Submission Title: The Switch Model of Split-brain Consciousness
Author: Elizabeth Schechter
Affiliation: Faculty of Philosophy at the University of Oxford
Contact: lizschechter@gmail.com
The Switch Model of Split-brain Consciousness
Abstract
The attempt to model the structure of consciousness in split-brain subjects is on-going. This paper
concerns the recently proposed switch model of split-brain consciousness, according to which a splitbrain subject possesses only a single stream of consciousness, unified at and across time, that shifts from
one hemisphere to the other from moment to moment. The paper argues that while the central explanatory
element of the switch model may account for some aspects of split-brain consciousness, the best general
picture of split-brain consciousness is still offered by some version of the conscious duality model.
1
Introduction
The corpus callosum connects the two cerebral hemispheres in placental mammals and is the largest fiber
tract in the human brain. A callosotomy is a surgery sectioning this structure. Under the experimental
conditions characteristic of the “split-brain” experiment, sensory information is directed only to one
hemisphere of the subject at a time, to the extent possible. This paradigm allows researchers to investigate
the abilities of each hemisphere somewhat independently.1
Many split-brain experiments suggest, though not definitively, that each hemisphere of a splitbrain subject is associated with a distinct stream of consciousness. While such experimental results would
in and of themselves suffice to make the split-brain phenomenon a real life “hard case” for theorizing
about consciousness, they constitute only one half of the puzzle of the split-brain phenomenon. The other
half is that, outside of experimental contexts, split-brain subjects generally behave more or less like “nonsplit” human subjects, i.e. subjects in whom the corpus callosum is intact, and of course we believe that
non-split subjects have only a single stream of consciousness apiece. In other words, experimental and
non-experimental evidence seems to point to different answers to the question: What is the true structure
of split-brain consciousness?
For many who have grappled with the split-brain cases, the central problem they raise concerns
the unity of the person or the self, of which the unity of consciousness is widely viewed as one important
aspect. As Nagel once wrote:
There are actually a variety of types of “split-brain” surgery. In this paper when I refer to “split-brain” subjects I
mean to refer to subjects who have undergone either full callosotomy (sectioning the entire corpus callosum) or
commissurotomy (in which some additional commissures, usually the anterior commissure, are also sectioned).
There are significant differences between split-brain subjects with respect to the dissociations they manifest, and the
“disconnection syndrome” typically becomes less pronounced as time (since surgery) passes; I ignore these
complications in what follows because even some cases of conscious disunity are sufficient to raise the
philosophical issues raised here.
Good introductions to the split-brain phenomenon can be found in Gazzaniga and LeDoux, 1978, and
Gazzaniga, 2000. The best introduction to some of the philosophical problems the phenomenon raises may still be
Nagel, 1971, though see also Marks, 1981.
1
1
Lack of interaction at the level of a preconscious control system would be
comprehensible. But lack of interaction in the domain of visual experience and
conscious intention threatens assumptions about the unity of consciousness
which are basic to our understanding of another individual as a person. These
assumptions are associated with our conception of ourselves, which to a
considerable extent constrains our understanding of others. (1971: 407)
On the whole, split-brain subjects do not seem less unified, as individuals, than do “non-split”
subjects. The possibility that this unity is in some sense or at some level illusory raises some
uncomfortable possibilities regarding the rest of us.2
Despite such worries, neuropsychologists appeared to converge, fairly early on, on some version
of the “conscious duality model” (e.g., Sperry, 1968, and Gazzaniga and LeDoux, 1978), according to
which the two hemisphere of a split-brain subject are associated with different streams of consciousness.
Philosophers have been more reluctant to accept this conclusion, and have proposed several more
conservative models of split-brain consciousness. Lockwood’s “partial unity model” (1989) limited the
extent of independent right and left hemisphere consciousness. According to this model, co-consciousness
is not a transitive relation: each hemisphere sustains experiences that are not unified with those in the
other, but all these experiences are at least mutually unified with a shared set of non-cortically sustained
experiences, as well. Marks (1981) and Tye (2003) have instead sought to limit the time periods in which
right hemisphere and left hemisphere are separately conscious, arguing that split-brain consciousness is
only divided under highly artificial experimental conditions that deny the hemispheres access to the same
perceptual information. Hurley (1998) meanwhile suggested that over time, subtle forms of behavioral
cross-cuing in split-brain subjects might become so automatic and “sub-personal” as to eventually serve
as a means of unifying split-brain consciousness. Still, while not going so far as to attribute two wholly
distinct streams of consciousness to a split-brain subject, all of these models do nonetheless allow that the
split-brain phenomenon involves some breakdown in conscious unity.
It is in this light that Bayne’s (2008) recently proposed “switch model” of split-brain
consciousness is particularly striking. According to this model, while each hemisphere of a split-brain
subject is associated with distinct conscious experiences, we still do not need to attribute conscious
duality, nor indeed any failure of conscious unity, to such a subject. Rather, the split-brain subject has a
single stream of consciousness whose physical basis shifts from one hemisphere to the other, from
moment to moment, as a function of which hemisphere has momentarily won the battle for attentional
2
These possibilities were fully embraced by Puccetti (1981), who argued that each normally functioning human
hemisphere is associated with a unique person—even in non-split subjects, that is. Only hemispherectomized
subjects are single persons.
2
resources (Bayne, 2008). So, in this model, a split-brain subject has only a single stream of consciousness
both at a time and across time—just as a non-split subject does.3
Since the split-brain phenomenon provides one of the best studied, and arguably the most
compelling, cases of conscious “disunity,” the switch model constitutes a significant first step towards the
defense of an even bolder claim (defended in Bayne, 2010): that consciousness is always and even
necessarily unified. The truth of this claim would have great significance. The adequacy of the switch
model would also be deeply reassuring to all those who would like to insist that personhood remains
unitary in split-brain subjects, but who cannot conceive of a single person with a divided consciousness.
As Bayne himself says of the model, it “avoids the philosophical baggage that accompanies the twostreams and partial unity models” (2008: 299), and does so without implausibly denying right hemisphere
consciousness.
Fortunately or unfortunately, however, the switch model is untenable. The central explanatory
component of the model—according to which there are interhemispheric fluctuations in a split-brain
subject’s consciousness, as a result of interhemispheric fluctuations in attention—very likely accounts for
certain findings in the split-brain literature. That central explanatory component is still compatible with
split-brain subjects possessing two streams of consciousness apiece, and indeed I argue in what follows
that the structure of split-brain consciousness, both at a moment and over time, is still best characterized
by some version of the conscious duality model.
2
The Switch Model
This section clarifies the commitments of the switch model and the duality model, identifying what it
would take to make the case for the latter over the former.
Let us call “co-consciousness” the relation between those experiences that belong to a “unified”
or a “single” consciousness, and that gives consciousness its structure, and let us define streams of
consciousness in terms of the co-consciousness relation, so that a stream of consciousness is composed of
phenomena that are co-conscious with each other.
