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Chapter 12
Conscious Mind
The two final chapters of this
book are included to paint a more comprehensive picture of our modern minds
and how they exist in a sometimes uneasy alliance with bodies that were designed
for conditions of the Paleolithic. The preceding chapters have elaborated on
the theme that our brain is not a general-purpose problem solver, using one
common process for all tasks, but rather a collection of semi-independent devices
tailored to specific jobs. Large computational problems (such as seeing, hearing,
moving, and talking) are split into many parts, all processed by specific brain
regions. These regions and their tasks can be revealed when they are damaged
by brain lesions or genetic mutations. They can sometimes be seen directly
in living brains by using imaging techniques. The specialized modules interact
extensively to perform their functions. Some of these modules (for example,
the visual areas specializing in form, motion, or color) have been partially
dissected not just logically but also literally, because their anatomical locations
are fairly precise and well known. The anatomy and physiology---the where,
what, and how---of other modules in the brain are generally less clear.
This chapter considers some ideas
about how this all comes together to generate consciousness and our subjective
experience. We begin by returning briefly to some issues we first raised in
Chapter 1, to note some classical debates on the question of consciousness,
whether "mindstuff" is in the same category as the rest of the physical
reality we know and whether consciousness can ever be explained. Assuming a
positive answer to this question, we then return to the arena of cognitive
neuroscience to discuss our conscious awareness system more thoroughly, briefly
outlining some of the neural underpinnings of the attention and working-memory
systems of the frontal lobes. Then we review some experiments in cognitive
psychology that show how the players or agents inside our heads, of which we
are largely unconscious, render meaningless the "Where does it all come
together?" question. The next section considers consciousness by examining
its perturbations, as in sleep and other altered states. Finally, after reviewing
current work on the neural correlates of consciousness, we outline several
models and metaphors for consciousness, ideas of what our "operating system" is
like, and how the "I" that you are experiencing right now might emerge
from it. Our conscious awareness is a small fraction of the many operations
going on in our brains, and the bottom line is that we must be disabused of
any notion that "our" thoughts reflect, in any direct sense, the
real operations of our minds.
The Mind/Body Problem
Rene Descartes's famous formulation "I
think, therefore I am" posited a clear dividing line between the mind
and the brain. The accompanying self-experiment illustrates the depth to which
this idea has permeated our culture and the way we construct ourselves. If
you are a classical dualist, you might assign the "I" to the realm
of spirit and the brain to the realm of the physical body, thus taking issue
with the materialism of modern science and the points outlined in Chapter 1,
which argues that mind is the sum of the physical components of the brain---its
ions, molecules, and cells and their activities. However, if the self or "I" is
different from the stuff of the brain, what is it? Is it some special kind
of mindstuff or essence distinct from the physical matter of which you are
composed? Dualists have the problem of explaining how mind, as a nonphysical
entity with no mass or energy, could interact with the physical stuff of the
brain.
*******
DESIGN NOTE: SELF-EXPERIMENT
Try the following exercise, which
was suggested by the philosopher Daniel Dennett. Look away from this page while
you repeat to yourself, "I have a brain." Now look away while you
think or speak, "This brain has itself." The first statement probably
feels comfortable to you, whereas the second feels a bit more alien, even though,
as far as we know, both statements are saying the same thing. Thinking "I
have a brain" corresponds to our daily experience of a narrative self,
a little person or "I" inside our heads, distinct from our body,
always seeing and commenting on what is taking place.
**********
There is another way in which you
might say that mind is different from brain without having to invent some sort
of spiritual or nonmaterial essence. You could believe that mind is ultimately
made up atoms and molecules, like everything we know about, but distinguish
what mind does---the functions it carries out---from the machinery that carries
out those functions. Thus you take mind or consciousness to be more than just
the electrical activities of cells in your brain in the same way a movie is
different from a description of the moving parts of the movie projector and
the path of the light through it, or the way writing is more than small pieces
of graphite spread on a paper surface in a continuous line. You could argue
that the input-analysis-output functions of our minds (taking in information,
analyzing it, then acting on it) could be carried out by different kinds of
hardware---perhaps by computers, Martians, or zombies. In this book, we have
not been dealing with these more hypothetical options but rather have focused
on the biology of mind in humans and other animals as we know them. From this
perspective, the analyses of what a mind does and what it is made of begin
to converge as we learn more and more from experiments in basic neuroscience
and cognitive psychology.
Thinking that you are just the
physical stuff of your brain and that a "chemistry of consciousness" specifies
your behavior may not feel quite right to you. But consider how profoundly
your mood can be altered by just a bit of coffee or alcohol or by the brain
chemistry underlying emotions that we discussed in Chapter 10. Consider the
numerous cases in the psychiatric literature of people who, with no previous
history of mental illness, became psychotic and were miraculously cured when
a bacterial infection of their cerebrospinal fluid was diagnosed and antibiotics
applied. Where does the body stop and the mind start in these cases? It makes
more sense to think of them as a unit. Chemistry can alter our behavior, and
our behavior can alter our chemistry. As an example of the latter, remember
a time when you have moved quickly to avoid sudden danger---and then remained
excited for a while as adrenaline continued to pump into your bloodstream.
Can the Problem of Consciousness
Be Solved?
Some modern commentators despair
of ever explaining consciousness. They just cannot grant that nerve signals
are the kind of stuff that can make up the qualitative feel or content of consciousness.
How can understanding nerve signals ever translate to what an apple tastes
like? The philosopher Owen Flanagan describes these individuals as the new
mysterians, the old mysterians being the Cartesian dualists described in the
previous section. The new mysterians don't deny that mind and consciousness
exist and operate in accordance with natural principles, but they argue that
our very cognitive constitution prevents us from achieving a complete description
of them. Asking ourselves to understand consciousness might be like asking
a television set to report on its own inner workings. Just as we don't expect
a dog's brain to understand quantum mechanics, we can't necessarily expect
our own brains to understand everything. However,
a useful approach is to act as though we can understand everything, because
there is no way of knowing that we can't understand something until after we
have tried.
It could be that, just as physicists
and mathematicians have acknowledged the limits to our understanding that are
imposed by Heisenberg's Uncertainty Principle and Godel's Incompleteness Theorem,
we are closed off from knowing what sort of natural phenomenon consciousness
is. It seems premature, however, to say that our brains can never understand
themselves; after all, they can do any number of other unusual things. We should
avoid arguments from ignorance, such as "neuroscience doesn't know how
to explain X (consciousness, for instance) in terms of the nervous system;
therefore it cannot be so explained. Rather it can eventually be explained
in terms of Y (pick your favorite thing... quantum wave packets, psychons,
ectoplasmic retrovibrations, etc.)." What
we can do is pursue descriptions from cognitive psychology of how mental life
works---descriptions given by neuroscience research---and also just listen
carefully to what individuals have to say about how things seem. In
some ways, we are similar to the subject of the familiar joke about the drunk
looking for car keys he lost near some dark bushes. He is looking for them
under a nearby street light "because the light is better here." We
can mimic this drunk when we are looking for an explanation of our minds, and
remain infatuated with our ideas of what a mind should be (by insisting, for
example, that brains can be modeled on the architecture of modern computers)
rather than trying to find out how the conscious human mind actually works.
