Selector inputs to action systems (Shallice 1988)
There are a few intriguing commonalities among the proposals
on this list. A number of them give a central role to interactions
between the thalamus and the cortex, for example. All the same,
the sheer number and diversity of the proposals can be a little
overwhelming. I propose to step back a little and try to make
sense of all this activity by asking some foundational questions.
A central question is this: how is it, in fact, that one can
search for the neural correlate of consciousness? As we all
know, there are problems in measuring consciousness. It's not
a directly and straightforwardly observable phenomenon. It would
be a lot easier if we had a way of getting at consciousness
directly; if we had, for example, a consciousness meter.
If we had a consciousness meter, searching for the NCC would
be straightforward. We'd wave the consciousness meter and measure
a subject's consciousness directly. At the same time, we'd monitor
the underlying brain processes. After a number of trials, we'd
say OK, such-and-such brain processes are correlated with experiences
of various kinds, so that's the neural correlate of consciousness.
Alas, we don't have a consciousness meter, and there seem to
be principled reasons why we can't have one. Consciousness just
isn't the sort of thing that can be measured directly. So: What
do we do without a consciousness meter? How can the search go
forward? How does all this experimental research proceed?
I think the answer is this: we get there through principles
of interpretation. These are principles by which we interpret
physical systems to judge whether or not they have consciousness.
We might call these pre-experimental bridging principles.
These are the criteria that we bring to bear in looking at systems
to say (a) whether or not they are conscious now, and (b) what
information they are conscious of, and what information they
are not. We can't reach in directly and grab those experiences
and "transpersonalize" them into our own, so we rely
on external criteria instead.
That's a perfectly reasonable thing to do. But in doing this
we have to realize that something interesting is going on. These
principles of interpretation are not themselves experimentally
determined or experimentally tested. In a sense they are pre-experimental
assumptions. Experimental research gives us a lot of information
about processing; then we bring in the bridging principles to
interpret the experimental results, whatever those results may
be. They are the principles by which we make inferences
from facts about processing to facts about consciousness, so
they are conceptually prior to the experiments themselves. We
can't actually refine them experimentally (except perhaps through
first-person experimentation!), because we don't have any independent
access to the independent variable. Instead, these principles
will be based on some combination of (a) conceptual judgments
about what counts as a conscious process and (b) information
gleaned from our first-person perspective on our own consciousness.
I think we are all stuck in this boat. The point applies whether
one is a reductionist or an anti-reductionist about consciousness.
A hard-line reductionist might put some of these points slightly
differently, but either way, the experimental work is going
to require pre-experimental reasoning to determine the criteria
for ascription of consciousness. Of course such principles are
usually left implicit in empirical research. We don't usually
see papers saying "Here is the bridging principle, here
are the data, and here is what follows." But it's useful
to make them explicit. The very presence of these principles
has some strong and interesting consequences in the search for
the NCC.
In a sense, in relying on these principles we are taking a
leap into the epistemological unknown. Because we don't measure
consciousness directly, we have to make something of a leap
of faith. It may not be a big leap, but nevertheless it suggests
that everyone doing this sort of work is engaged in philosophical
reasoning. Of course one can always choose to stay on solid
ground, talking about the empirical results in a neutral way;
but the price of doing so is that one gains no particular insight
into consciousness. Conversely, as soon as we draw any conclusions
about consciousness, we have gone beyond the information given,
so we need to pay careful attention to the reasoning involved.
So what are these principles of interpretation? The first and
by far the most prevalent such principle is a very straightforward
one: it's a principle of verbal report. When someone says "Yes,
I see that table now", we infer that they are conscious
of the table. When someone says "Yes, I see red now",
we infer that they are having an experience of red. Of course
one might always say "How do you know?" -- a philosopher
might suggest that we may be faced with a fully functioning
zombie - but in fact most of us don't believe that the people
around us are zombies, and in practice we are quite prepared
to rely on this principle. As pre-experimental assumptions go,
this is a relatively "safe" one -- it doesn't require
a huge leap of faith -- and it is very widely used.
So the principle here is that when information is verbally
reported, it is conscious. One can extend this slightly, as
no one believes that an actual verbal report is required
for consciousness; we are conscious of much more than we report
on any given occasion. So an extended principle might say that
when information is directly available for verbal report,
it is conscious.
Experimental researchers don't rely only on these principles
of verbal report and reportability. These principles can be
somewhat limiting when we want to do broader experiments. In
particular, we don't want to just restrict our studies of consciousness
to subjects that have language. In fact just this morning we
saw a beautiful example of research on consciousness in language-free
creatures. I'm referring to the work of Nikos Logothetis and
his colleagues (e.g. Logothetis & Schall 1989; Leopold &
Logothetis 1996). This work uses experiments on binocular rivalry
in monkeys to draw conclusions about the neural processes associated
with consciousness. How do Logothetis et al manage to
draw conclusions about a monkey's consciousness without getting
any verbal reports? What they do is rely on a monkey's pressing
bars: if a monkey can be made to press a bar in an appropriate
way in response to a stimulus, we'll say that that stimulus
was consciously perceived.
