APPARENT POSITION OF VISUAL TARGETS DURING REAL AND SIMULATED SACCADIC EYE-MOVEMENTS

It is now well established that briefly flashed single targets are mis localized in space, not only during saccades but also before them. We show here by several techniques (including a vernier judgment that did not require absolute location in space) that errors appear up to 100 msec before saccades are made and are maximal just before they start. The size and even the sign of errors depend strongly on position in th e visual field, the complete pattern of errors suggesting a compressio n of visual space around the initial fixation point and the target of the impending saccade. The compression was confirmed by displaying mul tiple rather than single targets and was found to be powerful enough t o reduce or even to remove vernier offset for pairs of bars shown simu ltaneously and to create offsets for colinear bars separated in time b y 75 msec. It also reduced the apparent number of parallel bars. When saccades were simulated by moving the display at saccadic speed, there were sometimes errors of location, but only for tasks requiring absol ute judgment of position. The pattern of errors differed greatly from that during saccades and, in particular, showed no signs of compressio n. We can model our saccade results by assuming a shift in the point i n space associated with eye position compression of eccentricity along the axis of saccades.