Reprints 121-144

Date:
1992-2001
Reference:
PP/CRI/M/1/7
Catalogue details

Licence: In copyright

Credit: Reprints 121-144. Source: Wellcome Collection.

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    visual signals. If attention is indeed cru cial for visual awareness, the brain could form representations by attending to just one object at a time, rapidly mov ing from one object to the next. For ex ample, 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 sym bolized the implications of that object but also temporarily strengthen the rel evant synapses so that this particular pattern of firing could be quickly re called—a form of short-term memo ry. (If only one representation needs to be held in short-term memory, as in remembering a single task, the neurons involved may contìnue to fire for a pe riod, as described by Patricia S. Gold- man-Rakic in Working Memory and the Mind, page 110.) A problem arises if it is necessary to be aware of more than one object at ex actly the same time. If all the attributes of two or more objects were represent ed by neurons firing rapidly, their at tributes might be confused. The color of one might become attached to the shape of another. This happens some times in very brief presentations. Some time ago Christoph von der Malsburg, now at the Ruhr-Universi tät Bochum, 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 out of synchrony with those represent ing other objects. More recently, two groups in Germany reported that there does appear to be correlated firing be tween 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 y, oscillation. Von der Malsburg's proposal prompt 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 in differ ent coiticeli areas concerning the same object. 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 dis tinguish figure from ground [see The Legacy of Gestalt Psychology, by Irvin Rock and Stephen Palmer; Scientific American , December 1990] or assist the mechanism of attention. Are there some particular types of neurons, distributed over the visual neo cortex, whose firing directly symbolizes the content of visual awareness? One BRIEF FLASHES of colored light enable researchers to infer the minimum time re quired for visual awareness. A disk of red light is projected for 20 milliseconds (a), followed immediately by a 20-millisecond flash of green li ght (b). But the observer reports seeing a single flash of yellow (c), the color that would be apparent if red and green were projected simultaneously. The subject does not become aware of red fol lowed by green until the length of the flashes is extended to 60 to 70 milliseconds. very simplistic hypothesis is that the activities in the upper layers of the cor tex are largely unconscious ones, where as the activities in the lower layers (lay ers 5 and 6) mostly correlate with con sciousness. 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 re sults. Moreover, the neurons in layer 5 show a rather unusual propensity to fire in bursts. The idea that the layer 5 neurons may directly symbolize visu al awareness is attractive, but it still is too early to tell whether there is any thing 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 ex perimental paradigm. We need to con struct neurobiological theories of visu al awareness and test them using a com bination of molecular, neurobiological and clinical imaging studies. We believe that once we have mas tered the secret of this simple form of awareness, we may be close to under standing 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 sensa tions—that is, how the brain relates to the mind. FURTHER READING Perception . Irvin Rock. Scientific Ameri can Library, 1984. Consciousness and the Computation al Mind . Ray Jackendoff. The MIT Press/Bradford Books, 1987. Cold Spring Harbor Symposia on Quantitative Biology , Vol. LV: The Brain . Cold Spring Harbor Laboratory Press, 1990. Towards a Neurobiological Theory of Consciousness . Francis Crick and Christof Koch in Seminars in the Neuro sciences, Vol. 2, pages 263-275; 1990. The Computational Brain . Patricia S. Churchland and Terrence J. Sejnowski. The MIT Press/Bradford Books, 1992.
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