In the River of Consciousness part 2/2
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. |