3
In Bayne’s version of the model, interhemispheric shifts in conscious status result from interhemispheric
competition for attentional resources. Hurley (1998), drawing most strongly on Trevarthen’s work (see especially
Trevarthen 1990 and 1974), at times describes split-brain consciousness in ways that appeal to a possible version of
the switch model, one in which interhemispheric changes in motor intentions are the mechanism driving
interhemispheric shifts in consciousness. Bayne also credits Levy with offering a version of the model in 1977 (see
also Levy 1990), since she at times characterizes split-brain subjects’ conscious perceptual experience in terms of
interhemispherically shifting hemi-inattention. It is not clear, however, whether Levy, Trevarthen or Hurley believe
that interhemispherically shifting, unihemispheric consciousness offers the best general picture of split-brain
consciousness. I therefore focus on Bayne’s version of the model, because it is Bayne who has most clearly
articulated what can properly be called the (general) switch model of split-brain consciousness.
3
When two experiences are co-conscious, they are experienced together: there is something it is
like for their subject to undergo both of them at once, something different from what it would be like for
one subject to undergo one of them while another subject undergoes the other; or, their subject
experiences, or can be aware of, their co-occurrence (or of the co-occurrence of their contents). This is a
characterization of synchronic or simultaneous co-consciousness, anyway, to which the next two sections
will be devoted; there is perhaps less agreement about the proper characterization of diachronic coconsciousness, or co-consciousness across time, to which I will turn in Section 5. I will assume that these
two characterizations of synchronic co-consciousness—in terms of experienced togetherness, and in terms
of co-awareness—are co-extensive, and I will also assume that the co-awareness in question could be
cashed out either in higher-order or in first-order terms (as in Tye, 2003).
Early on in the history of theorizing about split-brain consciousness there were some attempts to
deny normal human consciousness to the “disconnected” right hemisphere, and to therefore attribute only
a single, structurally normal stream of consciousness to the split-brain subject, associated uniquely with
the left hemisphere.4 Over time, however, this early “unity model” of split-brain consciousness came to
lose its empirical plausibility. Indeed, so far as I am aware, current models of split-brain consciousness,
which still diverge on the structure of split-brain consciousness, all allow that both hemispheres of a splitbrain subject are associated with conscious phenomena.
This convergence has been made possible in part by the emergence of some dominant
conceptions of what consciousness is, as well as growing agreement on some methodological principles
for its investigation. The term “consciousness” will be used, here, as it increasingly is, to refer both to
phenomenality—to the fact that some of our mental events feel like something to us—and to accessibility
by persons—to the fact that some of our mental events are ones we have some kind of “direct”, firstperson access to, and can introspect, report, use in our practical reasoning, and so forth. Bayne’s central
concern is with the structure of phenomenal consciousness in particular. Fortunately, while the precise
relationship between access consciousness and phenomenal consciousness remains in dispute, they are
widely believed to be, at a minimum, significantly coextensive.5
4
See, for example, Eccles 1973, 1965; Popper and Eccles, 1977; DeWitt, 1975. Technically these authors ascribed
what they called “consciousness” to the right hemisphere, but at that point in time the term was used in an
unhelpfully broad and heterogeneous manner to include, among other things, what we would now call “nonconscious perceptual representation and sensation.” (DeWitt, for instance, attributed consciousness not just to the
“disconnected” right hemisphere but also to earthworms (cf. DeWitt 1975: 42).) What these authors denied to the
right hemisphere was whatever kind or level of self-consciousness would require identifying right hemisphere
mental states with the mental states of a person. (See Eccles 1981, on this distinction.) Although self-consciousness
is not given specific attention in this paper, several of the studies I mention here (particularly Schiffer et al., 1998,
and Sperry, Zaidel, and Zaidel, 1979) do seem to provide evidence of at least a significant degree of right
hemisphere-sustained self-consciousness.
5
On the general conceptual distinction between access and phenomenal consciousness, see Block, 1995. On their
empirical relationship, see Chalmers, 1997; Block, 2007.
4
This is fortunate because access consciousness is functionally defined, and clearly admits of
empirical investigation. Bayne himself appears to allow that, at least for the time being, the argument for
the unity of phenomenal consciousness in split-brain subjects is only as good as the argument for the
unity of access consciousness in those subjects (see Bayne, 2008: p. 285; see also Bayne, 2010). I, too,
will mostly assume that the structure of access consciousness also reflects that of phenomenal
consciousness, and will therefore speak only of consciousness and co-consciousness, tout court,
ultimately citing evidence that I think shows two functionally distinct streams of consciousness and
therefore two distinct streams of phenomenal consciousness as well. The issue of phenomenal relations
versus other relations between conscious phenomena will be returned to briefly in Section 5, however.
Both the switch model and the duality model attribute distinct conscious experiences to each
hemisphere of a split-brain subject. The duality model attributes distinct conscious experiences to the two
hemispheres simultaneously, however, and also specifies that these experiences are only
intrahemispherically co-conscious. According to the switch model, in contrast, the two hemispheres are
not associated with distinct conscious experiences at one and the same time. Because, in this model,
distinct right and left hemisphere experiences don’t occur at the same time, they are not properly
describable as failing to be synchronically co-conscious. The model additionally specifies that right and
left hemisphere experiences are diachronically co-conscious, incorporated into a single stream of
consciousness over time.
The switch model, then, is committed to both synchronic and diachronic conscious unity in splitbrain subjects, just as the duality model is committed to both synchronic and diachronic conscious
duality. Still, one might propose that consciousness is synchronically singular but diachronically dual, or
vice versa, in the split-brain subject. It will therefore often be helpful to speak of the synchronic versus
the diachronic switch model, and similarly of the synchronic versus the diachronic duality model, in what
follows.6
The switch model gains support from a number of developments that have occurred in the last
two decades, a time frame that has seen the quest for an understanding of consciousness emerge as a
major pursuit in neuropsychology (e.g., Koch 2004, Crick and Koch 1990). Within this time frame,
models of consciousness that propose a vital role for attention have risen to prominence (e.g., Baars,
1988, 2007; Posner, 1994; Prinz, 2000; Dehaene and Naccache, 2001). This is in part because there is a
good deal of behavioral evidence suggesting that the absence of attention can prevent perceptual
processing from reaching conscious awareness (e.g., Mack and Rock, 1998; Raymond, Shapiro, and
6
Alternatively one could speak of the switch model as committed both to the unity and to the continuity of splitbrain consciousness; I will speak instead of the synchronic and of the diachronic aspect of the switch model, but
don’t think anything hinges, here, on this choice.
5
Arnell, 1992). At the neurophysiological level, moreover, attending has been associated with increased
neural activity (see Knudsen, 2007, for a helpful review), while there is simultaneously evidence that the
neurophysiological difference between conscious and non-conscious perceptual representation is at least
partly a matter of degree of activation (e.g., Moutoussis and Zeki, 2002). In light of such findings, the
switch model gains some plausibility from experimental results suggesting interhemispherically shared
attentional processes in split-brain subjects.
