Split Brain Patients and Consciousness Brian Copeland December 18, 2006 Senior Seminar Jason Ford

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I Cerebral commissurotomies provide philosophers of mind an interesting perspective into consciousness. In cerebral commissurotomy patients, the corpus callosum (the strip of brain connecting the two cerebral hemispheres) is cut, leaving the left and right hemispheres no direct link with which to communicate. Experimental results seem to indicate that these patients have two streams of consciousness. In this paper, I will describe the procedure, layout an argument that claims cerebral commissurotomy side effects show that all humans have two minds, and then discuss objections and problems with the argument. In section V, I will discuss implications of commissurotomies on same order theories of consciousness. Finally, I will conclude the paper with criterion for all theories of consciousness based on cerebral commissurotomies.

II Let’s begin with the brain. The brain is divided into two hemispheres, the left and the right. The right hemisphere dominates control over the left half of the body, likewise for the left hemisphere and the right half of the body. This includes motor control and vision, but not typically hearing. The corpus callosum is the strip of nerve fibers located between the two hemispheres of the brain (Baillie 115). In a complete cerebral commissurotomy, this strip of nerve fibers is severed, along with other underlying connections between the two hemispheres of the brain. Mad scientist intentions aside, this procedure has a medicinal purpose. It was discovered that epileptic seizures that started in one hemisphere and spread to the other would create a grand mal seizure. The cerebral commissurotomy essentially destroys the seizure’s path, stopping it in its tracks (Marks 4). A patient whom has undergone a complete cerebral

Copeland 3 commissurotomy is often referred to as a split brain patient. This is because the two hemispheres of the brain have no other direct link with which to communicate. Cutting the corpus callosum destroys the communication channel between the two, essentially splitting the brain into two. Shockingly, there are no obvious side effects. A split brain patient will not exhibit any personality changes, cognitive deficiencies, etc. Side effects can only be observed in experimental circumstances. Experimenters instruct a split brain patient (hereby affectionately referred to as a split brainer) to focus on a point located at the center of their vision. In the left half of the split brain’s vision, the word “key” is flashed for 250 microseconds. This is long enough for “key” to be perceived by the split brainer, but not long enough for eye movement to occur. On the right side, the word “ring” is flashed similarly. Because each word is flashed so momentarily, each hemisphere only receives its half of the visual field. If asked what he or she saw, the split brain patient will claim to have seen the word “ring,” as the left hemisphere (which controls the right eye, which only saw “ring”) has a greater degree of control over the verbal center of the brain. If asked to find the object displayed with his or her left hand, he or she will retrieve a key, rejecting any and all rings. After all, the right hemisphere controls the left side of the body and only saw “key” (Marks 4). Neither hemisphere is aware of what the other hemisphere saw. Additionally, the split brainer will not realize immediately that they have contradicted themselves. When this experiment is repeated for a non-split brainer (someone whose corpus callosum is intact), the person will report seeing “key ring” and will retrieve a key ring, regardless of what hand is used (For more experimental results, see Marks and Baillies).

III

Copeland 4 The key philosophical issue coming from a split brainer is the contradiction that they make during experimentation. It seems like our split brainer perceived the words “key” and “ring” independently. But how could this be? Let’s look at what we know. Without offering a positive theory on consciousness yet, let’s consider a response to a query as evidence for a conscious entity. We know that there was a conscious entity that perceived the word “key” because our split brainer retrieved a key when asked to retrieve what they saw with his or her left hand. We also know that there was a conscious entity that perceived the word “ring.” The split brainer told us so. So there (probably) is a conscious perception of both “key” and “ring.” Can we find conscious experience of “key ring?” Consider the following argument: 1)

If “key ring” was in X’s conscious experience, X would not report merely “key” or “ring.”

2)

X reported merely “key” or “ring.”

Therefore: C)

“Key ring” was not in X’s conscious experience.

