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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    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 half a billion axons called the corpus callosum. It is well known that if the corpus cal losum is cut, 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 right-hand ed person) appears not to be aware of visual information received exclusively by the right side. This shows that none of the information re quired 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 nor mal person, such informa tion can get to the other side only by using the axons in the corpus callosum. A different part of the brain—the hippocampal sys tem—is involved in one-shot, or episodic, memories that, over weeks and months, it passes on to the neocortex, as described in the article by Eric R. Kandel and Robert D. Hawkins. This system is so placed that it receives inputs from, and projects to, many parts of the brain. Thus, one might suspect that the hip pocampal 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, al though naturally a patient lacking one, such as H.M., is severely handicapped in everyday life because he cannot remem ber 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 redun dant wiring, produced by our genes and by embryonic processes [see The De veloping Brain, by Carla J. Shatz, page 60], 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 neocor tex (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 com binations of active neurons that, on the has seen to it that they form as fast as possible; otherwise, no animal could survive. The brain is handicapped in fo rming neuronal coalitions rapidly be cause, by computer standards, neurons act very slowly. The brain compensates for this relative slowness partly by us ing very many neurons, simultaneous ly and in parallel, and partly by arrang ing the system in a roughly hierarchical WILLIAM JAMES, the father of American psychology, ob served that consciousness is not a thing but a process. basis of its past experience, are most likely to represent the relevant objects and events in the visual world at that moment. The formation of such coali tions of active neurons may also be in fluenced 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 al ways changing. These rapidly formed coalitions occur at different levels and interact to form even broader coali tions. They are transient, lasting usual ly for only a fraction of a second. Be cause coalitions in the visual system are the basis of what we see, evolution 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 happen to be firing above their back ground rate at a particular moment. We would expect that, theoretically, at least some of these neurons would be involved in doing compu tations—trying to arrive at the best coalitions—while others would express the re sults of these computations, in other words, what we see. Fortunately, some exper imental evidence can be found to back up this theoretical conclusion. A phenomenon called binoc ular rivalry may help identi fy the neurons whose firing symbolizes awareness. This phenomenon can be seen in dramatic foim in an exhibit prepared by Sally Duensing and Bob Miller at the Explo- ratorium in San Francisco. Binocular rivalry occurs when each eye has a differ ent 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 con verse 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 mir ror [see box on next two pages], 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