From: Ed Halter @ NYUFF (email suppressed)
Date: Wed Jan 07 2004 - 10:27:48 PST
Hi
Thought many on this list would be interested in this article...
Best
Ed
Volume 51, Number 1 · January 15, 2004
Review
In the River of Consciousness
By Oliver Sacks
BOOKS MENTIONED IN THIS ARTICLE
The Principles of Psychology
by William James
Dover, two volumes, $16.95 each (paper)
Creative Evolution
by Henri Bergson
Dover, 432 pp., $14.95 (paper)
The Organization of Behavior: A Neuropsychological Theory
by Donald Hebb
Erlbaum, 368 pp., $45.00
Neural Darwinism: The Theory of Neuronal Group Selection
by Gerald M. Edelman
Basic Books, 371 pp. (out of print)
Wider Than the Sky: The Phenomenal Gift of Consciousness
by Gerald M. Edelman
Basic Books, 371 pp. (out of print)
The Physiology of Truth: Neuroscience and Human Knowledge
by Jean-Pierre Changeux
Harvard University Press, 336 pp., $45.00 (to be published in April 2004)
The Astonishing Hypothesis: The Scientific Search for the Soul
by Francis Crick
Scribner, 336 pp., $15.00 (paper)
The Quest for Consciousness: A Neurobiological Approach
by Christof Koch, foreword by Francis Crick.
Roberts & Company Publishers, 448 pp., $45
A Natural History of Vision
by Nicholas J. Wade
MIT Press, 486 pp., $85.00; $37.00 (paper)
1.
"Time," says Jorge Luis Borges, "is the substance I am made of. Time is a
river that carries me away, but I am the river...." Our movements, our
actions, are extended in time, as are our perceptions, our thoughts, the
contents of consciousness. We live in time, we organize time, we are time
creatures through and through. But is the time we live in, or live by,
continuous ‹like Borges's river? Or is it more comparable to a chain or a
train, a succession of discrete moments, like beads on a string?
David Hume, in the eighteenth century, favored the idea of discrete moments,
and for him the mind was "nothing but a bundle or collection of different
perceptions, which succeed each other with an inconceivable rapidity, and
are in a perpetual flux and movement."
For William James, writing his Principles of Psychology in 1890, the "Humean
view," as he called it, was both powerful and vexing. It seemed
counterintuitive, as a start. In his famous chapter on "the stream of
thought," James stressed that to its possessor, consciousness seems to be
always continuous, "without breach, crack, or division," never "chopped up,
into bits." The content of consciousness might be changing continually, but
we move smoothly from one thought to another, one percept to another,
without interruption or breaks. For James, thought flowed ; hence his
introduction of the term "stream of consciousness." But, he wondered, "is
consciousness really discontinuous... and does it only seem continuous to
itself by an illusion analogous to that of the zoetrope?"
Before 1830 (short of making an actual working model, or toy theater), there
was no way of making representations or images that had movement. Nor would
it have occurred to anyone that a sensation or illusion of movement could be
conveyed by still pictures. How could pictures convey movement if they had
none themselves? The very idea was paradoxical, a contradiction. But the
zoetrope proved that individual images could be fused in the brain to give
an illusion of continuous motion, an idea that was soon to give rise to the
motion picture.
Zoetropes (and many other similar devices, with a variety of names) were
extremely popular in James's time, and few middle-class Victorian households
were without one. All of these instruments contained a drum or disc on which
a series of drawings‹of animals moving, ball games, acrobats in motion,
plants growing‹was painted or pasted. The drawings could be viewed one at a
time through radial slits in the drum, but when the drum was set into
motion, the separate drawings flicked by in rapid succession, and at a
critical speed, this suddenly gave way to the perception of a single, steady
moving picture. When one slowed the drum again, the illusion vanished.
Though zoetropes were usually seen as toys, providing a magical illusion of
motion, they were originally designed (often by scientists or philosophers)
with a sense that they could serve a very serious purpose: to illuminate the
mechanisms both of vision and of animal motion.
Had James been writing a few years later, he might indeed have used the
analogy of a motion picture. A movie, with its taut stream of thematically
connected images, its visual narrative integrated by the viewpoint and
values of its director, is not at all a bad metaphor for the stream of
consciousness itself. And the technical and conceptual devices of
cinema‹zooming, fading, dissolving, omission, allusion, association and
juxtaposition of all sorts‹rather closely mimic (and perhaps are designed to
mimic) the streamings and veerings of consciousness.