I think we have a valid argument. The only grounds I can think of to reject premise 1 as false would imply that key and ring should be considered separately. But our split brainer shows no problems in day to day tasks. In “key ring,” both “key” and “ring” seem easily essential to “key ring,” as opposed to “cute dog.” I can’t think of any grounds as to why one would only report ring when prompted with “key ring.” If the argument is sound, we might have grounds to accept two independent streams of consciousness in our split brainer. One conscious stream perceived “key,” the other “ring.” We call them independent because the left hand of our split brainer does not ever point to a ring. Likewise, the split brainer never reports seeing “key.” If

Copeland 5 we do entertain the possibility of two independent streams of consciousness, Marks is very happy. I quote his argument from Page 7 of “Consciousness and Unity of Mind:” 1)

S exhibits the split-brain syndrome.

2)

S has two independent streams of consciousness […].

3)

S has two minds.

Marks motivates premise three by appealing to the capabilities of each hemisphere. Granted, the right hemisphere can’t process language and the left hemisphere is less than skilled with spatial tasks. But both hemispheres have adequate faculties to constitute a mind. So what constitutes separate minds? Well, I have access to my own experiences, beliefs, desires, etc. I don’t have access to Bob Dole’s experiences, beliefs, desires etc. It seems like this is good grounds to accept that Bob Dole and myself have separate minds. But this is exactly what separates the two hemispheres in our split brainer. Each hemisphere has access to its own stuff, but not to the other. Thus, the split brainer does have two minds. Marks takes Premise three and applies it to all humans, split brainers or not. Suppose person X had one mind before a commissurotomy. After the commissurotomy, X now has two. Where did this second mind come from? It seems impossible that cutting the corpus callosum could create a new mind: destroying nerve tissue won’t create new cognitive abilities. Thus we need to have supposed that there were two minds before the commissurotomy. The only difference between normal humans and split brainers is inter-hemispheric communication. But how is such a conclusion even feasible, given my seemingly unified consciousness? Puccetti believes that instead of having one integrated mind, we have two minds communicating to duplicate what we experience. Our consciousness is unified because our two minds have learned to work together without issue (76). Picture a duplex with a tenant in each. One looks out the

Copeland 6 window and sees “key.” The other looks out the window and sees “ring.” If the corpus callosum is intact, the doors in the duplex separating the two living quarters are open and the tenants can communicate. They yell back and forth “What’d you see?” “I saw key.” “Guess there was really key ring, then.” “You betcha.” Both individual minds have their own unified experience and no problems arise when we are asked what we saw. For the split brainer, however, the doors are closed. Tenant left hemisphere says he saw a ring, tenant right hemisphere points to a key. Neither knows what the other is doing and split brainer gives contradictory results. The argument is unsettling. Apparently, we all have two minds and two conscious experiences. We just don’t realize it, possibly because both are identical. The experimental results reveal something about consciousness that isn’t accounted for at all in our phenomenological experiences of consciousness. Thus, any theory of consciousness needs to address the issue. That the only time the walls of our two minds are visible is when we become a split brainer isn’t a philosophical issue: a truly unified consciousness wouldn’t have the walls at all. Even during experimentation, the split brainer retains the illusion of a single mind, despite giving contradictory results. Marks discusses cross-cuing as the reason a split brain person doesn’t show signs of two minds in day to day life. Though the two minds cannot communicate directly, they can typically compensate. Given more than 250 milliseconds, the eyes can move, giving both hemispheres access to “key ring.” Presumably, there are very little 250 millisecond stimuli in a split brainer’s day to day world.

