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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

University of Chicago evolutionary biologist Jerry Coyne concludes from studies of post-operative epilepsy patients whose brains have been split that “‘will’, ‘volition’ and ‘consciousness’ are the results of purely physical processes in the brain… perhaps the notion of consciousness and of will are things that merely report to us after the fact the deterministic actions of our brain, and are not in any way part of a causal chain.” He writes:

The interesting bit to me is how this division of the brain seems to divide consciousness or awareness as well as volition. This is based on some stuff we already knew: for example, that visual information from the left eye goes to the right side of the brain and vice versa, and also that the language center resides on the left side of the brain.

So here’s one experiment about consciousness. You present the word “key” to the subject’s left eye only. That visual information goes to the right side of the split brain. When you ask the subject what word she saw, she says “I saw nothing”, because the ability to formulate language is on the other side of the brain, the left side. This apparently means that the consciousness of having seen the word has been split.

Jerry Coyne, “The weirdness of split-brain experiments” at Why Evolution Is True

Not so fast. Like most commentators on this important research, Coyne misunderstands its implications.

Split surgery, called commissurotomy by neurosurgeons, is an operation that treats certain kinds of seizures. I’ve performed that operation myself and have taken care of the patients before and after the surgery. Beforehand, they are often incapacitated — they may have 20 or 30 seizures per day. In the surgery, we cut a portion (occasionally all) of the corpus callosum, which is a bundle of fibers that connects the two hemispheres of the brain. This procedure prevents seizures from moving across hemispheres and usually greatly reduces their severity.

What is most remarkable about these patients — what spurred Roger Sperry to do his landmark Nobel Prize-winning research — is that after the surgery they are unaffected in everyday life, except for the diminished seizures. They are one person after the surgery, as they were before. They are basically the same, even after their brain has been functionally cut in half. They feel the same, act the same, and think the same, for all intents and purposes.

In his meticulous research, Sperry found neurological effects from the surgery, but they were subtle. The hemispheres of the brain tend to act independently in some perceptual and motor activities. A patient could only name an object if it is shown to the speech hemisphere (usually the left hemisphere), and could move limbs only in accordance with the hemisphere to which information is presented. All of the disabilities Sperry found were perceptual and motor, not intellectual and abstract.

It’s interesting stuff, but the most important conclusion of the research is usually missed — just as Coyne misses it — or misinterpreted.

Let me repeat: Split-brain surgery doesn’t split the mind. People after split brain surgery remain one person, with one consciousness, one intellect, and one will. They have perceptual disabilities caused by the surgery but those disabilities are subtle and not noticed in everyday life. Their abstract intellect remains unified and the will that follows on that intellect remains unified. Split-brain surgery doesn’t split logic or mathematics or abstract reasoning or moral decisions based on abstract reasoning.

The results of split-brain surgery are strong arguments for dualism and for the immateriality of the intellect and will. It is a sad fact that an evolutionary biologist like Coyne, together with many neuroscientists, fails to understand the most important implication of this research—that the higher faculties of the mind cannot be split even by splitting the brain in half.

In a rational scientific community in which evidence and reason held sway, split-brain surgery would be hailed as compelling evidence for dualism and the immateriality of the intellect and will. In science, as in all endeavors, materialism is a kind of blindness that obscures the truth about nature.

If you enjoyed this piece, you may want to look at some of Dr. Egnor’s other recent articles on the immateriality of the mind:

How can mind interact with matter? Nature itself provides examples of how the immaterial interacts with the material.

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

An Oxford neuroscientist explains Mind vs. Brain. Sharon Dirckx explains the fallacies of materialism and the logical and scientific strengths of dualism


What is abstract thought? A reply to Dr. Ali. Abstract thoughts cannot arise from material things because a cause cannot give what it does not have.

Michael Egnor

Senior Fellow, Center for Natural & Artificial Intelligence
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 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.

Yes, Split Brains Are Weird, But Not the Way You Think