Debate: Do split-brain patients have two minds?

I went to the NYU Consciousness site this morning hoping to see if the recent debate on the relationship of prefrontal activity to consciousness had been posted yet.  It hasn’t, and based on what I can see, it might be a while.

But I did find this interesting debate from last year on whether split brain patients have two minds.

Just a reminder: split-brain patients were people who, in order to control severe epileptic seizures, had their corpus callosum, the connections between the two cerebral hemispheres of their brain separated.  The procedure left them remarkably functional in day to day life, but careful tests show that the two sides of their brain have limited if any communication with each other, although recent experiments by one of the debaters complicates that understanding.

This debate includes David Chalmers as the MC, Elizabeth Schechter, who argues that split-brain patients do have two minds, Yair Pinto, who argues that they don’t, and Joseph Ledoux, who argues that the issue is complicated and depends on the specific patients and which definitions we’re using.

Just so you know what you’re getting into, the video is two hours long, although the initial statements from the participants are done within the first hour.

I’m not sure if I’d heard of Schechter before.  I found her views interesting and might have to explore them at some point.  Unfortunately, her book appears to be pricey.  I noted above that she argues for two minds, but her view turns out be nuanced and not really too different from Ledoux’s.  She sees there being two minds, but one person.

My own conclusions on this is that I don’t think it’s productive to talk about there being two separate minds.  All of the tests do show that communication between the hemispheres are limited to some degree or another, but I think it’s more accurate to talk about fragments of a mind whose communications are disrupted.

Pinto makes much about the fact that the two hemispheres don’t seem to notice or be bothered by the separation.  But I think that is only an issue if we regard each hemisphere as its own separate self.  However, the hemispheres didn’t evolve that way.  They evolved to be a portion of a self, and it’s clear that’s what they expect to be.

V.S. Ramachandran in his book, The Tell-Tale Brain, relates the power of the brain’s expectations of its body plan.  When that expectation, due to some brain injury, becomes disrupted, in a condition known as apotemnophila, it can lead to conditions where people no longer feel a limb is theirs, with an intensity that can lead them seek to have it amputated.

If each hemisphere has a plan for its side of the body, then it wouldn’t expect to receive signals from the other side, much less be able to control it.  Indeed, if it suddenly found that it could, it might result in the apotemnophila condition.

So we end up with fragments of a mind whose communications have become limited.  Over time, split-brains either recruit remaining subcortical pathways, or learn to use subtle external behavioral cues to make up for the limitation.  This seems to bridge the results from the old Sperry / Gazzaniga / Ledoux results and the newer ones from Pinto.

It’s also a stark reminder of just how married mental processes are to their embodiment.

8 thoughts on “Debate: Do split-brain patients have two minds?

  1. I think you are making a mistake that many (most) make in thinking of “mind” as a monolithic thing instead of as a collection of processes. The question shouldn’t be is there one or two minds. The question should be what kind of mind (what collection of mental capabilities) does this (arbitrarily designated) system have. First you have to decide what is a mental process, and then decide if any given system (rock, neuron, subortical assembly, human, family, corporation, nation, species, planet, etc.) is capable of that process.

    So what counts as a mind, or a unit, is a matter of convention.


    Liked by 2 people

    1. Actually, I’m very aware that the mind is not a monolithic thing. It’s why I think we can talk about fragments of it existing. But you’re right, definitions matter a great deal here. The definition I used for my remarks is the suite of capabilities that process sensory information and deliberate future movements of the body.

      In that sense, since there’s still only one body, and both hemisphere’s sense of self appear to be tangled up with it, both thinking of themselves as the original person, and neither sense of self is consciously aware of the other, yet they still seem to work together, the idea that it’s fragments of the same mind seems right.

      Of course, you could put forth a different definition that would change that conclusion. For example, you could use Schechter’s definition of an integrated information system, in which case, at least at a cognitive level, there does appear to be two of them, at least in many cases. But even Schechter recognizes that it all still amounts to one person.

