Partial Unity of Consciousness: A Preliminary Defense

E. Schechter
Partial Unity of Consciousness

15

Partial Unity of Consciousness: A Preliminary Defense


Elizabeth Schechter

1 Introduction


Under the experimental conditions characteristic of the "split-brain"
experiment, a split-brain subject's conscious experience appears oddly
dissociated, as if each hemisphere is associated with its own stream of
consciousness. On the whole, however, split-brain subjects appear no
different from "normal" subjects, whom we assume have only a single stream
of consciousness. The tension between these impressions gives rise to a
debate about the structure of consciousness: the split-brain consciousness
debate.1
That debate has for the most part been pitched between two
possibilities: that a split-brain subject has a single stream of
consciousness, associated with the brain (or with the subject) as a whole,
or that she has two streams of consciousness, one associated with each
hemisphere. Considerably less attention has been paid to the possibility
that a split-brain subject has a single but an only partially unified
stream of consciousness, a possibility that has been articulated most
clearly by Lockwood (1989) (see also Trevarthen, 1974; Moor, 1982).
The partial unity model of split-brain consciousness is interesting
for reasons that extend beyond the split-brain consciousness debate itself.
Most saliently, the model raises questions about subjects of experience and
phenomenal perspectives, about the relationship between phenomenal
structure and the neural basis of consciousness, and about the place for
the type/token distinction in folk and scientific psychology.
This chapter examines two objections that have been raised to the
partial unity model, objections that presumably account for how relatively
little attention the model has received. Because I argue that neither of
these objections impugns the partial unity model in particular, the chapter
constitutes a preliminary defense of the partial unity model, working to
show that it is on par with its clearest contender, a version of the
conscious duality model.

2 The Split-Brain Consciousness Debate


The split-brain experimental paradigm typically involves carefully
directing perceptual information to a single hemisphere at a time, to the
extent possible. (See Lassonde & Ouiment, 2010, for a recent review.) This
is relatively simple to understand in the case of tactile perception.
Suppose you blindfold a split-brain subject (or in some other way obscure
his hands from his sight) and put an object in his left hand, say, a pipe.
Since patterned touch information transmits from each hand only to the
contralateral (opposite side) hemisphere (Gazzaniga, 2000, 1299), tactile
information about the pipe will be sent from the subject's left hand to his
right hemisphere (RH). In a "non-split" subject, the corpus callosum would
somehow transfer this information to, or enable access by, the left
hemisphere (LH) as well. In the split-brain subject, however, this tactile
information more or less stays put in the initial hemisphere that received
it. Meanwhile, in a large majority of the population, the right hemisphere
is mute. A split-brain subject is therefore likely to say, via his LH, that
he cannot feel and doesn't know what he is holding in his left hand. A few
minutes later, however, using the same left hand, and while still
blindfolded, the subject can select the object he was holding a minute ago
from a box of objects—showing that the object was not only felt but
recognized and remembered. The subject may even draw a picture of a pipe,
again using the left hand, which is under dominant control of the right
hemisphere (Levy, 1969). Visual, auditory, olfactory, pain, posture, and
temperature information may all be lateralized, to varying degrees, under
some conditions.
What makes such findings interesting for thinking about conscious
unity is this: On the one hand, a split-brain subject can respond to
stimuli presented to either hemisphere in ways that we think generally
require consciousness. On the other hand, a subject can't respond to
stimuli in the integrated way that we think consciousness affords, when the
different stimuli are lateralized to different hemispheres (or when a
response is elicited not from the hemisphere to which the stimulus was
presented, but from the other). For example, a very basic test for the
"split-brain syndrome" is a simple "matching" task in which the subject is
first required to demonstrate ability to recognize both RH-presented
stimuli and LH-presented stimuli by pointing to a picture of the referents
of the presented words, by drawing a picture, and so on. After
demonstrating this capacity, the subject is then finally asked to say
whether the two lateralized stimuli are the same or different. In the
paradigmatic case, the subject can perform the former, apparently much more
complex sort of task, but not the second, apparently simpler task. This is
what first suggests (obviously not conclusively), that the hemispheres
somehow have different streams of consciousness: after all, I could
demonstrate what I was conscious of and you could demonstrate what you were
conscious of, without either of us having any idea whether we were
conscious of the same thing.
Such results notwithstanding, a number of philosophers have defended
some kind of unity model (UM) of split-brain consciousness, according to
which a split-brain subject (at least typically) has a single stream of
consciousness. In the only version of the unity model invariably mentioned
in the split-brain consciousness literature, a split-brain subject has a
single stream of consciousness whose contents derive exclusively from the
left hemisphere. It's actually not clear that anyone ever defended this
version of the model; a couple of theorists (Eccles, 1973, 1965; Popper &
Eccles, 1977) are widely cited as having denied RH "consciousness," but
they may have been using the term to refer to what philosophers would call
"self-consciousness" (see especially Eccles, 1981). The simple difficulty
with that version of the UM is that a lot of RH-controlled behavior so
strongly appears to be the result of conscious perception and control. As
Shallice once said of RH-controlled performance on the Raven's Progressive
Matrices task (Zaidel, Zaidel, & Sperry, 1981):

If this level of performance could be obtained unconsciously, then it
would be really difficult to argue that consciousness is not an
epiphenomenon. Given that it is not, it is therefore very likely, if
not unequivocally established, that the split-brain right hemisphere
is aware. (Shallice, 1997, 264)