We can become so impressed by the richness of what we know that we lose sight
of the much larger area of the unknown.
*******
DESIGN NOTE: IMPORTANT POINT
We are like the map makers of the
16th century who were very pleased with the sophistication of their efforts
to depict the globe, especially when compared with Greek and Roman attempts.
The large errors and unknowns that we now see in these old maps are revealed
only in hindsight. We surely can expect our descendants to be acutely aware
of the limitations of our 20th-century efforts to understand the brain.
**********
The Machinery of Awareness
Let's turn now from this brief
discussion of what we might or might not be able to know to scientific studies
on the nature of our awareness and attention. Our daily life is a constant
checking of what is happening right now against what just happened a few moments
ago. We hold information, anticipations, or goals in our short-term working
memory for a time and then release them if they become irrelevant. Should we
wish to retain knowledge of a face or a phone number, the mechanisms of episodic
memory described in Chapter 6 are activated. If we later need to remember a
name or number, we can fetch it from that long-term memory and place it back
in our working memory.
The classical paradigm for engaging
the interaction between current awareness and short-term memory is the delayed-choice
test, in which a human volunteer or an animal subject is rewarded for recognizing
a previously presented image or situation. Electrical recordings from animals
and brain imaging experiments on human subjects show activation of the prefrontal
cortex during such tests. This
activation persists though a delayed-choice test even if distracting and irrelevant
auditory and visual stimuli are presented. Prefrontal cortex is linked to all
of the perceptual systems, and it apparently orchestrates an emphasis on cues
that are relevant to current needs over irrelevant stimuli. This is why we
are more likely to notice the smell of food when we are hungry than when we
have just eaten.
It has been proposed that working
memory has a central controller and a group of "slave systems," the
two main slave systems being one for silent speech or verbal memory and the
other for visuospatial information. Imaging
studies, however, do not reveal an obvious activity corresponding to a central
controller. Rather, different kinds of working memory (such as for spatial
locations, object identification, facial memory, and verbal memory) may correlate
with activation of different prefrontal regions. Moving from simple delayed-choice
tests to more complicated tasks (such as articulating a random list of each
number from 1 to 10 with no repetition, or saying which words in a list read
aloud were vegetable names) causes activation to spread from a limited part
of the right frontal cortex to higher dorsolateral areas of both right and
left prefrontal cortex.
Phonological memory and subvocal
rehearsal activate Broca's area of the left hemisphere as well as several other
areas. Our visual working memory recruits areas of the frontal, parietal, and
occipital lobes of the right hemisphere. PET scan studies demonstrate that
while working memory is performing "what" tasks, different loci are
active in our prefrontal cortex than when "where" tasks are being
performed. Higher resolution
magnetic resonance imaging shows sustained activation of different areas of
frontal cortex for face memory and letter memory. Tasks that require working
memory for locations activate mainly right brain regions, whereas letter-memory
tasks activate primarily left brain areas. When
a main goal is being kept in mind while concurrent subgoals are being performed,
activation of the most anterior part of the frontal lobes is observed. In
both human and monkey brains, the ventral stream of "where" visual
information mentioned in Chapter 8 moves into dorsolateral prefrontal cortex;
the dorsal "what" stream enters mainly ventrolateral areas. This
is shown in Figure 12-1. Interestingly, there appears to have been a displacement
of these areas in the human to make room for the expansion of prefrontal areas
serving language and higher cognition. Recordings from individual cells in
monkey prefrontal cortex show that they monitor expectancy and goal-directed
behavior. Some cells fire while "what" information
is being held, others fire while "where" information is being held,
and still others register a combination of "what" and "where" activity. Working
memory is impaired by lowered levels of the neurotransmitter dopamine (this
happens in patients with Parkinson's disease) and is enhanced by increasing
dopamine levels.
Figure 12-1
Projection of spatial perception
and object recognition into the frontal lobe working-memory areas of the brain.
*******
DESIGN NOTE: IMPORTANT POINT
The "what" and "where" streams
of information sent forward from parietal and temporal lobes project into the
working-memory areas of the frontal cortex.
**********
The contents of our awareness and
working memory reflect only one of many parallel tracks that deal with perception
and action. Many studies have documented this, such as experiments that have
demonstrated and mapped brain regions that are responsive to novelty without
awareness. Subjects were asked
to press a keypad to indicate numbers (1, 2, or 3) appearing on a screen in
what appeared to be random order but in fact followed a complex sequence. Improvement
in performance indicated that subjects had learned the sequences even though
they were unaware of the existence of any order. If the order of the sequence
was then changed, changes in right prefrontal and other areas were noted, even
though subjects remained unaware of the changes in number presentation.
The Brain's Time and Space: The
Disappearance of "I"
What we are holding in our working
memory---our current awareness and recall of our immediate past---are central
components of our experience of an "I." This "I" is the
basis for our common-sense notion that our mind has some central point where
sensations and perceptions ultimately are noticed, and where commands and actions
ultimately begin. Naturally, this conviction leads us to look for an actual
place in the brain where it all comes together. But then, who would be watching
that central place? And wouldn't that watcher need yet another to watch it?
It is one of the main messages of this book that there is no place where everything
comes together and that looking for the place in the brain where the "I" is
makes no more sense than looking for the place in a tornado where "the
storm" is. The "I" is a construct---an idea that we employ to
make sense of what many unconscious processes finally lead us to do---much
as "the storm" is the sum of what the turbulent, spinning masses
of air in a tornado do.
We have already noted that the
distinction between perception and action grows very fuzzy as we work our way
further into the brain and shift to the time interval of 50--500 milliseconds
after a sensory stimulus or before a movement. This is because there is no
single point in the brain through which all information funnels. The reason
it seems natural to make the distinction is that most of our conscious experience
of sensing or acting unfolds over a period of seconds rather than milliseconds,
as when we see a ball tossed and look to where it will land, or when we perform
the motions of peeling potatoes. But in the central brain processing that occurs
in the interface between our acting and reacting, most of the action is very
fast---so fast that auditory input, for example, can influence a visual search
on the time scale of 100--300 milliseconds.
*******
DESIGN NOTE: LONG SELF-EXPERIMENT
To give you a feel for the lengths
of time we are talking about, it takes about 1000 milliseconds to say "one
Mississippi," we can start and stop a stopwatch in roughly 175 milliseconds,
one frame of a movie film projects for about 40 milliseconds, most nerve cells
work on a time scale of 1--10 milliseconds. If you can drum all the fingers
of one of your hands on the desk five times while you are saying "one
Mississippi" a finger is hitting the surface every 40 milliseconds.