It is an analogy that Henri Bergson used twenty years later, in his 1908
book Creative Evolution , where he devoted an entire section to "The
Cinematographic Mechanism of Thought, and the Mechanistic Illusion":
We take snapshots, as it were, of the passing reality, and...we have only to
string these on a becoming, ...situated at the back of the apparatus of
knowledge, in order to imitate what there is that is characteristic in this
becoming itself.... We hardly do anything else than set going a kind of
cinematograph inside us.... The mechanism of our ordinary knowledge is of a
cinematographical kind .
Were James and Bergson intuiting a truth in comparing visual perception ‹and
indeed, the flow of consciousness itself‹to such a mechanism? Are the brain
mechanisms that give coherence to perception and consciousness somehow
analogous to motion picture cameras and projectors? Does the eye/brain
actually "take" perceptual stills and somehow fuse them to give a sense of
continuity and motion? No clear answer was forthcoming during their
lifetimes.
There is a rare but dramatic neurological disturbance that a number of my
patients have experienced during attacks of migraine, when they may lose the
sense of visual continuity and motion and see instead a flickering series of
"stills." The stills may be clear-cut and sharp, and succeed one another
without superimposition or overlap, but more commonly they are somewhat
blurred, as with a too-long photographic exposure, and they persist for so
long that each is still visible when the next "frame" is seen, and three or
four frames, the earlier ones progressively fainter, are apt to be
superimposed on each other. While the effect is somewhat like that of a film
(albeit an improperly shot and presented one, in which each exposure has
been too long to freeze motion completely and the rate of presentation too
slow to achieve fusion), it also resembles some of E.J. Marey's
"chronophotographs" of the 1880s, in which one sees a whole array of
photographic moments or time frames superimposed on a single plate. [1]
I heard several accounts of such visual effects while working in the late
1960s with a large number of migraine patients, and when I wrote about this
in my 1970 book Migraine , I noted that the rate of flickering in these
episodes seemed to be between six and twelve per second. There might also
be, in cases of migraine delirium, a flickering of kaleidoscopic patterns or
hallucinations. (The flickering might then accelerate to restore the
appearance of normal motion or of a continuously mod- ulated hallucination.)
Finding no good accounts of the phenomenon in the medical literature‹perhaps
not entirely surprising, for such attacks are brief, rare, and not readily
predicted or provoked‹I used the term "cinematographic" vision for them; for
patients always compared them to films run too slow.
This was a startling visual phenomenon, for which, in the 1960s, there was
no good physiological explanation. But I could not help wondering then
whether visual perception might in a very real way be analogous to
cinematography, taking in the visual environment in brief, instantaneous,
static frames, or "stills," and then, under normal conditions, fusing these
to give visual awareness its usual movement and continuity‹a "fusion" which,
seemingly, was failing to occur in the very abnormal conditions of these
migraine attacks.
Such visual effects may also occur in certain seizures, as well as in
intoxications (especially with hallucinogens such as LSD). And there are
other visual effects that may occur. Moving objects may leave a smear or
wake in the direction they move; images may repeat themselves; and
afterimages may be greatly prolonged. I have experienced this myself,
following the drinking of sakau, a hallucinogen and intoxicant popular in
Micronesia. I described some of these effects in a journal, and later in my
book The Island of the Colorblind :
Ghost petals ray out from a flower on our table, like a halo around it; when
it is moved...it leaves a slight train, a visual smear...in its wake.
Watching a palm waving, I see a succession of stills, like a film run too
slow, its continuity no longer maintained.
I heard strikingly similar accounts in the late 1960s from some of my
post-encephalitic patients, when they were "awakened," and especially
overexcited, by taking the drug L-DOPA. Some patients described cinematic
vision; some described extraordinary "standstills," sometimes hours long, in
which not only visual flow was arrested, but the stream of movement, of
action, of thought itself.
These standstills were especially severe with one patient, Hester Y. Once I
was called to the ward because Mrs. Y. had started a bath, and there was now
a flood in the bathroom. I found her standing completely motionless in the
middle of the flood.