Contemporary versions of the unity model (Marks, 1981; Hurley, 1998; Tye,
2003; Bayne, 2008) in fact all assume that conscious contents derive from
both hemispheres. I will make this same assumption in this paper.2
The major alternative to the unity model is the conscious duality
model (CDM). According to the CDM, a split-brain subject has two streams of
consciousness, each of whose contents derive from a different hemisphere.
This model appealed particularly to neuropsychologists (e.g., Gazzaniga,
1970; Sperry, 1977; LeDoux, Wilson, & Gazzaniga, 1977; Milner, Taylor, &
Jones-Gotman, 1990; Mark, 1996; Zaidel et al., 2003; Tononi, 2004), but
several philosophers have defended or assumed it as well (e.g., Dewitt,
1975; Davis, 1997).
Since both the CDM and contemporary versions of the UM allow that
conscious contents derive from both hemispheres, what is at issue between
them is whether or not RH and LH experiences are unified or co-conscious
with each other—that is, whether they belong to one and the same or to two
distinct streams of consciousness. Unsurprisingly, there is disagreement
about what co-consciousness (or conscious unity) is, and whether there is
even any single relation between conscious phenomena that we mean to refer
to when speaking of someone's consciousness as being "unified" (Hill, 1991;
Bayne & Chalmers, 2003; Tye, 2003; Schechter, forthcoming b). It is
nonetheless possible to articulate certain assumptions we make about a
subject's consciousness—assumptions concerning conscious unity—that appear
to somehow be violated in the split-brain case. As Nagel says, we assume
that, "for elements of experience … 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,
407). We might express this assumption by saying that we assume that all of
the (simultaneously) conscious experiences of a subject are co-accessible.
Marks, meanwhile, notes that we assume that two experiences "belong to the
same unified consciousness only if they are known, by introspection, to be
simultaneous" (1981, 13). That is, we assume that any two simultaneously
conscious experiences of a subject are ones of which the subject is (or can
be) co-aware. Finally, we assume that there is some single thing that it is
like to be a conscious subject at any given moment, something that
comprises whatever multitude and variety of experiences she's undergoing
(Bayne, 2010). We assume, that is, that at any given moment, any two
experiences of a subject are co-phenomenal.
Although the split-brain consciousness debate and this paper are most
centrally concerned with co-phenomenality, I will basically assume here
that whenever two (simultaneously) phenomenally conscious experiences are
either co-aware or co-accessible, then they are also co-phenomenal. (This
assumption may be controversial, but its truth or falsity does not affect
the central issues under consideration in this chapter, so long as we view
these relations as holding of experiences rather than contents; see
Schechter, forthcoming a.) For simplicity's sake, I will focus only on
synchronic conscious unity—the structure of split-brain consciousness at
any given moment in time—to the extent possible. Accordingly, I will speak
simply of the co-consciousness relation (or conscious unity relation) in
what follows.3
Let us say that streams of consciousness are constituted by
experiences and structured by the co-consciousness relation.4 According to
the unity model of split-brain consciousness, a split-brain subject has a
single stream of consciousness: right and left hemisphere experiences are
co-conscious, in other words. According to the conscious duality model, co-
consciousness holds intrahemispherically but fails interhemispherically in
the split-brain subject, so that the subject has two streams of
consciousness, one "associated with" each hemisphere.
Despite their disagreements, the CDM and the UM share a very
fundamental assumption: that co-consciousness is a transitive relation. In
this one respect, these two models have more in common with each other than
either of them does with the partial unity model (PUM). The PUM drops the
transitivity assumption, allowing that a single experience may be co-
conscious with others that are not co-conscious with each other. Streams of
consciousness may still be structured by co-consciousness, but it is not
necessary that every experience within a stream be co-conscious with every
other. In this model, then, conscious unity admits of degrees: only in a
strongly unified stream of consciousness is co-consciousness transitive.
According to both the UM and the CDM, then, a split-brain subject has some
whole number of strongly unified streams of consciousness, while according
to the PUM, a split-brain subject has only a single but only partly (or
weakly) unified consciousness.
Note that because there are several possible notions of conscious
unity, there are other possible partial unity models. The truth is that
conscious unity is (to borrow Block's [1995] term) a "mongrel concept"
(Schechter, forthcoming b); when we think of what it is to have a "unified"
consciousness, we think of a whole host of relations that subjects bear to
their conscious experiences and that these experiences bear to each other
and to action. Talk of a "dual" consciousness may connote a breakdown of
all these relations simultaneously. In reality, though, these relations may
not stand or fall all together; in fact, upon reflection, it's unlikely
that they would. One intuitive sense of what it means to have a partially
unified consciousness, then, is a consciousness in which some of these
unity relations still hold, and others do not (Hill, 1991).
This is not what I mean by a "partially unified consciousness,"
however. In one possible kind of partial unity model, some conscious unity
relations, but not others, hold between experiences. In the kind of partial
unity model under consideration here, conscious unity relations hold
between some experiences, but not between others. This point will be
crucial to understanding the choice between the PUM and the CDM.5
The PUM of split-brain consciousness has several prima facie
strengths. Most obviously, it appears to offer an appealingly intermediate
position between two more extreme models of split-brain consciousness. The
UM must apparently implausibly deny failures of interhemispheric co-
consciousness; the CDM is apparently inconsistent with the considerable
number of cases in which it is difficult or impossible to find evidence of
interhemispheric dissociation of conscious contents.
The PUM that I will consider also makes some kind of
neurophysiological unity the basis for conscious unity. Against those who
would claim that splitting the brain splits the mind, including the
conscious mind, some philosophers argued that a putatively single stream of
consciousness can be "disjunctively realized" (Marks, 1981; Tye, 2003).
Lockwood's defense of the PUM in contrast appeals explicitly to the fact
that the "split" brain is not totally split, but remains physically intact
beneath the cortical level: the cortically disconnected right and left
hemisphere are therefore associated with distinct conscious experiences
that are not (interhemispherically) co-conscious; nonetheless, these are
all co-conscious with a third set of subcortically exchanged or
communicated conscious contents. Many will be attracted to a model that
makes the structure of consciousness isomorphic to the neurophysiological
basis of consciousness in this way (Revonsuo, 2000).6
Another significant source of the PUM's appeal is its empirical
sensitivity or flexibility, in a particular sense. Lockwood sought to
motivate the PUM in part by considering the possibility of sectioning a
subject's corpus callosum one fiber at a time, resulting in increasing
degrees of (experimentally testable) dissociation. Would there be some
single fiber that, once cut, marked the transition from the subject's
having a unified to a dual consciousness? Or would the structure of
consciousness change equally gradually as did the neural basis of her
conscious experience? Lockwood implies that nothing but a pre-theoretic
commitment to the transitivity of co-consciousness would support the first
answer, and simply notes that "there remains something deeply
unsatisfactory about a philosophical position that obliges one to impose
this rigid dichotomy upon the experimental and clinical facts: either we
have just one center, or stream, of consciousness, or else we have two (or
more), entirely distinct from each other" (Lockwood, 1989, 86).
Lockwood's thought experiment is in fact not wholly fictitious:
callosotomy became routinely performed in stages, with predictable degrees
and sorts of dissociation evident following sections at particular callosal
locations (e.g., Sidtis et al., 1981). "Partially split" subjects really do
seem somehow intermediate between "nonsplit" and (fully) "split-brain"
subjects. Surely one appealing characterization of such subjects is that
the structure of their consciousness is intermediate between (strongly)
unified and (wholly) divided or dual.
In light of the apparent strengths of the PUM, it should be puzzling
how little philosophical attention it has received. Those who have
discussed the model, however, have not been enthusiastic. Hurley (1994)
suggested that there could be no determinate case of partial unity of
consciousness; Nagel suggested that even if empirical data suggested
partial unity, the possibility would remain inconceivable (and thus
unacceptable) from the first-person and folk perspective (1971, 409–410);
Bayne (2008, 2010) has questioned whether the model is even coherent.
Indeed, Lockwood himself at one point admitted that "in spite of having
defended it in print, I am still by no means wholly persuaded that the
concept of a merely weakly unified consciousness really does make sense"
(1994, 95).7
Of the philosophers just mentioned, Nagel, Bayne, and Lockwood (as
well as Dainton, 2000,) have been concerned, first and foremost, with what
I call the inconceivability challenge. Their charge is, at minimum, that a
partially unified consciousness is not possibly imaginable. Hurley's
indeterminacy charge, meanwhile, is that "no … factors can be identified
that would make for partial unity" (1998, 175) as opposed to conscious
duality.
At a glance, these two objections to the PUM look to be in some
tension with each other: the indeterminacy challenge suggests that the PUM
is in some sense equivalent to (or not distinguishable from) the CDM,
while, according to the inconceivability objection, the PUM is somehow
uniquely inconceivable. Deeper consideration, however, reveals that the two
objections are importantly related. The inconceivability objection is
rooted in the fact that there is nothing subjectively available to a
subject that makes her consciousness partially unified as opposed to dual;
the indeterminacy challenge adds that there is nothing objective that would
make it partially unified either. Taken together, these concerns may even
imply that there is no such thing as a partial unity model of
consciousness.
Sections 4 and 5 address these twin objections, ultimately arguing
that they do not and cannot work against the PUM in the way its critics
have thought. The conclusion of the chapter is that the PUM is a distinct
model, and one that deserves the same consideration as any other model of
split-brain consciousness. In the next section, I will lay out what is most
centrally at issue between these models.

