Characteristics of "anti" saccades in man.

Four subjects - all made large numbers of Express saccades in the normal gap task - were instructed to make saccades in the direction opposite to the side where a visual stimulus appeared ("anti" task). Gap and overlap trials were used. Saccadic reaction time (SRT), velocity and amplitude of the corresponding eye movements were analysed and compared to those of saccades made in the normal task. The velocity of "anti saccades" was found to be slightly (up to 15%) but significantly slower in two subjects. The distributions of SRTs in normal gap tasks show a small group of anticipatory saccades (with SRT below 80 ms and slower velocities) followed by a group of saccades with fast reaction times between 80 ms and 120 ms (Express saccades) followed by another large group ranging up to 180 ms (regular saccades). In the gap anti task there are anticipatory saccades and saccades with SRTs above 100 ms; Express saccades are missing. The distribution of SRTs obtained in the overlap anti task was unimodal with a mean value of 231 ms as compared to 216 ms in the normal task. The introduction of the gap therefore clearly decreases the reaction times of the anti saccades. Control experiments show that the delay of anti saccades is not due to an interhemispheric transfer time but must be attributed to the saccade generating system taking more time to program a saccade to a position where no visual stimulus appears. These data are discussed as providing further evidence for the existence of a reflex-like pathway connecting the retina to the oculomotor nuclei mediating the Express saccade.