Roland Puccetti (1993) Dennett on the Split-brain. Psycoloquy: 4(52) Split Brain (1)

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Psycoloquy 4(52): Dennett on the Split-brain

Target Article by Dennet on Split-Brain

Roland Puccetti
Philosophy Department
Dalhousie University
Halifax, Nova Scotia CANADA


In "Consciousness Explained," Dennett (1991) denies that split-brain humans have double consciousness: he describes the experiments as "anecdotal." In attempting to replace the Cartesian "Theatre of the Mind" with his own "Multiple Drafts" view of consciousness, Dennett rejects the notion of the mind as a countable thing in favour of its being a mere "abstraction." His criticisms of the standard interpretation of the split-brain data are analyzed here and each is found to be open to objections. There exist people who have survived left ["dominant"] cerebral hemispherectomy; by Dennett's criteria, they would not have minds.


cartesianism, cell death, cerebral dominance, consciousness, hemispherectomy, lateralization, mental duality, mental unity, multiple drafts, split brain.


1. In his already influential book, Consciousness Explained (1991), Daniel Dennett disputes the claim that split-brain patients exhibit double consciousness. We get an intimation of his view early on in the book, where he describes the rationale for the surgery and how split-brain subjects recover some functions despite permanent damage to the nerve circuits involved:

    A particularly suggestive anecdote comes from the research with
    split-brain patients (Gazzaniga, 1978) [sic: Dennett must mean
    Gazzaniga and Ledoux, 1978]. The left and right hemispheres are
    normally connected by a broad range of fibres called the corpus
    callosum. When this is surgically severed [for the treatment of
    epilepsy], the two hemispheres lose their major direct 'wires' of
    interconnection, and are practically incommunicado. If such a
    patient is asked to identify an object -- such as a pencil -- by
    reaching inside a bag and feeling it, success depends on which hand
    does the reaching. Most of the wiring in the body is arranged
    contralaterally, with the left hemisphere getting its information
    from -- and controlling -- the right side of the body, and
    vice-versa. Since the left hemisphere normally controls language,
    when the patient reaches in the bag with his right hand, he can
    readily say what is in the bag, but if the left hand does the
    reaching, only the right hemisphere gets the information that the
    object is a pencil, and is powerless to direct the voice to express
    this. But occasionally, it seems, a right hemisphere will hit upon
    a clever stratagem; by finding the point of a pencil, and digging
    it into his palm, he causes a sharp pain to be sent up the left
    arm, and some pain fibres are ipsilaterally wired. The left,
    language-controlling hemisphere gets a clue: it is something sharp
    enough to cause a pain. 'It's sharp -- it's perhaps a pen? a
    pencil?' the right hemisphere, overhearing this vocalization, may
    help it along with some hints -- frowning for pen, smiling for
    pencil -- so that by a brief bout of 'Twenty Questions' the left
    hemisphere is led to the correct answer (p. 198).

2. Since this kind of elaborate cross-cuing of the speaking left hemisphere by the mute right one is well known in the literature, it is surprising to read Dennett's sceptical remarks on it immediately following:

    There are more than a few anecdotes about such ingenious jury-rigs
    invented on the spot by patients with split brains, but we should
    treat them with caution. They might be what they appear to be:
    cases exhibiting the deftness with which the brain can discover and
    implement autostimulatory strategies to improve its internal
    communications in the absence of the 'desired' wiring. But they
    might also be the unwittingly embroidered fantasies of researchers
    hoping for just such evidence. That's the trouble with anecdotes

3. Why does Dennett say this? Perhaps because there being two centers of consciousness in split-brain patients would be problematic for his own theory, which denies that there is a "Cartesian Theater of the Mind" in which perceptions jointly come together to consciousness [forming a single mind]; rather, such perceptions consist of "Multiple Drafts" occurring throughout the whole brain [better, the entire cerebral cortex]. If the mind is only an abstraction derived from these multiple drafts, then it is not based in anything readily countable [one or two or however many], such as cerebral hemispheres. Hence the effects of cerebral commissurotomy, revealing a potential duality of mind in the normal intact human brain, need to be discounted.

4. But theoretical motivation is not yet demonstration by force of argument. It is only much later in Dennett's book that he attempts this, launching three lines of attack in just two pages of text. I turn to these arguments now.


5. Dennett claims that a right hemisphere-based center of consciousness [in a right-handed split-brain patient] exists as such only transitorily after the surgery:

    For brief periods during carefully devised experimental procedures,
    a few of these patients bifurcate in their response to a
    predicament, temporarily creating a second center of narrative
    gravity [Dennett's term for the self]. A few effects of the
    bifurcation may linger on in mutually inaccessible memory traces,
    but aside from these actually quite primitive traces of the
    bifurcation, the life of a second rudimentary self lasts a few
    minutes at most, not much time to accrue the sort of autobiography
    of which fully fledged selves are made (p. 425).

