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.