3 Experience Types and Token Experiences


The central challenge for the CDM has always been to account for the
variety of respects in which split-brain subjects appear to be "unified."
First of all, split-brain subjects don't seem that different from anyone
else: while their behavior outside of experimental conditions isn't quite
normal (Ferguson, Rayport, & Corrie, 1985), it isn't incoherent or wildly
conflicted. Second of all, even under experimental conditions,
bihemispheric conscious contents don't seem wholly dissociated. Via either
hemisphere, for instance, a split-brain subject can indicate certain
"crude" visual information about a stimulus presented in a given visual
field (Trevarthen & Sperry, 1973; though see also Tramo et al., 1995).
Similarly, although finely patterned tactile information from the hand
transmits only contralaterally, "deep touch" information (sufficient to
convey something about an object's texture, and whether it is, say, rounded
or pointed) transmits ipsilaterally as well. As a result, in such cases,
one apparently speaks of what the subject (tout court) sees and feels,
rather than speaking of what one hemisphere or the other sees or feels, or
of what the subject sees and feels via one hemisphere or the other.
Proponents of the CDM, however, have always viewed it as compatible
with the variety of respects in which split-brain subjects appear
"unified." Of course a split-brain subject seems to be a single thinker: RH
and LH have the same memories and personality by virtue of having the same
personal and social history, and so on. And of course split-brain subjects
typically behave in an integrated manner: especially outside of
experimental situations, the two streams of consciousness are likely to
have highly similar contents. In other words, proponents of the CDM long
appealed to interhemispheric overlap in psychological types, while
maintaining that the hemispheres are subject to distinct token mental
phenomena.
A primary reason for the persistence of the debate between the CDM and
the UM is that proponents of the CDM have readily availed themselves of the
type-token distinction in this way. Accordingly, the version of the CDM
that has been defended by neuropsychologists in particular is one in which
a split-brain subject has two entirely distinct streams of conscious
experiences, but with many type- (including content-) identical experiences
across the two streams. Call this the conscious duality (with some
duplication of contents) model, or CDM-duplication. Proponents of the UM
have meanwhile sometimes responded by arguing that there is no room for the
type-token distinction in this context. (See Schechter, 2010, responding to
Marks, 1981, and Tye, 2003, on this point; for a different version of this
objection to the CDM, see Bayne, 2010.) At around this point in the
dialectic, very deep questions arise about, among other things, the nature
of subjects of experience (Schechter, forthcoming a), and it is not clear
how to resolve them.
Let's look at an example. In one experiment, a split-brain subject had
an apparently terrifying fire safety film presented exclusively in her LVF
(to her RH). After viewing, V.P. said (via her LH) that she didn't know
what she saw—"I think just a white flash," she said, and, when prompted
further, "Maybe just some trees, red trees like in the fall." When asked by
her examiner (Michael Gazzaniga) whether she felt anything watching the
film, she replied (LH), "I don't really know why but I'm kind of scared. I
feel jumpy. I think maybe I don't like this room, or maybe it's you. You're
getting me nervous." Turning to the person assisting in the experiment, she
said, "I know I like Dr. Gazzaniga, but right now I'm scared of him for
some reason" (Gazzaniga, 1985, 75–76).
In this case, there appeared to be a kind of interhemispherically
common or shared emotional or affective experience. (And, perhaps, visual
experience.) But here the defender of the CDM will employ the type/token
distinction: what was common to or shared by V.P.'s two hemispheres was, at
most, a certain type of conscious emotional or affective (and perhaps
visual) experience—but each hemisphere was subject to its own token
experience of that type. Perhaps, for instance, interhemispheric transfer
of affect or simply bihemispheric access to somatic representations of
arousal meant that each hemisphere generated and was subject to an
experience of anxiety while V.P. (or her RH) watched the film—but if so,
then there were two experiences of anxiety.
Of course, if there really was an RH conscious visual experience of
the fire safety film that was not co-conscious with, say, an LH auditory
experience of a stream of inner speech that the LH was simultaneously
engaging in ("What's going on over there? I can't see anything?"), then
someone who accepts the transitivity principle has to resort to some kind
of strategy like this. If the RH experience and the LH experience are not
co-conscious, then they cannot belong to the same stream of
consciousness—even if both are co-conscious with an emotional or affective
experience of anxiety.
Because the PUM drops the transitivity principle, however, it can take
unified behavior and the absence of conscious dissociation at face value.
According to the PUM, the reason V.P. was able to describe, via her left
hemisphere, the feeling the anxiety that her RH was (presumably) also
experiencing was because V.P. really had a single token experience of
anxiety, co-conscious with all of her other token experiences at that time.
More generally, wherever the CDM posits two token experiences with a
common content (figure 15.1), the PUM posits a single token experience with
that content (figure 15.2). To put it differently, where there is no
qualitative difference between contents, the PUM posits no numerically
distinct experiences.

[Insert Figures 15.1 and 15.2 near here]