The criterion at play seems to require that the information
be available for an arbitrary response. If it turned out that
the monkey could press a bar in response to a red light but
couldn't do anything else, we would be tempted to say that it
wasn't a case of consciousness at all, but some sort of subconscious
connection. If on the other hand we find information that is
available for response in all sorts of different ways, then
we'll say that it is conscious. Actually Logothetis and his
colleagues also use some subtler reasoning about similarities
with binocular rivalry in humans to buttress the claim that
the monkey is having the relevant conscious experience, but
it is clearly the response that carries the most weight.
The underlying general principle is something like this: When
information is directly available for global control
in a cognitive system, then it is conscious. If information
is available for response in many different motor modalities,
we will say that it is conscious, at least in a range of relatively
familiar systems such as humans and primates and so on. This
principle squares well with the previous principle in cases
where the capacity for verbal report is present: availability
for verbal report and availability for global control seem to
go together in such cases (report is one of the key aspects
of control, after all, and it is rare to find information that
is reportable but not available more widely). But this principle
is also applicable more widely.
A correlation between consciousness and global availability
(for short) seems to fit the first-person evidence -- the evidence
gleaned from our own conscious experience -- quite well. When
information is present in my consciousness, it is generally
reportable, and it can generally be brought to bear in the control
of behavior in all sorts of different ways. I can talk about
it, I can point in the general direction of a stimulus, I can
press bars, and so on. Conversely, when we find information
that is directly available in this way for report and other
aspects of control, it is generally conscious information. I
think one can bear this out by consideration of cases.
There are some interesting puzzle cases to consider, such as
the case of blindsight, where one has some kind of availability
for control but arguably no conscious experience. Those cases
might best be handled by invoking the directness criterion:
insofar as the information here is available for report and
other control processes at all, it is available only indirectly,
by comparison to the direct and automatic availability in standard
cases. One might also stipulate that it is availability for
voluntary control that is relevant, to deal with certain
cases of involuntary unconscious response, although that is
a complex issue. I discuss a number of puzzle cases in more
detail elsewhere (Chalmers 1996, forthcoming), where I also
give a much more detailed defense of the idea that something
like global availability is the key pre-empirical criterion
for the ascription of consciousness.
But this remains at best a first-order approximation of the
functional criteria that come into play. I'm less concerned
today to get all the fine details right than to work with the
idea that some such functional criterion is required and indeed
is implicit in all the empirical research on the neural correlate
of consciousness. If you disagree with the criterion I've suggested
here - presumably because you can think of counterexamples --
you may want to use those counterexamples to refine it or to
come up with a better criterion of your own. But the point I
want to focus on here is that in the very act of experimentally
distinguishing conscious from unconscious processes, some such
criterion is always at play.
So the question I want to ask is: if something like
this is right, then what follows? That is, if some such bridging
principles are implicit in the methodology of the search for
the NCC, then what are the consequences? I will use global availability
as my central functional criterion in the discussion that follows,
but many of the points should generalize.
The first thing one can do is produce what philosophers might
call a rational reconstruction of the search for the
neural correlate of consciousness. With a rational reconstruction
we can say, maybe things don't work exactly like this in practice,
but the rational underpinnings of the process have something
like this form. That is, if one were to try to justify
the conclusions one has reached as well as one can, one's justification
would follow the shape of the rational reconstruction. In this
case, a rational reconstruction might look something like this:
(1) Consciousness <-> global availability (bridging principle)
(2) Global availability <-> neural process N (empirical
work)
so
(3) Consciousness <-> neural process N (conclusion).
According to this reconstruction, one implicitly embraces some
sort of pre-experimental bridging principle that one finds plausible
on independent grounds, such as conceptual or phenomenological
grounds. Then one does the empirical research. Instead of measuring
consciousness directly, we detect the functional property. One
sees that when this functional property (e.g. global availability)
is present, it is correlated with a certain neural process (e.g.
40-hertz oscillations). Combining the pre-empirical premise
and the empirical result, we arrive at the conclusion that this
neural process is a candidate for the NCC.
Of course it doesn't work nearly so simply in practice. The
two stages are very intertwined; our pre-experimental principles
may themselves be refined as experimental research goes along.
Nevertheless I think one can make a separation, at least at
the rational level, into pre-empirical and experimental components,
for the sake of analysis. So with this sort of rational reconstruction
in hand, what sort of conclusions follow? There are about six
consequences that I want to draw out here.
