Mind Matters Natural and Artificial Intelligence News and Analysis
Eye close up

What if only part — not all — of your brain were transplanted?

You might end up seeing double out of an eye a thousand miles away

Recently, we’ve been discussing the concept of total or partial human brain transplants. What about transplanting an eye and the parts of the visual cortex it needs from one person to another? Which of the two people would be seeing out of that eye? The answer is not simple.

As noted earlier, researchers may never succeed in transplanting both an eye and the hemisphere brain parts that the eye needs to function from one human being to another. But let’s assume a science fiction scenario — a thought experiment — in which there is an exchange. Jack gets Mary’s right eye/hemisphere and Mary gets Jack’s right eye/hemisphere. Both parties, who live on different parts of the planet, survive. For simplicity, we will focus on Jack’s right eye/hemisphere, which is now literally inside Mary’s head.

open eye in space

Jack’s right eye/hemisphere is surgically connected to Mary’s blood vessels, etc., so it remains alive. But it’s no longer connected with Jack’s body. It also can’t connect with Mary’s body because central nervous system tissue doesn’t regrow when cut. As a result, Jack’s right eye/hemisphere wouldn’t communicate via neurons with either Jack’s body or Mary’s.

That doesn’t necessarily mean that Jack’s right eye/hemisphere would do nothing. What if it simply does what it is capable of doing — mediating Jack’s visual perception from his right eye?

Jack would see out of his left eye/hemisphere. But he might also see something out of the transplanted right eye in Mary’s skull, even if he and Mary are thousands of miles apart. He would have something like double vision — two superimposed scenes, a thousand miles separated.

But how is this possible, even in a thought experiment? How does the right eye/hemisphere communicate with Jack? By what medium?

We tend to assume that there must be a medium of communication both between our eyes and our whole brain in order to see. But people who have had split brain surgery see quite well even though their hemispheres have been separated (thus there is no direct connection). If the eyes (and hemispheres) are separated by 4000 miles, would the principle be any different? On this view, we see where our eyes are, not matter how far they are apart.

This sounds strange, but it’s not really. Our eyes are already separated by about 4 inches (10 cm), which is what gives us depth perception. With transplantation, our eye separation would be variable and much greater. So Jack wouldn’t be able to fuse the different images and thus would have double vision.

We are not machines. Physical continuity between some brain regions seems necessary for some neural abilities. But physical connections between eye/brain and “self/soul” are apparently not always necessary.

While perceptual abilities do depend to a degree on physical connections (if you cut the optic nerve we cannot see through that eye), there is no reason to infer that Jack’s transplanted right eye/brain needs a medium to connect it to Jack’s self or soul. He would, on this view, see through both eyes, whether they are in the same skull or not. The same would be true of Mary, of course.

There are instances of perception without the obvious medium we might expect. For example, there is a phenomenon called blindsight, by which some people who are totally blind can navigate around objects in their environment despite the fact that they have no visual perception of the object.

More remarkably, neuroscientist Benjamin Libet became famous for his work on free will. But his most fascinating research was on perception. He pricked volunteers’ fingers and measured the nerve impulses from the finger to the brain and timed everything. Libet found that it took about a half second for any electrical activity to register in the brain after the finger prick. But the volunteer reported feeling the finger prick the moment it happened. In other words, the volunteers felt the prick a half second before the brain showed any activity corresponding to it.

This response floored Libet, who couldn’t explain how someone could feel something a half second before the brain got the message. He actually a theory — he called it “post-dating” — according to which the volunteer didn’t feel the prick when it happened, but only when the brain responded, and that the brain tricked the person into thinking he felt it at the moment it happened. (See Mind Time, Chapter 2, Harvard University Press, 2004)

The post-dating theory is unnecessary if we adopt the philosopher Aristotle’s view of the self/soul. Sensations occur in the sense organ and occur when the sense organ is stimulated. The skin, in this case, is the sense organ. The volunteers felt the prick the moment the sense organ was pricked. The brain is where perception occurs, which is the interpretation of the sensation. That took a half second to get started. So the sensation at the skin was instantaneous and the perceptual understanding in the brain took a little longer. That explains Libet’s results beautifully.

But then we must drop the implicit belief that the soul “lives” in the brain (somewhere near the pineal gland, according to another philosopher, René Descartes). The soul lives where we live, where we act. There is nothing more to it than that.

According to this view, sensations and perceptions occur in sense organs and perceptual organs. If an eye is cut and transplanted, the original owner of the eye will sense vision with it, no matter where it is. Because the eye is no longer attached to the brain, only raw visual sensation would occur — this might be analogous to “blindsight.” Interpretation of vision requires the interpretative organ (the brain), which would not be attached to a transplanted organ.

Organs that are transplanted do what they ordinarily do. Transplanted hearts pump blood, transplanted kidneys excrete urine, transplanted eyes/hemispheres perceive vision. But note one key difference:

Perception has a power that blood circulation and urine excretion lack: There’s an “I” to perception. If Aristotle is right, the “I” that perceives after the transplant wouldn’t change. Jack would see with his eyes and his brain — two superimposed scenes — no matter where his right eye/hemisphere is.

Aristotle’s principle that the soul is the set of powers of a living organism and that the powers act where the organs that mediate them act provides a framework, however uncomfortable, for dealing with these devilish thought questions. If we listened to Aristotle, 90% of our conundrums in neuroscience would disappear.

I also believe that the central nervous system is designed specifically to preserve personal identity — that’s why it (unlike other body parts) won’t heal when cut. We are not permitted by our Creator to mess with souls.

Our thought experiment has ended. One must hope, no live Frankenstein experiments of this kind will test the proposition. Some things are best left to the imagination.

Note: Another interesting question is what would happen if only one of Jack’s eyes (and not the hemisphere) were transplanted. The retina is brain tissue so it would be like transplanting a tiny part of the brain. The question of what Jack would experience is more subtle here. Grist for more thought and discussion.

You may also wish to read: Are human brain transplants even possible? What would be the outcome if one person received transplants from the brains of others? If it’s not possible, there may be a good reason why not. If tiny bits of the brains from all the people in my neighborhood were transplanted into my brain, would there be a neighborhood in my skull? (Michael Egnor)


Researchers: our conscious visual perception lies outside our visual cortex. They concluded that the end step of perceiving where objects are occurs in the frontal lobes, a finding they describe as “radical”

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.

What if only part — not all — of your brain were transplanted?