6. This is is a rather nonstandard interpretation of the split-brain evidence. Dennett seems to be confusing the time it takes to demonstrate the cognitive independence of the disconnected right hemisphere under testing conditions with the postsurgical span of consciousness of that hemisphere. The logic seems analogous to that of the following hypothetical conversation:

    YOU:      Does Puccetti speak French?
    DENNETT:  A little.
    YOU:      That's funny. I thought he got his doctorate from the Sorbonne.
    DENNETT:  Well, I heard him arguing with the waiter in a Parisian cafe
              once, but that lasted only a few minutes.


7. Dennett quotes approvingly the following argument (adapted from Ronald De Sousa, 1976), which he takes to support his own view that it is the distinctness of different "narratives" which accounts for there being distinct selves:

    When Dr. Jekyll changes into Mr. Hyde, that is a strange a
    mysterious thing. Are they two people taking turns in a single
    body? But here is something stranger: Dr. Juggle and Mr. Boggle
    [standing for the left and right cerebral hemispheres of a single
    body] too, take turns in one body. But they are as like as
    identical twins! Why then say that they have changed into one
    another? Well, why not: if Dr. Jekyll can change into a man as
    different as Hyde, surely it must be all the easier for Juggle to
    change into Boggle, who is exactly like him. We need conflict or
    strong difference to shake our natural assumption that to one body
    there corresponds at most one agent (ibid).

8. Note first that, on the surface of things, this second (adopted) argument conflicts with Dennett's first argument. For in the first he contended that the disconnected right hemisphere has only a transitory consciousness. If that were so, and its consciousness were identical with that of the left hemisphere, then its consciousness would be equally transitory. But in fact we already know that the consciousness of the two hemispheres is profoundly nonidentical (from Dennett's own description of a typical split-brain experiment, given above). For only the left, speaking hemisphere can name the object being palpated out of sight in a bag by the left hand; the mute right hemisphere knows what it is, but cannot name it, because it can't talk. And as for De Sousa's [and presumably Dennett's] demand for conflict or strong difference in behavior emanating from the disconnected cerebral hemispheres, there are indeed cases of intermanual conflict in the split-brain literature (Gazzaniga, 1970; Bogen, 1985).


9. Dennett asks what it is like to be a right-hemisphere-based self in a split brain patient:

    This is the most natural question in the world, and it conjures up
    a mind-boggling -- and chilling -- image: there you are, trapped
    in the right hemisphere of a body whose left side you know
    intimately (and still control) and whose right side is now as
    remote as the body of a passing stranger. You would like to tell
    the world what it is like to be you, but you can't! You're cut off
    from all verbal communication by the loss of your indirect phone
    lines to the radio station in the left hemisphere. You do your best
    to signal your existence to the outside world, tugging your half of
    the face into lopsided frowns and smiles, and occasionally (if you
    are a virtuoso right hemisphere self) scrawling a word or two with
    your left hand (pp.  425-426).

10. This is supposed to show, one infers, that there is something dreadfully amiss with the split-brain literature. But in fact there seems to be some misinterpretation on Dennett's part here. For example, if Dennett is a right-hander, it is his left hemisphere asking left-hemisphere readers to imagine being trapped in a disconnected right hemisphere. None of us left-hemisphere-based speakers and readers can do that, for we do not share the right hemisphere's typical life-long mutism. How could a disconnected right hemisphere possibly want to tell the world what it is like to be him or her, since a center of consciousness based there would not have a prior conception of what it is like to tell anyone anything? Dennett's misconception here is based on the assumption that split-brain surgery necessarily implies splitting a single self into two. Many followers of the split-brain literature do indeed make that assumption, but it needs independent argument, something Dennett does not supply.

11. There is an implication also in this passage that prior to the surgery the right hemisphere used its left cerebral companion to effect verbal communication. This is almost certainly incorrect, because adult left hemispherectomy to halt tumour invasion reveals the residual right hemisphere's verbal repertoire limited to simple epithets like "Shit!" and "Goddammit!" (Smith, 1966).

12. The parody of right-hemisphere alienation and helplessness following surgical disconnection provided by Dennett in the passage quoted is also strikingly contradicted by the apparently cheerful and persistent cross-cuing Dennett himself described earlier (e.g., by causing the face to frown at an incorrect left hemisphere verbal response, or to smile at a correct one). Why does the right hemisphere-based center of consciousness do this, especially in view of the left hemisphere's explicit denial, under direct questioning by the examiner, that another center of consciousness in the same head even exists (Levy, 1989)? In a game of charades where you are the silent partner, would you continue to cooperate if your verbose partner announced that you were a mere robot?

13. The explanation for this may lie in the observation that if lifelong mutism is characteristic of the right hemisphere, then it has been aware from a tender age of the mental duality because, since long before the surgery, it has been aware that it is not doing the talking and reading and writing that emanates from its own body.