Try the experiment of holding a
3-x-5 card just above a friend's thumb and forefinger and ask your friend to
catch the card when you let it go. He or she can't do it if the card is immediately
above the thumb and forefinger. The drop takes 100--150 milliseconds, faster
than the nerve signal that it is dropping can go from eye to brain to fingers.
If you hold the card 3 inches above the fingers, however, your friend can catch
it; 300--400 milliseconds is enough time.
*******
Plastic Representations of Time
and Space
Two well-known cognitive psychology
experiments show that in these small time intervals, our brain is an assembly
of agents with minds of their own (so to speak) that can play with even the
most basic representations of time and space in ways to which we are normally
oblivious. What happens during these intervals shows the impossibility of the "I" acting
as boss or command central. These experiments emphasize perception, but a corresponding
list could be drawn up of experiments dealing with the output of the brain,
or action.
The first experiment involves a
phenomenon that has been named the phi effect; it is depicted in Figure 12-2.
Two stationary spots of light separated by as much as 4 degrees of visual angle
are lit in succession. First one spot is on for 150 milliseconds, then both
are dark for 50 milliseconds, and then the other spot is on for 150 milliseconds.
They appear to be a single spot moving in a continuous line. If the first spot
is red and the second is green, the moving spot appears to have changed its
color from red to green abruptly in the middle of its passage, during the period
of darkness. Why is it our perception that the color changed to green before
the green light actually came on? There is virtually no difference in the response
time if a subject is asked to press a button as soon as the red spot comes
on, whether or not the green spot follows! It can be shown that button-pressing
responses would have to have been initiated before the discrimination of the
second stimulus, the green spot. Thus we did not hold up our conscious experience
for at least 200 milliseconds and revise history. Rather, processing is continually
going on. A revision occurs that leads us to think the spot changed color in
mid-flight. At the level of basic brain operations, this could translate into
something very straightforward. For example, the V5 region in the cortex (see
Chapter 8) responds to motions and to apparent motion. Perhaps some activity
in area V5 is the brain concluding that an intervening motion occurred.
Figure 12-2
The phi effect. If a red and a
green spot of light are flashed on a screen, as shown, in rapid succession,
they appear to be a single moving spot whose color changes in the middle of
the motion.
We must distinguish here between
the representation and the represented: The brain's representation of time
needn't use time-in-the-brain. The objective sequence of events occurring in
the brain of an observer can be different from the subjective sequence of events
reported. The mis-discrimination of red-turning-to-green occurred in the brain
after the discrimination of the green spot (in objective time), but the subjective
or narrative sequence is red spot, then red turning to green, then finally
green spot. Another way of putting it is to say that a unit of subjective time
can incorporate several units of physical time, as shown in Figure 12-3, and,
if appropriate, rearrange them.
Figure 12-3
Space and time in the brain. The
brain can store events that occur during sequential units of physical time
(indicated by the Roman numerals) in temporary holding spaces, or buffers (indicated
by the Arabic numerals). These buffers can rearrange the events in a way that
past experience indicates makes the most "sense" before a subjective
report is generated. In the case of the phi effect, the brain appears to be
following a rule that "two dots appearing in rapid succession near each
other probably represent a single moving spot of light."
A similar situation occurs for
the representation of space in the brain, which, as we saw in Chapter 8, does
not always use space-in-the-brain to represent space. These discrepancies are
no different in principle from shooting scenes in a movie in a different sequence
from the one in which they finally appear, or reading "Bill arrived at
the party after Sally, but Jane came earlier than both of them." You learn
of Bill's arrival before you learn of Jane's earlier arrival. (In fact, the
representation of time needn't be temporal at all. Think of the way in which
different points of time are represented by a wall calendar.)
A second classical experiment involves
what is called metacontrast. A display device that can flash a stimulus for
very brief intervals, around 1--500 milliseconds, is used to show the two stimuli
represented in Figure 12-4 for 30 milliseconds each, about as long as a single
frame of television, the second stimuli immediately following the first. In
the figure, the disc appears to the side of the ring, but in the experiment
the ring is flashed right on top of the disc, and its inner diameter is exactly
the same as the outer diameter of the disc.
Figure 12-4
Metacontrast. If the first and
second stimuli are flashed in rapid succession, only the second (the ring)
is observed, even though the first (the disc) is observed if it is flashed
by itself. (In the experiment, the disc to the left is actually flashed in
the center of the ring.) Thus the brain takes a while to decide what is "really" there,
and an initial stimulus can be overwritten by another that has features in
common with the first.
Individuals report seeing only
the second stimulus, the ring, although they can report the disc if it is shown
separately. The brain, initially informed that something with a circular contour
in a particular place happened, swiftly receives confirmation that there was
indeed a ring, with an inner and outer contour. Without further supporting
evidence that there was a disc, the brain arrives at the conservative conclusion
that there was only a ring. The disc was briefly in a position---functional
and spatial---to contribute to a later report, but this state lapsed. There
is no reason to insist that the state was inside the charmed circle of consciousness
until it got overwritten, or to insist that it never quite achieved this state.
Preliminary versions that were composed at particular times and places in the
brain were later withdrawn from circulation, replaced by more refined versions.
Multiple preliminary versions might be withdrawn, but they also can influence
a final version, even if they are not experienced. For example, very brief
presentation of a priming word can influence the judged meaning of a target
word that follows within 100 milliseconds, even though the priming word is
not experienced or reported.
*******
DESIGN NOTE: IMPORTANT POINT
Our intuition, what we think is
happening, is that information gets moved around the brain with events occurring
in sequence, like railroad cars on a track. The order in which the information
passes some point, or station, will be the order in which it arrives in consciousness.
But that isn't what we observe in the experiments described in this section,
and there is a good reason. The brain's job is to use incoming information
swiftly to "produce a future," staying one step ahead of disaster,
and it has to do this efficiently. The processes that do this are spatially
distributed across functional regions of a large brain, and communication between
different regions is relatively slow. Why wait for everything to come together
if appropriate action could be taken faster without that delay?
*******
The Futility of Asking "Where
Does It All Come Together?"
The philosopher Daniel Dennett
uses an analogy to drive home the futility of the question "Exactly when
do you become conscious of a stimulus?" Imagine your brain to be like
the British Empire, and rephrase the question as follows: "Exactly when
did the British Empire become aware of the truce in the War of 1812?" News
of the truce, signed in Belgium on December 24, 1814, reached different parts
of the British Empire (in America, India, Africa, and so on) at different times,
and it did not reach the most far-flung outposts until mid-January 1815. There
is no way to pin down a day and hour when "the Empire" knew. The
battle of New Orleans was fought 15 days after the truce that made it moot.
