In modern times it has usually come to be considered quite unscientific to base very much just on how things look to our eyes. But the fact remains that despite all the various methods of mathematical and other analysis that have been developed, our visual system still represents one of the most powerful and reliable tools we have. And certainly in writing this book I have relied heavily on our ability to make all sorts of deductions on the basis of looking at visual representations.
So how does the human visual system actually work? And what are its limitations? There are many details yet to be resolved, but over the past couple of decades, it has begun to become fairly clear how at least the lowest levels of the system work. And it turns out—just as in so many other cases that we have seen in this book—that much of what goes on can be thought of in terms of remarkably simple programs.
In fact, across essentially every kind of human perception, the basic scheme that seems to be used over and over again is to have particular kinds of cells set up to respond to specific fixed features in the data, and then to ignore all other features.
Color perception provides a classic example. On the retina of our eye are three kinds of color-sensitive cells, with each kind responding essentially to the level of either red, green or blue. Light from an object typically involves a whole spectrum of wavelengths. But the fact that we have only three kinds of color-sensitive cells means that our eyes essentially sample only three features of this spectrum. And this is why, for example, we have the impression that mixtures of just three fixed colors can successfully reproduce all other colors.
So what about patterns and textures? Does our visual system also work by picking out specific features of these? Everyday experience suggests that indeed it does. For if we look, say, at the picture on the next page we do not immediately notice every detail. And instead what our visual system seems to do is just to pick out certain features which quickly make us see the picture as a collection of patches with definite textures.