Volume 51, Number 6 · April 8, 2004
'In the River of Consciousness': An Exchange
By Steve J. Heims, David Ingle, Benjamin Libet, Reply by Oliver Sacks
In response to In the River of Consciousness
(January 15, 2004)
To the Editors:
Oliver Sacks ["In the River of Consciousness," NYR, January 15] writes that John M. Stroud's "perceptual moment" hypothesis, published in the 1950s, "was virtually ignored for the next half-century," as indeed it was. Yet Stroud had done an important piece of research showing that perceptions are not continuous as they seem, but come in discrete steps of between 1/5th and 1/20th of a second. He had surveyed all available experiments bearing on the subject and performed new ones: using the best electronic measuring techniques known at that time he observed the patterns of perception of light in some experiments and of sound in others, in each case corroborating his general result.
It is not the only instance where Stroud's work was ignored: in the early years of the cold war he thought about the possibility of colonizing space, designed tin-can-shaped units for interplanetary habitation, each the locus of one city-state, and worked out many of the construction and design details. A question his work raised is, whether we understand human requirements sufficiently well to provide for all the physical needs, as well as the psychological and social requirements, of such a city in space with a population of thousands. His challenging research was known only to a small group of people, and never published where other interested researchers might see it. More than two decades later physicist Gerard O'Neill at Princeton, and also Freeman Dyson, not knowing of Stroud's work, for different reasons made similar suggestions and calculations about space colonization, all in essential agreement with Stroud's earlier results. They generated considerable discussion on the topic, and O'Neill even interested NASA in it.
I learned of Stroud during my historical studies of published scientific discussions held a few years either side of 1950, but found that neither he nor his work, and much less the circumstances which led to his being ignored for so long, had been noticed by any historian of science. He was devoted to engaging in research, satisfying his curiosity, and seemed indifferent to professional recognition. Lest he be lost to the history and the sociology of science altogether, I communicated with him at length, and devoted about seven pages to him in The Cybernetics Group (MIT Press, 1991). He emerges as someone independent in his ideas, outside of the usual network for communicating scientific research results, and tending to see his research within a large philosophical and historical framework, rather than primarily in relation to contemporary work of others.
The phenomenon of a researcher's work being ignored only to be acknowledged decades later than others who have been thinking along similar lines may seem mysterious. However it is fully understandable, at least in the case of Stroud, not in terms of models of the rationality or logic of progress in the sciences, but solely in human, social, and institutional terms. From that perspective the phenomenon, not so uncommon, is natural.
Steve J. Heims
Gloucester, Massachusetts
To the Editors:
In his article "In the River of Consciousness," Oliver Sacks—as usual—dances through the brain in an engaging and provocative way. I feel almost guilty in pointing out one instance where he stumbles badly. Sacks tells us that the frog has "no visual following of events." And therefore no capacity for consciousness. On the contrary, in 1975 (Science, Vol. 188, p. 1033) I showed that common frogs pay selective attention to moving prey objects that lasts up to four seconds. When an object moves for only 1/2 second, it elicits a feeding strike only 10 percent of the time. Yet, when it moves again after a delay of up to four seconds, the animal now strikes 85 percent of the time. This "attention" is selective, since strikes at a second object three inches away are not facilitated. Moreover, while recording neural activity within the frog's optic tectum (the system that mediates visual feeding) I found a type of neuron which continued to discharge for three or four seconds after the prey-like stimulus had stopped moving.
Sacks quotes the 1959 observations of Jerome Lettvin as evidence that frogs only see moving prey objects. While Lettvin's recording of "bug detecting" neurons in the frog's optic tectum was a landmark discovery of "natural image" processing and showed that the frog's eye did not operate like a television camera, it said nothing at all about the frog's ability to see stationary contours. In 1973 (Science, Vol. 181, p. 1053) I showed that while the tectum was needed to react to moving prey, another visual area processed stationary edges, allowing frogs to dodge stationary barriers and to avoid tumbling into holes in the ground.
By 1990 I had shown that frogs not only perceive a rich 3-D realm of stationary contours, but that they remember the locations of barriers for at least sixty seconds after these have been whisked away and neatly dodge around these remembered edges while fleeing from looming black disks (BioScience, Vol. 40, p. 284). These simple creatures maintain a robust "image" of the locations of large stationary objects despite their lacking a cerebral cortex. I found that the brain region needed to remember the location of a barrier is the striatum, the evolutionary ancestor of the human "basal ganglia." This is the same area that was impaired by loss of dopamine in the encephalitic patients that Sacks so vividly described in Awakenings. And I well recall Sacks's descriptions of how these patients, frozen in their tracks, could be induced to step forward simply by placing a barrier on the floor before their feet. Here is another example of how human behaviors are sometimes prefigured in the brains of much simpler creatures. Yet, I will not argue that selective attention and memory of the environment in frogs are themselves strong evidence for consciousness.
David Ingle, Ph.D.
Framingham, Massachusetts
To the Editors:
The article by Oliver Sacks, "In the River of Consciousness," is another example of the interesting presentations by Dr. Sacks on mental functions. This piece centers on the question of whether consciousness is actually discontinuous, even though it appears to flow continuously, as in William James's "stream of consciousness."