Even if we can give precise times for the various moments at which various
officials of the Empire became informed, none of these moments can be singled
out as the time the Empire itself was informed. This is entirely analogous
to asking exactly when we become conscious of a stimulus. Because cognition
and control, and hence consciousness, are distributed throughout the brain,
no one moment can count as the precise moment at which each conscious event
happens.
In Dennett's "multiple drafts" theory
of consciousness, all varieties of perception---of thought or mental activity---are
accomplished in the brain by parallel multitrack processes of interpretation
and elaboration of sensory inputs. Information entering the nervous system
is subject to continuous "editorial revision." The brain adjusting
for movements of our head and eyes so that we experience a stable visual world
is an example of such revision. Revision lets us watch a French film dubbed
in English, not noticing the discrepancy between lip motions and sounds heard.
All these editorial processes are occurring over large fractions of a second,
during which time various additions, incorporations, emendations, and overwriting
of contents can occur.
Spatially and temporally distributed
content fixations in the brain (such as visual, auditory, and locomotor) are
precisely locatable in both space and time, but their onsets do not mark the
onset of consciousness of their content. These content discriminations produce
something rather like a narrative stream, or sequence, subject to continuous
editing by many processes distributed around the brain. There are ongoing multiple
drafts. There is never a final draft, just a current version. This applies
not only to input and sensing but also to acting, moving, speaking, and responding
to the question "What do I do next?"
*******
DESIGN NOTE: IMPORTANT POINT
The "common-sense" idea
has been that a where-it-all-comes-together hierarchy responds to a watcher
and then instructs a director who commands output. This view has been decisively
replaced by the idea of many competing parallel streams of input and output
that are constantly being compared, sorted, and tested for appropriateness.
The interpretations and actions that "work" or "are appropriate" rise
to the surface to constitute our subjective experience.
*******
The "extended present"---during
which multiple drafts can be tested, expanded, contracted, and rearranged to
make subjective time---is different from physical time. Different
states of vigilance, or mind-altering drugs, appear to change the depth of
the conscious present. In his description of being under the influence of mescaline,
Aldous Huxley mentions greatly enhanced visual and auditory impressions, as
though sensory activity were continuing to reverberate more strongly beyond
its normal limits, perhaps changing the duration of the conscious present.
Depressed people sometimes describe loss of visual intensity, colors appearing
flat and washed out, as though sensory activity had been curtailed and the
conscious present shrunk. The ultimate damping down might occur in sleep, when
the conscious present effectively shrinks to nothing, and subjective time becomes
the same as the stream of physical time.
Sleep and Other Altered States
of Consciousness
One route to getting a handle on
the experience of consciousness is to contrast it with what might be going
on when we either are not conscious or are in another state that feels very
different. During a substantial fraction of all our lives, our brain is not
computing an "I" but is doing something else: sleeping and dreaming.
The two systems that alternate to control waking and sleeping roughly mirror
the division of labor between the sympathetic (noradrenergic) and parasympathetic
(cholinergic) branches of our autonomic nervous system. In the waking state,
aminergic neurons in the brainstem that radiate to the cortex cause alertness
by literally "spritzing" the brain with neuromodulators (see Figure
12-5) at the same time that the sympathetic (arousal) part of the peripheral
autonomic nervous system is activated. Sleep occurs when this system is suppressed
as a cholinergic spritzing system, also shown in Figure 12-5, brings up parasympathetic
(resting) nervous activity. Nerve cells in the rostral hypothalamus also play
a central role in this sleep generation. During
sleep, the brainstem is instructing your spinal cord that you can't feel and
you can't move, and the cortex
is disconnected from sensory input and motor output.
*******
DESIGN NOTE: IMPORTANT POINT
Waking and sleeping are controlled
by two hierarchical systems spanning the brainstem to the cortex, systems so
reliable that it is as rare for us to gain awareness that we are dreaming when
we are dreaming as it is for us to hallucinate when we are awake.
*******
Figure 12-5
During the waking state, a noradrenergic
system is activated to maintain arousal of the cortex (solid lines). During
sleep, this system is suppressed as a cholinergic system predominates and induces
periods of REM sleep (dashed lines).
Stages of Sleep and Dreaming
A typical night of sleep proceeds
in four or five cycles, each usually described as having four stages. A cycle
starts with light dreaming (called REM sleep, for rapid eye movement), during
which heart and respiration are active and vivid hallucinations and emotions
are occurring. It then progresses through two intermediate stages to stage
4, wherein there is no dreaming, and body respiration and temperature are at
their lowest point. This non-REM sleep is known to boost immune function. The
cycle ends with a return to REM sleep. REM sleep appears to be essential for
long-term maintenance of the brain; animal and human subjects deprived of it
eventually die. Brain imaging experiments show that REM sleep correlates with
the activation of several areas, particularly the amygdala.
Why do we dream? No one is sure,
but one theory is that dreaming plays some role in the consolidation of memories,
experiences, and emotions. One
particularly fascinating study shows that in rats, hippocampal cells that show
place-specific activity during waking are selectively activated in subsequent
sleep, possibly participating in a memory consolidation process. During
dreaming, the brain may be making up stories to make sense of emotions. (Human
studies show that the emotions are twice as likely to be unpleasant than pleasant.)
The sleep researcher Alan Hobson offers an "activation-synthesis hypothesis" that
suggests that cholinergic and other activation rising from the brainstem stimulates
the cortex relatively nonspecifically, causing it to confabulate the storylines
of dreams, which are frequently bizarre and disconnected, to go with the underlying
chemistry. Such a model is not unreasonable, given the examples of confabulation
exhibited by split-brain and anosognosic patients mentioned in earlier chapters.
The brain is essentially insane during dreams, exhibiting bizarre cognitive
features such as discontinuity, incongruity, and hallucinations. Anyone who
displayed the sort of mental activity during wakefulness that goes on during
dreams would be judged psychotic.
What does all of this have to do
with understanding consciousness? One interesting suggestion is that wakefulness
and REM sleep may be very similar states, subserved by an internal thalamocortical
loop that proceeds during both. The
idea is that the thalamus and cortex are expending most of their effort talking
back and forth with each other. At no time does sensory input constitute more
than a fraction of what is going on. The major difference between wakefulness
and sleep lies in the weight given to incoming sensory input. In this model,
consciousness is essentially a closed loop, driven by intrinsically active
cells. Thus we have the image of a central churning processor that can proceed
in a manner either dependent on or independent of sensory input---an internal
babbler that keeps on going in dreams. Our brains would be continuously spinning
a self, automatically generating a web of perceptions, words, and deeds. During
waking, this spinning would be guided by the environment.