Recent decades have also seen the development and refinement of neuroimaging technologies
that make quite salient that consciousness, at the neural level, is less an object than an ongoing, dynamic
process or activity. If consciousness is realized in such an active, ever-changing manner in the brain, then
why couldn’t a single stream of consciousness switch back and forth between the hemispheres from
moment to moment? Indeed, against the backdrop of such developments, the classic conscious duality
model—with its seeming pronouncement, “Two hemispheres, therefore two streams of consciousness”—
can be made to appear rather flat-footed.
The adequacy of the switch model of course depends upon the actual relationship between
consciousness and attention in general, and upon the interhemispheric operation of attention in split-brain
subjects in particular. Bayne’s switch model need not make attention the mechanism of consciousness
per se; it need be committed, at most, only to the necessity of attention for consciousness, since it “paints
the split-brain patient as suffering from a kind of fluctuating perceptual extinction: when the left
hemisphere is activated stimuli in the RVF [right visual field] win the competition for entry into
consciousness at the expense of LVF [left visual field] stimuli, and the converse happens when the right
hemisphere is activated” (Bayne, 2008: 294). At present, the switch model is compatible with at least two
different proposals about what it takes to “win” the interhemispheric competition for consciousness. (See
Kanwisher, 2001, for a related discussion.) According to one proposal, the “winner” is simply the
hemisphere that garners more attentional resources: there is no threshold level of activation that is always
sufficient for consciousness; rather, whether a given level of attentional activation obtained in one
hemisphere suffices for consciousness depends upon the level of activation concurrently obtained in the
other.7 Alternatively, a defender of the switch model might assume that there is an absolute threshold
level of activation that is at least normally sufficient for consciousness—but that attentional resources are
shared or divided in the “split” brain in such a manner that the two hemispheres cannot in practice both
exceed that threshold at once.
In conversation Bayne has suggested that this was nearest his own idea, and noted that it recalls Dennett’s “fame”
model of consciousness: “just as in the wider world, whether or not you become famous can depend on what is
going on elsewhere at the same time” (Dennett, 2001: 226).
7
6
This model of split-brain consciousness receives some degree of support from empirical studies
on the operation of attention in split-brain subjects. There have been several studies suggesting that spatial
attention systems remain undivided in the split-brain subject or shared between the two hemispheres (for
example, Gazzaniga, 2000, 1987; Holtzman, Volpe, and Gazzaniga 1984; Holtzman et al. 1981; ReuterLorenz and Fedrich,1990; see also Corballis, 1995), and it is unsurprising that the brain has a finite degree
of processing resources for which the hemispheres are therefore ultimately in some degree of competition
(Holtzman and Gazzaniga, 1982). At the same time, there are also studies suggesting that each
hemisphere can attend at least partially independently of the other (for example, Luck et al. 1989 ; Luck et
al. 1994; Arguin et al. 2000; Mangun et al. 1994), and that the corpus callosum in fact plays a role in
creating a unified spatial attentional system (Hines, Paul, and Brown, 2002). Furthermore, while attention
and consciousness are widely recognized to be importantly connected, the precise mechanistic
relationship between them remains uncertain. There is at least some evidence that consciousness is
possible in the absence of highly focused, endogenous attention, for instance (see discussion in Koch and
Tsuchiya, 2006; Hardcastle, 2003), though this is compatible with consciousness requiring other sorts of
attention (as defended by, e.g., De Brigard and Prinz, 2010).
Because there are still many unknowns concerning the relationship between consciousness and
attention, and concerning the operation of attention in split-brain subjects, behavioral evidence regarding
conscious unity or disunity is especially important. Here the switch model might appear to enjoy the clear
advantage. The duality model faces the serious challenge of accounting for the generally unified-seeming
nature of split-brain subjects’ behavior, especially outside of experimental situations. The switch model
can account for such basically normal-seeming behavior in terms of a split-brain subject’s possessing a
structurally normal consciousness; any disunified or dissociated behavior, meanwhile, can be explained,
using the model, in terms of rapid shifts in what the subject is conscious of, and via which hemisphere’s
processing.
I believe that underneath the surface, however, the conscious duality model is more empirically
adequate than the switch model; indeed I believe that there is a good deal of data that the switch model
simply cannot account for. Of course, to the extent that the duality model and the switch model are
idealizations, each can tolerate a certain number of deviations from the structure it strictly postulates. The
switch model and the duality model are in an asymmetric position with respect to such deviating
instances, however. The conscious duality model is compatible with one hemisphere’s sometimes ceasing
to be, or failing to become, conscious of a thing, due to activity ongoing in the other hemisphere. In
contrast, the switch model—which is, after all, a version of the conscious unity model—is on its face
incompatible with non-co-conscious experience.
7
Note, moreover, that even occasional instances of simultaneously conscious but not co-conscious
experiences in the split-brain subject might suffice to undercut a good deal of the motivation for the
switch model. The paper in which Bayne proposes and defends the switch model begins:
According to conventional wisdom, the split-brain syndrome puts paid to the
thesis that consciousness is necessarily unified. The aim of this paper is to
challenge that view.
(Bayne, 2008: 277)
Even occasional instances, even a single instance, of simultaneously conscious experiences in the
split-brain subject that are not co-conscious with each other would show that consciousness is not always
or necessarily unified.8 Those who debate the structure of consciousness in split-brain subjects are, after
all, probably typically less interested in the split-brain phenomenon per se than they are in what the
phenomenon can tell us about consciousness in general. We already believe that consciousness is
sometimes unified, but it would be interesting to discover that it always is—and it would be interesting to
discover that it sometimes is not. So even a small number of cases of simultaneous conscious duality
would present a serious challenge to the switch model, especially given the philosophical motivations
Bayne provides for it. The next section examines the positive case for simultaneous conscious duality in
split-brain subjects.
It is important to note that the synchronic switch model need not be incompatible with merely
simultaneous conscious experience in the two hemispheres: what the model denies is simultaneous and
distinct (non-shared) experiences in the two hemispheres. One could develop a version of the switch
model according to which the hemispheres are simultaneously subject to a shared set of conscious mental
tokens concerning, for example, the visual periphery (see, e.g., Trevarthen and Sperry, 1973), with focal
visual consciousness switching between hemispheres from moment to moment. To support the duality
model, then, it is insufficient to provide cases in which the hemispheres are both conscious at once; what
is wanted is simultaneous and independent, non-co-conscious conscious experience. The next section
looks at evidence for such experience.
3
Evidence supporting the duality model9
8
At least, not within a single conscious creature. It would remain possible that consciousness is necessarily unified
relative to a single subject of experience, since the two hemispheres of a split-brain subject might be associated with
distinct subjects of experience. Bayne (2008) expresses some sympathy for this view, but nonetheless ultimately
assumes that a split-brain subject is associated with a single subject of experience. (See also Bayne, 2010.)
9
Most of what I will say in defense of conscious duality in split-brain subjects will in fact be equally consistent with
the partial unity model. I regard the two models as importantly similar with respect to the issues raised in this paper,
but for simplicity’s sake, I focus on the contrast between the switch model and the duality model exclusively.
