Does anyone that wants to read this series on conciousness NOT have a
PDF reader? If everyone has a pdf reader I'll just post it as a pdf
file. These are a special series I bought from Scientific American that
just came out.
There are about 12 more such related "brain/mind/consciousness" articles
I still have to post.
My conversion to text files leaves out the paragraph breaks. If anyone
knows of a program that will take the pictures out of pdf files and
properly convert what's left to text with paragraph breaks intact, let
me know PLEASE!
Enjoy...
Sorry for no par. breaks!!
Walter
PS--Moderator--should I decide to post these as pdf files, do I send
inline or as an attachment???
WARNING: the following is seven non-stop text pages--but worth it.
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the problem of consciousness
10 SCIENTIFIC AMERICAN IT IS NOW BEING EXPLORED THROUGH THE VISUAL
SYSTEM- REQUIRING A CLOSE COLLABORATION AMONG PSYCHOLOGISTS,
NEUROSCIENTISTS AND THEORISTS BY FRANCIS CRICK AND CHRISTOF KOCH
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. The overwhelming question in
neurobiology today is the relation between the mind and the brain.
Everyone agrees that what we know as mind is closely related to certain
aspects of the behavior of the brain, not to the heart, as Aristotle
thought. Its most mysterious aspect is consciousness or awareness, which
can take many forms, from the experience of pain to self-consciousness.
In the past the mind (or soul) was often regarded, as it was by
Descartes, as something immaterial, separate from the brain but
interacting with it in some way. A few neuroscientists, such as the late
Sir John Eccles, have asserted that the soul is distinct from the body.
But most neuroscientists now believe that all aspects of mind, including
its most puzzling attribute-consciousness or awareness-are likely to be
explainable in a more materialistic way as the behavior of large sets of
interacting neurons. As William James, the father of American
psychology, said a century ago, consciousness is not a thing but a
process. Exactly what the process is, however, has yet to be discovered.
For many years after James penned The Principles of Psychology,
consciousness was a taboo concept in American psychology because of the
dominance of the behaviorist movement. With the advent of cognitive
science in the mid-1950s, it became possible once more for psychologists
to consider mental processes as opposed to merely observing behavior. In
spite of these changes, until recently most cognitive scientists ignored
consciousness, as did almost all neuroscientists. The problem was felt
to be either purely "philosophical" or too elusive to study
experimentally. It would not have been easy for a neuroscientist to get
a grant just to study consciousness. In our opinion, such timidity is
ridiculous, so some years ago we began to think about how best to attack
the problem scientifically. How to explain mental events as being caused
by the firing of large sets of neurons? Although there are those who
believe such an approach is hopeless, we feel it is not productive to
worry too much over aspects of the problem that cannot be solved
scientifically or, more precisely, cannot be solved solely by using
existing scientific ideas. Radically new concepts may indeed be
needed-recall the modifications of scientific thinking forced on us by
quantum mechanics. The only sensible approach is to press the
experimental attack until we are confronted with dilemmas that call for
new ways of thinking. There are many possible approaches to the problem
of consciousness. Some psychologists feel that any satisfactory theory
should try to explain as many aspects of consciousness as possible,
including emotion, imagination, dreams, mystical experiences and so on.
Although such an all-embracing theory will be necessary in the long run,
we thought it wiser to begin with the particular aspect of consciousness
that is likely to yield most easily. What this aspect may be is a matter
of personal judgment. We selected the mammalian visual system because
humans are very visual animals and because so much experimental and
theoretical work has already been done on it. It is not easy to grasp
exactly what we need to explain, and it will take many careful
experiments before visual consciousness can be described scientifically.
