Can You Split the Mind by Splitting the Brain?
New findings cast doubt on the two-mind view.
Posted October 5, 2021 | Reviewed by Abigail Fagan
- The brain has a pair of neocortices, one on the left and one on the right, each with its own visual, auditory, attentional, and motor system.
- Previous research has suggested that severing these cortices leads to "two minds."
- Recent evidence casts doubt on that conclusion.
This is a guest post by Yair Pinto, Ph.D.
The main part of your brain is your neocortex. This is where, supposedly, your conscious awareness resides. The neocortex is a dual organ, just like your lungs and kidneys: There is really a pair of neocortices, one on the left and another one on the right. The left part and the right part of the neocortex each have their own visual, auditory, attentional, and motor systems. Your skull thus appears to house "two brains." But if so, why does it not contain two conscious persons, lefty and righty?
Crucially, your "two brains" have a major connection containing hundreds of millions of axonal fibers — the corpus callosum. More than 95 percent of all information exchange between the left and the right part of the neocortex passes through the corpus callosum.
Intuitively, it seems that this interconnectedness is the key to the unity of your mind. After all, if there are two brains in your skull that barely communicate with each other, how can they ever create one person?
Interestingly, this idea is empirically testable. In "split-brain" patients, the corpus callosum is surgically severed to alleviate otherwise intractable epileptic seizures. What happens in these split-brain patients to the unity of mind? When you split their brains, do you split the person as well, thereby creating two people?
Sperry and colleagues investigated this and found evidence that in a split-brain, the mind is split as well, confirming our core intuition that two brains cannot create one person. Sperry won the Nobel prize for his split-brain research in 1981. Not only that, as Sperry pointed out himself, his findings seem to vindicate the materialistic worldview. If you can split the mind by splitting the brain, then this suggests that the mind can indeed be equated to the brain.
How did Sperry show that the mind is split? His main evidence requires a bit of neural understanding. The visual system in the left part of the brain processes stimuli in the right visual field (everything to the right of where you fixate with your eyes) while the right part of the brain processes the information in the left visual field. In addition, we know that the left part of the neocortex controls language and the right hand, whereas the right part controls the left hand.
Now what happens if a picture is presented in the left visual field? The picture is seen by the right part of the brain, but not by the left part. In a healthy adult, this does not matter. Both parts together house one person that perceives the entire visual field and controls the whole body. However, what would happen if the two brains house two different persons? Now only “righty” can see the picture while “lefty” cannot. However, since the left part of the neocortex controls language, the patient would not be able to say he saw anything. He might, however, be able to indicate he saw the image with the left hand, controlled by "righty."
Thus, the split-mind hypothesis has a clear prediction: If a picture is presented in the left visual field, then “lefty” should verbally indicate that he saw nothing, yet “righty,” who cannot speak but can move the left hand, should be able to indicate using that hand that he saw the picture! Sperry and Gazzaniga tested this prediction, and lo and behold, this is exactly what they found. Lefty would say he saw nothing but righty would draw a picture of the image on the screen.
It is probably fair to say that the idea we can create two minds by severing a person’s corpus callosum became orthodoxy in the subsequent decades. It’s been taught to thousands of college students and many people who are not neuroscientists have heard that you can create two people by splitting the brain. Case closed?
Not so fast. There are several reasons to question the conclusions Sperry, Gazzaniga, and others sought to draw. First, both split-brain patients and people closest to them report that no major changes in the person have occurred after the surgery. When you communicate with the patient, you never get the sense that the there are now different people living in the patient's head.
This would be very puzzling if the mind was really split. Currently, you are the only conscious person in your neocortex. You consciously perceive your entire visual field, and you control your whole body. However, if your mind splits, this would dramatically change. You would become two people: “lefty” and “righty.” Lefty would only see what is in the right visual field and control the right side of the body while “righty” would see what’s in the left visual field and control the left side of the body. Both "lefty" and "righty" would be half-blind and half-paralyzed. It would seem to each of them that another person is in charge of half of the body.
Yet, patients never indicate that it feels as though someone else is controlling half of the body. The patients’ loved ones don’t report noticing a dramatic change in the person after the surgery either. Could we all — patients themselves, their family members, and neutral observers — miss the signs that a single person has been replaced by two people? If you suddenly lost control of half of your body, could you fail to notice? Could you fail to notice if the two halves of your spouse’s or child’s body are controlled by two different minds?
Second, we (and others) have recently found strong evidence against the claimed findings of Sperry and Gazzaniga. We ran an experiment in which two split-brain patients saw an image appear in the left visual field. If the patient’s mind was split into “lefty” and “righty”, only “righty” would see this image while “lefty” would not. Since “lefty” would control language as well as the right hand, the patient would not be able to either verbally indicate that he saw an image or do so with the right hand. However, that is not what we found. Quite the opposite, if an image appeared in the left visual field, the patient would, both verbally and with the right hand, indicate that he saw the picture.
Moreover, false alarms were nearly absent — if no picture appeared, the patient almost never falsely claimed to have seen something. We did find that the patients struggled with the task of naming objects in the left visual field. Initially, this led one patient to sometimes indicate that he saw nothing when an image appeared to the left of fixation, presumably because he could not name what he had seen. However, after instructing the patient to only say he saw nothing when he really saw nothing as opposed to seeing an object he couldn’t name, this ceased to occur.
So what is going on? Does splitting the brain lead to splitting the mind? We believe that the question is far from settled, and further research is needed. Split-brain operations are no longer performed, so the window of opportunity for finding the answer is closing. However, our findings cast serious doubt on the view that split-brain patients have two minds.
Yair Pinto is a neuroscientist at the University of Amsterdam. He is interested in how the brain creates consciousness and in whether we have free will.
Pinto, Y., de Haan, E. H. F. & Lamme, V. A. F. (2017). The Split-Brain Phenomenon Revisited: A Single Conscious Agent with Split Perception. Trends Cogn. Sci. (Regul. Ed. ) 21, 835-851.
Pinto, Y. et al. (2017). Split brain: divided perception but undivided consciousness. Brain 140, 1231-1237.