8
One simple example of non-co-consciousness in split-brain subjects is that “in tests involving righthemisphere performance, the vocal hemisphere often tends to offer throughout a running commentary
based on those aspects of the situation not restricted to the mute hemisphere” (Sperry 1977: 105). In other
words, simultaneous (and, Sperry makes clear in context, conscious) activity in both hemispheres at one
time. Granted, “this parallel activity of the second hemisphere may often interfere with the performance
of the test hemisphere” (ibid), and as a result, Sperry says, subjects are typically asked to keep quiet. This
suggests that the hemispheres do compete for attentional (or at least general processing) resources, to the
detriment of one hemisphere’s or the other’s conscious performance. There is a good deal of evidence,
however, that the victory is not always one-sided.
To begin with, although split-brain subjects generally behave in an integrated fashion (even
within experimental situations, as Nagel (1971) noted), it is still important to acknowledge those cases in
which the hemispheres appear not just separately, and simultaneously, conscious, but also in some
significant degree of conflict with each other. Outside of experimental conditions, split-brain subjects
have been reported to slap their own faces with their left hands (right hemisphere?) when oversleeping
(Dimond, 1980) and to violently shake and come to the aid of a loved one simultaneously (Gazzaniga,
1970). One split-brain subject complained (left hemisphere) that his left hand (right hemisphere?) kept
plucking the lit cigarettes from his mouth or from his right hand and putting them out before he was
finished with them (cited in Joseph, 1990, p. 29). One of those split-brain subjects apparently capable of
speech via both hemispheres would frequently get into verbal disputes with herself, especially under
experimental conditions, after lateralized perceptual input. (“On one occasion she mentioned that she did
not have feelings in her left hand. When I echoed the statement, she said that she was not numb, and then
the torrent of alternating ‘Yes!’ and ‘No!’ replies ensued, followed by a despairing ‘I don’t know!’” Mark
1996: p.191).)
That said, the hemispheres are usually not at war with each other. Even some of the mutually
cooperative behavior of the hemispheres suggests conscious duality, however. For instance, Stuart Butler
(in personal communication to Bogen; see Bogen 1990 and 1983) described an experiment in which he
(Butler and Norrsell, 1968) would put a wooden object—sometimes a sphere, sometimes a cube with
rounded edges, sometimes a rounded pyramid—in split-brain subject L.B.’s left hand (right hemisphere),
while his hand was hidden behind a screen so that he couldn’t see the objects, and then ask him which of
the three objects he was holding. To their surprise, L.B. was naming them accurately most of the time.
Then they noticed that he was looking around the room in a systematic way during these trials: whenever
they put the sphere in his hand he would look at the wall clock and then say, “It’s the round one.” When
they put the cube in his hand he would look at the door and then say, “That’s the square one.” And
whenever they put the pyramid in his hand he would stare up at the ceiling for a while before saying, “It
9
must be that triangular shape.” Once they blindfolded him, his performance fell to chance. Although most
cross-cuing in split-brain subjects looks quite different from this, this particular cross-cuing strategy
appears so deliberate (and so deviously clever) that it seems likely that L.B.’s right hemisphere was
consciously engaging in it. (L.B. is also prone to attempting to spell out answers to questions only his
right hemisphere knows the answer to on his right hand, using a finger of his left. See for instance Sperry,
Zaidel, and Zaidel, 1979.)
Of course someone deeply committed to the switch model could say that in all such cases, the
hemispheres were merely taking turns being conscious—that conscious experience was switching, very
rapidly, back and forth and back and forth again, between the hemispheres, and that, in order to make any
genuine case for simultaneous experiences in the two hemispheres of a split-brain subject, we need
instances of strictly simultaneous right hemisphere and left hemisphere (consciously) guided actions.
Bayne’s defense of the switch model does indeed exploit what is, in this context, a genuine limitation of
the methodology characterizing most split-brain experiments. Most of these experiments were designed to
examine whether either of the two hemispheres were conscious of a particular stimulus, and whether they
were both conscious of that particular stimulus, but were not designed to examine whether the two
hemispheres were conscious of anything at all simultaneously. It is therefore reasonable to question
whether such studies can really provide evidence relevant to evaluating the possibility that the two
hemispheres weren’t simultaneously, separately conscious. Admittedly, to object that the behaviors I’ve
just cited in support of the conscious duality model weren’t engaged in strictly simultaneously, and to
interpret them, instead, in the switch model’s light, might seem a bit ad hoc, mainly because in such cases
(and unlike in the chimeric matching experiment I will describe in Section 4) it is difficult to see what
would be driving the interhemispheric switches in consciousness and attention to begin with. Hurley is
furthermore correct that, “Simultaneous conscious events may not be manifested simultaneously” (1998:
199; emphasis added). Still, it would be nice to find some cases in which right and left hemisphere
conscious events were simultaneously manifested.
Fortunately, although they may be the exceptions rather than the rule, there are at least some splitbrain experiments in which the hemispheres do simultaneously, yet independently, participate in what
looks to be consciously-driven behavior. Consider, for instance, an experiment by Schiffer et al. (1998),
designed to determine whether the two hemispheres of a split-brain subject might differ in their emotional
propensities. In this study, the subject A.A. (who had earlier mentioned to the authors a bad experience
with bullies as a child) was required to respond to a series of verbally presented questions. These were of
two basic types: general questions (e.g., “How much on the scale from none to extreme, do you believe in
God?”), and more emotionally sensitive questions (e.g., “How much on the scale from none to extreme,
do you think the bullies were cruel?”).The subject’s hands were hidden from his own view behind a
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screen, but before each hand lay a row of five pegs, the peg furthest to the left representing “none” and
the peg furthest to the right representing “extreme” (with middle pegs representing “mild,” “moderate,”
and “quite a bit”). This allowed the two hemispheres to simultaneously answer each question by touching
the appropriate peg. (As it turned out, his hemispheres largely gave the same answers to the general
questions and gave significantly different answers to emotionally sensitive questions.)
In an experiment with a very different aim (Ellenberg and Sperry, 1980), split-brain and non-split
subjects were asked to engage in a variety of sorting tasks, to see how well they could perform with the
two hands, and thus presumably the two hemispheres, at once. All four conditions involved taking small
objects from a central container, and then putting each object onto either a top or a bottom shelf, right of
the central container for the right hand, left of the central container for the left hand. The objects used in
the study were cylindrical beads, spherical beads, wing nuts and hex nuts, and the various trial types
varied with respect to the sorting task required of each hand, so that the subjects might be asked to put
cylindrical beads on the top shelf and spherical beads on the bottom shelf with both hands in one trial type
(“bimanual-same”), or they might to be asked to perform the opposite sorting task with one hand as with
the other (“bimanual-reverse”), or they might be asked to simply perform different sorting tasks with each
hand—one hand to sort beads, one hand to sort nuts (“bimanual-different”). Needless to say, these tasks
(and perhaps the objects themselves) were novel to the subjects, and thus required their conscious
attention (at least until they were well-learned).