4 The Inconceivability Objection


As Bayne points out (2008, 2010), the inconceivability objection says more
than that we cannot imagine what it's like to have a partially unified
consciousness. After all, there may be all kinds of creatures whose
conscious experience we cannot imagine (Nagel, 1974) simply because of
contingent facts about our own perceptual systems and capacities. According
to the inconceivability objection, there is nothing that would even count
as successfully imagining what it is like to have partially unified
consciousness.
Why should this objection face the PUM uniquely? After all, we cannot
imagine what it would be like to have (simultaneously) two streams of
consciousness, either. This follows from the very concept of co-
consciousness: two experiences are co-conscious when there is something it
is like to undergo them together. Failures of co-consciousness, in general
then, are not the kinds of things for which there is anything that it's
like to be subject to them (see Tye, 2003, 120). (More on subjects of
experience below.)
As Tye (2003, 120) notes, there is of course a qualified sense in
which one can imagine having two streams of consciousness: via two
successive acts of imagination. That is, one can first imagine what it's
like to have the one stream of consciousness and then imagine what it's
like to have the other. There is just no single "experiential whole"
encompassing both imaginative acts, for the experiences in the two streams
aren't "together" in experience, in the relevant, phenomenological sense.
We could say, if we wanted, that having multiple streams of consciousness
is sequentially but not simultaneously imaginable.
These same remarks apply to the PUM as well, however. Consider figure
15.2 again. We can first imagine what it's like to undergo experiences E1
and E2 together, and can then imagine what it's like to undergo E2 and E3
together. There is just no single "experiential whole" encompassing E1, E2,
and E3, because neither E1 and E3 nor their contents, A and C, are together
in experience in the relevant, phenomenological sense. Thus having
partially unified consciousness is also sequentially if not simultaneously
imaginable.
On the face of it, then, the inconceivability objection should face
the PUM and the CDM equally. The objection concerns what it's like to be
conscious—a subjective matter—and there is nothing in the phenomenology of
conscious duality or partial unity to distinguish them. The PUM and the CDM-
duplication differ with respect to whether the experience that is carrying
the content B and that is co-conscious with the experience that is carrying
the content A, is the very same experience as the experience that is
carrying the content B that is co-conscious with the experience that is
carrying the content C. They differ, that is, with respect to whether the
experience that is co-conscious with E1 is the very same (token) experience
as the experience that is co-conscious with E3. This is a question about
the token identities of experiences, and as Hurley (1998) notes, the
identities of experiences are not subjectively available to us.8
The inconceivability objection concerns the phenomenality or
subjective properties of experience, but there is no phenomenal, subjective
difference between having two streams of consciousness and having a single
but only weakly unified stream of consciousness. Why, then, have critics of
the PUM—and even its major philosophical proponent (Lockwood, 1994)—found
the PUM somehow uniquely threatened by the objection?
I think that the reason has to do with personal identity. In ordinary
psychological thought, the individuation of mental tokens, including
conscious experiences, is parasitic upon identifying the subject whose
experiences they are, so that if there is a single subject, for example,
feeling a twinge of pain at a given time, there is one experience of pain
at that time; if there are two subjects feeling (qualitatively identical)
pains at that time, then there are two experiences of pain, and so on. The
problem is that our thinking about experience is so closely tied to our
thinking about subjects of experience that whether or not the "divided"
hemispheres are associated with distinct subjects of experience seems just
as uncertain as whether or not they share any (token) conscious
experiences.
Precisely because we ordinarily individuate conscious experiences by
assigning them to subjects, one natural interpretation of the CDM has
always been that the two hemispheres of a split-brain subject are
associated not only with different streams of consciousness but with
different subjects of experience (or "conscious selves," e.g., Sperry,
1985). If that interpretation is correct, then no wonder split-brain
consciousness is only sequentially imaginable: when we imagine a split-
brain human being's consciousness, we must in fact imagine the perspectives
of two different subjects of experience in turn. The PUM has instead been
interpreted as positing a single subject of experience with a single stream
of consciousness—but one whose consciousness is not (simultaneously)
imaginable.
There must at least be two subjective perspectives in the conscious
duality case (figure 15.3) because the co-consciousness relation is itself
one that appeals to falling within such a perspective. (Think about the
origins of this "what it's like" talk!; Nagel, 1974.) An experience is
conscious if and only if it falls within some phenomenal perspective or
other; two experiences are co-conscious if and only if they fall within the
same phenomenal perspective, if there is some perspective that "includes"
them both. Now, either subjects of experience necessarily stand in a one-to-
one with phenomenal perspectives, or they do not. We might understand
subjects of experience in such a way that a subject of experience
necessarily has a (single) phenomenal perspective at a time. If this is the
case, then the CDM posits two subjects of experience, each of whose
perspectives is (it would seem) perfectly imaginable. Alternatively we
might let go of the connection between subjects of experience and
phenomenal perspectives. If so, then the CDM may posit a single subject of
experience with two phenomenal perspectives. If we pursue this second
course, then we cannot imagine what it is like to be such a subject of
experience—but this is unsurprising, since we have already forgone the
connection between being a subject of experience and having a phenomenal
perspective.
As before, however, these remarks apply equally to the PUM. The PUM
also posits two phenomenal perspectives, for again failures of co-
consciousness—even between two experiences that are mutually co-conscious
with a third—mark the boundaries of such perspectives. Only and all those
experiences that are transitively co-conscious with each other fall within
a single phenomenal perspective (figure 15.4).

[Insert Figure 15.3 and 15.4 near here]

(As before, the solid lines signify co-consciousness; each dashed oval
circumscribes those experiences that fall within a single subjective
perspective.)
Once again, we can relinquish the connection between being a subject
of experience and having a single phenomenal perspective, in which case we
can't imagine what it's like to be the subject with the partially unified
consciousness, but in which case, again, we've already forgone the
commitment to there being something it's like to be her. Alternatively, we
can insist upon a necessary connection between being a subject of
experience and having a phenomenal perspective—but then the PUM must also
posit two subjects of experience within any animal that has a partially
unified consciousness. And we can imagine the perspective of either of
these subjects of experience.9
Whichever model we accept—that shown in figure 15.3 or in figure
15.4—and whether we identify, for example, the split-brain subject as a
whole with a subject of experience or not, the entity to which we would
ascribe E1 and E3 in the figures above—the subject in the organismic
sense—is not something that has a phenomenal perspective—not in the
ordinary sense in which we speak of subjects "having" such perspectives.
These remarks suggest an attenuated sense in which the two models can
be distinguished on subjective grounds. On the one hand, there is no
difference between what it's like to have a partially unified consciousness
versus what it's like to have two streams of consciousness because there is
nothing—no one thing—that it is like to have either of those things. But
there is a difference between the models with respect to the role they make
for phenomenal perspectives in individuating experiences. Because streams
of consciousness are strongly unified, according to the CDM, an
experience's token identity may depend upon the phenomenal perspective that
it falls within (or contributes to). The PUM forgoes this dependence: there
can be multiple phenomenal perspectives associated with the same stream of
consciousness, and a single experience can fall within multiple phenomenal
perspectives.
The strength of the conceptual connection between experiences and
phenomenal perspectives is certainly a consideration that speaks against
the PUM. What remains open, however, is whether other considerations could
outweigh this one. For the reasons I go on to explain in the next section,
I agree with Lockwood that this is at least possible.
For now, the important point is that the distinction between subjects
of experience and subjective perspectives undercuts the force of the
inconceivability objection. Consider figure 15.2 again. According to the
PUM, the experience that is co-conscious with the experience of A (with E1,
in other words) and the experience that is co-conscious with the experience
of C (with E3, in other words) is one and the same experience. Since the
experience nonetheless contributes to two distinct phenomenal perspectives,
there is nothing subjective that makes it true that there is just one
experience with that content. It must therefore be an objective fact or
feature that makes it the case that the experience that is co-conscious
with E1 is one and the same as the experience that is co-conscious with E3.
So long as there are properties of experiences that are not
subjectively available to us, there is, on the face of it, no reason to
think that there could not be any such feature or fact. According to the
indeterminacy objection, however, this is just the situation that the PUM
is in. That is, there is no fact or feature—subjective or objective—that
could make it true that the experience that is co-conscious with E1 is the
experience that is co-conscious with E3. I turn to this objection next.

5 The Indeterminacy Objection


Where the CDM posits two token experiences with a common content, the PUM
posits a single token experience with that content. This is where the
threat of indeterminacy gets its grip: what would make it the case that a
subject had a single token experience that was co-conscious with others
that were not co-conscious with each other (figure 15.6)—rather than a case
in which the subject had two (or more) streams of consciousness, but with
some overlap in contents (figure 15.5)?