We did not attempt to de- fine consciousness itself because of the
dangers of premature definition. (If this seems like a copout, try
defining the word "gene"-you will not find it easy.) Yet the
experimental evidence that already exists provides enough of a glimpse
of the nature of visual consciousness to guide research. In this
article, we will attempt to show how this evidence opens the way to
attack this profound and intriguing problem. Describing Visual
Consciousness VISUAL THEORISTS AGREE that the problem of visual
consciousness is ill posed. The mathematical term "ill posed" means that
additional constraints are needed to solve the problem. Although the
main function of the visual system is to perceive objects and events in
the world around us, the information available to our eyes is not
sufficient by itself to provide the brain with its unique interpretation
of the visual world. The brain must use past experience (either its own
or that of our distant ancestors, which is embedded in our genes) to
help interpret the information coming into our eyes. An example would be
the derivation of the three-dimensional representation of the world from
the two-dimensional signals falling onto the retinas of our two eyes or
even onto one of them. Visual theorists would also agree that seeing is
a constructive process, one in which the brain has to carry out complex
activities
(sometimes called computations) in order to decide which interpretation
to adopt of the ambiguous visual input. "Computation" implies that the
brain acts to form a symbolic representation of the visual world, with a
mapping (in the mathematical sense) of certain aspects of that world
onto elements in the brain. Ray Jackendoff of Brandeis University
postulates, as do most cognitive scientists, that the computations
carried out by the brain are largely unconscious and that what we become
aware of is the result of these computations. But while the customary
view is that this awareness occurs at the highest levels of the
computational system, Jackendoff has proposed an intermediate- level
theory of consciousness. What we see, Jackendoff suggests, relates to a
representation of surfaces that are directly visible to us, together
with their outline, orientation, color, texture and movement. In the
next stage this sketch is processed by the brain to produce a
three-dimensional representation. Jackendoff argues that we are not
visually aware of this three-dimensional representation. An example may
make this process clearer. If you look at a person whose back is turned
to you, you can see the back of the head but not the face. Nevertheless,
your brain infers that the person has a face. We can deduce as much
because if that person turned around and had no face, you would be very
surprised. The viewer-centered representation that corresponds to the
visible back of the head is what you are vividly aware of. What
www.sciam.com Updated from the September 1992 issue 11 ©1997 C.
HERSCOVICI, BRUSSELS /ARTISTS RIGHTS SOCIETY (ARS), NEW YORK VISUAL
AWARENESS primarily involves seeing what is directly in front of you,
but it can be influenced by a three-dimensional representation of the
object in view retained by the brain. If you see the back of a person's
head, the brain infers that there is a face on the front of it. We know
this is true because we would be very startled if a mirror revealed that
the front was exactly like the back, as in this painting, Reproduction
Prohibited (1937), by René Magritte. COPYRIGHT 2002 SCIENTIFIC AMERICAN,
INC.
12 SCIENTIFIC AMERICAN THE HIDDEN MIND your brain infers about the front
would come from some kind of three-dimensional representation. This does
not mean that information flows only from the surface representation to
the three-dimensional one; it almost certainly flows in both directions.
When you imagine the front of the face, what you are aware of is a
surface representation generated by information from the
three-dimensional model. It is important to distinguish between an
explicit and an implicit representation. An explicit representation is
something that is symbolized without further processing. An implicit
representation contains the same information but requires further
processing to make it explicit. The pattern of colored dots on a
television screen, for example, contains an implicit representation of
objects (say, a person's face), but only the dots and their locations
are explicit. When you see a face on the screen, there must be neurons
in your brain whose firing, in some sense, symbolizes that face. We call
this pattern of firing neurons an active representation. A latent
representation of a face must also be stored in the brain, probably as a
special pattern of synaptic connections between neurons. For example,
you probably have a representation of the Statue of Liberty in your
brain, a representation that usually is inactive. If you do think about
the statue, the representation becomes active, with the relevant neurons
firing away. An object, incidentally, may be represented in more than
one way-as a visual image, as a set of words and their related sounds,
or even as a touch or a smell. These different representations are
likely to interact with one another. The representation is likely to be
distributed over many neurons, both locally and more globally. Such a