Mystical Experience
The uncoupling of the brain from
sensory input and motor output during sleep is reminiscent of another altered
state: mystical experience. Mystical experiences involving a very different
sense of "I" have been reported throughout history by individuals
in different religious traditions. They
frequently are accompanied by feelings of deep conviction, ineffability, and
cosmic unity. The science journalist Timothy Ferris suggests possible correlations
between these experiences and processes in the brain: The conviction is that
something powerful has been learned beyond the world of the senses (an uncoupling
of what the brain is doing from sensory input, as in sleep?). Ineffability
is the experience being beyond words (reflecting a core symbolic device lying
at deeper levels than the modules of language that provide it with input and
output?). The cosmic sense of unity is that everything contains the seed of
everything else (is this the mechanism that makes a unified mind out of the
disparate parts of the human brain?). Might enlightenment correspond to breaking
through the barrier of language, to awareness of the mental modules responsible
for representing the many functions of the brain as a whole? Or could meditation
systematically reduce mental activity to the point of being devoid of content
while still aware---a state independent of perceptions and actions?
*******
DESIGN NOTE: IMPORTANT POINT
Perhaps mystical phenomena have
more to do with the internal architecture of the brain than with the phenomena
of the outer world, corresponding to a mind detached from the thoughts that
are its normal objects.
*******
We enter another sort of state
in which we are not computing an "I" when we are made unconscious
by anesthesia. Interestingly,
human subjects show implicit memory of a random word list read while they are
unconscious; thus, if presented with another random list, they are more likely
to select the words that were read without knowing why. When the brain activity
of unconscious and conscious subjects is imaged during reading of the first
list, both show increased activity in verbal memory areas, but activity in
the mediodorsal thalamic nucleus is much higher in conscious subjects. This
suggests that distinctions between conscious and unconscious memory may depend
on the presence or absence of correlated thalamic activity, and it reinforces
the idea that thalamic activity is a necessary component of the conscious state.
Humor and Laughter
Does laughter give us a window
on the mind? Ferris, whom we just mentioned, offers the interesting notion
that laughter arises from interaction of two programs in the brain: an Apollo
program, which constructs plausible models of reality, and a Pan program, which
challenges such models. The
Apollo program is sober, responsible, and creative, but the models it creates
always have flaws. The Pan program is irreverent, skeptical, and playful. Hence
we have a god that builds models and a god that mocks and tears them down.
A laugh is the energy released when the Pan program spots a potentially dangerous
error in a model crafted by Apollo, but the error turns out to be harmless.
The stress first aroused is then quickly dispelled through the physical manifestations
of laughter. The idea is that we take pleasure in laughter both because we
enjoy the release from anxiety that it provides and because the brain feels
pleasure in discovering incongruities between perception and reality.
Possible locations for the Apollo
and Pan programs are suggested by observations on anosognosic patients (see
Chapter 7). Patients with right hemisphere damage who deny the possession of
some parts of their body are much more likely to confabulate rationalizations
of their situation. It is as though a right hemisphere locus of emotions involved
in detecting and reacting to anomalies and discrepancies has been disabled.
The left hemisphere, whose job is to make up plausible stories, is no longer
edited or monitored by the right hemisphere. Another
dissociation into component selves is observed in split-brain patients who
confabulate with their linguistic left hemisphere the reason for a behavior
that is initiated by an instruction directed to their nonverbal right hemisphere.
Observations such as these remind us again that consciousness is far from being
a unitary phenomenon.
Neural Correlates and Models of
Consciousness
Much as we earlier asked about
the neuronal correlates of our visual experience (see Chapter 8), we would
like to know the explicit neuronal correlates of other aspects of our conscious
experience. Are there specific neurons whose activity mediates consciousness,
whose firing gives rise to the current content of our consciousness? We are
looking for mechanisms that support the brain's awareness of a small fraction
of all the information available to it---information that includes our experience,
identity, sensations, perceptions, action, attention, memory, emotional states,
and so on. Conscious awareness is just the tip of a large, unconscious iceberg.
Its most common metaphor is that of a spotlight surrounded by a fringe, or
halo, of lesser illumination.
You might suppose, given the increasing
respectability of efforts to look for neural correlates of consciousness, that
a well-defined problem was being pursued. This is far from the truth, because
in most cases we lack a thorough or rigorous description of what it is we are
trying to explain. Before we can make much headway regarding neural correlates
of consciousness we need to specify what varieties of consciousness we're talking
about. At a minimum we must distinguish, as outlined in Chapter 1, "consciousness" from "self
consciousness" from "reflexive self consciousness." The latter
consists of the internal simulation of self-reference---of thinking about thinking---that
appears to be unique to humans. Humans have a variety of "meta" levels
of cognition, of conscious states that think about each other (such as the
ego states mentioned in Chapter 7) or about lower levels (such as the primary
and secondary emotions described in Chapter 10). Furthermore, our consciousness
is a constant mingling of our present experience with the past memories evoked
by that experience. An underlying self-system appears to interpose itself as
an editor of our perceptions, as noted in Chapter 8, a system that can alter
or delete items that threaten it.
Determining what nerve-firing patterns
correlate with even the simplest of these varieties of consciousness is a daunting
task. Many so-called neuronal codes have yet to be cracked. At a fundamental
level we have to know, for example, what the firing of action potentials by
single nerve cells in our brain means. It has commonly been supposed that the
information is contained just in the firing rate, the number of action potentials
sent down its axons in any given period. Recent work has suggested, however,
that firing patterns may have clustered substructures, or temporal codes, that
communicate several different meanings at once. It is also likely that the
meaning of an action potential train is affected by whether it is in synchrony
with the action potentials of other cells.
**********
DESIGN NOTE: IMPORTANT POINT
We have the technical problem that
we can record the detailed activity of only one neuron or a small number of
neurons at the same time. This is like trying to decipher a television video
image one pixel at a time while the image is constantly changing. Remarkably
enough, despite this, the activity of some cells in the visual cortex can be
correlated with subjective visual perception. We
also can use imaging techniques (such as EEG, PET, and MRI) that sample the
activity of many nerve cells without giving much detail about each, but these
do not yet provide the resolution required to define functional splotches,
blobs, or columns of cells that have dimensions of less than 1 millimeter.
What they can give us is gross neuronal correlates such as those showing activation
of motion-processing regions of the visual cortex.
**********
Brain Structures Required for Conscious
Awareness
What, then, is it that binds together,
and selects from the many assemblies of neurons, the awareness of the moment?
Modern answers to this question of how brain cells might generate behavior
or consciousness often starts
with a suggestion by the Canadian psychologist Donald Hebb: that nerve cells
related to a particular function form a so-called cell assembly, a sort of "three-dimensional
fishnet" that can be dispersed widely through the brain. As
an analogy at the level of human culture, consider the residents of a large
city who organize themselves into different interest groups: families, the
Lutheran Church, a motorcycle gang, a university's alumni club, an environmental
group, bridge foursomes, and so on. These social assemblies are interconnected
by overlapping memberships. Any particular assembly reaches the threshold of
being active as a group (in other words, it meets) only a small fraction of
the time. And some of these groups may occasionally join together in larger
associations to address common interests. Most individuals belong to many such
groups.
