29 Nov
1993
29 Nov
'93
4:17 a.m.
Roland Puccetti: DENNETT ON THE SPLIT-BRAIN
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TARGET ARTICLE AUTHOR'S RATIONALE FOR SOLICITING COMMENTARY:
The standard interpretation of split-brain patients is that they have
double consciousness. Dennett rejects this interpretation in support of
his "multiple drafts" model of consciousness. Important problems with
this alternative interpretation are pointed out here. Psychologists,
neuropsychologists, neurologists, cognitive scientists and
philosophers are invited to comment.
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psycoloquy.93.4.52.split-brain.1.puccetti Tuesday, 23 November 1993
ISSN 1055-0143 (22 paragraphs, 9 references, 344 lines)
PSYCOLOQUY is sponsored by the American Psychological Association (APA)
Copyright 1993 Roland Puccetti
DENNETT ON THE SPLIT-BRAIN
Roland Puccetti
Philosophy Department
Dalhousie University
Halifax, Nova Scotia CANADA
DALPHIL(a)ac.dal.ca
ABSTRACT: 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.
KEYWORDS: cartesianism, cell death, cerebral dominance,
consciousness, hemispherectomy, lateralization, mental duality,
mental unity, multiple drafts, split brain.
I. INTRODUCTION
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
(ibid).
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.
II. FIRST ARGUMENT
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.
III. SECOND ARGUMENT
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).
IV. THIRD ARGUMENT
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.
V. DENNETT'S CONCLUSION
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.
VI. LEFT HEMISPHERECTOMY AND NEOCORTICAL DEATH
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.
VII. REFERENCES
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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