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![formation available to our eyes is not sufficient by itself to provide the brain with its unique interpretation of the vi sual world. The brain must use past ex perience (either its own or that of our distant ancestors, which is embedded in our genes) to help interpret the infor mation coming into our eyes. An exam ple would be the derivation of the three- dimensional representation of the world from the two-dimensional signals falling onto the retinas of our two eyes or even onto one of them. Visual theorists also would agree that seeing is a constructive process, one in which the brain has to carry out com plex activities (sometimes called com putations) in order to decide which in terpretation to adopt of the ambiguous visual input. Computation implies that the brain acts to form a symbolic rep resentation of the visual world, with a mapping (in the mathematical sense) of certain aspects of that world onto ele ments in the brain. Ray Jackendoff of Brandeis Univer sity postulates, as do most cognitive scientists, that the computations car ried out by the brain are largely uncon scious and that what we become aware of is the result of these computations. But while the customary view is that this awareness occurs at the highest lev els of the computational system, Jack endoff has proposed an intermediate- level theory of consciousness. What we see, Jackendoff suggests, re lates to a representation of surfaces that are directly visible to us, together with their outline, orientation, color, texture and movement. (This idea has similari ties to what the late David C. Marr of the Massachusetts Institute of Technology called a 2 V2 -dimensional sketch. It is more than a two-dimensional sketch be cause it conveys the orientation of the visible surfaces. It is less than three-di mensional because depth information is not explicitly represented.) In the next stage this sketch is processed by the brain to produce a three-dimensional representation. Jackendoff argues that we are not visually aware of this three- dimensional representation. An example may make this process clearer. If you look at a person whose back is turned to you, you can see the back of the head but not the face. Nev ertheless, your brain infers that the person has a face. We can deduce as much because if that person turned around and had no face, you would be very surprised. The viewer-centered representation that corresponds to the visible back of the head is what you are vividly aware of. What your brain infers about the front would come from some kind of three-dimensional representation. This does not mean that information flows only from the surface representation to the three-dimensional one; it almost cer tainly flows in both directions. When you imagine the front of the face, what you are aware of is a surface represen tation generated by information from the three-dimensional model It is important to distinguish be tween an explicit and an implicit representation. An explicit repre sentation is something that is symbol ized without further processing. An im plicit representation contains the same information but requires further pro cessing to make it explicit. The pattern of colored dots on a television screen, for example, contains an implicit rep resentation of objects (say, a person's face), but only the dots and their loca tions are explicit. When you see a face on the screen, there must be neurons in your brain whose firing, in some sense, symbolizes that face. We call this pattern of firing neu rons an active representation. A latent representation of a face must also be stored in the brain, probably as a spe cial pattern of synaptic connections be tween neurons [see How Neural Net works Learn from Experience, by Geoff rey E. Hinton, page 144]. For example, you probably have a representation of the Statue of Liberty in your brain, a representation that usually is inactive. If you do think about the Statue, the representation becomes active, with the relevant neurons firing away. An object, incidentally, may be repre sented in more than one way—as a vi sual image, as a set of words and their related sounds, or even as a touch or a smell. These different representations are likely to interact with one another. The representation is likely to be dis tributed over many neurons, both lo cally, as discussed in Geoffrey E. Hin- ton's article, and more globally. Such a representation may not be as simple and straightforward as uncritical intro spection might indi t . re. There is sug gestive evidence, pa; ' from studying how neurons fire in various parts of a monkey's brain and partly from ex amining the effects of certain types of brain damage in humans, that different aspects of a face—and of the implica tions of a face—may be represented in different parts of the brain. First, there is the representation of a face as a face: two eyes, a nose, a mouth and so on. The neurons involved are usually not too fussy about the exact size or position of this face in the visual field, nor are they very sensitive to small changes in its orientation. In monkeys, there are neurons that respond best when the face is turning in a particular direction, while others seem to be more concerned with the direction in which the eyes are gazing. Then there are representations of the parts of a face, as separate from those for the face as a whole. Further, the im plications of seeing a face, such as that person's sex, the facial expression, the familiarity or unfamiliarity of the face, and in particular whose face it is, may each be correlated with neurons firing in other places. What we are aware of at any moment, in one sense or another, is not a simple matter. We have suggested that there may be a very transient form of fleeting awareness that represents only rather simple features and does not require an attentional mechanism. From this brief awareness the brain constructs a viewer- centered representation—what we see vividly and clearly—that does require attention This in turn probably leads to three-dimensional object representa tions and thence to more cognitive ones. Representations corresponding to viv id consciousness are likely to have spe cial properties. William James thought that consciousness involved both at tention and short-term memory. Most psychologists today would agree with this view. Jackendoff writes that con sciousness is enriched by attention, implying that while attention may not be essential for certain limited types of consciousness, it is necessary for full consciousness. Yet it is not dear exactly which forms of memory are involved. Is long-term memory needed? Some forms of ac quired knowledge are so embedded in the machinery of neural processing that they are almost certainly used in becom ing aware of something. On the other hand, there is evidence from studies of brain-damaged patients (such as H.M., described in The Biological Basis of Learning and Individuality, by Eric R. Kandel and Robert D. Hawkins, page 78) that the ability to lay down new long-term episodic memories is not es sential for consciousness. It is difficult to imagine that anyone could be conscious if he or she had no memory whatsoever of what had just happened, even an extremely short one. Visual psychologists talk of iconic mem ory, which lasts for a fraction of a sec ond, and working memory (such as that used to remember a new telephone number) that lasts for only a few sec onds unless it is rehearsed. It is not clear whether both of these are essential for consciousness. In any case, the divi sion of short-term memory into these two categories may be too crude.](https://iiif.wellcomecollection.org/image/b18169946_PP_CRI_M_1_7_0003.jp2/full/800%2C/0/default.jpg)