What evidence is there for such
nerve cell assemblies? The closest we come is in observations that neurons
activated by the same object in the world discharge in synchrony. Recall
that this correlated firing was proposed in Chapter 8 (in the section"A
Binding Process Underlies Visual Perception") as a solution to the binding
problem of how different aspects of a visual stimulus are organized into a
perception. Several observations suggest that the thalamus is needed to bring
these bound assemblies to awareness. It sits at the center of the radial organization
of the brain, much like the hub of a bicycle wheel, which connects to the rim
of the wheel with a large number of spokes (see Figure 12-6). Large areas of
cortex can be removed without abolishing awareness (as in patients who have
lost most of their frontal, occipital, temporal, or parietal lobes yet remain
aware), but bilateral lesions that occur in the relatively small intralaminar
nuclei of the thalamus when its blood supply is interrupted do cause loss of
conscious awareness. This area makes reciprocal connections with nearly all
parts of the cortex.
**********
DESIGN NOTE: IMPORTANT POINT
Different cortical areas have different
weights in contributing to consciousness and to subjective "what it feels
like" experience. The temporal lobes, and in particular the amygdala,
appear to be central. The most profound disturbances in consciousness are those
that generate temporal lobe seizures. Nothing dramatic happens to consciousness
if you damage the frontal lobes, or portions of higher sensory or motor areas.
The temporal lobes could be viewed as being an interface between perception
and action, where the significance or meaning of things to the organism is
being determined.
**********
How might the intralaminar nuclei
interact with other cortical areas in generating consciousness? The neurophysiologist
Rodolfo Llinas has suggested that the intralaminar nucleus is the seat of a
scanning mechanism that operates every 12.5 milliseconds to send out a series
of overlapping 40-cycle-per-second (hertz) signals (these are bursts of action
potentials occurring every 25 milliseconds that sweep across the brain). These
entrain synchronized cells in the cortex that are currently recording sensory
information to their own rhythm (see Figure 12-6). The synchronized cells then
fire a coherent wave of messages back to the thalamus. The phase of the 40-hertz
signal can be reset by new input. Llinas likens the wave of nervous impulses,
radiating out from around the intralaminar nucleus to overlying parts of the
cortex, to the central arm of an old-fashioned radar screen, illuminating each
object in its path as it moves. The responses received by the thalamus within
one cycle of its scan are perceived as a single moment of consciousness. A
succession of such moments are created so fast that they seem to be continuous.
In this model, consciousness is the dialog between the thalamus and the cerebral
cortex, as mediated by the senses. The
thalamus playing the central role seems logical, because it is the structure
through which all of the sensory information coming to the brain must pass.
Figure 12-6
One model for how different aspects
of a conscious experience may be bound together. Two systems linking thalamus
and cortex are proposed. The first (dashed lines) is a series of closed oscillating
loops between specific sensory or motor nuclei of the thalamus and the regions
of cortex to which they report via layer 4 of the cortex. Within each modality
(visual, auditory, somatosensory, and so on), the firing of cells reflecting
the relevant percept are coordinated. Each of these is then bound or recruited
into a larger unison by a second, more global assembly of loops between the
thalamus and layer 1 of the cortex (solid lines). These sweep as waves of activity
from the front to the back of the cortex, as indicated by the arrow at the
top, starting every 12.5 millisecond. Each of the waves has a frequency of
about 40 hertz, corresponding to bursts of action potentials occurring every
25 millisecond. Each of the sweeps that start every 12.5 millisecond corresponds
to one "quantum" of conscious experience.
It is interesting that coherent,
40-hertz electromagnetic activity is strong in people who are awake or in rapid
eye movement (REM) dreaming sleep but is very reduced during deep sleep. It
is reset by sensory stimuli in the waking state, but not during either REM
or deep sleep. This is one
basis for the suggestion that REM sleep might be a correlate of mental awareness,
reflecting resonance between thalamocortical-specific and nonspecific loops.
The specific loops may give the content of cognition, and a nonspecific loop
the temporal binding required for the unity of cognitive experience. This system
would be modulated by an arousal system, known since the 19th century, that
originates in the brainstem reticular core and projects through relays in the
thalamus to the cortex. Electrical stimulation of this core can enhance the
40-hertz signal as well as the synchronization of responses in the visual cortex. A
word of caution is necessary here: We don't really have evidence yet that these
40-hertz signals are any more significant than the 60-hertz hum of the power
transformer on your stereo sound system or personal computer. These signals
may just be saying that the system is operational, not telling us much about
its specific states.
Generating an Apparent "I"
There is a growing consensus on
what models for the mind would be appropriate; a view consensus shared by workers
in psychology, neurobiology, artificial intelligence, anthropology, and philosophy.
All see the mind as a "pandemonium"---a collection of specialized
agents acting in parallel, communicating and competing with each other to generate
perceptions and behaviors. The unifying idea is that the brain is essentially
a Darwin Machine, an array of many modular processors working all at once,
without any single center or processor (the "self") running the whole
show, from which control and
consciousness emerge. Previous chapters have already mentioned Darwin Machine
models for skilled action and language generation.
What is being sought is a plausible
model for how we generate an apparent "I." Many have remarked on
the relatively slow, awkward pace of conscious mental activity, and on how
little can be held in awareness at one time, compared with vast amount of processing
we know goes on. And some have long suspected that this might be because the
brain was not really designed---hardwired---for such activity. Human narrative
consciousness in particular might then be the activity of some sort of serial
virtual machine implemented on the parallel hardware of the brain. This presumably
would have been accomplished by the mythic mind that developed language and
thus finally tamed the parallel pandemonium of the present-centered archaic
sapiens brain so that very long sequences of verbal meaning could be developed
and retained.
In this model, the contents of
our awareness at each moment are the current winners from among a horde of
competing specialists' reports. Most of these specialists were not designed
for specifically human actions such as speaking but for the myriad components
of perception, action, and emotions characteristic of higher vertebrates. Perhaps
the most accessible metaphor is William Calvin's railroad marshaling yard,
described in Chapter 9, in which parallel tracks all connect to one exit track.
Each of the parallel tracks is a candidate sequence for perception or action,
and a competition determines which one will make it to the exit. The brain
produces a virtual center---the illusion of a central viewpoint---just as physicists
find it useful to represent the earth's gravity as an attraction to the center
of the earth, whereas in fact it is a function of interactions between individual
atoms across the whole planet. Our consciousness is composed of many functional
subcomponents, but nevertheless we have a unity of conscious experience and
the virtual equivalent of a narrator.