She jumped when I touched her, and said, "What happened?"
"You tell me," I answered.
She said that she had started to run a bath for herself, and there was an
inch of water in the tub...and then I touched her, and she suddenly realized
that the tub must have run over and caused a flood. But she had been stuck,
transfixed, at that perceptual moment when there was just an inch of water
in the bath.
Such standstills showed that consciousness could be brought to a halt,
stopped dead, for substantial periods, while automatic, nonconscious
function ‹maintenance of posture or breathing, for example‹continued as
before.
Another striking example of perceptual standstill could be demonstrated with
a common visual illusion, that of the Necker cube. Normally, when we look at
this ambiguous perspective drawing of a cube, it switches perspective every
few seconds, first seeming to project, then to recede, and no effort of will
suffices to prevent this switching back and forth. The drawing itself does
not change, nor does the retinal image. The switching is a cortical process,
a conflict in consciousness itself, as it vacillates between alternative
perceptual interpretations. This switching is seen in all normal subjects,
and can be observed with functional brain imaging. But a post-encephalitic
patient, during a standstill state, may see the same unchanging perspective
for minutes or hours at a time.
The normal flow of consciousness, it seemed, could not only be fragmented,
broken into small, snapshot-like bits, but could be suspended
intermittently, for hours at a time. [2] I found this even more puzzling and
uncanny than cinematic vision, for it has been accepted almost axiomatically
since the time of William James that consciousness, in its very nature, is
ever-changing and ever-flowing; but now my own clinical experience had to
cast doubt on even this.
Thus I was primed to be further fascinated when, in 1983, Josef Zihl and his
colleagues in Munich published a single, very fully described case of motion
blindness: a woman who became permanently unable to perceive motion
following a stroke. (The stroke had damaged the highly specific areas of the
visual cortex which physiologists have shown in experimental animals to be
crucial for motion perception.) In this patient, whom they call L.M., there
were "freeze frames" lasting several seconds, during which Mrs. M. would see
a prolonged, motionless image and be visually unaware of any movement around
her, though her flow of thought and perception was otherwise normal. For
example, Mrs. M. might begin a conversation with a friend standing in front
of her, but not be able to see her friend's lips moving or facial
expressions changing. And if the friend moved around behind her, Mrs. M.
might continue to "see" him in front of her, even though his voice now came
from behind. She might see a car "frozen" a considerable distance from her,
but find, when she tried to cross the road, that it was now almost upon her;
she would see a "glacier," a frozen arc of tea coming from the spout of the
teapot, but then realize that she had overfilled the cup, and that there was
now a puddle of tea on the table. Such a condition was utterly bewildering,
and sometimes quite dangerous.
There are clear differences between cinematic vision and the sort of motion
blindness described by Zihl; and perhaps between these and the very long
visual and sometimes global freezes experienced by some post-encephalitic
patients. These differences imply that there must be a number of different
mechanisms or systems for the perception of visual motion and the continuity
of visual consciousness‹and this accords with evidence obtained from
perceptual and psychological experiments. Some or all of these mechanisms
may fail to work as they should in certain intoxications, some attacks of
migraine, and some forms of brain damage‹but can they also reveal themselves
under normal conditions?
An obvious example springs to mind, which many of us have seen and perhaps
puzzled over when watching evenly rotating objects‹fans, wheels, propeller
blades‹or when walking past fences or palings, when the normal continuity of
motion seems to be interrupted. Thus, occasionally, as I lie in bed looking
up at my ceiling fan, the blades seem suddenly to reverse direction for a
few seconds, and then to return equally suddenly to their original forward
motion. Sometimes the fan seems to hover or stall, and sometimes to develop
additional blades or dark bands broader than the blades.
It is similar to what happens when, in a film, the wheels of stagecoaches
sometimes appear to be going slowly backward or scarcely moving. This
wagon-wheel illusion, as it is called, reflects a lack of synchronization
between the rate of filming and that of the rotating wheels. But I can have
a real-life wagon-wheel illusion even when I look at my fan with the morning
sun flooding into my room, bathing everything in a continuous, even light.
Is there, then, some flickering or lack of synchronization in my own
perceptual mechanisms‹analogous, again, to the action of a movie camera?
