The Global
Workspace Hypothesis
The studies we’ve looked at so
far tell us something about where in the brain we can find activity crucial for
consciousness; and as we’ve seen, there plainly is no single brain site that
serves as the “seat of consciousness.” The studies also tell us something about
when the brain changes that give rise
to consciousness actually occur. However, we wantto go beyond merely specifying
this where and when. We also want to learn how these brain regions might work
together to produce the properties we associate with consciousness and to
provide a biological base for the claims we made earlier about the function of consciousness.
In approaching these issues, we
need to start with some basic facts. As we saw, each area of the brain has a
highly specialized function. One brain region is specialized for controlling
hand movements, another for processing auditory inputs, and still another for
analyzing visual stimuli. Then, within each of these regions, we find fur-ther
specialization. For example, the various aspects of visual perception depend on
dis-tinct brain areas, one specialized for perceiving color, another for
perceiving movement, and so on. Each of these areas does its own job, and the
activity in each area is highly transient: The neurons respond to the current
inputs and then, when their immediate task is done, they cease responding so
they’re ready for the next input or the next task.
For many purposes, though, we
need to sustain the activity in these various systems (e.g., for prolonged
scrutiny). We also need to integrate the activity of the various systems into a
coherent whole. What makes both of these steps possible is attention—probably implemented through mechanisms in the prefrontal
cortex, but then influencing activity in many other brain sites (Maia &
Cleeremans, 2005). The mechanisms of attention can amplify the activity within
a specific neural system, and they can also sustain that activity.
Attention also seems to link the
activity of different neural systems, binding them into a single
representation. Thus, for example, a red moving object in front of our eyes
will trigger a response in one brain area in which cells are sensitive to
motion as well as in an area in which cells are sensitive to color. If we
aren’t paying attention to this object, these two neural responses will be
independent of each other. But if we are paying attention to it, the neurons in
these two systems fire in synchrony . And when neurons fire in this coordinated
way, the brain seems to register this as a linkage among the dif-ferent
processing areas. As a result, these attributes are bound together so that we
correctly perceive the stimulus as a unified whole
This coordination of separate
neural systems requires communication among distinct brain areas, and this
communication is made possible by “workspace neurons” that literally connect
one area of the brain to another. However, the communication, and thus the
information carried by the workspace neurons, is selective, and so it’s
certainly not the case that every bit of neural activity gets linked to every
other bit. Instead, it’s typi-cally the signals from more active areas that are
transmitted to other sites. Bear in mind, though, that attention can be used to
amplify activity and so can govern which brain areas are more activated and
which are less. In this way, the information flow is controllable by virtue of
what the person chooses to pay attention to.
These points are the backdrop for
the global workspace hypothesis
about consciousness. According to this hypothesis the integrated neural
activity made possible by the workspace neurons provides the biological basis
for consciousness. Activity in these neurons does not specify the content of consciousness; that content
is instead represented in more specialized brain areas. Thus, when you’re aware
of the blue sky overhead, that “blueness” is represented by the appropriate
pattern of firing in your visual cortex; when you’re thinking about how cold
your toes are, the “coldness” is represented by firing in somatosensory areas.
What the workspace neurons do, however, is glue these bits together, creating a
unified experience and allowing the exchange of information from one module to
the next. (For several variants of this general hypothesis, see Baars, 2002;
Baars & Franklin, 2003; Cooney & Gazzaniga, 2003; F. Crick & Koch,
2003; Dennett, 2001; Engel & Singer, 2000; Maia & Cleeremans, 2005;
Roser & Gazzaniga, 2004; Tononi, 2004.)
The broad proposal, therefore, is
that stimuli or ideas become conscious when they’re linked to each other in a
dynamic, coherent representation made possible by the workspace neurons and
supported by attention. This hypothesis helps us understand why our con-scious
experience feels unitary and coherent. We aren’t, after all, separately aware
of redness and movement and roundness. Instead, we’re aware of a single
experience in which the red apple rolls slowly by. This coherence, of course,
is precisely what the workspace allows—one representation, constructed from the
coordinated activity of many processing components.
Likewise, we aren’t conscious of
every aspect of our experience—we can focus on the rose’s color and fail to
notice its thorns. But, of course, we can usually choose what we’re going to
focus on (so that, when picking up the rose, we might decide to pay attention
to those thorns). These observations are easily accommodated by the workspace
hypothesis. As we’ve said, the information carried by the workspace neurons is
selective (so might not carry the information, at least at the start, about the
thorns) but is shaped by how some-one focuses their attention (and so, with a
change in focus, would highlight the thorns).
Also bear in mind that the
workspace neurons allow us to maintain a mental representation in an active
state for an extended period of time, so that we can continue thinking about a
stimulus or an idea after the specific trigger is removed. In this way, we can
link the workspace to a form of memory known as working memory—the mem-ory that you keep ideas in while you’re
working with them . We can also link it to the brain areas associated with this
type of memory—specifically, the prefrontal cortex (Goldman-Rakic, 1987; also
see McIntosh, Rajah, & Lobaugh, 1999).
Finally, the workspace hypothesis
may also help with other puzzles—including the variations in consciousness that we sometimes experience. For
example, when we’reasleep (and not dreaming), we aren’t conscious of time
passing, of any ongoing stream of thought, or of many events taking place in
our vicinity. Why is this? Evidence sug-gests that when we’re in sleep (without
dreaming), the communication breaks down between different parts of the cortex
so that, even though the sleeping brain is intensely active, the various
activities aren’t coordinated with each other. The sugges-tion, of course, is
that this communication (mediated by the neuronal workspace) is crucial for
consciousness, so it makes sense that sleeping people, having temporarily lost
this communication, aren’t conscious of their state or their circumstances
(Massimini et al., 2005).
Likewise, when someone is
anesthetized (e.g., before surgery), their brain remains active. Nonetheless,
the person is not conscious—after all, that’s the point of anesthe-sia! So how should we think about the anesthetized
brain? According to one recent review, the “loss of consciousness is associated
with a breakdown of cortical connectiv-ity and thus of integration” (Alkire,
Hudetz, & Tononi, 2008, pg. 879)—a proposal, again, in line with the idea
that consciousness depends on communication and coordi-nation among distinct
neural systems.
Overall, then, the global
workspace hypothesis—rooted in psychology, philosophy, and neuroscience—links
several lines of argument. These include claims about the subjective
“coherence” of consciousness, research on the neural correlates of consciousness,
and evidence from people who are sleeping or anesthetized. We should
acknowledge that there is still controversy about some of these points (e.g.,
N. Block, 1997, 2001; Chalmers, 1995; Kinsbourne, 2000); but even so, the
workspace hypothe-sis appears enormously promising as an account of how our
brains make consciousness possible.
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