**********
DESIGN NOTE: IMPORTANT POINT
The apparent unity and continuity
of our experience may be generated like the emergent behavior of a mob, much
as some flocks of birds maintain cohesion without having any single leader,
or like the "hive mind" of bees, who become increasingly activated
and then swarm when enough scouts have returned to the nest reporting a favorable
destination. We could also
invoke the image of a choir of individual voices coalescing into a synchronous
chorus, having risen from the cacophony.
**********
Introspection and the Self System
At several intervals throughout
this book, we have tried simple exercises on sensing, moving, thinking, and
feeling that involve time scales of less than 1 second. They may have permitted
you to notice some brain processes that you are normally unaware of. This chapter
has reinforced the point that during this period, many normally unconscious
agents can massage the self or "I" that emerges as our subjective
experience. We are far from being human analogs of a camcorder, using rote
neural hardware to link the onset of stimulation and the appearance of a conscious
image. A variety of experiments have demonstrated that unconscious processes
can bias our perceptions in the interval before they become conscious. After
rote processing of elemental chunks of information, as described in Chapter
8 for visual input, we appear to interpose an additional strategic delay that
allows for checking and editing the developing sketch or draft, so that elements
that might threaten our underlying self system can be altered or deleted.
**********
DESIGN NOTE: IMPORTANT POINT
When incoming information is threatening
to the self system, a dissociation can appear between our conscious report,
which denies or ignores the potential threat, and behavioral indicators of
unconscious processing (such as changes in skin resistance, indicating autonomic
arousal). Denial reaches an extreme in anosognosic patients, who, in defense
of their self system, might report that nothing is wrong with a paralyzed left
arm.
**********
Most of us, after the first 100--200
milliseconds of processing a new input or idea, permit an additional delay
of 400--500 milliseconds for strategic editing and revision. This is the time
window we were tuning in to during the exercises in Chapter 9 on movement and
those in Chapter 10 on dissociating thoughts and feelings. By slowing down
enough to notice quietly the flickerings in our thinking and body sensations
during this period, we can engage in a sort of "attentional training" in
which self operations during the strategic delay can be noted rather than ignored
(see Figure 12-7). Such training
is a central component of some meditation techniques. The reason why introspective
techniques are a core of many cognitive therapies for modifying behaviors is
that the observation of feelings associating with thoughts, or the coloring
of perceptions by the self-system permits "editorial" modifications
of these processes that are not possible if they remain unobserved. Like the
Russian dolls that nest inside each other, we can ascend to the level of metacognition,
a state of "thinking about thinking."
Figure 12-7
A depiction of time frames of unconscious
and conscious experience, suggesting that introspection techniques can expand
conscious awareness into the time interval during which unconscious editing
normally occurs, making it possible to observe such editing. The time frames
shown as very approximate.
Human efforts to think about thinking
have a long history, and it is interesting that what modern psychology and
cognitive neuroscience say about how the mind works can be taken as a restatement,
in modern form, of what Buddhist psychology has been saying about the nature
of cognition for the past 1500 years. The labels and lists that are supplied
by Buddhists and cognitive neuroscientists differ, but many of the underlying
fundamentals appear to be similar. Buddhist texts carefully describe the "co-dependent
arising" of observer and object, and of sensing and acting, and emphasize
the futility of trying to explain one apart from the other. Points about the
relativity of knowledge that we have raised here are explicitly addressed.
Meditation techniques best described as "mindfulness" or "awareness" are
used to sense the transitory, shifting nature of all phenomena and the various
guises of the ego self, "I," that we take to be who we are. An observer
is established that is aware of the succession and variety of ego states that
we have considered. The attitude
of just observing is cultivated: observing the successive graspings or attachments
to this or that goal or desire, observing the successions of radically different
personalities that can inhabit our bodies from moment to moment. From this
state of mindfulness or awareness, one grows to appreciate more and more the
relativity of all things, which is called groundlessness or "sunyata" in
the Buddhist tradition. This state of non-ego attachment is said to allow energy
for compassion, empathy, and service to arise spontaneously.
This position, poise, or process
is a logical stance for addressing and dealing with the sorts of questions
raised in this book on how the mind works. In the absence of mindfulness we
are likely to act from some sort of unconscious ego position as we study and
evaluate phenomena. This can include having a set of unconscious starting assumptions
or positions that bias how we look at things without our being aware of it
(such as assuming the clean separateness of sensing and acting or of subject
and object). We don't have to look far for examples of this. Even though many
neuroscientists know Cartesian dualism to be discredited, they still are practicing
dualists. Their research does not fully integrate an appreciation of the constructive
interchange that is involved in cognition, the constant co-development and
co-shaping of a nervous system and its environment. Being aware of the many
selves, egos, or biases that we generate is a better basis for action in the
world than being unconsciously wrapped up in one of those selves and unaware
of the processes that generated it. Put more simply, it can be more useful
to be conscious, than unconscious, of the relativity of the self that is driving
you---the mind in place at any one time.
Computer Metaphors
No discussion of models of the
brain would be complete without mention of what has become a most pervasive
model: the metaphor of the brain as a giant computer, with neurons being the
hardware circuit elements. It has been postulated that the way computers carry
out functions might be relevant to how the brain does similar things and that
conversely, what brains do might serve as a model for designing more intelligent
machines. A recent approach has been to ask how what we know about the workings
of a brain and the organism it serves might inform the construction of computers
or robots that have similar properties. In particular, because we know that
the development and behavior of an organism are shaped through an evolutionary
process of replicating what trial and error has shown to work, why not build
computers that "try" to behave in a complicated way using these same
Darwinian mechanisms?
The study of artificial neural
nets modeled on computers is one example of this approach. An army of neuron-like
elements are linked together in a parallel array with many interconnections.
Such nets can be taught to recognize words, generate speech, distinguish male
and female faces and their emotions, diagnose heart conditions from EKG data,
or drive a truck down a freeway. During a training phase, intermediate results
are fed back into the system to let it know how it i doing. Such
neural nets have even been used to simulate the effect of brain damage on reading
and to model the development of language capabilities. Because
there is a lot of redundancy in the network, many of its elements can be removed
with only a small effect on overall performance.
Another approach has been to play
with the design of "creatures," small mobile robots attempting to
mimic the behavior of a simple animal such as a cockroach. These are constructed
from a number of activity-producing subsystems, each of which can individually
connect sensation to action. The creatures are tested in the real world. (One
example is a simulated cockroach with a neural net of 78 neurons and 156 connections ).
Learning rules are applied that select and reinforce the connections that work
best in guiding the insect to wander, follow edges, and find and consume patches
of food. This approach to trying to understand whole embodied agents interacting
with a world, whether these agents are ourselves or simpler machines that we
build, is an antidote to placing undue emphasis on the brain by itself. The
generation of action and the problem solving done by the brain simply can't
be separated from the environment it must continually exploit, interact with,
and be transformed by. Mind includes the interactive loops an organism makes
with its environment---a point raised in several previous chapters and developed
further in the final chapter.