Copeland 7 IV Opponents of the dual mind argument have a few grounds to attack. Perhaps there has been an ambiguity in what we mean by mind. Marks differed between streams of consciousness and the mind, so perhaps he needs to account for why two streams of consciousness cannot be contained within the same mind. Marks responds with the challenge to define mind without appealing to consciousness. This is tough, but Baillie has an answer. Baillie doesn’t think that the unity of the mind needs to be an all or nothing matter. Consider the number of minds present as one issue and the integration of a mind (“the degree of internal coherence, or by contrast, compartmentalization, in a single mind.”) a second (135). Baillie claims that calling the two hemispheres in our brains two different minds confuses the fact that the two hemispheres can simply be considered separate parts of the same mind. The only difference between split brainers and non split brainers is that split brainers have a lower degree of integration in their mind. The brain can be considered a modular entity (136). Baillie may be right, in that different parts of the brain handle different tasks, but I’m not sure this is the real issue. If the two hemispheres are one mind that simply doesn’t integrate itself well, why can’t two people working on a project be considered one mind that is very slowly integrating (through meeting, emails, etc)? The level of integration (or synthesis, as I think Baillie is arguing) does seem to matter in this case. I highly doubt that Baillie is willing to accept the two person team as one mind. I also fear that he may be question begging by including “in a single mind” in his definition of mental integration. Baillie can’t use this definition of integration when he is describing why integration allows there to be one mind. DeWitt has a different definition scheme that may clarify things. He sees three levels: consciousness, the basic phenomenal awareness that we’ve seen in each hemisphere; mind, the

Copeland 8 ability to prioritize actions that each hemisphere has demonstrated by responding to a query; and self-consciousness, the ability to apprehend one’s self as distinct from others. DeWitt is willing to grant that the two hemispheres of our brains each have separate consciousness. He might even accept that the two hemispheres have a mind (though doubt is cast upon this, as each hemisphere does have some role in movement and other functions). But he believes that it is confounding the issue by not discussing self-consciousness. This is what unites everything together into one general package and what we refer to when we use the word “I.” DeWitt has some power to this punch. Despite the contradicting responses to queries, we have no evidence that the hemispheres act as a separate person. On the contrary, the hemispheres normally have identical information and would always be considered to correspond to the same person. DeWitt is willing to concede separate minds, but not separate persons. But let’s look at this a bit more critically. Consider the following two sentences describing our split brainer in the experimental settings: 1) I am pointing to a key. 2) I saw the word ring. Can the “I” in both sentences refer to the same thing? The left hemisphere is responsible for the “I” in sentence 2, likewise for the right hemisphere with the “I” in sentence 1. But we’ve got a problem here. “I” is an indexical. It would have strange philosophical implications if “I” could fluctuate wildly. Davis asks us to consider the sentence “I pointed at a key, but I don’t know why.” Here, “I” must have two meanings within the same sentence. The second “I” clearly refers to the left hemisphere, as the left hemisphere is doing the talking and didn’t control the pointing. But the first “I” isn’t just the right hemisphere. Both hemispheres have some control over the other side’s motor abilities. Both hemispheres will claim ownership of the body. By uttering the phrase, the left hemisphere is confessing ownership of the arm. But at the same

Copeland 9 time, it admits that it doesn’t know the motivations of the arm. We’ve got something that is separate at times and unified at times, even within the same sentence. There has to be a sense with which the two minds can refer to the collective identity of the body. Davis proposes a third entity, “the whole.” He doesn’t want this to be called “the whole person,” as this would be question begging. But perhaps this “whole” is what both hemispheres will profess membership to. We’ve still got a strange philosophical situation on our hands: we’re allowing that the hemispheres of the brain can have two self references; the hemisphere themselves and the whole. But wait: if we allow that the hemispheres can say “I”, haven’t we given them self consciousness? I don’t know. Accepting this as a possibly still means we are forced to admit that everyone of us has two streams of consciousness going on. The split brain theorists are winning. A third objection to the argument at hand comes from the split brainers themselves. Some split brainers stop exhibiting the key ring symptom. In fact, most do twenty years after their surgery. The two hemispheres seem to have the ability to cue the other as to what it is doing (Marks 25). Even more interesting, humans born without the corpus callosum typically won’t show the symptom, either (Baillie 116). Combine this with the strong testimony that split brainers patients don’t report any feeling of duality in their consciousness and these objections raise serious problems for the split-brain argument. Perhaps the fact that both hemispheres are still seated in the same subcortical cortex (lower brain stem, etc) allows them to remain united in one mind. This propensity to communicate and work together seems like solid evidence that the two hemispheres are part of the same mind. But the lack of sensing a duality to consciousness is not conclusive evidence that it isn’t there. Proprioception is the sense of one’s body and self. As seen in Sachs book, while one may not be aware of prioprioception in our normal consciousness,