(1) The first conclusion is a characterization of the neural
correlates of consciousness. If the NCC is arrived at through
this sort of methodology, then whatever it turns out to be,
it will be a mechanism of global availability. The presence
of the NCC wherever global availability is present suggests
that it is a mechanism that subserves the process of
global availability in the brain. The only alternative that
we have to worry about is that it might be a symptom
rather than a mechanism of global availability; but that
possibility ought to be addressable in principle by dissociation
studies, by lesioning, and so on. If a process is a mere symptom
of availability, we ought to be able to empirically dissociate
it from the process of global availability while leaving the
latter intact. The resulting data would suggest to us that consciousness
can be present even when the neural process in question is not,
thus indicating that it wasn't a perfect correlate of consciousness
after all.
(A related line of reasoning supports the idea that a true
NCC must be a mechanism of direct availability for global
control. Mechanisms of indirect availability will in principle
be dissociable from the empirical evidence for consciousness,
for example by directly stimulating the mechanisms of direct
availability. The indirect mechanisms will be "screened
off" by the direct mechanisms in much the same way as the
retina is screened off as an NCC by the visual cortex.)
In fact, if one looks at the various proposals that are out
there, this template seems to fit them pretty well. For example,
the 40-hertz oscillations discussed by Crick and Koch were put
forward precisely because of the role they might play in binding
and integrating information into working memory, and working
memory is of course a central mechanism whereby information
is made available for global control in a cognitive system.
Similarly, it is plausible that Libet's extended neural activity
is relevant precisely because the temporal extendedness of activity
is what gives certain information the capacity to dominate later
processes that lead to control. Baars' global workspace is a
particularly explicit proposal of a mechanism in this direction;
it is put forward explicitly as a mechanism whereby information
can be globally disseminated. All of these mechanisms and many
of the others seem to be candidates for mechanisms of global
availability in the brain.
(2) This reconstruction suggests that a full story about the
neural processes associated with consciousness will to do two
things. Firstly, it will explain global availability
in the brain. Once we know all about the relevant neural processes,
we will know precisely how information is made directly available
for global control in the brain, and this will be an explanation
in the full sense. Global availability is a functional property,
and as always the problem of explaining the performance of a
function is a problem to which mechanistic explanation is well-suited.
So we can be confident that in a century or two, global availability
will be straightforwardly explained. Secondly, this explanation
of availability will do something else: it will isolate the
processes that underlie consciousness itself. If the
bridging principle is granted, then mechanisms of availability
will automatically be correlates of phenomenology in the full
sense.
Now, I don't think this gives us a full explanation
of consciousness. One can always raise the question of why it
is that these processes of availability should give rise to
consciousness in the first place. As yet we have no explanation
of why this is, and it may well be that the full details concerning
the processes of availability still won't answer this question.
Certainly, nothing in the standard methodology I have outlined
answers the question; that methodology assumes a relation
between availability and consciousness, and therefore does nothing
to explain it. The relationship between the two is instead
taken as something of a primitive. So the hard problem still
remains. But who knows: somewhere along the line we may be led
to the relevant insights that show why the link is there, and
the hard problem may then be solved. In any case, whether or
not we have solved the hard problem, we may nevertheless have
isolated the basis of consciousness in the brain. We
just have to keep in mind the distinction between correlation
and explanation.
(3) Given this paradigm, it is likely that there are going to
be many different neural correlates of consciousness. I take
it that this is not going to surprise many people; but the rational
reconstruction gives us a way of seeing just why such a multiplicity
of correlates should exist. There will be many neural correlates
of consciousness because there may well be many different mechanisms
of global availability. There will be mechanisms of availability
in different modalities: the mechanisms of visual availability
may be quite different from the mechanisms of auditory availability,
for example. (Of course they may be the same, in that
we could find a later area that integrates and disseminates
all this information, but that's an open question.) There will
also be mechanisms at different stages of the processing path
whereby information is made globally available: early mechanisms
and later ones. So these may all be candidates for the NCC.
And there will be mechanisms at many different levels of description:
for example, 40-hertz oscillations may well be redescribed as
high-quality representations, or as part of a global workspace,
at a different level of description. So it may turn out that
a number of the animals in the zoo, so to speak, can co-exist,
because they are compatible in one of these ways.
I won't speculate much further on just what the neural correlates
of consciousness are. No doubt some of the ideas in the
initial list will prove to be entirely off-track, while some
of the others will prove closer to the mark. As we philosophers
like to say, humbly, that's an empirical question. But I hope
the conceptual issues are becoming clearer.
(4) This way of thinking about things allows one to make sense
of a idea that is sometimes floated: that of a consciousness
module. Sometimes this notion is disparaged; sometimes it
is embraced. But this picture of the methodology in the search
for an NCC suggests that it is at least possible that there
could turn out to be such a module. What would it take? It would
require that there turns out to be some sort of functionally
localizable, internally integrated area, through which all global
availability runs. It needn't be anatomically localizable, but
to qualify as a module it would need to be localizable in some
broader sense. For example, the parts of the module would have
to have high-bandwidth communication among themselves, compared
to the relatively low-bandwidth communication that they have
with other areas. Such a thing could turn out to exist.