representation may not be as simple and straightforward as uncritical
introspection might indicate. There is suggestive evidence, partly from
studying how neurons fire in various parts of a monkey's brain and
partly from examining the effects of certain types of brain damage in
humans, that different aspects of a face-and of the implications of a
face-may be represented in different parts of the brain. First, there is
the representation of a face as a face: two eyes, a nose, a mouth and so
on. The neurons involved are usually not too fussy about the exact size
or position of this face in the visual field, nor are they very
sensitive to small changes in its orientation. In monkeys, there are
neurons that respond best when the face is turning in a particular
direction, while others seem to be more concerned with the direction in
which the eyes are gazing. Then there are representations of the parts
of a face, as separate from those for the face as a whole. Further, the
implications of seeing a face, such as that person's sex, the facial
expression, the familiarity or unfamiliarity of the face, and in
particular whose face it is, may each be correlated with neurons firing
in other places. What we are aware of at any moment, in one sense or
another, is not a simple matter. We have suggested that there may be a
very transient form of fleeting awareness that represents only rather
simple features and does not require an attentional mechanism. From this
brief awareness the brain constructs a viewer-centered
representation-what we see vividly and clearly-that does require
attention. This in turn probably leads to threedimensional object
representations and thence to more cognitive ones. Representations
corresponding to vivid consciousness are likely to have special
properties. William James thought that consciousness involved both
attention and short-term memory. Most psychologists today would agree
with this view. Jackendoff writes that consciousness is "enriched" by
attention, implying that whereas attention may not be essential for
certain limited types of consciousness, it is necessary for full
consciousness. Yet it is not clear exactly which forms of memory are
involved. Is long-term memory needed? Some forms of acquired knowledge
are so embedded in the machinery of neural processing that they are
almost certainly part of the process of becoming aware of something. On
the other hand, there is evidence from studies of brain-damaged patients
that the ability to lay down new long-term episodic memories is not
essential for consciousness to be experienced. It is difficult to
imagine that anyone could be conscious if he or she had no memory
whatsoever, even an extremely short one, of what had just happened.
Visual psychologists talk of iconic memory, which lasts for a fraction
of a second, and working memory (such as that used to remember a new
telephone number) that lasts for only a few seconds unless it is
rehearsed. It is not clear whether both of these are essential for
consciousness. In any case, the division of short-term memory into these
two categories may be too crude. If these complex processes of visual
awareness are localized in parts of the brain, which processes are
likely to be where? Many regions of the brain may be involved, but it is
almost certain that the cerebral neocortex plays a dominant role. Visual
information from the retina reaches the neocortex mainly by way of a
part of the thalamus (the lateral geniculate nucleus); another
significant visual pathway from the retina is to the superior
colliculus, at the top of the brain stem. The cortex in humans consists
of two intricately folded sheets of nerve tissue, one on each side of
the head. These sheets are connected by a large tract of about
200,000 axons called the corpus callosum. It is well known that if the
corpus callosum is cut in a split-brain operation, as is done for
certain cases of intractable epilepsy, one side of the brain is not
aware of what the other side is seeing. In particular, the left side of
the brain (in a righthanded person) appears not to be aware What we are
aware of at any moment, in one sense or another, is not a simple matter.
COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. of visual information received
exclusively by the right side. This shows that none of the information
required for visual awareness can reach the other side of the brain by
traveling down to the brain stem and, from there, back up. In a normal
person, such information can get to the other side only by using the
axons in the corpus callosum. A different part of the brain-the
hippocampal system-is involved in oneshot, or episodic, memories that,
over weeks and months, it passes on to the neocortex. This system is so
placed that it receives inputs from, and projects to, many parts of the
brain. Thus, one might suspect that the hippocampal system is the
essential seat of consciousness. This is not the case: evidence from
studies of patients with damaged brains shows that this system is not
essential for visual awareness, although naturally a patient lacking one
is severely handicapped in everyday life because he cannot remember
anything that took place more than a minute or so in the past. In broad
terms, the neocortex of alert animals probably acts in two ways. By
