EFFECT OF REMOTE DISTRACTORS ON SACCADE PROGRAMMING - EVIDENCE FOR ANEXTENDED FIXATION ZONE

In a series of experiments, we examined the increase in saccade latenc y that is observed consistently when distracter stimuli are presented simultaneously with the saccade target at various nontarget locations. In the first experiment, targets and distracters were presented on th e horizontal axis. We found that saccade latency was increased when di stracters appeared at fixation and in the contralateral nontarget hemi field (at eccentricities less than or equal to 10 degrees). In contras t, latency was unaffected by distracters presented along the ipsilater al target axis, but amplitude was increased as saccades tended to land al intermediate locations between the two stimuli (global effect). Th e effect of presenting distracters at various two-dimensional location s in both the target and nontarget hemifields then was examined, and t he maximum latency increase again was observed when distracters appear ed at fixation. Distractors presented on any of the eight principal ax es in either hemifield, other than on the horizontal target axis, also increased latency. The relationship between the effects of distracter s on latency and amplitude was reciprocal. Within similar to-20 degree s of the target axis itself, distracters affected saccade amplitude bu t not latency. In contrast, distracters presented outside this ''windo w'' increased saccade latency without affecting amplitude. A systemati c quantitative relationship was revealed between the increase in laten cy and the ratio between target and distracter eccentricities. The lat ency increase was largest with small values of the ratio and reached a peak with distracters at the fixation location. The finding that the increase observed for more eccentric distracter locations fitted the s ame function as that at fixation shows that inhibitory effects operate over large areas of the visual field. The increase in latency under d istracter conditions is interpreted in light of recent neurophysiologi cal findings of inhibitory processes operating in the rostral region o f the superior colliculus. Our results suggest that these inhibitory p rocesses are not restricted to the central foveal region alone but ope rate over wider regions of the visual field.