The Inhibition of Voluntary Saccadic Eye Movement Commands by Abrupt Visual Onsets

Saccadic eye movements are made both to explore the visual world and to react to sudden sensory events. We studied the ability for humans to execute a voluntary (i.e. nonstimulus driven) saccade command in the face of a suddenly appearing visual stimulus. Subjects were required to make a saccade to a memorized location when a central fixation point disappeared. At varying times relative to fixation point disappearance, a visual distractor appeared at a random location. When the distractor appeared at locations distant from the target virtually no saccades were initiated in a 30-40 ms interval beginning 70-80 ms after the distractor appearance. If the distractor was presented slightly earlier relative to saccade initiation then saccades tended to have smaller amplitudes, with velocity profiles suggesting that the distractor terminated them prematurely. In contrast, distractors appearing close to the saccade target elicited express saccade-like movements 70-100 msec after their appearance, though saccade endpoint was generally little affected by the distractor. An additional experiment showed that these effects were weaker when the saccade was made to a visible target in a delayed task and still weaker when the saccade itself was made in response to the abrupt appearance of a visual stimulus. A final experiment revealed that the effect is smaller, but quite evident, for very small stimuli. These results suggest that the transient component of a visual response can briefly but almost completely suppress a voluntary saccade command, but only when the stimulus evoking that response is distant from the saccade goal.