Neurosurgery and Pediatrics, State University of New York, Stony Brook
Michael R. Egnor, MD, is a Professor of Neurosurgery and Pediatrics at State University of New York, Stony Brook, has served as the Director of Pediatric Neurosurgery, and is an award-winning brain surgeon. He was named one of New York’s best doctors by the New York Magazine in 2005. He received his medical education at Columbia University College of Physicians and Surgeons and completed his residency at Jackson Memorial Hospital. His research on hydrocephalus has been published in journals including Journal of Neurosurgery, Pediatrics, and Cerebrospinal Fluid Research. He is on the Scientific Advisory Board of the Hydrocephalus Association in the United States and has lectured extensively throughout the United States and Europe.