EXPRESS SACCADES ELICITED DURING VISUAL SCAN IN THE MONKEY

Monkeys trained to saccade to visual targets can develop separate ''ex press'' and ''regular'' modes in their distribution of saccadic latenc ies. The purpose of this study was to determine whether this occurs un der more natural viewing conditions, when targets are suddenly present ed in a structured visual field during visual scan. It was found that scanning saccades stopped appearing 60 msec after a target's onset, an d subsequent saccades, which were directed toward the suddenly appeari ng target, had a bimodal distribution of latencies. Express saccades w ere more likely to occur as the target was presented later in a fixati on. Regular mode saccades were more likely to occur with longer target durations. Scanning saccades made to stimuli of the structured visual field always had unimodal inter-saccadic interval distributions. All these effects were apparent after only 2-3 days of training. These fin dings, taken together with recent physiological results, suggest that the visuomotor cells of the superior colliculus mediate latency bimoda lity.