Chapter One

1.1 Introduction—The Senses

Of the five senses—vision, hearing, smell, taste, and touch—vision is undoubtedly the one that we have come to depend upon above all others, and indeed the one that provides most of the data we receive. Not only do the input pathways from the eyes provide megabits of information at each glance, but the data rate for continuous viewing probably exceed 10 megabits per second. However, much of this information is redundant and is compressed by the various layers of the visual cortex, so that the higher centers of the brain have to interpret abstractly only a small fraction of the data. Nonetheless, the amount of information the higher centers receive from the eyes must be at least two orders of magnitude greater than all the information they obtain from the other senses.

Another feature of the human visual system is the ease with which interpretation is carried out. We see a scene as it is—trees in a landscape, books on a desk, widgets in a factory. No obvious deductions are needed, and no overt effort is required to interpret each scene. In addition, answers are effectively immediate and are normally available within a tenth of a second. Just now and again some doubt arises—for example, a wire cube might be "seen" correctly or inside out. This and a host of other optical illusions are well known, although for the most part we can regard them as curiosities—irrelevant freaks of nature. Somewhat surprisingly, it turns out that illusions are quite important, since they reflect hidden assumptions that the brain is making in its struggle with