[Insert Figure 15.5 and 15.6 near here]

The conscious duality model and the partial unity model agree that
wherever there is a dissociation between contents, there is a failure of co-
consciousness between the vehicles or experiences carrying those contents.
The models differ with respect to what they say about nondissociated
contents: according to the PUM, interhemispherically shared contents are
carried by interhemispherically shared experiences; according to the CDM-
duplication, they are not.
Neuropsychologists apparently recognized these as distinct
possibilities. Sperry, for instance, once commented, "Whether the neural
cross integration involved in … for example, that mediating emotional tone,
constitutes an extension of a single conscious process [across the two
hemispheres] or is better interpreted as just a transmission of neural
activity that triggers a second and separate bisymmetric conscious effect
in the opposite hemisphere remains open at this stage" (Sperry, 1977, 114).
Sperry implies, here, that whether a subject like V.P. (sec. 3) has one or
two experiences of anxiety is something we simply have yet to discover.
Hurley (1998), however, suggested that the difficulty of
distinguishing between partial unity of consciousness and conscious duality
with some duplication of contents is a principled one. According to Hurley,
the problem is not at base epistemic, but metaphysical: there is nothing
that would make a subject's consciousness partially unified, as opposed to
dual but with some common contents. The PUM thus stands accused, once
again, of unintelligibility:

What does the difference between these two interpretations [partial
unity of consciousness versus conscious duality with some duplication
of contents] amount to? There is no subjective viewpoint by which the
issue can be determined. If it is determined, objective factors of
some kind must determine it. But what kind? … Note the lurking threat
of indeterminacy. If no objective factors can be identified that would
make for partial unity as opposed to separateness with duplication,
then there is a fundamental indeterminacy in the conception of what
partial unity would be, were it to exist. We can't just shrug this off
if we want to defend the view that partial unity is intelligible.
(1998, 175)

The difficulty of conceptualizing the difference between partial unity and
conscious duality with some duplication of contents is rooted in the
purposes to which the type/token distinction is ordinarily put.
Generalizations in psychology—whether folk or scientific—are
generalizations over psychological types, including contents (Burge, 2009,
248). Mental tokens are just the instantiation of those properties or types
within subjects. We assume that two subjects can't share the same mental
token, so if they both behave in ways that are apparently guided by some
mental content, we must attribute to each of them a distinct mental token
with that content. That is: what entokenings of contents explain is the
access that certain "systems"—in ordinary thought, subjects—have to those
contents.
The problem is that both the PUM and the CDM-duplication allow that
the right and left hemisphere of a split-brain subject have access to some
of the same contents. Indeed, while disagreeing about how to individuate
tokens, the PUM and the CDM-duplication could in principle be in perfect
agreement about which systems have access to which information, and about
what role this shared access to information plays in behavioral control. In
that case, there would be no predictive or explanatory work, vis-à-vis
behavior, for the type/token distinction to do.
Suppose, for the sake of argument, that this is right, and that the
two models are predictively equivalent vis-à-vis behavior. I have already
argued that they are subjectively indistinguishable as well. Are there any
other grounds for distinguishing partial unity from conscious duality with
some duplication of contents?
The most obvious possibility is that some or other neural facts will
"provide the needed objective basis for the distinction" (Hurley, 1998,
175). In the early days of the split-brain consciousness debate,
consciousness was usually assumed to be a basically cortical phenomenon so
that the neuroanatomy of the callosotomized brain was taken to support the
conscious duality model. Tides have changed, however, and by now the
"split" brain, which of course remains physically intact beneath the
cortical level, might be taken to provide prima facie support for the claim
that split-brain consciousness is partially unified as well.
Although my reasons for thinking so differ from hers, I agree with
Hurley that the structure of consciousness cannot be read off
neuroanatomical structure so straightforwardly. To start with, although
subcortical structures are (usually) left intact by split-brain surgery,
subcortico-cortical pathways may still be largely unilateral. Indeed so far
as I know, this is largely the case for individual pathways of, for
example, individual thalamic nuclei, though subcortico-cortical pathways
taken collectively may still ultimately terminate and originate
bilaterally.10
Furthermore, although structural connectivity is a good guide to
functional connectivity, the latter is what we are really interested in.
Now, given how intimately subcortical activities are integrated with
cortical activities in the human brain, it is of course natural to
hypothesize that the physical intactness of subcortical structures in the
"split" brain provides the basis for whatever kind or degree of
interhemispheric functional connectivity is needed for conscious unity. On
the other hand, one could apparently reason just as well in the opposite
direction: given how intimately subcortical activities are integrated with
cortical activities, it is reasonable to suspect that a physical (surgical)
disruption of cortical activities creates a functional disruption or
reorganization of activity even at the subcortical level. Johnston et al.
(2008), for instance, found a significant reduction in the coherence of
firing activity not just across the two hemispheres of a recently
callosotomized subject, but across right and left thalamus, despite the
fact that the subject's thalamus was structurally intact.
What we will ultimately need, in order to determine the side on which
the neural facts lie in this debate, is a developed theory of the phenomena
of interest—consciousness and conscious unity—including a theory of their
physical basis. It is only against the background of such a theory that the
relevance of any particular neural facts can be judged, and, of course,
only against the background of such a theory that those facts could make it
intelligible that the experience that is co-conscious with E1 is the
experience that is co-conscious with E3. Suppose, for instance, that we
found the neural basis of the co-consciousness relation: suppose we find
the neurophysiological relation that holds between neural regions
supporting co-conscious experiences, and found that that relation holds
between the region supporting consciousness of B on the one hand and the
regions supporting consciousness of A and of C on the other. That discovery
would weigh in favor of the PUM. But we would first have needed to have a
theory of the co-consciousness relation, and we would need to have had some
prior if imperfect grip on when experiences are and aren't co-conscious.
Thus, for example, Tononi (2004), who views thalamocortical interactions as
a crucial part of the substrate of consciousness, also believes that the
split-brain phenomenon involves some conscious dissociation, and this is
because Tononi makes the integration of information the basis (and purpose)
of consciousness. Behavioral evidence meanwhile strongly suggests that
there is more intrahemispheric than interhemispheric integration of
information in the split-brain subject. Depending on whether conscious
unity requires some absolute degree of informational integration or instead
just some relatively greatest degree, split-brain consciousness could be
revealed to have been dual or partially unified.
In her discussion of whether the PUM can appeal to neural facts to
defeat the indeterminacy objection, Hurley considers neuroanatomical facts
are the only class of neural facts that Hurley considersalone. I think
there is a dialectical explanation for this: Lockwood himself motivates the
PUM by appealing to neuroanatomical facts specifically, and of course the
(very gross) neuroanatomy of the "split" brain is relatively simple to
appreciate. In the long run though, we will have various facts about neural
activity to adjudicate between the PUM and the CDM as well. Consider recent
fMRI research investigating the effects of callosotomy on the bilateral
coherence of resting state activity. Now as it happens, these studies have
thus far yielded conflicting results. Johnston et al. (2008) (cited above)
found a significant reduction in the coherence of firing activity across
the two hemispheres following callosotomy, while Uddin et al. (2008) found
a very high degree of bihemispheric coherence in a different subject.11
Suppose, however, that one or the other finding were replicated across a
number of subjects. This is just the kind of finding that could weigh in
favor of one model or the other—assuming some neurofunctional theory of
consciousness according to which internally generated, coordinated firing
activity across wide brain regions serves as the neural mechanism of
consciousness.
Hurley herself has fundamental objections to the notion that neural
structure might make it the case that a subject's consciousness was
partially unified. On the basis of considerations familiar from the
embodied/extended mind view, she argues that the very same neuroanatomy may
be equally compatible with a dual and a unified consciousness. (Though I
don't know if she would say that all neural properties—not just those
concerning anatomy—are so compatible!) A discussion of the
embodied/extended mind debate would take us too far afield here. Suffice it
to say that the position Hurley espouses is controversial from the
perspective of the ongoing science of consciousness, and, as for a science
of conscious unity, "it seems to me that the physical basis of the unity of
consciousness should be sought in whatever we have reason to identify as
the physical substratum of consciousness itself" (Lockwood, 1994, 94).
Still, whether our best-developed theory of consciousness will necessarily
be a theory of the brain is admittedly itself an empirical question.