Summary
We have taken several different
trajectories in this chapter, trying to frame the central issue of explaining
our self consciousness. Even to engage the issue requires that we sidestep
a number of arguments-in-principle that the problem of consciousness can never
be solved and that we assert, instead, that it doesn't hurt to keep trying
until a more convincing case for the impossibility of our quest has been stated.
We can't help but feel that we are getting closer as we observe the activity
of our prefrontal cortexes while they carry out tasks involving attention and
working memory, or when we devise clever experiments that show how our brains
can rearrange time and space. We can see, through studies of thalamus-cortical
interactions during sleep and waking, as well as through brain lesion and brain
imaging studies, some plausible candidates for neural correlates of consciousness.
Where does all this leave us, in
trying to think about the "I" of our self consciousness? It seems
perfectly natural for us to think that our brains see and feel the world as "we" do---that
they chunk concepts and perceptions according to the linguistic conventions "we" use.
We encourage this inclination when we play with introspective exercises of
the sort that have been sprinkled throughout this book. What we have to keep
in mind is that such exercises may be dealing with only a very limited kind
of access, for our brains basically are survival machines whose sophisticated
perceptual and motor modules were shaped by evolution long before the recent
add-on of linguistic abilities. The awareness we try to explain with these
abilities is probably just that small fraction of our total brain/body activities
that has proved useful for survival and reproduction. There would be no point
in our consciously attending to the vast number of subterranean activities
that massage and process our perception and action, for example, to directly
sense knowledge of animals, persons, or tools being stored in different parts
of our temporal lobes. What our brains display to our awareness is like the
display panel on our car's dashboard: indicators that inform us and can make
a difference in how we drive the car. Our
linguistic habits for "chunking" reality bear no deep resemblance
to the parallel and distributed brain mechanisms used in information storage
and retrieval. Our main hope is that through new techniques such as more sophisticated
monitoring of brain activity, we can gain better insight into the relationship
among the local environment, brain activity, and the patchwork construction
of our sense of self---between our "I" and the array of background
processes going on. We have to accept, for the present, the jarring idea that
the "I" inside our heads is a virtual "I" constructed anew
in each moment from a cacophonous interaction among brain, body, and world.
At this point, you may feel you
have been left with something like the proverbial Chinese meal that doesn't
stick to your ribs. There is no clear "So that's how it works!" The
chapters on perceiving, acting, emotional, linguistic, and now conscious minds
have left us with bits and pieces waiting to be assembled into a whole. Many
of us take it as an article of faith that it will someday be possible to do
this; others think not. We do have some models for our consciousness that seem
worth further pursuit and testing. More immediately, the information we have
assembled thus far has consequences for how we view and comport ourselves in
the modern world, and this is the subject of the next chapter.
Questions for Thought
1. Our prefrontal lobes play a
central role in awareness, attention, and working memory. Are awareness and
attention the same thing? If not, how would you distinguish between them? (Think
about keeping your eyes directed at the middle of a clock face while you are
being asked to describe one of the specific numbers for a time of day.)
2. Cite some of the evidence that
supports the following contention: Our subjective experience of being a central
observer in a constant world actually reflects the construction of a "best
guess" of what that world is like from a series of fleeting "snapshots" (of
visual, auditory, proprioceptive, and other sensory inputs)---snapshots that
sample very narrow windows of time and are constantly renewed. What are some
of the ways in which this process can be misled?
3. Consciousness can be abolished
by lesions in a very small region of the thalamus. Does this mean that these
regions are the seat or locus of consciousness?
4. This chapter outlines a "Darwin
Machine" model for generating the apparent unity and continuity of our
experience. In this model, our current thread of awareness is the winner that
has emerged from a competition among many alternative options. Can you think
of any other models---models that do not have the flaw of ultimately suggesting
a little human, or homunculus, inside our heads that in turn has to be explained?
Suggestions for further general
reading
Dennett, D.C. 1991. Consciousness
Explained. Boston: Little, Brown. This chapter's discussion of time and space
in the brain (noting the phi and metacontrast effects), and its account of
the futility of asking "Where does it all come together?" was drawn
largely from Chapters 5 and 6 of Dennett's book.
Flanagan, O. 1992. Consciousness
Reconsidered. Cambridge, MA: M.I.T. Press. This book is a broad introduction
to philosophy of mind and general models of brain mechanisms of consciousness.
It discusses the idea of consciousness as arising from Darwin Machine--like
mechanisms.
Hobson, J.A. 1994. The Chemistry
of Conscious States: How the Brain Changes Its Mind. Boston: Little, Brown.
This account of Hobson's sleep research also gives his more general views on
how both "bottom-up" (from brainstem to cortex) and "top-down" (from
cortex to brainstem) processes shape our cognition.
Ferris, T. 1992. The Mind's Sky---Human
Intelligence in a Cosmic Context. New York: Bantam. This chapter notes some
interesting speculations on altered states of consciousness that are offered
in Part 2 of Ferris's book.
Churchland, P.M. 1995. The Engine
of Reason, the Seat of the Soul: A Philosophical Journey into the Brain. Cambridge,
MA: M.I.T. Press. This book presents the brain as a neurocomputational device
and gives an accessible account of using artificial neural networks, simulated
by computers, to model brain processes.
Reading on more advanced or specialized
topics
Gazzaniga, M.S., Ivry, R.B., & Mangun,
G.R. 1998. Cognitive Neuroscience. New York: Norton. Chapter 11 of this book
deals with the role of the frontal lobes in awareness, executive functions,
and complex behaviors.
Ungerleider, L.G., Courtney, S.M., & Haxby,
J.V. 1998. A neural system for human visual working memory. Proceedings of
the National Academy of Sciences USA 95:883--890. A comparison of pathways
in the monkey and human brains that regulate visual working memory, with different
areas of prefrontal cortex serving spatial and object memory.
Llinas, R., & Ribary, U. 1993.
Coherent 40-Hz oscillation characterizes dream state in humans. Proceedings
of the National Academy of Sciences USA 90:2078--2081. A proposal that an oscillation
in cortical activity that occurs in the awake and rapid eye movement (REM)
sleep states is a correlate of cognition.
Newman, J. 1997. Putting the puzzle
together. Towards a general theory of the neural correlates of consciousness.
Journal of Consciousness Studies 4:47--66. This article expands on a model
that makes the thalamus a central part of an extended activating system that
links cortex and lower brain centers to generate the conscious state.
Ramachandran, V.S., & Hirstein,
W. 1998. Three laws of Qualia: what neurology tells us about the biological
functions of consciousness. Journal of Consciousness Studies 4:429--457. This
is a fascinating account of neurological syndromes in which consciousness seems
to malfunction, suggesting regions of the brain that are central to maintaining
a conscious state.
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