One interesting finding in this experiment was that in non-split subjects, the two hands moved at
the same rate, regardless of the task type. Moreover, when non-split subjects made a mistake with one
hand, they tended to simultaneously slip up with the other. And, unsurprisingly, they performed the
“bimanual-same” task more quickly than the “bimanual-different” task, which they performed more
quickly than the “bimanual-reverse” task. But fully callosotomized subjects performed all the sorting
tasks at about the same rate, and with the same number of errors. Most interestingly, each hand seemed to
perform its given task independently of the other: often the hands would “drift,” and come to sort at
different rates, and when they committed an error with one hand, the other hand often didn’t miss a beat
(see Ellenberg and Sperry 1979 as well).
Finally, there are a number cases in which the hemispheres seem to be simultaneously
undergoing conscious experiences for extended and continuous periods of time. A study by Sperry,
Zaidel, and Zaidel (1979) presents several nice examples of this: the authors describe the right and left
hemispheres of their subjects interacting with each other, apparently consciously, in a number of complex
ways, via the subjects’ behavioral and emotional responses, over the course of seconds and minutes.
Given the fairly fluid nature of this interhemispheric exchange, and of the responses emanating from each
11
hemisphere individually throughout, it is difficult to imagine this fluidity resulting from consciousness
switching on and off over and over again in each hemisphere.
4
Two challenges to the duality model
In the previous section I tried to provide a range of examples suggestive of conscious duality in the splitbrain subject. This section deals with two of the challenges facing the duality model of split-brain
consciousness. The first challenge I will discuss is that of accounting for cases in which fluctuating
inattention really does appear to characterize split-brain consciousness. The second is that of accounting
for the generally unified nature of split-brain subjects’ behavior if split-brain consciousness is in fact dual.
4.1
The challenge from inattention
The first challenge I will discuss comes from the switch model itself—or, rather, from experimental
results that appear best accounted for in terms of interhemispheric shifts in conscious status, just as the
switch model postulates. One fine source of examples is the chimeric matching experiment of Levy,
Trevarthen, and Sperry (1972), on which Bayne relies in his own defense of the switch model.10
In each trial of the experiment, a chimeric image, centered at the vertical midline, was presented
to a split-brain subject; a distinct half-image was therefore presented in each visual field, such that each
hemisphere would presumably receive a distinct half-image. Under these conditions, each hemisphere
apparently typically engages in “perceptual completion”, so that, using either hemisphere, a split-brain
subject will typically indicate having seen a normal and complete version of the half-image presented in
one visual field (rather than half an image, or a whole but chimeric image).11
Levy et al. found that the particular hemisphere that guided a subject’s response on a given trial
was primarily a factor of the kind of processing that the subject could use to complete the task required.
On some trials, following presentation of the target image, subjects were presented with an array of nonchimeric images as possible answer choices—an array that always contained the completed version of
each visual field’s half-image—and were asked to point to the image they’d just seen. This task could be
performed using a kind of perceptual matching, and in such cases subjects typically indicated the left
visual field’s (i.e. presumably the right hemisphere’s) target image. On other trials, subjects were not
presented with an array of images, and were instead simply asked to name the target they’d seen; under
such conditions subjects usually indicated the right visual field’s (i.e. presumably the left hemisphere’s)
10
All references to Levy et al. will be to this (1972) experiment.
Somewhat different results obtain for non-split subjects, who easily recognize the same chimeric images as
chimeras. Milner and Dunne (1977), however, showed that by removing the very central (vertical) portion of the
chimeras, so that there is no asymmetry visible just around the midline (because there is nothing visible at the
midline), non-split subjects, too, can be made to engage in perceptual completion, rather than immediately
recognizing the chimeras as chimeras. (After a time, non-split subjects will typically indicate having seen the two,
asymmetric halves of these midline-missing images, but it takes them a number of trials.)
11
12
target image, seemingly by attempting “a visual recognition based on nameable analytical features of the
stimulus” (Levy et al. 1972: 76). In this way, by varying the manner in which subjects were permitted to
respond, and thereby the kind of processing that could be used to perform the task, Levy and colleagues
could manipulate which hemisphere’s target image the subjects indicated having seen.
Under such conditions, and over the course of multiple trials, a split-brain subject’s behavior will
therefore suggest conscious duality. But were the two hemispheres ever conscious of their own visual
field’s image both at once? Or were the hemispheres merely taking turns being conscious—left
hemisphere conscious of its percept on those trials in which the subject is engaging in a processing task
for which that hemisphere dominates, and similarly for the right hemisphere? If the latter interpretation is
correct, then the apparent conscious duality manifested over the course of multiple trials would be mere
illusion, the result of a single conscious perspective with a varying content and physical “location.”
One might be inclined to argue, however, that a different trial type in the experiment provides
support for the duality model’s interpretation instead. In contrast to the trials just described (call those the
“non-interruption” trials), on some trials, just as a subject was about to respond in a fashion that had been
shown to bias processing towards one hemisphere, he or she would be interrupted by the experimenter
and asked to respond in a manner that biased processing towards the other hemisphere instead. (Call these
the “interruption” trials.) Typically, after being interrupted in this way, the subject would indicate the
second hemisphere’s target image. The subject would then be asked to go ahead and respond in the
manner he or she had initially intended. In three quarters of these trials, the subject would now indicate
the first hemisphere’s image. So on each of these “double-response,” interruption trials, each hemisphere
was apparently conscious of a percept to which the other hemisphere was not subject. At first glance, this
seems strong support for the duality model.
This first glance is too quick, however, for subjects were never asked to engage in both
processing types simultaneously—only sequentially. Right and left hemisphere consciousness of their
respective stimuli may therefore have been similarly sequential, rather than simultaneous. Moreover, on
these double-response trials, in which each hemisphere indicated its own percept, “the conflict between
the verbal and the manual responses became evident to the subjects and resulted in a considerable
perplexity and confusion” (Levy et al. 1972: 67). Yet in the large majority of non-interruption trials, trials
in which no double-response was given, no such perplexity was evidenced. On those trials, too, shouldn’t
each hemisphere have exhibited some displeasure, consternation, or confusion when the other hemisphere
picked what was, from its perspective, the wrong target? Unless, that is, only a single hemisphere was
13
ever conscious of its stimulus in these non-interruption trials—since in each of these trials the task could
be performed using only a single hemisphere’s preferred processing method.12
Unfortunately, we are provided with no detailed information about the time course of the
subjects’ responses on these crucial “interruption” trials. In order to support the “switching” interpretation
of subjects’ performance on these trials, according to which the first hemisphere is conscious of its
percept only before being interrupted and after being asked to continue, but not during the interruption
period, Levy (1990) cites the results of Sperling’s (1960) famous memory experiment, which some
believe show that a non-conscious perceptual representation of an item can become conscious once the
item is no longer perceptually available, as a result of subsequent attentional activation of the (heretofore
unconscious) representation. But (putatively) non-conscious visual representations can be attentionally
activated and thus made conscious for only a limited time period. It seems likely that this time period
would have been easily exceeded, in the Levy et al. experiment, during the interruptions, in which arrays
of answer choices are suddenly presented or removed, new requests made of and understood by the
subject, new responses prepared and then given. The continued availability of the first hemisphere’s
percept for conscious behavioral control after such interruptions, then, suggests that some degree of
independent right and left hemisphere attentional focus was maintained throughout these interludes. Yet it
was the absence of such independent attentional focus that was supposed to make plausible the absence of
dual conscious to begin with.