6 Principles of Conscious Unity


According to the indeterminacy objection, there is nothing that would make
it the case that a subject's consciousness was partially unified.
Unfortunately, it is not possible, at present, to respond to the objection
by stating what would. I have argued that if we had an adequate theory of
the phenomenon of interest, we could use it to adjudicate the structure of
consciousness in hard cases. Because we don't yet have such a theory, this
response, however persuasive in principle, is not fully satisfying at
present.
I will therefore conclude by offering a very different kind of
response to the indeterminacy objection. The basic thought will be that the
indeterminacy objection is neutral or asymmetric between the PUM and the
CDM-duplication: that is, the PUM is no more vulnerable to the objection
than is the CDM-duplication. If that is right, then the objection cannot
work to rule out the PUM since it can't plausibly rule out both models
simultaneously.
Even on the face of things, it is puzzling that the PUM should be
uniquely vulnerable to the indeterminacy objection, since what is
purportedly indeterminate is whether a given subject's consciousness is
partially unified or dual with some duplication of contents. In that case,
shouldn't the CDM be just as vulnerable to the objection? Why does Hurley
(apparently) think otherwise?
Hurley might respond that there are at least hypothetical cases
involving conscious dissociation for which the PUM isn't even a candidate
model, cases that are thus determinately cases of conscious duality. These
are cases in which there are no contents common to the two streams. Perhaps
this suffices to make the CDM invulnerable (or less vulnerable somehow) to
the indeterminacy objection.
The version of the CDM under consideration here, however—and the
version that has been popular among neuropsychologists—is one that does
posit some duplicate contents. Moreover, although there may not be any
candidate cases of partial unity for which the CDM-duplication is not a
possible model as well, there are at least hypothetical cases that look to
be pretty strong ones for the PUM. Imagine sectioning just a tiny segment
of the corpus callosum, resulting in, say, dissociation of tactile
information from the little fingers of both hands, and no more. Now
consider a proposed account of the individuation of experiences: for a
given content B, there are as many experiences carrying the content B as
there are "functional sets" of conscious control systems to which that
content is made available. What makes a collection of control systems
constitute a single functional set, meanwhile, is that they have access to
most or all of the same contents. (The prima facie appeal of this account
is that it is, I think, consistent with some accounts of the architecture
of the mind, according to which all that "unifies" conscious control
systems is their shared access to a limited number of contents [Baars,
1988].) In the imagined case, in which we section only one tiny segment of
the corpus callosum, there is (arguably) a single functional set of
conscious control systems, and thus just one vehicle carrying the content
B.12, 13
Is there any other reason to think that the indeterminacy challenge
faces the PUM uniquely? Hurley's thought seems to be that the CDM-
duplication skirts the indeterminacy challenge by offering a constraint
according to which a partially unified consciousness is impossible. The
constraint in question is just that co-consciousness is a transitive
relation:

What does the difference between these two interpretations [partial
unity of consciousness versus conscious duality with some duplication
of contents] amount to? … In the absence of a constraint of
transitivity, norms of consistency do not here give us the needed
independent leverage on the identity of experiences … note the lurking
threat of indeterminacy. (Hurley, 1998, 175; emphasis added)

This is a threat, Hurley means, to the intelligibility of the PUM in
particular.
The transitivity constraint in effect acts as a principle of
individuation for the CDM-duplication and makes rules out the possibility
of a partially unified consciousness impossible. If the PUM comes with no
analogous constraint or principle of individuation, then the most a
proponent of the PUM can do is simply stipulate that a subject has a
partially unified consciousness. Such stipulation would of course leave
worries about metaphysical indeterminacy intact; the PUM would thus be
uniquely vulnerable to the indeterminacy challenge.
There is a constraint that plays an individuating role for the PUM,
however, one analogous to that played by the transitivity constraint for
the CDM-duplication. For the PUM, the individuating role is played by the
nonduplication constraint. This constraint might say simply that, at any
moment in time, an animal cannot have multiple experiences with the same
content. Such a nonduplication principle falls out of the account of the
tripartite account of experiences offered by Bayne (2010), for instance, at
least one version of which identifies an experience only by appeal to its
content, time of occurrence, and the biological subject or animal to which
it belongs. Or the constraint might be formulated in terms of a (prominent
though still developing) functional theory of consciousness (Baars, 1988;
Dehaene & Naccache, 2001): perhaps there is but a single experience for
each content that is available to the full suite of conscious control
systems within an organism. Whatever the ultimate merits of such a
nonduplication constraint, it can at least be given a principled defense
(see Schechter, forthcoming a).
I cannot see a reason, then, to conclude that the indeterminacy
objection faces the PUM uniquely. If that is so, then the objection cannot
work in quite the way Hurley suggests. My reasoning here takes the form of
a simple reductio: if the indeterminacy objection makes the PUM an
unacceptable model of consciousness, then it should make the CDM-
duplication model equally unacceptable, and on the same a priori grounds.
Yet a priori grounds are surely the wrong grounds upon which to rule out
both the PUM and the CDM-duplication for a given subject: whether there are
any animals in whom some but not all conscious contents are integrated in
the manner characteristic of conscious unity is surely at least in part an
empirical question.
For all the reasons I have discussed, it seems possible that there
should be determinate cases of partially unified consciousness. Of course,
I have not addressed how we (that is, neuropsychologists) should determine
whether a subject has a partially unified stream of consciousness or two
streams of consciousness with some duplication of contents. The question is
difficult in part because it is, as I have suggested throughout, heavily
theoretical rather than straightforwardly empirical. But that is true for
many of the most interesting unanswered questions in psychology.

Figure 15.1
Conscious duality with partial duplication of contents.

Figure 15.2
Partial unity of consciousness.

Figure 15.3
Conscious duality with partial duplication.

Figure 15.4
Partial unity of consciousness.

Figure 15.5
Conscious duality with partial duplication of contents.

Figure 15.6
Partial unity of consciousness.




Notes

1. Throughout the chapter I use the term "split-brain subject" (in place of
"split-brain patient") to be synonymous with "split-brain human animal." I
mean the term to be as neutral as possible with respect to personal
identity concerns. How many subjects of experience there are within or
associated with a split-brain subject will be addressed separately.

2. Marks (1981) and Tye (2003) believe that a split-brain subject usually
has one stream of consciousness but occasionally—under experimental
conditions involving perceptual lateralization—two. It does not matter here
whether we view this as a unity or a duality model. Because Marks and Tye
make common contents the basis of conscious unity, their models are
interestingly related to the partial unity model, but the version of the
partial unity model that I consider also makes some kind of
neurophysiological unity the basis of conscious unity, which their models
do not.

3. Restricting our attention to synchronic co-consciousness in this way of
course yields, at best, a limited view of split-brain consciousness.
Moreover, co-accessibility, co-awareness, and co-phenomenality relations
are probably more likely to diverge diachronically than synchronically
(Schechter, 2012). I still hope that the restricted focus is justified by
the fact that the objections to the partial unity model that I treat here
don't particularly concern what's true across time in the split-brain
subject.

4. This way of talking suggests what Searle calls a "building-block" model
of consciousness (Searle, 2000; see also Bayne, 2007). If one assumes a
unified field model of consciousness, then the distinction between the
partial unity model (PUM) and the CDM is, at a glance, less clear, for
reasons that will emerge in sec. 4. It nonetheless seems possible to me
that the kinds of considerations I discuss in sec. 5 could be used to
distinguish partial unity from conscious duality (with some duplication of
contents).

