Control of fixation duration in visual search and memory search: another look.

In visual search a variable delay (up to 150 msec) between the beginning of each fixation and the onset of a search stimulus reduces the time (oculomotor latency) between stimulus onset and the subject's next saccadic eye movement. Two hypotheses for this effect of stimulus onset delay (SOD) were compared: first, process monitoring, that SOD simply serves as a warning interval to facilitate saccadic responses; and second, preprogramming, that saccades are preprogrammed at short SODs. In the first experiment SOD produced a decline in oculomotor latency in search similar to that seen in previous studies. In the second and third experiments, the size of the memory set in a Sternberg memory search paradigm was varied, or a mask flanking some of the search stimuli was used, to vary the processing time of each stimulus. Partial preprogramming of saccades at short delays would predict that increasing the processing time of individual stimuli would increase oculomotor latency at only short SODs. However, oculomotor latency increased equally at all SODs. In this search task, then, the SODs appeared to facilitate saccade initiation.