Meanwhile, while uncommon, double responses on non-interruption trials did occur. (The authors
note that three of four (right-handed) subjects “correctly described or named both left and right halves of
the composite stimuli” at a rate “well above a chance level” (Levy et al. 1972: 75); a fifth, left-handed
split-brain subject (P.D.) actually pointed to both visual field’s target patterns on 25% of (noninterruption) trials when using his right hand.) Obviously such trials suggest either both right and left
hemisphere consciousness even without experimentally-evoked interhemispheric switches in attention, or
intrahemispheric consciousness of both left visual field and right visual field targets (though in the latter
case one would then expect, among other things, that the subjects would be aware that the targets were
chimeras).
Some might then argue that in the final accounting it is simply unclear whether the central
findings in this experiment are best accounted for in terms of interhemispheric shifts of consciousness. At
least for the sake of argument, however, let us accept, going forward, that the experimental results
12
One might interject that even if this were correct, further argument would be needed to show that these sequential
right and left hemisphere percepts belonged to a single stream of consciousness across time—as the diachronic
aspect of Bayne’s switch model has them do. See Section 5.
14
described above are best accounted for in terms of interhemispherically switching consciousness of each
chimeric image. What challenges would this pose to the duality model?
This question is pressing because we can identify an explanatory component of the switch model
that is indeed plausible. That explanatory component is one according to which there are interhemispheric
fluctuations in split-brain consciousness as a result of interhemispheric fluctuations in attention. It seems
very likely that such fluctuations account for certain findings within the split-brain literature. For one
thing, the experimental results described above do appear to make a reasonably good case for this central
explanatory component of the switch model; they also don’t constitute the only data that it is tempting to
explain in terms of interhemispheric shifts in consciousness.13 Even Sperry himself—a famous proponent
of the conscious duality (or at times the partial unity) model—once remarked that:
Occasionally the commissurotomy subject may become so absorbed in a righthemisphere task that speech and other left-hemisphere functions are temporarily
depressed to the extent that one questions whether consciousness may not have
been shifted entirely to the one working hemisphere.
(Sperry 1977: 105)
This central explanatory component of the switch model is moreover made independently
plausible by evidence that the two “disconnected” hemispheres at least partly share a common pool of
attentional and processing resources (as mentioned in Section 2), since there is of course reason to believe
that there are important links between attention and consciousness (see Coslett, 1997, for review).
It is, however, essential to distinguish the central explanatory component of the switch model—
which postulates interhemispheric fluctuations in consciousness as a result of interhemispheric
competition for attention—from the switch model itself, according to which, as a result of
interhemispheric competition for attention, there are no failures of co-consciousness in the split-brain
subject. I have used the term“fluctuate” because it connotes intermediacy: fluctuations in consciousness
need not involve periods in which consciousness is totally absent, and accepting that such
interhemispheric fluctuations occur in the split-brain subject need not commit us to accepting that the two
hemispheres are never simultaneously yet separately conscious. Perhaps not just attention but
consciousness, too, admits of gradations, perhaps requiring some level of attentional activation (whether
an absolute threshold level, or a level sufficiently great relative to that obtaining elsewhere in the brain),
without necessarily requiring whatever level is typically associated with highly focused endogenous
attention, for instance. Alternatively or in addition, the hemispheres may be “unified” with respect to
I don’t take the time to discuss further such experimental results here, mainly in the interest of space, but also
because these experiments tend to have a good deal in common.
13
15
some attentional processes, or under some conditions or at some time scales, but not others, as mentioned
in Section 2. Studies on attentional mechanisms in split-brain subjects are complex and at times
contradictory, and of course we do not yet possess a clear model of the operation of attention in general.
It is also worth noting the possibility that the results obtained in experiments like that of the Levy
et al. study are to some extent artefactual. By design, the experiment created a situation in which, trial
after trial, one hemisphere was disposed to dominate performance over and over again. (Many split-brain
experiments have this feature.) That is, trials in which subjects were to respond verbally, predisposing a
left hemisphere response, were not mixed with trials in which they were to respond by pointing,
predisposing a right hemisphere response, except in occasional “interruption” trials. Instead, the trials
were grouped by response type. Such conditions—creating prolonged periods in which one hemisphere is
essentially encouraged to remain passive and just let the other hemisphere do its job—might effectively
encourage conscious processing in one hemisphere to basically shut down.
This feature of the experiment makes questionable the extent to which its results would generalize
to less artificial conditions. As Sperry noted, the apparent suppression of consciousness in one
hemisphere is most likely under experimental conditions involving “prolonged use of a single
hemisphere”:
In ordinary unrestricted behavior, on the other hand, it is rare that conditions
would thus selectively restrict sensory input or central processing to one
hemisphere for an extended period. Thus, typically, the two disconnected
hemispheres appear to be actively, but separately, conscious in parallel, each
working and contributing in its own way to the performance on which attention is
focused.
(1977: 114)
Of course, sensory input, though perhaps minimal, wasn’t restricted to only a single hemisphere
in this experiment, but central processing may well have been, and for an extended period of time.
Ordinary life presents a much richer, multi-sensory environment, however, and a much richer set of
behavioral options, and indeed a much richer set of perceptual, behavioral, and cognitive requirements. In
particular, ordinary life often presents us with situations that require both right hemisphere and left
hemisphere skills and specializations simultaneously, and for at least some stretch of time. These
considerations suggest that simultaneous and non-co-conscious experience is more likely to be the rule
than the exception, in split-brain subjects.
4.2
The challenge from unified behavior
Someone sympathetic to the line of argument I have been pushing, in favor of some sort of (synchronic)
conscious duality model, and moved by the empirical data I have cited in its support, might still worry
16
that the model is empirically inadequate with respect to split-brain behavior as a whole. “If both
hemispheres are generally simultaneously conscious, and yet independently so,” such a person might
object, “then how is one to account for the basically integrated nature of the subjects’ behavior?” In fact,
although the resistance to the conscious duality model has many sources, one of the simplest is the
generally “normal”-seeming nature of split-brain subjects’ behavior—something that the duality model in
particular might seem ill-equipped to explain, however well it accounts for behavior inside the laboratory.
Critics of the duality model have often supposed that the only significant resource defenders of
the duality model have available to them to meet this “challenge from unified behavior” is the
(hypothesized) duplication of conscious contents across the two hemispheres (especially in nonexperimental contexts). That they draw this conclusion is reasonable, since defenders of the conscious
duality model have in fact relied a good deal on this “conscious duplication” strategy of explanation.
Unfortunately, as Bayne notes, the strategy is a dubious one in certain respects, since it is arguable that
the two hemispheres of a split-brain subject will not be identified with the very same or even with highly
similar conscious contents, even in non-experimental situations.