building on crude and somewhat redundant wiring, produced by our genes
and by embryonic processes, the neocortex draws on visual and other
experience to slowly "rewire" itself to create categories
(or "features") it can respond to. A new category is not fully created
in the neocortex after exposure to only one example of it, although some
small modifications of the neural connections may be made. The second
function of the neocortex
(at least of the visual part of it) is to respond extremely rapidly to
incoming signals. To do so, it uses the categories it has learned and
tries to find the combinations of active neurons that, on the basis of
its past experience, are most likely to represent the relevant objects
and events in www.sciam.com THE HIDDEN MIND 13 ©1997 DEMART PRO ARTE
(R), GENEVA/ARTISTS RIGHTS SOCIETY (ARS), NEW YORK; © SALVADOR DALÍ
MUSEUM, INC., ST. PETERSBURG, FLA. FRANCIS CRICK and CHRISTOF KOCH share
an interest in the experimental study of consciousness. Crick is the
co-discoverer, with James Watson, of the double helical structure of
DNA. While at the Medical Research Council Laboratory of Molecular
Biology in Cambridge, England, he worked on the genetic code and on
developmental biology. Since 1976 he has been at the Salk Institute for
Biological Studies in San Diego. His main interest lies in understanding
the visual system of mammals. Koch was awarded his Ph.D. in biophysics
by the University of Tübingen in Germany. After a stint at M.I.T., he
joined the California Institute of Technology, where he is Lois and
Victor Troendle Professor of Cognitive and Behavioral Biology. He
studies how single brain cells process information and the neural basis
of motion perception, visual attention, and awareness in mice, monkeys
and humans. THE AUTHORS AMBIGUOUS IMAGES were frequently used by
Salvador Dalí in his paintings. In Slave Market with the Disappearing
Bust of Voltaire (1940), the head of the French philosopher Voltaire is
apparent from a distance but transforms into the figures of three people
when viewed at close range. Studies of monkeys shown ambiguous figures
have found that many neurons in higher cortical areas respond to only
the currently "perceived" figure; the neuronal response to the "unseen"
image is suppressed. COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.
14 SCIENTIFIC AMERICAN THE HIDDEN MIND the visual world at that moment.
The formation of such coalitions of active neurons may also be
influenced by biases coming from other parts of the brain: for example,
signals telling it what best to attend to or high-level expectations
about the nature of the stimulus. Consciousness, as James noted, is
always changing. These rapidly formed coalitions occur at different
levels and interact to form even broader coalitions. They are transient,
lasting usually for only a fraction of a second. Because coalitions in
the visual system are the basis of what we see, evolution has seen to it
that they form as fast as possible; otherwise, no animal could survive.
The brain is handicapped in forming neuronal coalitions rapidly because,
by computer standards, neurons act very slowly. The brain compensates
for this relative slowness partly by using very many neurons,
simultaneously and in parallel, and partly by arranging the system in a
roughly hierarchical manner. If visual awareness at any moment
corresponds to sets of neurons firing, then the obvious question is:
Where are these neurons located in the brain, and in what way are they
firing? Visual awareness is highly unlikely to occupy all the neurons in
the neocortex that are firing above their background rate at a
particular moment. We would expect that, theoretically, at least some of
these neurons would be involved in doing computations-trying to arrive
at the best coalitions-whereas others would express the results of these
computations, in other words, what we see. Fortunately, some
experimental evidence can be found to back up this theoretical
conclusion. A phenomenon called binocular rivalry may help identify the
neurons whose firing symbolizes awareness. This phenomenon can be seen
in dramatic form in an exhibit prepared by Sally Duensing and Bob Miller
at the Exploratorium in San Francisco. Binocular rivalry occurs when
each eye has a different visual input relating to the same part of the
visual field. The early visual system on the left side of the brain
receives an input from both eyes but sees only the part of the visual
field to the right of the fixation point. The converse is true for the
right side. If these two conflicting inputs are rivalrous, one sees not
the two inputs superimposed but first one input, then the other, and so
on in alternation. In the exhibit, called "The Cheshire Cat," viewers
put their heads in a fixed place and are told to keep the gaze fixed. By
means of a suitably placed mirror, one of the eyes can look at another
person's face, directly in front, while the other eye sees a blank white
screen to the side. If the viewer waves a hand in front of this plain
screen at the same location in his or her visual field occupied by the
face, the face is wiped out. The movement of the hand, being visually
very salient, has captured the brain's attention. Without attention the
face cannot be seen. If the viewer moves the eyes, the face reappears.