Dale Purves and his colleagues at Duke University have explored wagon- wheel
illusions in great detail, and they have confirmed that this type of
illusion or misperception is universal among their subjects. Having excluded
any other cause of discontinuity (intermittent lighting, eye movements,
etc.), they conclude that the visual system processes information "in
sequential episodes," at the rate of three to twenty such episodes per
second. Normally, these sequential images are experienced as an unbroken
perceptual flow. Indeed, Purves et al. suggest, we may find movies
convincing precisely because we ourselves break up time and reality much as
a movie camera does, into discrete frames, which we then reassemble into an
apparently continuous flow.
In Purves's view, it is precisely this decomposition of what we see into a
succession of moments that enables the brain to detect and compute motion;
for all it has to do is to note the differing positions of objects between
successive "frames," and from these calculate the direction and speed of
motion. [3]
2.
But this is not enough. We do not merely calculate movement as a robot
might‹we perceive it. We perceive motion, just as we perceive color or
depth, as a unique qualitative experience that is vital to our visual
awareness and consciousness. Something beyond our understanding occurs in
the genesis of qualia, the transformation of an objective cerebral
computation to a subjective experience. Philosophers argue endlessly over
how these transformations occur, and whether we will ever be capable of
understanding them. Neuroscientists, by and large, are content for the
moment to accept that they do occur, and to devote themselves to finding the
underlying basis or "neural correlates" of consciousness, starting from such
elemental forms of consciousness as the perception of motion.
James dreamed of zoetropes as a metaphor for the conscious brain, Bergson of
cinematography ‹but these were, of necessity, no more than tantalizing
analogies and images. It has only been in the last twenty or thirty years
that neuroscience could even start to address such issues as the neural
basis of consciousness.
Indeed, from having been an almost untouchable subject before the 1970s, the
neuroscientific study of consciousness has now become a central concern, one
that engages scientists all over the world. Every level of consciousness is
now being explored, from the most elemental perceptual mechanisms
(mechanisms common to many animals besides ourselves) to the higher reaches
of memory, imagery, and self-reflective consciousness.
It is now possible to monitor simultaneously the activities of a hundred or
more individual neurons in the brain, and to do this in unanesthetized
animals given simple perceptual and mental tasks. We can examine the
activity and interactions of large areas of the brain by means of imaging
techniques like functional MRIs and PET scans, and such non-invasive
techniques can be used with human subjects, to see which areas of the brain
are activated in complex mental activities.
In addition to physiological studies, there is the relatively new realm of
computerized neural modeling, using populations or networks of virtual
neurons, and seeing how these organize themselves in response to various
stimuli and constraints.
All of these approaches, along with concepts not available to earlier
generations, now combine to make the quest for the neural correlates of
consciousness the most fundamental and exciting adventure in neuroscience
today. A crucial innovation has been "population-thinking," thinking in
terms that take account of the brain's huge population of neurons (a hundred
billion or so), and the power of experience to differentially alter the
strengths of connections between them, and to promote the formation of
func-tional groups or constellations of neurons throughout the brain‹groups
whose interactions serve to categorize experience. [4]
Instead of seeing the brain as rigid, fixed in mode, programmed like a
computer, there is now a much more biological and powerful notion of
"experiential selection," of experience literally shaping the connectivity
and function of the brain (within genetic, anatomical, and physiological
limits, of course).
Such a selection of neuronal groups (groups consisting of perhaps a thousand
or so individual neurons), and its effect on shaping the brain over the
lifetime of an individual, is seen as analogous to the role of natural
selection in the evolution of species; hence Gerald M. Edelman, who was a
pioneer in such thinking in the 1970s, speaks of "neural Darwinism." J.P.
Changeux, the French neuroscientist, is more concerned with the connections
of individual neurons, and speaks of "the Darwinism of synapses." Both
Changeux and Edelman will soon publish highly readable, general accounts of
their work.
William James himself always insisted that consciousness was not a "thing"
but a "process." The neural basis of these processes, for Edelman, is one of
dynamic interaction between neuronal groups in different areas of the cortex
(and between the cortex and the thalamus, and other parts of the brain). He
speaks here of "re-entrant" (i.e., reciprocal) interactions, and sees
consciousness as arising from the enormous number of such interactions
between memory systems in the anterior parts of the brain and systems
concerned with perceptual categorization in the posterior parts of the
brain.
