Event-related potentials and saccadic reaction times: effects of fixation point offset or change.

Previous studies have shown that saccadic reaction times (SRTs) are reduced if the initial fixation point (FP) disappears 200 ms (gap period) before a peripheral target is presented. This gap saccade task is associated with a negative cortical potential at the end of the gap period. To determine whether the neural processes underlying this potential account for the reduction of SRTs during gap saccade tasks, we recorded event-related potentials (ERPs) in 19 subjects performing a gap saccade task (gap duration 200 ms), a warning saccade task (the color of the FP changed 200 ms prior to target appearance) and an overlap task (the FP remained visible during the trial). SRTs were shortest during the gap task, longest during the overlap task and intermediate during the warning task. The gap and warning tasks were accompanied by the same widespread negative cortical potential with a maximum at the time of stimulus presentation. These findings indicate that the warning effect mediated by the disappearance of the FP during gap saccade tasks is responsible for the gap negativity which was observed by several authors. Our findings of shorter SRTs during the gap task than the warning task, however, suggest that the gap has an additional effect that probably depends on subcortical mechanisms.