ABSTRACT
The most important task of the eyes when exploring space is the identification of directions.The‘extensiveness,’whichwas consideredbyDescartes tobe the essence of space, is provided with two of its three spatial dimensions by the perception of directions.The appropriate system for the description of these two directions is the polar co-ordinate system with its centre approximately at the turning-point of the head (the visual egocentre). The primary direction is straight ahead when the eyes are in the centralposition.One co-ordinate is thedirection in relation to the primary direction (above-to the right-below-to the left), the other is the eccentricity in relation to the primary direction. These co-ordinates determine the surveyable visual space. The visual perception of space is the start of a sensorimotor process which endswith the strikeofa clawor the snap ofteeth. Inprimitiveanimals the surveyable part of space is identical with the ‘field of visually-directed motor action.’As the animal’s visuomotor system becomes better developed, more intermediate links appearbetween perception and action. Action canbe delayed, altered or cancelled.
