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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    in his or her visual field occupied by the face, the face is wiped out. The movement of the hand, being visually very salient, has captured the brain's attention. Without attention the face cannot be seen If the viewer moves the eyes, the face reappears. In some cases, only part of the face disappears. Sometimes, for example, one eye, or both eyes, will remain. If the viewer looks at the smile on the per son's face, the face may disappear, leaving only the smile. For this reason, the effect has been called the Cheshire Cat effect, after the cat in Lewis Car roll's Alice's Adventures in Wonderland. A lthough it is very difficult to re- /\ cord activity in individual neu- J- j L rons in a human brain, such studies can be done in monkeys. A simple example of binocular rivalry has been studied in a monkey by Nikos K. Logothetis and Jeffrey D. Schall, both then at M.I.T. They trained a macaque to keep its eyes still and to signal wheth er it is seeing upward or downward movement of a horizontal grating. To produce rivalry, upward movement is projected into one of the monkey's eyes and downward movement into the other, so that the two images overlap in the visual field. The monkey signals that it sees up and down movements alternatively, just as humans would. Even though the motion stimulus com ing into the monkey's eyes is always the same, the monkey's percept chang es every second or so. Cortical area MT (which Semir Zeki calls in his article V5) is an area mainly concerned with movement. What do the neurons in MT do when the monkey's percept is sometimes up and sometimes down? (The researchers studied only the monkey's first response.) The sim plified answer—the actual data are rath er more messy—is that whereas the fir ing of some of the neurons correlates with the changes in the percept, for oth ers the average firing rate is relatively unchanged and independent of which direction of movement the monkey is seeing at that moment. Thus, it is un likely that the firing of all the neurons in the visual neocortex at one particu lar moment corresponds to the mon key's visual awareness. Exactly which neurons do correspond remains to be discovered. We have postulated that when we clearly see something, there must be neurons actively firing that stand for what we see. This might be called the activity principle. Here, too, there is some experimental evidence. One ex ample is the firing of neurons in corti cal area V2 in response to illusory con tours, as described by Zeki. Another and perhaps more striking case is the filling in of the blind spot. The blind spot in each eye is caused by the lack of photoreceptors in the area of the retina where the optic nerve leaves the retina and projects to the brain Its lo cation is about 15 degrees from the fovea (the visual center of the eye). Yet if you close one eye, you do not see a hole in your visual field. Philosopher Daniel C. Dennett of Tufts University is unusual among phi losophers in that he is interested both in psychology and in the brain. This interest is much to be welcomed. In a recent book, Consciousness Explained, he has argued that it is wrong to talk about filling in He concludes, correctly, that an absence of information is not the same as information about an ab sence. From this general principle he argues that the brain does not fill in the blind spot but rather ignores it. Dennett's argument by itself, howev er, does not establish that filling in does not occur; it only suggests that it might not. Dennett also states that your brain has no machinery for [fill ing in] at this location This statement is incorrect. The primary visual cortex (VI) lacks a direct input from one eye, but normal machinery is there to deal with the input from the other eye. Ricardo Gattass and his colleagues at the Federal University of Rio de Janeiro have shown that in the macaque some of the neurons in the blind-spot area of VI do respond to input from both eyes, probably assisted by inputs from other parts of the cortex. Moreover, in the case of simple filling in, some of the neurons in that region respond as if they were actively filling in Thus, Dennett's claim about blind spots is incorrect. In addition, psycho logical experiments by Vilayanur S. Ra- machandran [see Blind Spots, Scien tific American , May] have shown that what is filled in can be quite complex depending on the overall context of the visual scene. How, he argues, can your brain be ignoring something that is in fact commanding attention? Filling in, therefore, is not to be dis missed as nonexistent or unusual. It probably represents a basic interpola tion process that can occur at many levels in the neocortex. It is, incidental ly, a good example of what is meant by a constructive process. How can we discover the neurons whose firing symbolizes a particular percept? William T. Newsome and his colleagues at Stanford University have done a series of brilliant experiments on neurons in cortical area MT of the macaque's brain. By studying a neuron The Cheshire Cat Experiment This simple experiment with a mirror illustrates one aspect of visual aware ness. It relies on a phenomenon called binocular rivalry, which occurs when each eye has a different input from the same part of the visual field. Motion in the field of one eye can cause either the entire image or parts of the image to be erased. The movement captures the brain's attention.