In some cases, only part of the face disappears. Sometimes, for example,
one eye, or both eyes, will remain. If the viewer looks at the smile on
the person's face, the face may disappear, leaving only the smile. For
this reason, the effect has been called the Cheshire Cat effect, after
the cat in Lewis Carroll's Alice's Adventures in Wonderland. Although it
is difficult, though not impossible, to record activity in individual
neurons in a human brain, such studies can be done in monkeys. A simple
example of binocular rivalry was studied in a monkey by Nikos K.
Logothetis and Jeffrey D. Schall, both then at M.I.T. They trained a
macaque to keep its eyes still and to signal whether it is seeing upward
or downward movement of a horizontal grating. To produce rivalry, upward
movement is projected into one of the monkey's eyes and downward
movement into the other, so that the two images overlap in the visual
field. The monkey signals that it sees up and down movements
alternatively, just as humans would. Even though the motion stimulus
coming into the monkey's eyes is always the same, the monkey's percept
changes every second or so. Cortical area MT (which some researchers
prefer to label V5) is an area mainly concerned with movement. What do
the neurons in area MT do when the monkey's percept is sometimes up and
sometimes down? (The researchers studied only the monkey's first
response.) The simplified answer-the actual data are rather more
messy-is that whereas the firing of some of the neurons correlates with
the changes in the percept, for others the average firing rate is
relatively unchanged and independent of which direction of movement the
monkey is seeing at that moment. Thus, it is unlikely that the firing of
all the neurons in the visual neocortex at one particular moment
corresponds to the monkey's visual awareness. Exactly which neurons do
correspond to awareness remains to be discovered. We have postulated
that when we clearly see something, there must be neurons actively
firing that stand for what we see. This might be called the activity
principle. Here, too, there is some experimental evidence. One example
is the firing of neurons in a specific cortical visual area in response
to illusory contours. Another and perhaps more striking case is the
filling in of the blind spot. The blind spot in each eye is caused by
the lack of photoreceptors in the area of the retina where the optic
nerve leaves the retina and projects to the brain. Its location is about
15 degrees from the fovea (the visual center of the eye). Yet if you
close one eye, you do not see a hole in your visual field. Philosopher
Daniel C. Dennett of Tufts University is unusual among philosophers in
that he is interested both in psychology and in the brain. This interest
is to be welcomed. In his 1991 book, Consciousness Explained, he argues
that it is wrong to talk about filling in. He concludes, correctly, that
"an absence of information is not the same as information When we
clearly see something, there must be neurons actively firing that stand
for what we see. COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. about an
absence." From this general principle he argues that the brain does not
fill in the blind spot but rather ignores it. Dennett's argument by
itself, however, does not establish that filling in does not occur; it
only suggests that it might not. Dennett also states that "your brain
has no machinery for [filling in] at this location." This statement is
incorrect. The primary visual cortex lacks a direct input from one eye,
but normal "machinery" is there to deal with the input from the other
eye. Ricardo Gattass and his colleagues at the Federal University of Rio
de Janeiro have shown that in the macaque some of the neurons in the
blind-spot area of the primary visual cortex do respond to input from
both eyes, probably assisted by inputs from other parts of the cortex.
