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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