There are a great number of things besides bi-hemispheric duplication of contents, however—and
besides interhemispheric co-consciousness—that could help account for the integrated nature of splitbrain subjects’ ordinary behavior. Simply having a single body significantly constrains the degree to
which an animal, regardless of the structure of its consciousness, can behave in a disunified fashion
(especially at a time). There are no doubt a great number of mental, but non-conscious, mechanisms that
contribute to unity in behavior as well (this may be the proper characterization of interhemispheric
response integration—on which, see discussion in Lambert and Naikar, 2000), and even sub-mental,
nervous system mechanisms that do the same (e.g., pattern generators in the spinal cord; Bogen 1990, pp.
220-221, offers further examples.) For that matter, a subject’s desire, sustained in either hemisphere or in
both, to behave in a rational and socially acceptable fashion, would also constrain the degree to which he
acts in a disunified manner.14 Then there is the possibility that one hemisphere’s behavioral commands
are sometimes simply suppressed, in favor of the other’s, in cases of conflict, without its conscious
experiences being similarly suppressed. Or the possibility (mentioned earlier) that neither hemisphere is
as active as it might otherwise be. Then there is the fact that the left hemisphere, anyway, appears
14
Actually I am reminded here of an anecdote Trevarthen relates regarding a split-brain baboon, whose two hands
were observed to engage in a “tug-of-war” over some food until the baboon looking down and, “startled,”
relinquished the food with one hand to the other (Trevarthen 1984, p. 333). I am obviously not suggesting that the
baboon was interested in behaving in a rational and socially acceptable manner, though these are interests that
human split-brain subjects do have, and that I believe do serve to integrate their behavior. So long as both
hemispheres of the baboon somehow identified with the baboon, however, either hemisphere’s coming to see that
the baboon already had the food in hand could serve to “re-establish” unified behavior (ibid)—regardless of how
many such events of identification there were.
17
strongly disposed to weave into a unified life story of its own making even conscious actions with origins
in exclusively right hemisphere experiences and intentions.
There can be no doubt that the integrated nature of split-brain subjects’ behavior poses a serious
challenge to the conscious duality model. I only mean to emphasize that the model still has a good
number of resources with which to attempt to meet that challenge—including, when necessary, instances
in which states of consciousness do in fact switch back and forth between the hemispheres.
This section and the previous were devoted to defending the synchronic conscious duality model.
Section 3 attempted to show that the synchronic switch model of split-brain consciousness cannot
accommodate a range of behavioral data on the subjects; the current section acknowledged that the
conscious duality model faces its own challenges, but sketched some of the ways a defender of that model
might endeavor to meet some of them. The next section turns to the structure of split-brain consciousness
across time, arguing that even if the synchronic aspect of the switch model were technically correct, the
best model of split-brain consciousness would still be offered by some version of the conscious duality
model.
5
Consciousness across time
I have up until this point addressed myself only to the synchronic aspect of the switch model, and
have argued that the model offers an inadequate picture of split-brain consciousness, considered at a
moment. In this section I explain why, even if the synchronic switch model were correct, and the two
hemispheres of a split-brain subject were never simultaneously, separately conscious, they would still
be associated with distinct streams of conscious experience.
The switch model, recall, is one according to which the two hemispheres of a split-brain subject
both contribute to or share a single stream of consciousness across time. In fact, the most salient feature of
the diachronic switch model is the likeness with which it depicts split-brain and “non-split”
consciousness: they are the same, structurally, so that, if a non-split subject has, over the course of a given
day, a single stream of consciousness whose contents change from moment to moment, then so, too, does
a split-brain subject. Bayne provides little in the way of positive argument for the diachronic aspect of his
switch model, but may believe (not unreasonably) that there is a strong general presumption in favor of
conscious unity, and that the argumentative burden therefore rests squarely on one who would deny that it
holds in some particular instance.
The unity relation between conscious phenomena that Bayne is concerned with remains a
phenomenal one (cf. Bayne 2008, p. 291), even when he turns his attention from synchronic to diachronic
co-consciousness. Other, non-phenomenal relations between conscious experiences over time are not
considered. While Bayne is of course free to examine what most interests him, I do have some prima facie
18
qualms about taking a purely phenomenal approach to the general question of the structure of split-brain
consciousness over time, in particular. We might expect two simultaneously phenomenally conscious
experiences that aren’t phenomenally unified to fail to be consciously “unified” in any sense. When
separated by a sufficient length of time, however, we do not expect two phenomenally conscious
experiences of, e.g., a single person, to be phenomenally unified, even if we expect there to be other
important “unity” relations between them.
The period of time over which co-consciousness does lend itself to a purely phenomenological
analysis may be an open question—and so, too, the existence and the extent of diachronic,
interhemispheric phenomenal unity in split-brain subjects. Both questions are of course largely empirical,
but there are also a host of subtle and complex theoretical issues here involving the individuation of
experiences that unfortunately cannot be treated satisfactorily in this work.15 Bayne’s thesis that splitbrain consciousness is generally unified over time does look to face at least one significant challenge even
on its own, purely phenomenal, terms, however: if consciousness switches between the two hemispheres
from moment to moment, even under conditions of perceptual lateralization, when the hemispheres
certainly sustain different conscious contents, and yet these different contents are all brought within a
single phenomenal stream of consciousness across time, then why don’t split-brain subjects report rapid
changes in what they’re conscious of? Why don’t the subjects say things like, “Well, a moment ago I
thought I saw a boat, but that’s gone now and instead I see a fork—oh, wait, no, there’s the boat again”?
Bayne acknowledges this difficulty, and does not claim to be able to dispose of it entirely, but
points out that there other disorders of consciousness, such as unilateral neglect, that are characterized in
part by an absence of awareness of any abnormality (cf. 2008: 298). This defensive move makes
particularly salient the analogy the switch model draws between the split-brain phenomenon and disorders
of attention, however. While the split-brain phenomenon may well come with impairments of attention, it
is questionable that it is, basically or at root, an attentional phenomenon, rather than a phenomenon
involving divisions in attentional as well perceptual, cognitive, and motor decisional processes—just as
the functional neuroanatomy of the “split” brain would suggest.16
If something like Lockwood’s (1989) partial unity model of split-brain consciousness is correct, for example, then
one aspect of diachronic phenomenal unity in split-brain subjects will be that such subjects undergo temporally
extended emotional experiences that are bihemispherically co-conscious. Yet whether Lockwood’s partial unity
model of split-brain consciousness does individuate experiences correctly is not obvious. (Shoemaker’s distinction
between core and total realizations, for instance, might raise doubts (1996: 194, fn 9).) See Hurley, 1998, 1994, for
discussion on the difficulty of choosing between the partial unity and the conscious duality model’s individuation of
experiences in the split-brain subject. Bayne (2008, 2010) discusses some of these same issues (see especially
Chapter 9 of his 2010).