Other pioneers in the study of the neural basis of consciousness are Francis
Crick (of the "double helix") and his younger colleague Christof Koch, who,
from their first collaborative work in the 1980s, have focused more narrowly
on elementary visual perception and processes. Koch gives a detailed but
vivid and personal history of their work, and of the search for the neural
basis of consciousness generally, in his new book, The Quest for
Consciousness . Mechanisms of visual consciousness, Crick and Koch feel, are
an ideal starting point, because they are the most amenable to investigation
at present, and can serve as a model for investigating and understanding
higher and higher forms of consciousness.
In a synoptic paper called "A Framework for Consciousness," published in
Nature Neuroscience in February 2003, Crick and Koch speculate on the neural
correlates of motion perception, how visual continuity is perceived or
constructed, and, by extension, the seeming continuity of consciousness
itself. They propose that "conscious awareness [for vision] is a series of
static snapshots, with motion 'painted' on them...[and] that perception
occurs in discrete epochs."
I was startled when I first came across this passage a few months ago,
because their formulation seemed to rest upon the same notion of
consciousness that James and Bergson had intimated a century ago, and that
had been in my mind since I first heard accounts of cinematic vision from my
migraine patients in the 1960s. Here, however, was something more, a
possible substrate for consciousness based in neuronal activity.
But the "snapshots" that Crick and Koch postulate are not uniform, like
cinematic ones. The duration of successive snapshots, they feel, is not
likely to be constant; moreover, the time of a snapshot for shape, say, may
not coincide with one for color. While this "snapshotting" mechanism for
visual sensory inputs is probably a fairly simple and automatic one, a
relatively low-order neural mechanism, each visual percept must include a
great number of visual attributes, all of which are bound together on some
pre-conscious level. [5] How, then, are the various snapshots "assembled" to
achieve apparent continuity, and how do they reach the level of
consciousness?
While a particular motion, for example, may be represented by neurons firing
at a particular rate in the motion centers of the visual cortex, this is
only the beginning of an elaborate process. To reach consciousness, this
neuronal firing, or some higher representation of it, must cross a certain
threshold of intensity and be maintained above it‹consciousness, for Crick
and Koch, is a threshold phenomenon. To do that, this group of neurons must
engage other parts of the brain (usually in the frontal lobes) and ally
itself with millions of other neurons to form a "coalition." Such
coalitions, they conceive, can form and dissolve in a fraction of a second,
and involve reciprocal connections between the visual cortex and many other
areas of the brain. These neural coalitions in different parts of the brain
"talk" to one another in a continuous back-and-forth interaction. A single
conscious visual percept may thus entail the parallel and mutually
influencing activities of billions of nerve cells.
Finally, the activity of a coalition, or coalition of coalitions, if it is
to reach consciousness, must not only cross a threshold of intensity, but
must be held there for a certain time‹roughly a hundred milliseconds. This
is the duration of a "perceptual moment." [6]
To explain the apparent continuity of visual consciousness, Crick and Koch
suggest that the activity of the coalition shows "hysteresis," that is, a
persistence outlasting the stimulus. This notion is very similar, in a way,
to the "persistence of vision" theories advanced in the nineteenth century.
[7] In his Physiological Optics of 1860, Hermann Helmholtz wrote, "All that
is necessary is that the repetition of the impression shall be fast enough
for the after-effect of one impression not to have died down perceptibly
before the next one comes." Helmholtz and his contemporaries supposed that
this aftereffect occurred in the retina, but for Crick and Koch it occurs in
the coalitions of neurons in the cortex. The sense of continuity, in other
words, results from the continuous overlapping of successive perceptual
moments. It may be that the forms of cinematographic vision I have described
‹with either sharply separated stills or blurred and overlapping ones‹
represent abnormalities of excitability in the coalitions, with either too
much, or too little, hysteresis. [8]
Vision, in ordinary circumstances, is seamless and gives no indication of
the underlying processes on which it depends. It has to be decomposed,
experimentally or in neurological disorders, to show the elements that com-
pose it. Thus it is decomposed vision ‹the flickering, perseverative,
time-blurred images experienced in certain intoxications or severe
migraines‹ which above all lends credence to the notion that consciousness
is composed of discrete moments.
