Perceptual illusions and brain models

Abstract An adequate theory of visual perception must explain how the fleeting patterns of light upon the retinas give knowledge of surrounding objects. The problem of how the brain ‘reads’ reality from images is acute, because images represent directly but few, and biologically unimportant, characteristics of objects. What matters biologically are such things as whether an object is poisonous or food, hard or soft, heavy or light, sharp or blunt, friend or foe. These are not properties of images. The owner of the eye cannot eat or be eaten by its images, and yet his life depends upon interpreting them in terms of quite different characteristics of objects. It follows that eyes are of little biological value unless there is an adequate brain to interpret their images; which raises the evolutionary problem : how did the eye‒brain combination arise? (cf. Gregory 1967a). To read reality from images is to solve a problem : a running set of very difficult problems throughout active life. Errors are illusions. Certain situations present special difficulty, giving rise to systematic errors : can these serve as clues to how the brain generally solves the problem of what objects are represented by which images? Illusions can occur in any of the sense modalities, and they can cross the senses. A powerful illusion crosses from the seen size of an object to its apparent weight, as judged by lifting. Small objects feel up to fifty percent heavier than larger objects of the same scale weight. Thus weight is evidently not judged simply by the input from the arm but also by prior expectation set by the previous handling of weights. When the density is unexpectedly great or small we suffer a corre­sponding illusion of weight.