16
The switch model, recall, “paints the split-brain patient as suffering from a kind of fluctuating perceptual
extinction” (Bayne, 2008: 294). In contrast, other researchers have suggested that one of the diagnostic differences
between “cortical disconnection syndromes” (such as the split-brain phenomenon) and the “much more commonly
occurring extinction or hemi-inattention deficits from a hemispheric lesion” is that the latter deficit is “typically one15
19
There is a deeper problem with taking a purely phenomenal approach to the general question of
split-brain consciousness across time, however. Such an approach appears to ignore a large part of what
matters in conscious unity over time, which has to do with certain kinds of causal or functional relations
that the conscious experiences of a person often stand in to each other, that they do not stand in to the
conscious experiences of another person. Those relations are surely not exclusively, or even primarily,
phenomenal. It is not even necessary to pinpoint very precisely what these relations are in order to
conclude that the structure of split-brain consciousness is importantly different from the structure of nonsplit consciousness.
Even if the hemispheres were never simultaneously conscious, each hemisphere would still
appear to be associated with a stream of conscious experience that was importantly psychologically
distinct from that of the other. To begin with, the two hemispheres would still be subject to different
conscious experiences. This would be not just for the trivial reason that (according to the synchronic
switch model) those experiences would exist at different times, but because those experiences are the
product of and are located within distinct mental systems, in this case distinct hemispheres—either one of
which could, on its own, support a mind “comfortably characterizable as human” (Marks, 1980: 47, fn
18)). The two hemispheres also differ with respect to cognitive and perceptual processing styles, in ways
that plausibly make a difference to the nature of the conscious experiences that each bears. They are
apparently capable of some independent agency; there are cases in which the left hemisphere literally
doesn’t know what the left hand is doing (see for example Sperry, 1968: pp. 725-726). There is evidence
that they have different emotional propensities, and that they have access to different memories. Perhaps
even different long-term dreams and goals in life.
Now presumably many of these differences are true of the hemispheres in a non-split brain as
well. In the non-split brain the hemispheres also appear to be integrated with respect to all of the features
just listed, however, in a way that they aren’t in the “split” brain; this is just what the results of the splitbrain experiments show.17 Thus even if we accepted the synchronic aspect of the switch model, it would
make less sense to speak of a split-brain subject possessing a single stream of consciousness whose neural
basis switches from one hemisphere to the other, from moment to moment, than it would to speak of the
sided” (Bogen, 1985: 315). Bogen also noted that, while the failure to respond to one of two bilaterally presented
stimuli is a feature of both the split-brain phenomenon and attentional neglect, the feature is thought to result, in the
former case, from a “relatively primitive sensory loss” (a lack of interhemispheric integration of visual information)
while “hemi-inattention is usually considered to be a higher-order derangement” (ibid).
17
Consider again the results of the Schiffer et al. (1998) experiment described in Section 3, for instance. Or consider
even the chimeric matching experiment, mentioned above (footnote 11), that Milner and Dunne (1977) ran using
non-split subjects. Perhaps on a typical trial using these “midline-blank” chimeric stimuli, a non-split subject is only
conscious of one visual field’s (and one hemisphere’s) stimulus at a time, just as is (plausibly) true of split-brain
subjects. It remains significant that when regular, “midline-complete” chimeric stimuli are used, the corpus callosum
appears to serve to create an interhemispheric, integrated conscious experience of a chimeric stimulus.
20
subject’s having two streams of consciousness, one associated with each hemisphere, that take turns even
existing. Or, to put it differently, there would still be two psychologically distinct streams of activity in
the “split” brain, even if (at any given moment) only one stream was running above the threshold of
consciousness.
This point about psychological distinctiveness explains why the split-brain cases have always
been taken to constitute a hard case not just for any account of the unity of consciousness, but also for any
account of their identity, and of ours, as persons. I do not argue that a split-brain subject is associated with
two persons, a claim whose truth would of course depend upon (among other things) the role of coconsciousness in personal identity. Something more than an exclusively phenomenal approach to
consciousness across time is necessary, however, to even explain why so many people have thought—
rightly or wrongly—that the puzzle of the split-brain phenomenon is partly one of personal identity.
They have thought so because a split-brain subject seems to live two conscious mental lives, each
with a degree of internal integrity and continuity not possessed by the two together. According to the
standard, i.e. the synchronic, conscious duality model, these are lived simultaneously. If the synchronic
switch model is correct, then they aren’t lived quite simultaneously: they occur over the same number of
days and months and years, but are not lived at the very same moments. They are nonetheless lives of
different emotional propensities, different memories, differing capacities for conscious reasoning; one is a
life of inner speech, a life of speaking and listening to oneself, and one is presumably a life of inner
silence, though perhaps rich in visual and other kinds of imagery (and perhaps auditory but non-verbal,
e.g. melodic, imagery). If Gazzaniga is right, one is a life narrated (or, a narrative life), and one is a life
simply lived (and perhaps lived more truthfully in certain respects; see discussion in Gazzaniga, 2000.)
This is extraordinary, and it requires psychological—and philosophical—accounting for.
If this is correct, it may significantly undercut the motivation for adopting even the synchronic
switch model of split-brain consciousness. (Though of course the model could still be correct for all that.)
A large part of the model’s appeal is that it ostensibly offers a way of avoiding having to provide such a
psychological accounting. To be sure, even if we accepted that consciousness perpetually switches back
and forth between the two “disconnected” hemispheres, such that it is never in both at once, we would
need to propose some neural mechanism to account for this. Compared to the difficulty of explaining
almost anything else about consciousness, however, that challenge does not appear great. What the switch
model implies is that there is no important psychological distinction between consciousness in the splitbrain subject and in anyone else: there is a single stream of consciousness per human being, any human
being, and it is realized dynamically in the brain, whatever brain you have, and what you are conscious of
depends upon the outcome of a competition for attentional resources, whoever you are, and. . . . And so
21
(the switch model seems to promise), there is no genuine psychological puzzle of split-brain
consciousness—thankfully, since we’ve already got quite enough puzzles of consciousness on our hands.
But we do have this puzzle on our hands, too: the possibility of two conscious mental lives
lived—either side by side, or neck and neck—within one body. And we have it whether it is correct that a
split-brain subject technically only has a single stream of conscious experience at a time, or, as I believe,
not.
6
Conclusion
Bayne has suggested that the split-brain phenomenon presents no counterexample to the claim that
consciousness is necessarily unified, and that split-brain consciousness is bihemispheric,
intrahemispherically unified at a moment, and interhemispherically unified across time. There is some
compelling evidence that interhemispheric fluctuations in consciousness do occur in split-brain subjects,
and the crucial explanatory element of the switch model will surely contribute to our ultimate
understanding of the split-brain phenomenon. The switch model does not obviate the attribution of
conscious duality to split-brain subjects, however. It is implausible to think that conscious activity in one
hemisphere consistently precludes independent conscious experience in the other. Moreover, even if the
hemispheres were never simultaneously conscious, each would still be associated with a psychologically
distinct stream of consciousness. One way or the other, then, the split-brain phenomenon is not just
another case of conscious unity.
22
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