Differential effects of non-target stimuli on the occurrence of express saccades in man.

The effect of competing non-target stimuli (distractors) presented together with a target for saccades was investigated with special emphasis on saccadic reaction time. Using the gap task (fixation point offset preceding target onset by 200 msec) high numbers of express saccades were obtained from four human subjects. A single non-target stimulus in the visual hemifield contralateral to the target reduced the number of express saccades. The extend of reduction increased with increasing stimulus size leading to a situation where express saccades were replaced entirely with saccades having a latency mode 30 msec later. Non-target stimuli occurring in the hemifield ipsilateral to the saccade target could also reduce the number of express saccades in favour of fast regular saccades. The effect of ipsilateral non-target stimuli depended mainly on their position in the visual field: ipsilateral non-target stimuli outside the dead zone for express saccades caused only a slight reduction of express saccades. Non-target stimuli invading the dead zone, however, suppressed express saccades almost completely, even though the saccade target appeared well outside the dead zone. The onset of a whole field of bars of one orientation (serving as distractors) simultaneously with the target (consisting of a single bar of another orientation = pop-out condition), suppressed express saccades completely. The same result, however, was obtained with only two distractor bars presented ipsilateral and contralateral within the dead zone for express saccades. Express saccades were present, however, when the whole field of distracting bars was continuously visible throughout the whole trial.