It doesn't strike me as especially likely that things
will turn out this way; it seems just as likely that there will
be multiple independent mechanisms of global availability in
the brain, scattered around without any special degree of mutual
integration. If that's so, we will likely say that there doesn't
turn out to be a consciousness module after all. But that's
another one of those empirical questions.
If something like this does turn out to exist in the brain,
it would resemble Baars' conception of a global workspace: a
functional area responsible for the integration of information
in the brain and for its dissemination to multiple nonconscious
specialized processes. In fact I should acknowledge that many
of the ideas I'm putting forward here are compatible with things
that Baars has been saying for years about the role of global
availability in the study of consciousness. Indeed, this way
of looking at things suggests that some of his ideas are almost
forced on one by the methodology. The special epistemological
role of global availability helps explain why the idea of a
global workspace provides a useful way of thinking about almost
any empirical proposal about consciousness. If NCC's are identified
as such precisely because of their role in global control, then
at least on a first approximation, we should expect the global
workspace idea to be a natural fit.
(5) We can also apply this picture to a question that has been
discussed frequently at this conference: are the neural correlates
of visual consciousness to be found in V1, in the extrastriate
visual cortex, or elsewhere? If our picture of the methodology
is correct, then the answer will presumably depend on which
visual area is most directly implicated in global availability.
Crick and Koch have suggested that the visual NCC is not to
be found within V1, as V1 does not contain neurons that project
to the prefrontal cortex. This reasoning has been criticized
by Ned Block for conflating access consciousness and phenomenal
consciousness (see Block, this volume); but interestingly, the
picture I have developed suggests that it may be good reasoning.
The prefrontal cortex is known to be associated with control
processes; so if a given area in the visual cortex projects
to prefrontal areas, then it may well be a mechanism of direct
availability. And if it does not project in this way, it is
less likely to be such a mechanism; at best it might be indirectly
associated with global availability. Of course there is still
plenty of room to raise questions about the empirical details.
But the broader point is that for the sort of reasons discussed
in (2) above, it is likely that the neural processes involved
in explaining access consciousness will simultaneously
be involved in a story about the basis of phenomenal
consciousness. If something like this is implicit in their reasoning,
Crick and Koch might escape the charge of conflation. Of course
the reasoning does depend on these somewhat shaky bridging principles,
but then all work on the neural correlates of consciousness
must appeal to such principles somewhere, so this can't be held
against Crick and Koch in particular.
(6) Sometimes the neural correlate of consciousness is conceived
of as the Holy Grail for a theory of consciousness. It will
make everything fall into place. For example, once we discover
the NCC, then we'll have a definitive test for consciousness,
enabling us to discover consciousness wherever it arises. That
is, we might use the neural correlate itself as a sort of consciousness
meter. If a system has 40-hertz oscillations (say), then it
is conscious; if it has none, then it is not conscious. Or if
a thalamocortical system turns out to be the NCC, then a system
without that system is unlikely to be conscious. This sort of
reasoning is not usually put quite so baldly as this, but I
think one finds some version of it quite frequently.
This reasoning can be tempting, but one should not succumb
to the temptation. Given the very methodology that comes into
play here, there is no way to definitely establish a given NCC
as an independent test for consciousness. The primary criterion
for consciousness will always remain the functional property
we started with: global availability, or verbal report, or whatever.
That's how we discovered the correlations in the first place.
40-hertz oscillations (or whatever) are relevant only
because of the role they play in satisfying this criterion.
True, in cases where we know that this association between the
NCC and the functional property is present, the NCC might itself
function as a sort of "signature" of consciousness;
but once we dissociate the NCC from the functional property,
all bets are off. To take an extreme example, if we have 40-hertz
oscillations in a test tube, that almost certainly won't yield
consciousness. But the point applies equally in less extreme
cases. Because it was the bridging principles that gave us all
the traction in the search for an NCC in the first place, it's
not clear that anything follows in cases where the functional
criterion is thrown it away. So there's no free lunch here:
one can't get something for nothing.
Once one recognizes the central role that pre-experimental
assumptions play in the search for the NCC, one realizes that
there are some limitations on just what we can expect this search
to tell us. Still, whether or not the NCC is the Holy Grail,
I hope that I have said enough to make it clear that the quest
for it is likely to enhance our understanding considerably.
And I hope to have convinced you that there are important ways
in which philosophy and neuroscience can come together to help
clarify some of the deep problems involved in the study of consciousness.
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a Science of Consciousness II: The Second Tucson Discussions
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