I would like to contribute to that discussion by referring to our experimental studies of brain mechanisms in conscious experience. These were carried out on awake human subjects. They provide experimental evidence to help answer the question of discontinuity of consciousness.[1]
Several lines of our findings showed that up to about 500 milliseconds of cerebral activity precedes the appearance of conscious awareness. That meant that awareness is delayed by a period of unconscious processes. This was demonstrated directly for the appearance of a conscious intention to perform a voluntary act,[2] and also for the production of awareness of a signal in a forced choice experiment,[3] i.e., an experiment in which the subject is forced to make a choice.
If conscious awareness for an event, including a thought, is substantially delayed, then there would have to be a silent (unconscious) interval before each conscious event. The evidence indicates, therefore, that consciousness is discontinuous. The fact that consciousness seems continuous may be explainable in a number of ways, as described by Dr. Sacks. One simple possibility would be to have succeeding conscious events overlapping, so that the delay for each event is obscured by the tail end of the preceding conscious event.[4] Crick and Koch have evidently proposed a similar mechanism. We should add that it was shown, experimentally,[5] that a delayed conscious sensation is subjectively antedated so that it seems to occur almost immediately (within ten to thirty milliseconds) after the actual stimulus.
It should be recognized, then, that some of these curious timing questions about consciousness can be investigated experimentally. Further, the delays for awareness result in profound inferences for our mental and physical activities, including one that conscious voluntary acts are initiated unconsciously. One must also be careful to distinguish between detection of a signal and becoming conscious of it.[6] Detection, without awareness, can occur with very brief cerebral activation and can account for quick behavioral responses (as in "a reaction time") to occur unconsciously (as in sports actions, etc.).
Benjamin Libet
Professor Emeritus of Physiology, University of California, San Francisco
Member of the Center for Neuroscience, University of California, Davis
Oliver Sacks replies:
I am very grateful to Drs. Ingle, Heims, and Libet for their letters. I had an uncomfortable feeling that I was doing frogs (of which I am very fond) a disservice, for, of course, I have seen them leaping around, avoiding obstacles, negotiating complex environments, and could hardly imagine how they could do this without a more sophisticated and dynamic visual system. All animals have "minds" of a sort, and even nematodes—with only 300-odd neurons—show complex and unpredictable behaviors. So this is certainly the case for frogs, which have 16 million neurons (while octopuses, which can learn from observation, and are strikingly intelligent and playful, have 300 million neurons).
Dr. Heims's description of how all of Stroud's work was ignored for so long is of great social interest, and makes fascinating, though rather painful, reading. I am reminded of Gunther Stent's notion of "prematurity" in the sciences (see Hidden Histories of Science, edited by Robert Silvers, New York Review Books, 1995), but clearly in the case of Stroud, highly personal considerations are paramount too. One wonders how many Strouds there are in the world at any time, and how much society and science may lose by their reticence. I cannot help thinking of Cavendish in this regard, who was wholly indifferent to the recognition of his peers, and who published his astounding discoveries inconspicuously, or sometimes not at all. In the 1780s, in effect, he discovered argon in the atmosphere, but his work was so ignored that it was only a century later, in the 1890s, that argon was rediscovered, a rediscovery greatly assisted by the "discovery" of his long-forgotten experiments.
On the subject of forgetting, I should have, in my original article, drawn readers' attention to the beautiful descriptions of cinematic vision induced by mescaline in Heinrich Klüver's 1928 book Mescal. I read this, as I read Dr. Ingle's paper, many years ago, but then, with the fallibility of memory, they were forgotten.
Professor Libet was the first physiologist to attack the problem of consciousness experimentally, and his startling experiments, which do seem to indicate other forms of discontinuity in consciousness, have aroused discussion for almost forty years. There are, indeed, many forms of perceptual discontinuity. We think we take in a face, a place, a complex scene, in a single glance, but it can readily be shown that there may be a dozen or more separate eye movements or fixations of gaze, a dozen "takes" (especially clustered about the salient features of a scene), of which we are entirely unconscious, which are then assembled or synthesized to yield our "single" perception. We do not, in this sense, take simple "snapshots"; we create composites, reminiscent of Hockney's marvelous photomontages, which give a feeling of perceptual reality that no single photograph could achieve. Whether it is color or motion, let alone anything more complex, a double process of breaking down and building up, or decomposition and recomposition—whatever one likes to call it—seems to be unavoidable when our nervous systems represent our world.
Notes
[1] See B. Libet, Mind-Time (Harvard University Press, March 2004); B. Libet, in Brain and Conscious Experience, edited by J.C. Eccles (Springer Verlag, 1966), pp. 165–181; and B. Libet, Journal of Consciousness Studies, Vol. 6 (1999), pp. 47–57.
[2] B. Libet, Behavioral & Brain Sciences, Vol. 89 (1985), pp. 567–615.
[3] B. Libet et al., Brain, Vol. 114 (1991), pp. 1731–1759.
[4] See Libet, Mind-Time.
[5] B. Libet et al., Brain, Vol. 102 (1979), pp. 193–224.
[6] See Libet et al., Brain, Vol. 114.