Copeland 10 it is clearly there (if only in the periphery), as it is very evident when it is gone. As for the subcortical cueing, a lack of data will force me to remain silent.

V Same order theories of consciousness all refer to various forms of a model where on what makes a thought conscious is its representing of itself. Let’s consider the thoughts: a) “I see key” b) “I see ring” Based on a split brainer's responses to queries regarding what they’ve perceived, we regarded these thoughts as conscious. Let’s work backwards and see how these were made conscious, according to the different types of same order consciousness theories. To differ between different types theories of same order consciousness, I will refer the classification provided by Dr. Ford in his lectures. A Type 1 same order theory states that a conscious mental state presents its focal content and refers to itself. So thought (a) refers to itself in the right hemisphere and thought (b) refers to itself in the left hemisphere, as indicated by the fact that those are the specific areas where the thoughts are conscious. What are missing are duplicate thoughts of (a) and (b) in the other hemispheres. So we don’t have any inherent problems with Type 1 as to why a split brainer would show evidence of two streams of consciousness. Type 1 allows for a very modular approach to consciousness, so that if you have self representing thoughts, you have a stream of consciousness. Each hemisphere has its own self representing thoughts, and thus its own stream of consciousness.

Copeland 11 Type 1 fails to tell us why we don’t run into the trouble our split brainers are experiencing more often, even in non-commissurotomy patients. There is nothing about consciousness that allows for synchronizing of conscious streams. Suppose I have thought (a) and (b) and I am not a split brainer. I still realize that (a) and (b) are both occurring at the same time, despite being separate thoughts, and report that I see “key ring.” Somehow (a) and (b), through representing themselves and becoming conscious, must also include additional information about the thoughts (like the fact that (a) and (b) are happening together). But this kind of thing isn’t allowed when consciousness is entirely made up of self representing thoughts. According to Type 1, they should only be making themselves conscious and not performing any additional functions like synthesis. To need synthesis is to need something about consciousness not offered in Type 1. A Type 1 theorist has even more trouble should they want to maintain that there aren’t multiple streams of consciousness. As a split brainer begins the experiment, they would have one stream of consciousness. When “key ring” is flashed before them, they suddenly have two new thoughts, (a) and (b). The thoughts represent themselves, but somehow do so differently. There must be something about the representing of (a) that is different from the representing of (b), as they are not both conscious in the same way. A Type 1 theorist must now posit various ways in which a thought representing itself can differ, which is separate from merely making it conscious through representing itself. In Type 2, a conscious mental state presents its focal content and it presents with content the person who is having that mental state. Thus, we can explain why each thought (a and b) is located where it is. By representing themselves, they present information about the person experiencing the thoughts. In the same stream of consciousness, (a) can occur and represent itself

