TORSION DURING SACCADES BETWEEN TERTIARY POSITIONS

We systematically studied the effect of saccade direction and saccade starting position on the velocity profile of the saccade. Saccades wer e made between targets placed at optical infinity by dichoptic present ation. This arrangement was chosen to evoke conjugate eye movements. E ye movements were recorded binocularly, including torsion. Horizontal and vertical movements of the eyes are strongly correlated (r greater than or equal to 0.95) during the saccade, torsional movements are muc h less so (r similar or equal to 0.67). Listing's law would predict th at the three-dimensional versional velocity of the eye would be locate d in a plane that is tilted out of Listing's plane by an amount that d epends on the saccade's starting position (half angle rule). Taking to gether all saccades that started from the same initial position a plan e could be fitted through the velocity vectors. However, this plane wa s tilted less relative to Listing's plane than predicted by the half a ngle rule. The deviation was especially large for the yaw component of the tilt (56% of predicted). For the pitch component the prediction w as better (81% of predicted). In addition, we find that the torsional velocity during the fast ''intrasaccadic'' part of the motion can be u nequal in the two eyes. The implications for three-dimensional models of saccadic control are discussed.