Story of Psychology - Morton Hunt [303]
None of this, however, explains two other mysteries of motion perception. If we move our eyes or head to follow a flying bird or other moving object, we perceive movement even though the image remains at the center of the retina. Conversely, if we move our eyes, images sweep across the retina but we see the world as still.
There must, then, be some other source of information that confirms or corrects the information coming from the retina. Two possibilities have long been put forward: Either the brain’s commands to the eyes and head to move, in order to keep the image of a moving object on the center of the retina, or the eye and head movements themselves are relayed to the visual cortex and there interpreted as the object’s movement. Similarly, when we scan a still background, either the brain’s commands or the eye and head movements send signals to the visual cortex that enable it to recognize the moving retinal image as that of an unmoving scene.52
The matter has not been resolved; laboratory experiments with animals provide some evidence for each theory. By one means or another, eye and head movements provide part of the information essential to movement perception. Studies of afterimages prove the point. If subjects stare at a bright light for a little while, then look away toward a relatively dark area, they see an afterimage of the light. If they move their eyes, the afterimage moves in the same direction, although the source of the afterimage, the fatigued area of the retina, does not move. This means that the visual cortex, receiving messages that the eyes are moving but that the image is not moving across the retina, interprets them to mean that the eyes are tracking a moving image.53
Another possible explanation is the frame of reference effect. If you are looking at some background, such as the tennis court across the net, your opponent starts to move cross-court, and you turn your head to keep your eyes on him, the result is an array of images across your retinas. But you know that the other player moved, not the tennis court, which had been established in your brain as the frame of reference.54
Much recent neuroscientific research has investigated the neural pathways involved in various forms of disorders of motion perception. Thus far, it has neither confirmed nor amended the above hypotheses, but may well be on the brink of doing so.
Seeing Depth
In nature, unlike the laboratory, neither form nor movement exists apart from three-dimensionality; to understand form and motion perception in everyday life, it is essential to understand depth perception.55 Psychologists have always considered this a central puzzle about perception; a bibliography of all their writings on depth perception would fill more than a volume.56
The basic question has always been both obvious and simple: How do we see the world as three-dimensional when our source of information, the image on the retina, is essentially two-dimensional? Why do we not see the world as flat, like a color photograph in which distance and the three-dimensional qualities of every object are merely suggested by size, perspective, shading, and other cues?
Such cues are, in fact, the answer offered by a group of theories. These take many forms but all hold that depth perception is not automatic and innate. Some say that it comes about as a result of experiences that lead us to associate