CVRREm DIRECTIONS INj.PSYCHDLOGICAL SCIENCE Direction Constancy in the Oculomotor System

Whenever a visual image moves across the retina, the brain is faced with a dilemma: Did the image move because an object in the environment (or the environment itself) changed location or because the eyes changed location with respect to a stable environment? If the brain compares the retinal positions of individual objects within the visual image and finds that only one moved with respect to the others, then it can reasonably assume that the retinal motion was caused by motion of the object and not of the eyes. (This comparison of an object's position with the positions of other objects is known as exocentric, or allocentric, localization.) But how does the brain react to this dilemma when the entire visual image moves, whether the image is a fully structured scene or merely a single object In otherwise complete darkness? LJnder these conditions, exocentric localization techniques fail, and the brain must rely on its ability to localize the objects with respect to itself, the observer. For this egocentric localization, the brain must combine information concerning the retinal location of the visual im-