Whatever the mechanism, the fusing of discrete visual frames or snapshots is
a prerequisite for continuity, for a flowing, mobile consciousness. Such a
dynamic consciousness probably first arose in reptiles a quarter of a
billion years ago. It seems probable that no such stream of consciousness
exists in an amphibian, like a frog, which shows no active attention, and no
visual following of events. The frog does not have a visual world or visual
consciousness as we know it, only a purely automatic ability to recognize an
insect-like object if this enters its visual field, and to dart out its
tongue in response. It has been said that a frog's vision is, in effect, no
more than a fly-catching mechanism. [9]
If a dynamic, flowing consciousness allows, at the lowest level, a
continuous, active scanning or looking, it allows, at a higher level, the
interaction of perception and memory, of present and past. And such a
"primary" consciousness, as Edelman puts it, is highly efficacious, highly
adaptive, in the struggle for life. [10]
From such a relatively simple primary consciousness, we leap to human
consciousness, with the advent of language and self-consciousness and an
explicit sense of the past and the future. And it is this which gives a
thematic and personal continuity to the consciousness of every individual.
As I write I am sitting at a café on Seventh Avenue, watching the world go
by. My attention and focus dart to and fro‹a girl in a red dress goes by, a
man walking a funny dog, the sun (at last!) emerging from the clouds. These
are all events which catch my attention for a moment as they happen. Why,
out of a thousand possible perceptions, are these the ones I seize upon?
Reflections, memories, associations lie behind them. For consciousness is
always active and selective‹charged with feelings and meanings uniquely our
own, informing our choices and interfusing our perceptions. So it is not
just Seventh Avenue that I see, but my Seventh Avenue, marked by my own
selfhood and identity.
Christopher Isherwood starts his Berlin Diary with an extended photographic
simile: "I am a camera with its shutter open, quite passive, recording, not
thinking. Recording the man shaving at the window opposite and the woman in
the kimono washing her hair. Some day, all this will have to be developed,
carefully printed, fixed." But we deceive ourselves if we imagine that we
can ever be passive, impartial observers. Every perception, every scene, is
shaped by us, whether we intend it, know it, or not. We are the directors of
the film we are making‹but we are, equally, its subjects too: every frame,
every moment, is us, is ours‹ our forms (as Proust says) are outlined in
each one, even if we have no existence, no reality, other than this.
But how then do our frames, our momentary moments, hold together? How, if
there is only transience, do we achieve continuity? Our passing thoughts, as
James says (in an image which smacks of cowboy life in the 1880s) do not
wander round like wild cattle. Each one is owned, our own, and bears the
brand of this ownership, and each thought, in James's words, is born an
owner of the thoughts that went before, and "dies owned, transmitting
whatever it realized as its Self to its own later proprietor."
So it is not just perceptual moments, simple physiological moments‹though
these underlie everything else‹but moments of an essentially personal kind,
which seem to constitute our very being. Finally, then, we come around to
Proust's image, itself slightly reminiscent of photography (and even of
Hume), that we consist entirely of "a collection of moments," even though
these flow into one another like Borges's river. [11]
Notes
[1] Étienne-Jules Marey, in France, like Eadweard Muybridge in the United
States, pioneered the development of quick-fire, instantaneous, serial
photographs. While these could be arrayed around a zoetrope drum to provide
a brief "movie," they could also be used to decompose movement, to
investigate the temporal organization and biodynamics of animal and human
motion. This was Marey's special interest, as a physiologist, and for this
purpose he preferred to superimpose his images‹a dozen or twenty images, a
second's worth‹on a single plate. Such composite photographs, in effect,
captured a span of time; this is why he called them "chronophotographs."
Marey's photographs became the model for all subsequent scientific
photographic studies of movement, and chronophotography was an inspiration
to artists, too (one thinks of Duchamp's famous Nude Descending a Staircase
, which Duchamp himself referred to as "a static image of movement").
[2] Music, with its rhythm and flow, can be of crucial importance in such
freezings, allowing patients to resume their suddenly arrested flow of
movement, perception, and thought. Music sometimes seems able to act as a
sort of model or template for the sense of time and movement such patients
have temporarily lost, and which they need to regain. Thus a parkinsonian
patient in the midst of a standstill may be able to move when music is
played. Indeed, they may be completely unable to walk, but able to dance to
music. Neurologists intuitively use musical terms here, and speak of
parkinsonism as a "kinetic stutter" and normal movement as "kinetic melody."
