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 were made between targets placed at optical infinity by dichoptic presentation. This arrangement was chosen to evoke conjugate eye movements. Eye movements were recorded binocularly, including torsion. Horizontal and vertical movements of the eyes are strongly correlated (r > or = 0.95) during the saccade, torsional movements are much less so (r approximately 0.67). Listing's law would predict that the three-dimensional versional velocity of the eye would be located in a plane that is tilted out of Listing's plane by an amount that depends on the saccade's starting position (half angle rule). Taking together all saccades that started from the same initial position a plane could be fitted through the velocity vectors. However, this plane was tilted less relative to Listing's plane than predicted by the half angle rule. The deviation was especially large for the yaw component of the tilt (56% of predicted). For the pitch component the prediction was better (81% of predicted). In addition, we find that the torsional velocity during the fast "intrasaccadic" part of the motion can be unequal in the two eyes. The implications for three-dimensional models of saccadic control are discussed.