Effect of remote distractors on saccade programming: Evidence for an extended fixation zone

In a series of experiments, we examined the increase in saccade latency 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 the horizontal axis. We found that saccade latency was increased when distracters appeared at fixation and in the contralateral nontarget hemifield (at eccentricities less than or equal to 10 degrees). In contrast, latency was unaffected by distracters presented along the ipsilateral target axis, but amplitude was increased as saccades tended to land al intermediate locations between the two stimuli (global effect). The effect of presenting distracters at various two-dimensional locations in both the target and nontarget hemifields then was examined, and the maximum latency increase again was observed when distracters appeared at fixation. Distractors presented on any of the eight principal axes in either hemifield, other than on the horizontal target axis, also increased latency. The relationship between the effects of distracters on latency and amplitude was reciprocal. Within similar to-20 degrees of the target axis itself, distracters affected saccade amplitude but not latency. In contrast, distracters presented outside this ''window'' increased saccade latency without affecting amplitude. A systematic quantitative relationship was revealed between the increase in latency and the ratio between target and distracter eccentricities. The latency 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 same function as that at fixation shows that inhibitory effects operate over large areas of the visual field. The increase in latency under distracter conditions is interpreted in light of recent neurophysiological findings of inhibitory processes operating in the rostral region of the superior colliculus. Our results suggest that these inhibitory processes are not restricted to the central foveal region alone but operate over wider regions of the visual field.