William Harvey, writing in 1627, referred to animal motion as "the silent
music of the body."
[3] Whether or not this is so, the brain can also create motion on its own:
one can "see" motion when, objectively, there is none, as in the well-known
waterfall illusion.
[4] No paradigms or concepts, however original, ever come totally out of the
blue. While population-thinking in relation to the brain only emerged in the
1970s, there was an important antecedent twenty-five years earlier, Donald
Hebb's famous 1949 book The Organization of Behavior . Hebb sought to bridge
the great gap between neurophysiology and psychology with a general theory
which could relate neural processes to mental ones, and, in particular, show
how experience could modify, in effect shape, the brain. The potential for
modification, Hebb felt, was vested in the synapses which connect brain
cells to each other‹a single cerebral neuron, we now know, can have up to
ten thousand synapses, and the brain as a whole has upward of a hundred
trillion, so the capacities for modification are practically infinite.
Hebb's original concept was soon to be confirmed, and set the stage for new
ways of thinking. Every neuroscientist who now thinks about consciousness is
thus indebted to Hebb.
[5] The mechanisms of binding seem to entail the synchronization of neuronal
firing in a range of sensory areas. Sometimes it may fail to occur, and
Crick cites a comic instance of this in his remarkable 1994 book The
Astonishing Hypothesis : "A friend walking in a busy street 'saw' a
colleague and was about to address him when he realized that the black beard
belonged to another passerby and the bald head and spectacles to another."
[6] The term "perceptual moment" was first used by the psychologist J.M.
Stroud in the 1950s, in his paper on "The Fine Structure of Psychological
Time." The perceptual moment represented for him the "grain" of
psychological time, that duration (about a tenth of a second, he estimated
from his experiments) which it took to integrate sensory information as a
unit. There was some thought at this time that the alpha rhythms of the
brain might be connected with the underlying neurological mechanism for such
perceptual moments, since its "ticks" also followed one another at intervals
of roughly a tenth of a second. But, as Crick and Koch remark, Stroud's
"perceptual moment" hypothesis was virtually ignored for the next
half-century.
[7] In his delightful book A Natural History of Vision , Nicholas Wade
quotes Seneca, Ptolemy, and other classical authors, who, observing that a
flaming torch swung rapidly in a circle appeared to form a continuous ring
of fire, realized that there must be a considerable duration or persistence
of vis- ual images (or, in Seneca's term, a "slowness" of vision). An
impressively accurate measurement of this duration ‹as 8/60 of a second‹was
made in 1765, but it was only in the nineteenth century that the persistence
of vision was systematically exploited in such instruments as the zoetrope.
It seems too that motion illusions akin to the wagon-wheel effect were well
known as much as two thousand years ago.
[8] An alternative explanation, Crick and Koch suggest (personal
communication), is that the blurring and persistence of snapshots is due to
their reaching short-term memory (or a short-term visual memory buffer) and
slowly decaying there.
[9] J.Y. Lettvin and his colleagues at MIT described the experiments
demonstrating this in a famous paper called "What the Frog's Eye Tells the
Frog's Brain."
[10] Edelman provides the following description in his latest book, Wider
Than the Sky: The Phenomenal Gift of Consciousness : "Imagine an animal with
primary consciousness in the jungle. It hears a low growling noise, and at
the same time the wind shifts and the light begins to wane. It quickly runs
away, to a safer location. A physicist might not be able to detect any
necessary causal relation among these events. But to an animal with primary
consciousness, just such a set of simultaneous events might have accompanied
a previous experience, which included the appearance of a tiger.
Consciousness allowed integration of the present scene with the animal's
past history of conscious experience, and that integration has survival
value whether a tiger is present or not. An animal without primary
consciousness might have many of the individual responses that the conscious
animal has and might even survive. But, on average, it is more likely to
have lower chances of survival‹in the same environment it is less able than
the conscious animal to discriminate and plan in light of previous and
present events."
[11] I would like to acknowledge the great help of Francis Crick, Christof
Koch, and Ralph M. Siegel, who have reviewed this article and made many
valuable comments.
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