5. The two kinds of partial unity models are of course interestingly
related, and Hurley (1998), for one, considers a kind of mixed model.
Although the objections to the PUM that I discuss here could be raised
against either version of the model, I think they emerge most starkly in
the context of the second.

6. There is a possible version of the PUM that is (at least on its face)
neutral with respect to implementation. I don't think that's the version
that Lockwood intended (see, e.g., Lockwood, 1994, 93), but nothing hinges
on this exegetical claim. A version that is neutral with respect to
implementation would be especially vulnerable to the indeterminacy
objection (and, thereby, the inconceivability objection), though I suggest
in sec. 5 that theoretical constraints and not just neural facts could be
brought to bear in support of the PUM.

7. Within the neuropsychological literature on the split-brain phenomenon,
the model is occasionally hinted at (e.g., Trevarthen, 1974; Sperry, 1977;
Trevarthen & Sperry, 1973), but, interestingly, these writings are on the
whole ambiguous—interpretable as endorsing either a model of split-brain
consciousness as partially unified or a model in terms of two streams of
consciousness with common inputs. Several explanations for this ambiguity
will be suggested in this paper.

8. Bayne (2010) disputes this, at least up to a point. See response in
Schechter (forthcoming a).

9. The language used in this section implies that we can choose whether and
how to revise our concepts, but I don't mean to commit myself to this
(Grice & Strawson, 1956). Perhaps our concept of a subject of experience is
basic, even innately specified, and perhaps there just is an essential
conceptual connection between it and the concept of a subjective
perspective.

10. Certainly this is the case if we read "subcortical" to mean
"noncortical," which most discussions of the role of "subcortical"
connections in the split-brain subject appear to do.

11. The subject Johnston et al. (2008) looked at had been very recently
callosotomized, while the subject Uddin et al. studied—"N.G."—underwent
callosotomy nearly fifty years ago. One possibility then is that in N.G.,
other, noncortical structures have come to play the coordinating role that
her corpus callosum once played. (Actually N.G. has always been a slightly
unusual split-brain subject, but then arguably each split-brain subject
is.) A distinct possibility is that the marked reduction in
interhemispheric coherence observed by Johnston et al. was simply an acute
consequence of undergoing major neurosurgery itself.

12. This particular approach to individuating conscious experiences makes
it possible for there to be subjects for whom it is genuinely indeterminate
(not just indeterminable) whether they have a dual or a partially unified
consciousness. This is because it views the identity of experiences and
streams of consciousness as in part a matter of integration, something that
comes in degrees. It isn't clear, for instance, whether a split-brain
subject has one or two "functional sets" of conscious control systems. So
the structure of split-brain consciousness could be genuinely indeterminate
without showing that there are no possible determinate cases of partial
unity.

13. It is worth noting that there is in fact some debate about the
structure of consciousness in the "normal," i.e., "nonsplit" case. How
certain are we that there won't turn out to be any failures of co-
consciousness in nonsplit subjects? Several psychologists believe that
there are (e.g., Marcel, 1993). If we discovered that there were any such
failures, my guess is that we would be inclined to conclude that our
consciousness was mostly unified, rather than dual—but to admit that our
consciousness is mostly unified would be to acknowledge that it is
partially not. Thus it is possible that even the normal case will end up
being one to which we confidently apply the PUM rather than the CDM-
duplication.





References

Y N "">" ""Baars, B. (1988). A
cognitive theory of consciousness. Cambridge: Cambridge University
Press.Y N "">" ""

Y N "">" " _issn="0022-
362X""Bayne, T. (2008). The unity of consciousness and the split-
brain syndrome. Journal of Philosophy, 105, 277–300.Y N
"">" ""

Y N "">" ""Bayne, T. (2010).
The unity of consciousness. Oxford: Oxford University Press.Y N
"">" ""

Y N "" >" ""
Bayne, T. (2007). Conscious states and conscious creatures:
Explanation in the scientific study of consciousness. Philosophical
Perspectives 21 (Philosophy of Mind): 1–22. Oxford: Wiley.Y N
"" >" ""

Y N "">" ""Bayne, T., &
Chalmers, D. (2003). What is the unity of consciousness? In A.
Cleeremans (Ed.), The unity of consciousness: Binding, integration,
and dissociation (pp. 23–58). Oxford: Oxford University Press.Y N
"">" ""

Y N "">" " _issn="0140-
525X""Block, N. (1995). On a confusion about a function of
consciousness. Behavioral and Brain Sciences, 18, 227–287.Y N
"">" ""

Y N "">" ""Burge, T. (2009).
Five theses on de re states and attitudes. In J. Almog & P. Leonardi
(Eds.), The philosophy of David Kaplan (pp. 246–316). Oxford: Oxford
University Press.Y N "">" ""

Y N "">" ""Dainton, B. (2000).
Stream of consciousness. London: Routledge.Y N "">" ""

Y N "">" " _issn="0031-
8116""Davis, L. (1997). Cerebral hemispheres. Philosophical
Studies, 87, 207–222.Y N "">" ""

Y N "">" " _issn="0010-
0277""Dehaene, S., & Naccache, L. (2001). Towards a cognitive
neuroscience of consciousness: Basic evidence and a workspace
framework. Cognition, 79, 1–37.Y N "">" ""

Y N "">" " _issn="0007-
0882""Dewitt, L. (1975). Consciousness, mind, and self: The
implications of the split-brain studies. British Journal for the
Philosophy of Science, 26, 41–47.Y N "">"
""

Y N "">" ""Eccles, J. (1965).
The brain and the unity of conscious experience. Nineteenth Arthur
Stanley Eddington Memorial Lecture. Cambridge: Cambridge University
Press.Y N "">" ""

Y N "">" ""Eccles, J. (1973).
The understanding of the brain. New York: McGraw-Hill.Y N
"">" ""

Y N "">" ""Eccles, J. (1981).
Mental dualism and commissurotomy. Brain and Behavioral Science, 4,
105.Y N "">" ""

Y N "">" ""Ferguson, S.,
Rayport, M., & Corrie, W. (1985). Neuropsychiatric observations on
behavioural consequences of corpus callosum section for seizure
control. In A. Reeves (Ed.), Epilepsy and the corpus callosum (pp.
501–514). New York: Plenum Press.Y N "">"
""

Y N "">" ""Gazzaniga, M.
(1970). The bisected brain. New York: Appleton-Century-Crofts.Y N
"">" ""

Y N "">" ""Gazzaniga, M.
(1985). The social brain. New York: Basic Books.Y N "">" ""

Y N "">" " _issn="0006-
8950""Gazzaniga, M. (2000). Cerebral specialization and
interhemispheric communication: Does the corpus callosum enable the
human condition? Brain, 123, 1293–1326.Y N "">"
""

Y N "">" " _issn="0031-
8108""Grice, H., & Strawson, P. (1956). In defense of a dogma.
Philosophical Review, 65, 141–158.Y N "">"
""

Y N "">" ""Hill, C. (1991).
Sensations: A defense of type materialism. Cambridge, MA: Cambridge
University Press.Y N "">" ""

Y N "">" ""Hurley, S. (1994).
Unity and objectivity. In C. Peacocke (Ed.), Objectivity, simulation,
and the unity of consciousness (pp. 49–77). Oxford: Oxford University
Press.Y N "">" ""