Moreover, in the case of simple filling in, some of the neurons in that
region respond as if they were actively filling in. Thus, Dennett's
claim about blind spots is incorrect. In addition, psychological
experiments by Vilayanur S. Ramachandran [see "Blind Spots," Scientific
American, May 1992] have shown that what is filled in can be quite
complex depending on the overall context of the visual scene. How, he
argues, can your brain be ignoring something that is in fact commanding
attention? Filling in, therefore, is not to be dismissed as nonexistent
or unusual. It probably represents a basic interpolation process that
can occur at many levels in the neocortex. It is a good example of what
is meant by a constructive process. How can we discover the neurons
whose firing symbolizes a particular percept? William T. Newsome and his
colleagues at Stanford University did a series of brilliant experiments
on neurons in cortical area MT of the macaque's brain. By studying a
neuron in area MT, we may discover that it responds best to very
specific visual features having to do with motion. A neuron, for
instance, might fire strongly in response to the movement of a bar in a
particular place in the visual field, but only when the bar is oriented
at a certain angle, moving in one of the two directions perpendicular to
its length within a certain range of speed. It is technically difficult
to excite just a single neuron, but it is known that neurons that
respond to roughly the same position, orientation and direction of
movement of a bar tend to be located near one another in the cortical
sheet. The experimenters taught the monkey a simple task in movement
discrimination using a mixture of dots, some moving randomly, the rest
all in one direction. They showed that electrical stimulation of a small
region in the right place in cortical area MT would bias the monkey's
motion discrimination, almost always in the expected direction. Thus,
the stimulation of these neurons can influence the monkey's behavior and
probably its visual percept. Such experiments do not, however, show
decisively that the firing of such neurons is the exact neural correlate
of the percept. The correlate could be only a subset of the neurons
being activated. Or perhaps the real correlate is the firing of neurons
in another part of the visual hierarchy that are strongly influenced by
the neurons activated in area MT. These same reservations also apply to
cases of binocular rivalry. Clearly, the problem of finding the neurons
whose firing symbolizes a particular percept is not going to be easy. It
will take many careful experiments to track them down even for one kind
of percept. Visual Awareness IT SEEMS OBVIOUS that the purpose of vivid
visual awareness is to feed into the cortical areas concerned with the
implications of what we see; from there the information shuttles on the
one hand to the hippocampal system, to be encoded (temporarily) into
long-term episodic memory, and on the other to the planning levels of
the motor system. But is it possible to go from a visual input to a
behavioral output without any relevant visual awareness? That such a
process can happen is demonstrated by a very small and remarkable class
of patients with "blindsight." These patients, all of whom have suffered
damage to their visual cortex, can point with fair accuracy at visual
targets or track them with their eyes while vigorously denying seeing
anything. In fact, these patients are as surprised as their doctors by
their abilities. The amount of information that "gets through," however,
is limited: blindsight patients have some ability to respond to
wavelength, orientation and motion, yet they cannot distinguish a
triangle from a square. It is of great interest to know which neural
pathways are being used in these patients. Investigators originally
suspected that the pathway ran through the superior colliculus.
Subsequent experiments suggested that a direct, albeit weak, connection
may be involved between the lateral geniculate nucleus and other visual
areas in the cortex. It is unclear whether an intact primary visual
cortex region is essential for immediate visual awareness. Conceivably
the visual signal in blindsight is so weak that the neural activity
cannot produce awareness, although it remains strong enough to get
through to the motor system. Normal-seeing people regularly respond to
visual signals without being fully aware of them. In automatic actions,
such as swimming or driving a car, complex but stereotypical actions
occur with little, if any, associated visual awareness. In other cases,
the information conveyed is either very limited or very attenuated.
www.sciam.com THE HIDDEN MIND 15 MELISSA SZALKOWSKI KNOWLEDGE about
visual systems is important in the study of consciousness. COPYRIGHT
2002 SCIENTIFIC AMERICAN, INC.
16 SCIENTIFIC AMERICAN THE HIDDEN MIND Thus, while we can function
without visual awareness, our behavior without it is rather restricted.
Clearly, it takes a certain amount of time to experience a conscious
percept. It is difficult to determine just how much time is needed for
an episode of visual awareness, but one aspect of the problem that can
be demonstrated experimentally is that signals that are received close
together in time are treated by the brain as simultaneous. A disk of red
light is flashed for, say,
20 milliseconds, followed immediately by a 20-millisecond flash of green
light in the same place. The subject reports that he did not see a red
light followed by a green light. Instead he saw a yellow light, just as
he would have if the red and the green light had been flashed
simultaneously. Yet the subject could not have experienced yellow until
after the information from the green flash had been processed and
integrated with the preceding red one. Experiments of this type led
psychologist Robert Efron of the University of California at Davis to
conclude that the processing period for perception is about
60 to 70 milliseconds. Similar periods are found in experiments with
tones in the auditory system. It is always possible, however, that the
processing times may be different in higher parts of the visual
hierarchy and in other parts of the brain. Processing is also more rapid
in trained, compared with naive, observers. Because attention appears to
be involved in some forms of visual awareness, it would help if we could
discover its neural basis. Eye movement is a form of attention, since
the area of the visual field in which we see with high resolution is
remarkably small, roughly the area of the thumbnail at arm's length.
