Models of Consciousness
in Split-brain Subjects
Today
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The split-brain phenomenon
Consciousness and conscious unity
The conscious duality model
3 Unity models
The partial unity model
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The Split-brain Phenomenon
• The surgery
• The experiments
• Their design:
1. perceptual lateralization
2. response control
Sperry, R., Vogel, P., and Bogen, J. 1967.
Syndrome of hemisphere deconnection.
Physiology of Behavior, 6th Ed.
From Carlson, N. 2004.
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The Split-brain Phenomenon
• The surgery
• The experiments
• Their design:
1. perceptual lateralization
2. response control
3. prevention of cross-cuing
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Consciousness and conscious unity
• The split-brain phenomenon has been
taken to be a real-life “hard case” for
accounts of conscious unity.
Three notions of consciousness
Perhaps a conscious mental state is…
•one of which its subject is aware.
•one that is not just used in local processing but
which is available to a wide suite of reasoning
systems simultaneously (or one that is “globally
available”).
•one for which there is something that it is like
to have it.
Three notions of conscious unity
Perhaps for any two simultaneous conscious
experiences of a subject…
•their subject is or can be aware of undergoing
them at once (co-awareness)
•their subject can reason about the pair, use their
conjunction in reasoning (co-accessibility)
•there is something it is like for the subject to
undergo the two together, something different from
what it is like to undergo one of them one day and
the other the next (co-phenomenality)
The unity of consciousness
Split-brain results violate intuitions about
the way consciousness is structured:
“Roughly, we assume that a single mind has sufficiently
immediate access to its conscious states so that, for
elements of experience or other mental events occurring
simultaneously or in close temporal proximity, the mind
which is their subject can also experience the simpler
relations between them if it attends to the matter.”
Nagel, 1971, “Brain bisection and the unity of
consciousness”
The unity of consciousness
Experimental results violate intuitions about the way
consciousness is structured.
“…. we assume that… for elements of experience or other
mental events occurring simultaneously or in close
temporal proximity, the mind which is their subject can
also experience the simpler relations between them….”
… vs. standard cross-comparison test for “callosal
disconnection syndrome”.
The unity of consciousness
All kinds of divisions within and breakdowns
of cognitive processing are now
recognized.
But it is still commonly believed that
consciousness is integrated.
– E.g. Tononi’s (2004) “information integration”
theory
– E.g. Baars’ (1988) “global workspace” theory
Co-consciousness and
streams of consciousness
How many streams of consciousness does a
split-brain subject have…
…. where a stream of consciousness is a
collection of conscious experiences that
are co-conscious (co-aware, coaccessible, co-phenomenal) with each
other?
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Duality Model
Basic idea: A split-brain subject has two streams of
consciousness, a RH stream and a LH stream.
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Duality Model
Basic idea: A split-brain subject has two streams of
consciousness, a RH stream and a LH stream.
Concerns:
1. A significant amount of conscious information
appears shared between or undivided across
the two hemispheres.
2. Split-brain subjects act pretty normally, most or
all of the time.
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Unity Models
“Classic” (or “skeptical”) unity model
“Unity through duplication” model
“Switch” model
Classic (skeptical) unity model
Basic idea: A split-brain subject has a single
stream of consciousness, associated with only
the left hemisphere.
Concerns:
1. RH controlled behavior seems the result of
genuinely conscious experience and
control. (E.g., Zaidel, Zaidel, and Sperry,
1981.)
“Both patients did as well or nearly as well with their
right hemispheres as with their left. Indeed, with his
right hemisphere, LB scored at a level equivalent to
an 11-year-old child. To perform correctly on this
test, the relation between two items has to be
abstracted and then extrapolated so as to infer the
third item in a progression; finally, the result much
be matched to one of a set of possible answers…. If
this level of performance could be obtained
unconsciously, then it would be really difficult to
argue that consciousness is not an
epiphenomenon.”
(Shallice 1997, Modularity and Consciousness 264)
Classic (skeptical) unity model
Basic idea: A split-brain subject has a single
stream of consciousness, associated with only
the left hemisphere.
Concerns:
1. RH controlled behavior seems the result of
genuinely conscious experience and
control. (E.g., Zaidel, Zaidel, and Sperry,
1981.)
2. In some cases the RH learns to speak!
Unity through duplication model
Basic idea: common contents suffice for
conscious unity; it doesn’t matter the routes or
mechanisms by which contents are
duplicated across the hemispheres.
Concerns:
1.Can the physical basis of conscious unity be
partially behavioral?
2.Isn’t it a little strange to think that simply e.g.
plugging a nostril alters the structure of
consciousness?
Switch model
Basic idea: A split-brain subject has a single
stream of consciousness whose contents
derive from both hemispheres in turn.
Figure from Levy, Trevarthen, and Sperry, 1972, “Perception of bilateral chimeric figures following
hemispheric deconnexion”, p. 68
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Unity models
Figure from Levy, Trevarthen, and Sperry, 1972, “Perception of bilateral chimeric figures following
hemispheric deconnexion”, p. 70
Manipulations
•
In each standard trial, subject is
asked to indicate the stimulus she’s just
seen in one of two ways, by naming or
by pointing to a match.
“An
eye.”
Basic results, non-split vs.
split-brain subjects.
(“Perceptual completion.”)
Conscious duality or interhemispheric
switching of conscious contents?
• Certainly seems to show that, across trials,
each hemisphere was subject to conscious
experiences to which the other was not
privy.
• But that’s compatible with the switch model
as well.
Switch model
Basic idea: A split-brain subject has a single
stream of consciousness whose contents derive
from both hemispheres in turn.
Concerns:
1. 2 hemispheres sometimes seem to be
separately conscious simultaneously (e.g.
Schiffer et al. 1997).
2. At most, means that (technically) no two
streams at any single moment in time; still
looks like two streams of consciousness
across time.
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Partial Unity Model
Basic idea: There are RH and LH experiences
that are not consciously unified (co-conscious)
with each other, even though they are all
unified with a set of (subcortically transferred or
sustained?) experiences.
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Partial Unity Model
Basic idea: There are RH and LH experiences
that are not consciously unified (co-conscious)
with each other, even though they are all
unified with a set of (subcortically transferred or
sustained?) experiences.
Concerns:
1. Difficult to empirically distinguish between a
partially unified stream of consciousness
and two streams of consciousness with
some common contents.
2. Appears to drop notion of unitary conscious
perspective.