14. Despite potential problems of the kind I have described, Dennett seems confident that he has demolished the standard view of cerebral commissurotomy, namely, that it creates two minds in place of the single mind that was based in the brain before:

    This exercise of imagination could go on in the obvious ways, but
    we know it is a fantasy -- as much a fantasy as Beatrix Potter's
    charming stories of Peter Rabbit and his anthropomorphic animal
    friends. Not because 'consciousness is only in the left hemisphere'
    and not because it couldn't be the case that someone found himself
    or herself in such a pickle, but simply because it isn't the case
    that commissurotomy leaves in its wake organizations both distinct
    and robust enough to support such a separate self (p. 426).

15. To this strong dismissal of the standard view, Dennett adds:

    It could hardly be a challenge to my theory of the self that it is
    'logically possible' that there is such a right hemisphere self in
    a split-brain patient, for my theory says that there isn't, and
    says why: the conditions for accumulating the sort of narrative
    richness [and independence] that constitutes a 'fully fledged' self
    are not present. My theory is similarly impervious to the claim --
    which I would not dream of denying -- that there could be talking
    bunny rabbits, spiders who write English messages in their webs,
    and for that matter melancholy choo-choo trains. There could be, I
    suppose, but there aren't -- so my theory doesn't have to explain
    them (ibid).

16. Thus, for Dennett, "narrative richness" is not merely characteristic of human selves, it is a necessary condition of coming to constitute a self. But if that were so then it would not just be the disconnected right hemisphere of the split-brain patient that would not qualify for selfhood. Let me bring this out by contrasting two very different syndromes of brain damage and their behavioral sequelae.


17. Surgical excision of the left cerebral hemisphere in adults [usually to prevent the spread of a malignant tumour] has the typical effect of provoking profound aphasia in the patient. This is hardly a surprising outcome, since the left hemisphere is almost always dominant for both speech and handedness. In other words, the healthy residual right hemisphere is, to say the least, language-poor (Smith, 1966). Call this syndrome LH for short.

18. The situation for patients succumbing to neocortical death without brain stem death is very different indeed. This can be caused by a temporary deprivation of oxygen and glucose to the brain, as in a near-drowning accident, so that within minutes the gray matter [neocortex] of the brain is destroyed, while the relatively hardy neurons making up the brain stem are spared. As a consequence, the patient can breathe spontaneously and does not need to be placed on a respirator. The bodies of such patients can then be kept alive indefinitely under intensive care. Call this syndrome ND for short.

19. With the gray matter gone, however, no one survives ND to appreciate this care, for the neocortex subserves conscious functions, and without it personal life ends [at least in this life, to avoid begging religious questions]. In a state of permanent coma, the ND patient is beyond both help and harm, so it can be argued that "breathing corpses" should be declared dead and the breathing stopped to prepare them for burial (Puccetti, 1988).

20. Now if Dennett were right about "narrative richness" being a necessary condition for constituting a human self, the LH patient is no better off than the ND patient. One could argue on this basis that the former should be disposed of too: there is "no one home" in either case. But this conclusion is grossly counterintuitive. Suppose the ND patient receives a visit from a loved one, say his granddaughter. He lies there supinely, breathing away, but nothing the granddaughter does or can do ever gets through to his conscious self, since there is none left.

21. Suppose now, by contrast, the LH patient receives a visit from her grandson. She is seated in a wheelchair. She cannot talk [though she may say "Hello" and give the grandson's name]. She understands only simple language. She is paralysed on the right side of her body and face. But she positively beams during the visit, delighted with photos of family members, the house and pets. And she may even reach out with her left hand to stroke her grandson's arm.

22. Who could possibly recommend medical disposal of the LH patient on a par with the ND patient? [Of course Dennett would not, either: he just hasn't thought through these implications of his own analysis.] There is, however, a lesson to be learned from this. New theories of mind are welcome, but they must not be allowed to override relevant behavioral evidence. Otherwise philosophy becomes a dangerous undertaking.


Bogen, J.E. (1985) The Callosal Syndromes. In: Clinical Neurology. 2nd Ed. Oxford University Press: Oxford: 295-338.

Dennett, D.C (1991) Consciousness Explained. Boston: Little, Brown.

De Sousa, R. (1976) Rational Homunculi. In: A.O. Rorty (ed.) The Identity of Persons. University of California Press: Berkeley: 217-238.

Gazzaniga, M.S. (1970) The Bisected Brain. Appleton-Century-Crofts: New York.

Gazzaniga, M.S. (1978) Is Seeing Believing: Notes on Clinical Recovery. In: S. Finger, (ed.) Recovery from Brain Damage: Research and Theory. Plenum Press: New York: 409-414.

Gazzaniga, M.S. and Ledoux, J.E. (1978) The Integrated Mind. Plenum Press: New York.

Levy, J. (1989) Address to the Department of Psychology. Dalhousie University.

Puccetti, R. (1988) Does Anyone Survive Neocortical Death? In: R. M. Zaner (ed.) Death: Beyond Whole-Brain Criteria. Kluver Academic Publishers: Dordrecht: 75-90.

Smith, A. (1966) Speech and Other Functions after Left (Dominant) Hemispherectomy. Journal of Neurology, Neurosurgery and Psychiatry 29: 467-471.

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