What the presentation of two visual targets with varied contrasts, sizes and temporal asynchronies tells us about the process of target selection in humans and monkeys

Abstract

Target choice and the decision times involved were determined in humans and monkeys using a two-target task in which the contrast, size and the temporal asynchrony between the targets were varied. Stimuli were presented in the left and right hemifields at eccentricities of 3–8 degrees. For comparison in performance, single targets were intermingled with the presentation of the paired targets. Targets were selected either by making a saccadic eye movement or by pressing a lever. In the two-target task, target choice was strongly influenced by relative stimulus contrast and by size as specified by the temporal offset required to have both targets chosen with equal probability. For equal probability choices in monkeys performing the two-target task saccadic reaction times were 12–47 ms longer than for single targets; in humans performing the lever press task reaction times to paired targets were 160–300 ms longer than those obtained to single targets. Targets were chosen with equal probability most frequently when the two targets were identical and were presented simultaneously. Reaction times dropped rapidly with increasing temporal asynchronies between the targets. With a temporal asynchrony of 34 ms, reaction times decreased by more than 125 ms in humans but only by 15–25 ms in monkeys. Unilateral frontal eye field (FEF) but not medial eye field lesions in monkeys produced major shifts in equal probability target choice that persisted even four years after FEF lesions. Equal probability choice in the paired target task after a unilateral FEF lesion required the presentation of the target in the affected hemifield 60–120 ms earlier than the target in the intact hemifield indicating a major decrease in the speed of processing as a result of the lesion. A study examining the effects of brain infarcts in human subjects on these tasks is under way.