Unequal amplitude saccades produced by aniseikonic patterns: effects of viewing distance.

This study describes differences in horizontal and vertical disconjugate saccades under far and close viewing conditions of two dichoptically presented aniseikonic random checkerboard patterns. At far viewing, disconjugacy of horizontal saccades requiring divergence was accomplished intrasaccadically after several minutes; for convergence the intrasaccadic disconjugacy was limited. Size differences partially persisted in open-loop trials. At close viewing intrasaccadic divergent changes in conjugacy were instantaneous, but motor storage during open-loop was absent. It is concluded that disconjugate saccades to targets at far distance lead to an adaptation process, whereas at close viewing distance horizontal disparity is a visual compensation process used directly to scale the relative amplitudes of both eyes, not leading to adaptation. The time-course of disconjugate vertical saccades was much slower, with mostly postsaccadic vergence. Nearby viewing enhanced the disconjugacy of vertical saccades.