Apparent position of visual targets during real and simulated saccadic eye movements

It is now well established that briefly flashed single targets are mislocalized 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 the visual field, the complete pattern of errors suggesting a compression of visual space around the initial fixation point and the target of the impending saccade. The compression was confirmed by displaying multiple rather than single targets and was found to be powerful enough to reduce or even to remove vernier offset for pairs of bars shown simultaneously and to create offsets for colinear bars separated in time by 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 absolute judgment of position. The pattern of errors differed greatly from that during saccades and, in particular, showed no signs of compression. We can model our saccade results by assuming a shift in the point in space associated with eye position compression of eccentricity along the axis of saccades.