Split Brains and the Compositional Metaphysics of Consciousness


Luke Roelofs- Postdoctoral Fellow in Philosophy at the Australian National University

The mammalian brain has an odd sort of redundancy: it has two hemispheres, each capable of supporting more-or-less normal human consciousness without the other. We know this because destroying, incapacitating, or removing one hemisphere leaves a patient who, despite some difficulties with particular activities, is clearly lucid and conscious. The puzzling implications of this redundancy are best brought out by considering the unusual phenomenon called the ‘split-brain’.

The hemispheres are connected by a bundle of nerve fibres called the corpus callosum, as well as both being linked to the non-hemispheric parts of the brain (the ‘brainstem’). To control the spread of epileptic seizures, some patients had their corpus callosum severed while leaving both hemispheres, and the brainstem, intact (Gazzaniga et al. 1962, Sperry 1964). These patients appear normal most of the time, with no abnormalities in thought or action, but when experimenters manage to present stimuli to sensory channels which will take them exclusively to one hemisphere or the other, strange dissociations appear. For example, when we show the word ‘key’ to the right hemisphere (such as by flashing it in the left half of the patient’s visual field), it cannot be verbally reported (because the left hemisphere controls language), but if we ask the patient to pick up the object they saw the word for, they will readily pick out a key – but only if they can use their left hand (controlled by the right hemisphere). Moreover, for example, if the patient is shown the word ‘keyring’, with ‘key’ going to the right hemisphere and ‘ring’ going to the left, they will pick out a key (with their left hand) and a ring (with their right hand), but not a keyring. They will even report having seen only the word ‘ring’, and deny having seen either ‘key’ or ‘keyring’.

Philosophical discussion of the split-brain phenomenon takes two forms: arguing in support of a particular account of what is going on (e.g. Marks 1980, Hurley 1998, Tye 2003, pp.111-129, Bayne & Chalmers 2003, pp.111-112, Bayne 2008, 2010, pp.197-220), or exploring how the case challenges the very way that we frame such accounts. A seminal example of the latter form is Nagel (1971) which reviews several ways to make sense of the split-brain patient – as one person, as two people, as one person who occasionally splits into two people, etc. – and rejects them all for different reasons, concluding that we have found a case where our ordinary concept of ‘a person’ breaks down and cannot be coherently applied. My work develops an idea in the vicinity of Nagel’s: that our ordinary concept of ‘a person’ can handle the split-brain phenomenon if we transform it to allow for composite subjectivity – something which we have independent arguments for.

Start with what Nagel says about one of the proposed interpretations of the split-brain patient: as two people inhabiting one body. Pointing out that when not in experimental situations, the patient shows fully integrated behaviour, he asks whether we can really refuse to ascribe all their behaviour to a single person, “just because of some peculiarities about how the integration is achieved”(Nagel 1971, p.406). Of course sometimes two people do seem to work ‘as one’, as in “pairs of individuals engaged in a performance requiring exact behavioral coordination, like using a two-handed saw, or playing a duet.” Perhaps the two hemispheres are like this? But Nagel worries that this position is unstable:

“If we decided that they definitely had two minds, then [why not] conclude on anatomical grounds that everyone has two minds, but that we don’t notice it except in these odd cases because most pairs of minds in a single body run in perfect parallel?” (Nagel 1971, p.409)

Nagel’s worry here is cogent: if we accept that there can be two distinct subjects despite it appearing for all the world as though there was only one, we seem to lose any basis for confidence that the same thing is not happening in other cases. He continues:

“In case anyone is inclined to embrace the conclusion that we all have two minds, let me suggest that the trouble will not end there. For the mental operations of a single hemisphere, such as vision, hearing, speech, writing, verbal comprehension, etc. can to a great extent be separated from one another by suitable cortical deconnections; why then should we not regard each hemisphere as inhabited by several cooperating minds with specialized capacities? Where is one to stop?” (Nagel 1971, Fn11)

Where indeed? If one apparently unified mind could be really a collection of interacting minds, why not think that all apparently unified minds are really such collections? What evidence could decide one way or the other? Taking this line seems to leave us with empirically undecidable questions about every mind we encounter.

What is striking is that this way of thinking isn’t problematic for anything other than minds – indeed it is platitudinous. Most things can be equally well understood as one or as many, because we are happy to regard them simultaneously as a collection of parts and as a single whole. What makes the split-brain phenomenon so perplexing is our difficulty in extending this attitude to minds.

Consider, for instance, the physical brain. Do we have one brain, or do we have several billion neurones, or even 8-or-so lobes? The answer of course is ‘all of the above’: the brain is nothing separate from the billions of neurones, in the right relationships, and neither are the 8 lobes anything separate from the brain (which they compose) or the neurones (which compose them). And as a result of the ease with which we shift between one-whole and many-parts modes of description, we can be sanguine about the question ‘how many brains does the split-brain patient have?’ There is some basis for saying ‘one’, and some basis for saying ‘two’, but it’s fine if we can’t settle on a single answer, because the question is ultimately a verbal one. There are all the normal parts of a brain, standing in some but not all of their normal relations, and so not fitting the criteria for being ‘a brain’ as well as they normally would. And there are two overlapping subsystems within the one whole, which individually fit the criteria for being ‘a brain’ moderately well. But there is no further fact about which form of description – calling the whole a brain or calling the two subsystems each a brain – is ultimately correct.

