Saccadic behavior in Rhesus monkeys performing a visual search task

Abstract

Monkeys performed a visual search task which began with their fixating a central point for 500 ms; after which an array appeared with a target (an upright or inverted T) and 7, 11, or 15 distractors (upright or inverted crosses).The stimuli subtended roughly 2 and were arranged in a symmetric array 10 from the fixation point. All stimuli were black, except in certain blocks in which the target or a distractor was brightly colored and therefore pop out from the remainder of the array. The monkey manually signaled the target orientation. For trials in which the target did not pop out the manual reaction time varied as a function of distractor number and also of the number of saccades. The probability of finding the target on any given fixation varied as a function of popout and distractor number, but not of saccade sequence, suggesting that there was no memory effect in this search paradigm. First saccades were most likely to be drawn to the popout distractor, but also slightly more likely to be drawn to the non-popout target These results suggest that three different factors influence the oculomotor system in this search paradigm: the popout, the target, and the pool of non-popout distractors. In multiple saccade trials (but not 1 saccade trials), the final fixation time (the time from the final saccade to the response) correlated strongly with reaction time. When the eye position just prior to the onset of the final saccade lay within a certain range of the target, it.strongly determined the time it would take for the monkey to make the discrimination after moving its eye to the target. This suggests that there was a zone within which visual discrimination of the stimulus orientation could take place before the saccade that captured the target. In addition, the properties of this zone varied as function of trial type and distractor number.