Copeland 12 while presenting the right hemisphere. Likewise, (b) can be conscious and present the left hemisphere. But how could (b) present the right hemisphere? The right hemisphere isn’t conscious of (b). Likewise for (a). The problem is that despite a single stream of consciousness, we have multiple conscious entities being presented in the same stream of consciousness. This may seem compatible with cases of thought insertion, but it doesn’t seem compatible with our experimental data. The split brainer doesn’t think (a) and (b) are experienced differently, or by someone different: the split brainer is separately not aware of (a) and (b). Let’s instead consider two streams of consciousness. When the left hemisphere is conscious of (a), (a) represents itself and in doing so presenting the self with it. Here’s the big question: can (b), a different conscious mental state, present the same self as (a) to the right hemisphere? To match empirical evidence, we want to say yes. Then we would have a mechanism (outside temporal proximity) to describe why both hemispheres of the brain think they are one individual person. Basically, they are both being presented with the same self. But it isn’t clear how this could occur. How could (a) and (b) present the same self when they are separate mental states being represented by themselves in two separate streams of consciousness? How could they possibly match? I think the answer is that they don’t. That is why the “I” discussed previously can fluctuate. Normally, they are almost entirely the same person, as they usually are conscious of the same mental states, which present the same self. Thus, in normal situations when a split brainer tells us “I pointed at the key,” the "I" refers to the self being presented to both hemispheres. But in experimental settings, small details are different, resulting in slightly different selves being presented. “I don’t know why” refers to the self being presented in the left hemisphere. The difference between split brainers and non split brainers is that it isn’t possible to present different mental content to the two streams of

Copeland 13 consciousness. The corpus callosum acts as a conduit to share mental states and thus the self is always presented identically to both hemispheres. Is this theoretically possible? I think a similar argument is made by intensionalists. At an abstract level, the content of my beliefs may remain the same over some period of time. But at a very detailed level, the content of my beliefs are changing constantly as I gather new experiences. This is the same reasoning made for the split brainer’s identity. At an abstract level, the two streams of consciousness have the same identity, as they are being presented with the same self. In our experimental settings, the split brainer’s conscious streams change identity when presented with different selves. What works for a Type 2 theorist backfires for a Type 3 theory of self representational consciousness. The Type 3 theory states that a mental state represents itself while presenting the person having the mental state, but the person presented is presented with no content. Without content being presented, we need to either accept that a mental state is presenting the same person to both conscious hemispheres despite the contradictions (requiring an explanation of how someone could so easily contradict themselves), or they present completely different persons. The first option seems very troublesome for a theory of identity; the second doesn’t fit with our experimental results (the left hemisphere simply wouldn’t accept that it was pointing to the key).

VI Cerebral commissurotomies provide startling conclusions. Clearly, there is only a minimal amount of interaction between the two hemispheres of the brain; else the corpus callosum would be a much larger portion of the brain. Somehow, consciousness arises from two

Copeland 14 separate streams to create the illusion that we have a unified consciousness. Thus, theories of consciousness can only be made stronger by rising to the occasion and incorporating what we know about split brainers as a fundamental part of the theory. The same order theory of consciousness that includes the self being presented through the mental states self-representation has a good grip on the situation. Higher order theories of consciousness, on the other hand, have an uphill battle in describing how they can incorporate the evidence presented by split brainers. How could a higher order theory of consciousness work with two separate streams of consciousness? This question remains open. Even more puzzling is the question of why we have the phenomenal experience one unified stream of consciousness if there are actually two separate streams. The answer to this question could hold the key to a great theory of consciousness.

Copeland 15 Bibliography Baillie, James. Problems in Personal Identity. 1st ed. New York: Paragon House, 1993. Davis, Lawrence H. "Cerebral Hemispheres.” Philosophical Studies 87 (1997): 207-222. Philosopher's Index. University of Minnesota, Duluth. 1 Nov. 2006. Keyword: Commissurotomy. Dewitt, Richard. "Consciousness, Mind and Self: the Implications of the Split-Brain Studies." The British Journal for the Philosophy of Science 26 (1975): 93-118. Jstor. University of Minnesota Duluth, Duluth. 5 Nov. 2006. Keyword: Split-Brain. Marks, Charles E. Commissurotomy Consciousness and Unity of Mind. 1st ed. Montgomery: Bradford Books, 1980. Moor, James. "Split Brains and Atomic Persons.” Philosophy of Science 49 (1982): 91-106. Philosopher's Index. University of Minnesota, Duluth. 1 Nov. 2006. Keyword: Commissurotomy. Puccetti, Roland. "Rigterink on Mental Division.” Canadian Journal of Philosophy XII (1982): 75-76.