Thus, we move our eyes to gaze directly at an object in order to see it
more clearly. Our eyes usually move three or four times a second.
Psychologists have shown, however, that there appears to be a faster
form of attention that moves around, in some sense, when our eyes are
stationary. The exact psychological nature of this faster attentional
mechanism is controversial. Several neuroscientists, however, including
Robert Desimone and his colleagues at the National Institute of Mental
Health, have shown that the rate of firing of certain neurons in the
macaque's visual system depends on what the monkey is attending to in
the visual field. Thus, attention is not solely a psychological concept;
it also has neural correlates that can be observed. A number of
researchers have found that the pulvinar, a region of the thalamus,
appears to be involved in visual attention. We would like to believe
that the thalamus deserves to be called "the organ of attention," but
this status has yet to be established. Attention and Awareness THE MAJOR
PROBLEM is to find what activity in the brain corresponds directly to
visual awareness. It has been speculated that each cortical area
produces awareness of only those visual features that are "columnar," or
arranged in the stack or column of neurons perpendicular to the cortical
surface. Thus, the primary visual cortex could code for orientation and
area MT for certain aspects of motion. So far experimentalists have not
found one region in the brain where all the information needed for
visual awareness appears to come together. Dennett has dubbed such a
hypothetical place "The Cartesian Theater." He argues on theoretical
grounds that it does not exist. Awareness seems to be distributed not
just on a local scale but more widely over the neocortex. Vivid visual
awareness is unlikely to be distributed over every cortical area,
because some areas show no response to visual signals. Awareness might,
for example, be associated with only those areas that connect back
directly to the primary visual cortex or alternatively with those areas
that project into one another's layer 4. (The latter areas are always at
the same level in the visual hierarchy.) The key issue, then, is how the
brain forms its global representations from visual signals. If attention
is indeed crucial for visual awareness, the brain could form
representations by attending to just one object at a time, rapidly
moving from one object to the next. For example, the neurons
representing all the different aspects of the attended object could all
fire together very rapidly for a short period, possibly in rapid bursts.
This fast, simultaneous firing might not only excite those neurons that
symbolized the implications of that object but also temporarily
strengthen the relevant synapses so that this particular pattern of
firing could be quickly recalled-a form of short-term memory. If only
one representation needs to be held in short-term memory, as in
remembering a single task, the neurons involved may continue to fire for
a period. A problem arises if it is necessary to be aware of more than
one object at exactly the same time. If all the attributes of two or
more objects were represented by neurons firing rapidly, their
attributes might be confused. The color of one might become attached to
the shape of another. This happens sometimes in very brief
presentations. Some time ago Christoph von der Malsburg, now at Ruhr
University Bochum in Germany, suggested that this dif- ficulty would be
circumvented if the neurons associated with any one object all fired in
synchrony (that is, if their times of firing were correlated) but were
out of synchrony with those representing other objects. Two other groups
in Germany reported that there does appear to be correlated firing
between neurons in the visual cortex of the cat, often in a rhythmic
manner, with a frequency in the 35- to
75-hertz range, sometimes called 40-hertz, or ă, oscillation. Von der
Malsburg's proposal prompt- The key issueis how the brain forms its
global representations from visual signals. COPYRIGHT 2002 SCIENTIFIC
AMERICAN, INC. ed us to suggest that this rhythmic and synchronized
firing might be the neural correlate of awareness and that it might
serve to bind together activity concerning the same object in different
cortical areas. The matter is still undecided, but at present the
fragmentary experimental evidence does rather little to support such an
idea. Another possibility is that the 40- hertz oscillations may help
distinguish figure from ground or assist the mechanism of attention.