Y N "">" ""Hurley, S. (1998).
Consciousness in action. Cambridge, MA: Harvard University Press.Y N
"">" ""

Y N "">" " _issn="0270-
6474""Johnston, J., Vaishnavi, S., Smyth, M., Zhang, D., He, B.,
Zempel, J., et al. (2008). Loss of resting interhemispheric functional
connectivity after complete section of the corpus callosum. Journal of
Neuroscience, 28, 6452–6458.Y N "">" ""

Y N "">" " _issn="1939-
5078""Lassonde, M., & Ouiment, C. (2010). The split-brain. Wiley
Interdisciplinary Reviews: Cognitive Science, 1, 191–202.Y N
"">" ""

Y N "">" " _issn="0364-
5134""LeDoux, J., Wilson, D., & Gazzaniga, M. (1977). A divided
mind: Observations on the conscious properties of the separated
hemispheres. Annals of Neurology, 2, 417–421.Y N "">" ""

Y N "">" ""Levy, J. (1969).
Information processing and higher psychological functions in the
disconnected hemispheres of human commissurotomy patients. Unpublished
doctoral dissertation. California Institute of Technology.Y N
"">" ""

Y N "">" ""Lockwood, M.
(1989). Mind, brain, and the quantum. Oxford: Blackwell.Y N
"">" ""

Y N "">" ""Lockwood, M.
(1994). Issues of unity and objectivity. In C. Peacocke (Ed.),
Objectivity, simulation, and the unity of consciousness (pp. 89–95).
Oxford: Oxford University Press.Y N "">" ""

Y N "">" ""Marcel, A. (1993).
Slippage in the unity of consciousness. In G. Bock & J. Marsh (Eds.),
Experimental and theoretical studies of consciousness (pp. 168–179).
Chinchester: John Wiley & Sons.Y N "">" ""

Y N "">" ""Mark, V. (1996).
Conflicting communicative behavior in a split-brain patient: Support
for dual consciousness. In S. Hameroff, A. Kaszniak, & A. Scott
(Eds.), Toward a science of consciousness: The first Tucson
discussions and debates (pp. 189–196). Cambridge, MA: MIT Press.Y N
"">" ""

Y N "">" ""Marks, C. (1981).
Commissurotomy, consciousness, and unity of mind. Cambridge, MA: MIT
Press.Y N "">" ""

Y N "">" ""Milner, B., Taylor,
L., & Jones-Gotman, M. (1990). Lessons from cerebral commissurotomy:
Auditory attention, haptic memory, and visual images in verbal-
associative learning. In C. Trevarthen (Ed.), Brain circuits and
functions of the mind (pp. 293–303). Cambridge: Cambridge University
Press.Y N "">" ""

Moor, J. 1982. Split-brains and atomic persons. Philosophy of Science 49:
91-106.

Y N "">" " _issn="0039-
7857""Nagel, T. (1971). Brain bisection and the unity of
consciousness. Synthese, 22, 396–413.Y N "">"
""

Y N "">" " _issn="0031-
8108""Nagel, T. (1974). What is it like to be a bat?
Philosophical Review, 83, 435–450.Y N "">"
""

Y N "">" ""Popper, K., &
Eccles, J. (1977). The self and its brain. New York: Springer
International.Y N "">" ""

Y N "">" ""Revonsuo, A.
(2000). Prospects for a scientific research program on consciousness.
In T. Metzinger (Ed.), Neural correlates of consciousness: Empirical
and conceptual questions. Cambridge, MA: MIT Press.Y N
"">" ""

Y N "">" " _issn="0031-
8000""Schechter, E. (2010). Individuating mental tokens: The
split-brain case. Philosophia, 38, 195–216.Y N "">" ""

Y N "">" " _issn="0951-
5089""Schechter, E. (2012). The switch model of split-brain
consciousness. Philosophical Psychology, 25, 203–226.Y N
"">" ""

Y N "" >" ""
Schechter, E. (forthcoming a). The unity of consciousness:
Subjects and objectivity. Philosophical Studies.Y N "" >" ""

Y N "" >" ""
Schechter, E. (forthcoming b). Two unities of consciousness.
European Journal of Philosophy.Y N "" >" ""


Y N "">" " _issn="0147-
006X""Searle, J. (2000). Consciousness. Annual Review of
Neuroscience, 23, 557–578.Y N "">" ""

Y N "">" " _issn="0036-
8075""Sidtis, J., Volpe, B., Holtzman, J., Wilson, D., &
Gazzaniga, M. (1981). Cognitive interaction after staged callosal
section: Evidence for transfer of semantic activation. Science, 212,
344–346.Y N "">" ""

Y N "">" ""Shallice, T.
(1997). Modularity and consciousness. In N. Block, O., Flanagan, and
G. Güzeldere (Eds.), The nature of consciousness (pp. 255–276).
Cambridge, MA: MIT Press.Y N "">" ""

Y N "">" " _issn="0360-
5310""Sperry, R. (1977). Forebrain commissurotomy and conscious
awareness. Journal of Medicine and Philosophy, 2, 101–126.Y N
"">" ""

Y N "">" " _issn="0028-
3932""Sperry, R. (1985). Consciousness, personal identity, and
the divided brain. Neuropsychologia, 22, 661–673.Y N "">" ""

Y N "">" " _issn="1471-
2202""Tononi, G. (2004). An information integration theory of
consciousness. BMC Neuroscience, 5, 42.Y N "">"
""

Y N "">" ""Tramo, M., Baynes,
K., Fendrich, R., Mangun, G., Phelps, E., Reuter-Lorenz, P., et al.
(1995). Hemispheric specialization and interhemispheric integration:
Insights from experiments with commissurotomy patients. In A. Reeves &
D. Roberts (Eds.), Epilepsy and the corpus callosum (Vol. 2, pp.
263–295). New York: Plenum Press.Y N "">"
""

Y N "">" ""Trevarthen, C.
(1974). Analysis of cerebral activities that generate and regulate
consciousness is commissurotomy patients. In S. Dimond and J. Beaumont
(Eds.), Hemisphere Function in the Human Brain (pp. 235–263). New
York: Halsted Press.Y N "">" ""

Y N "">" " _issn="0006-
8950""Trevarthen, C., & Sperry, R. (1973). Perceptual unity of
the ambient visual field in human commissurotomy patients. Brain, 96,
547–570.Y N "">" ""

Y N "">" ""Tye, M. (2003).
Consciousness and persons: Unity and identity. Cambridge, MA: MIT
Press.Y N "">" ""

Y N "">" " _issn="0959-
4965""Uddin, L., Mooshagian, E., Zaidel, E., Scheres, A.,
Margulies, D., Clare Kelly, A., et al. (2008). Residual functional
connectivity in the split-brain revealed with resting-state functional
MRI. Neuroreport, 19, 703–709.Y N "">" ""

Y N "">" ""Zaidel, E.,
Iacaboni, M., Zaidel, D., & Bogen, J. (2003). The callosal syndromes.
In K. M. Heilman & E. Valenstein, E. (Eds.), Clinical Neuropsychology
2002 (4th ed., pp. 347–403). New York: Oxford University Press. Y N
"">" ""

Y N "">" " _issn="0010-
9452""Zaidel, E., Zaidel, D., & Sperry, R. (1981). Left and right
intelligence: Case studies of Raven's Progressive Matrices following
brain bisection and hemi-decortication. Cortex, 17, 167–186.Y N
"">" ""