The challenge is to take the same relaxed attitude to the question ‘how many people?’ Here is what I would like to say: the two hemispheres are conscious, and the one brain that they compose is conscious in virtue of their consciousness and the relations between them. Under normal circumstances their interactions ensure that the composite consciousness of the whole brain is well-unified: in the split-brain experiments, their interactions are different and establish a lesser degree of unity. And each hemisphere is itself a composite of smaller conscious parts. This amounts to embracing what Nagel views as a reductio.

There is something very difficult to think through about the composite consciousness view. It seems as though if each hemisphere is someone, that’s one thing, and if the whole brain is someone, that’s another: they cannot be just two equivalent ways of describing the same state of affairs. And this intuitive resistance to seeing conscious minds as composed of others (call it the ‘Anti-Combination intuition’) goes well beyond the split-brain phenomenon. It has a long history in the form of the ‘simplicity argument’, which anti-materialist philosophers from Plotinus (1956, pp.255-­258, 342-­356) to Descartes (1985, Volume 2, p.59) to Brentano (1987, pp. 290­-301) have used to show the immateriality of the soul. In a nutshell, this argument says that since minds cannot be thought of as composite, they must be indivisible, and since all material things are divisible, the mind cannot be material (for further analysis see Mijuskovic 1984, Schachter 2002, Lennon & Stainton 2008). Nor is the significance of this difficulty just historical: many recent materialist theories either stipulate that no conscious being can be part of another (Putnam 1965, pp.163, Tononi 2012, pp.59­-68), or else advance arguments based on the intuitive absurdity of consciousness in a being composed of other conscious beings (Block 1978, cf. Barnett 2008, Schwitzgebel 2015).

All of the just-cited authors take the Anti-Combination intuition as a datum, and draw conclusions from it about the nature of consciousness – conclusions up to and including substance dualism. I prefer the opposite approach: to see the Anti-Combination intuition as a fact about humans which impedes our understanding of how consciousness fits into the natural world, and thus as something which philosophers should seek to analyse, understand, and ultimately move beyond. As it happens, there is a group of contemporary philosophers engaged in just this task: constitutive panpsychists. Panpsychists think that the best explanation for human consciousness is that consciousness is a general feature of matter, and constitutive panpsychists see human consciousness as constituted out of simpler consciousnesses just as the human brain is constituted out of simpler physical structures. The most pressing objection to this view, which has received extensive recent discussion, is the ‘combination problem’: can multiple simple consciousness really compose a single complex consciousness (Seager 1995, p.280, Goff 2009, Coleman 2013, Mørch 2014, Roelofs 2014, Forthcoming-a, Forthcoming-b, Chalmers Forthcoming)? And this is at bottom the same issue as we have been grappling with concerning the split-brain phenomenon. In my research, I try to explore the Anti-Combination intuition, its basis, and how to move past it, with an eye both to the general metaphysical questions raised by constitutive panpsychism, and to particular neuroscientific phenomena like the split-brain.



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2 thoughts on “Split Brains and the Compositional Metaphysics of Consciousness

  1. I was wondering if visual information was crosswired/contralateral like all the stuff connected via the spinal cord and discovered via web search it is crosswired but by visual field as you correctly put it not by say eye …

    I sort of thought the eyes might be different from sensory input that goes through the spine because I have a compromised left inner ear but have more trouble balancing on my right leg suggesting that it is the left hemisphere getting the reduced information. My quick search suggests balance is processed in the lower part of the brain and perhaps it is not cross wired like hearing (sort of is) and visual inputs are. Although no one says that….

    I read Nagel recently and have heard about the strange phenomena of brain splitting more informally a lot. My biggest worry with all this is that philosophers may be doing a bit too much just responding to each other, some checking in (going back to the well) with the neuro-scientists and the experience of split brain individuals seems in order since different research agendas etc. can yield very different perspectives on the same phenomenon. So some engagement with those more detailed and evolving perspectives seems in order, otherwise the whole thing seems in danger of becoming a potted example that fails to show much.

    I would tend to say that this kind of thing is evidence against the proposition that all mental life is somehow a irreducibly unity, but I see lots of thing like that…

    1. Yes, the specifics of sensory lateralisation are rather fiddly. Nostrils to the ipsilateral hemisphere, ears go to the contralateral hemisphere, and the eyes are split by visual field (and even that’s a simplification). Even worse, touch for most of the body goes contralateral, but touch for the head and neck seems to be handled by both hemispheres, and doesn’t show the same dissociations in the split-brain patient. I don’t know about balance, though – I think there’s some evidence linking it to the cerebellum, which is another whole kettle of fish.

      You’re right of course that philosophers should keep their eyes on the data and not simply on what other philosophers say. If you think the split-brain phenomenon is evidence against a specially irreducible unity in mental life, well then, we’re on the same side! But perhaps you don’t think that’s as important a point to labour as I do?

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