Correlates of Consciousness ARE THERE SOME particular types of neurons,
distributed over the visual neocortex, whose firing directly symbolizes
the content of visual awareness? One very simplistic hypothesis is that
the activities in the upper layers of the cortex are largely unconscious
ones, whereas the activities in the lower layers (layers 5 and 6) mostly
correlate with consciousness. We have wondered whether the pyramidal
neurons in layer 5 of the neocortex, especially the larger ones, might
play this latter role. These are the only cortical neurons that project
right out of the cortical system
(that is, not to the neocortex, the thalamus or the claustrum). If
visual awareness represents the results of neural computations in the
cortex, one might expect that what the cortex sends elsewhere would
symbolize those results. Moreover, the neurons in layer 5 show a rather
unusual propensity to fire in bursts. The idea that layer 5 neurons may
directly symbolize visual awareness is attractive, but it still is too
early to tell whether there is anything in it. Visual awareness is
clearly a difficult problem. More work is needed on the psychological
and neural basis of both attention and very short term memory. Studying
the neurons when a percept changes, even though the visual input is
constant, should be a powerful experimental paradigm. We need to
construct neurobiological theories of visual awareness and test them
using a combination of molecular, neurobiological and clinical imaging
studies. We believe that once we have mastered the secret of this simple
form of awareness, we may be close to understanding a central mystery of
human life: how the physical events occurring in our brains while we
think and act in the world relate to our subjective sensations-that is,
how the brain relates to the mind. Postscript THERE HAVE BEEN several
relevant developments since this article was first published in 1992. It
now seems likely that there are rapid "online" systems for stereotyped
motor responses such as hand and eye movement. These systems are
unconscious and lack memory. Conscious seeing, on the other hand, seems
to be slower and more subject to visual illusions. The brain needs to
form a conscious representation of the visual scene that it can then
employ for many different actions or thoughts. Why is consciousness
needed? Why could our brains not consist of a whole series of
stereotyped online systems? We would argue that far too many would be
required to express human behavior. The slower, conscious mode allows
time for the individual neurons to become sensitive to the context of
what typically excites them, so that a broader view of the current state
of affairs can be constructed. It would be a great evolutionary
advantage to be able to respond very rapidly to stereotyped situations
and also, more slowly, to more complex and novel ones. Usually both
these modes will act in parallel. Exactly how all these pathways work
and how they interact are far from clear. There have been more
experiments on the behavior of neurons that respond to bistable visual
percepts, such as binocular rivalry, but it is probably too early to
draw firm conclusions from them about the exact neural correlates of
visual consciousness. We have suggested on theoretical grounds based on
the neuroanatomy of the macaque that primates are not directly aware of
what is happening in the primary visual cortex, even though most of the
visual information flows through it. This hypothesis is supported by
some experimental evidence, but it is still controversial. www.sciam.com
THE HIDDEN MIND 17 JOHNNY JOHNSON SA Consciousness and the Computational
Mind. Ray Jackendoff. MIT Press/Bradford Books, 1990. The Visual Brain
in Action. A. David Milner and Melvyn A. Goodale. Oxford University
Press, 1995. Are We Aware of Neural Activity in Primary Visual Cortex?
Francis Crick and Christof Koch in Nature, Vol. 375, pages 121-123; May
11, 1995. Consciousness and Neuroscience. Francis Crick and Christof
Koch in Cerebral Cortex, Vol. 8, No. 2, pages 97-107; 1998. Vision
Science: From Photons to Phenomenology. Stephen E. Palmer. MIT
Press/Bradford Books, 1999. Principles of Neural Science. Eric R.
Kandel, James H. Schwartz and Thomas M. Jessell. McGraw-Hill, 2000. MOR
E TO E XPLORE OPTICAL ILLUSION devised by Vilayanur S. Ramachandran
illustrates the brain's ability to reconstruct missing visual
information that falls on the blind spot of the eye. When you look at
the patterns of broken green bars, the visual system produces two
illusory contours defining a vertical strip. Now shut your right eye and
focus on the white square in the green series of bars. Move the page
toward the eye until the dot disappears
(roughly six inches away). Most people see the vertical strip completed
across the blind spot, not the broken line. Try the same experiment with
the series of three red bars. The illusory vertical contours are less
well defined, and the visual system tends to fill in the horizontal bar
across the blind spot. Thus, the brain fills in differently depending on
the image. COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC.
--Walter Watts Tulsa Network Solutions, Inc.
"No one gets to see the Wizard! Not nobody! Not no how!"
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