      But my feel is that a mind, while a composite suite of processes and capabilities, shouldn’t be divorced so easily from its evolutionary role. It’s why I’m leery of talk of separation creating new minds, or linking brains creating new composite minds. Each of the individual capabilities has a role to play, and I don’t think they’re that fungible. (At least not in the short term. Long term, who knows what a brain’s plasticity can do.)


      1. [donning nit-finding goggles]

        Actually, I’m very aware that the mind is not a monolithic thing. It’s why I think we can talk about fragments of it existing.

        This statement is kinda self-contradictory. When you talk about fragments of a thing, you’re talking monolithicly, i.e., you’re talking about fragments of a monolith. It’s not clear that you mean that a fragment can be it’s own monolith.

        In that sense, since there’s still only one body,

        There’s that monolithic thinking again. The body is not a monolith either. Each hemisphere has direct connections to its own (partly same, partly opposite) side of the body. You could say each has its own body. The boundaries under consideration are chosen for expediency.

        and both hemisphere’s sense of self appear to be tangled up with it, both thinking of themselves as the original person,

        Not sure we have enough evidence to speak of the hemisphere’s sense of self, especially to say that any one hemisphere thinks of itself as the totality of the original whole as opposed to saying that each recognizes itself as part of a unit, and that there was easy coordination with an “other” before the split, but such coordination is harder after the split.

        the idea that it’s fragments of the same mind seems right.

        It seems right when you want to deal with everything inside the skull as a unit, which is most of the time. It doesn’t seem right when you want to find out which mental capabilities are associated with one hemisphere instead of the other.

        [admiring collection of nits]


        1. Usually the term “monolithic” implies indivisibility, at least at a functional level, which I’m not positing. But I also know you could find a monolithic interpretation of anything I might say about it, so whatever. 🙂

          “Not sure we have enough evidence to speak of the hemisphere’s sense of self,”

          In the debate, it’s mentioned that both hemispheres respond to the person’s name. (The right hemisphere typically can’t produce grammatical speech, but it has some language comprehension, and can answer with single words.)

          Not that that means each hemisphere thinks of itself as the whole self, but I don’t know that each hemisphere, at a primal level, even has a conception of the whole self. They only have a conception of their half, which they see themselves as being. Remember, at least at a conscious level, they don’t even miss the other half.

          “[Fragments] doesn’t seem right when you want to find out which mental capabilities are associated with one hemisphere instead of the other.”

          Why not? We know there is some specialization aside from just what side of the body is controlled. Why wouldn’t these be considered functional fragments of the whole system?


  2. Perhaps 2 or more people can, to some extent, act as a single mind, as when twins or long married couples act in a coordinated way, with great fluidity. Perhaps the same is seen in high performing teams at work, or in sport. An action of any individual in such a team is the consequence of things sensed and communicated by different members of the team, predictions are coordinated, and actions selected from the viewpoint of collective desires. Perhaps in such cases a single mind is created, overlaid over, and interacting with, individual minds. A split brain would then be an unusual case of the same phenomenon.

    Liked by 1 person

    1. Could be. It depends on how flexible we want to be with the mind concept. And we’re social creatures and will instinctively coordinate as a social unit. The large scale societies we build, which are unique in the animal kingdom, couldn’t exist without those instincts.

      However, I don’t think that’s what’s happening between the hemispheres. Social coordination is coordination between complete selves, each of which participate for their own selfish gene reasons. But the genetics of the two hemisphere are generally identical. I don’t think what’s happening there comes from social instinct. I think it’s far more primal. It seems like the parts of the mind that evolved to work together simply finding alternate ways to do so.


    1. Storage vs processing seems overly broad. Arguably they both do both. The specialization of the hemispheres, aside from controlling one side of the body, is a complex thing still being researched. We know that, in most people, the left handles language. There’s been a lot written about the left being analytical and the right more intuitive, but these designations are actually controversial among neuroscientists.

      Those controversies keep me away from discussing hemispherical specialization, except for the consensus ones of controlling a particular side of the body and language. Even in the case of language, the specialization is complex. The right can still understand a lot of language and can even emit single words intelligently. It just can’t do grammar. And in a small minority of people, language is actually on the right.

      Liked by 1 person

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