Cortical potentials during the gap prior to express saccades and fast regular saccades.

When a temporal gap is introduced between the offset of the central fixation point and the appearance of a new target, saccadic reaction time is reduced (gap effect) and a special population of extremely fast saccades occurs (express saccades). It has been hypothesized that the gap triggers a readiness signal, which is responsible for the reduced saccadic reaction times. Here, we recorded event-related potentials during the gap to investigate the central processes associated with the generation of fast regular saccades and express saccades. Prior to the execution of fast regular saccades, subjects produced a slow negative shift, with a maximum at frontal and central channels that started 40 ms after fixation offset. This widespread negativity is similar to a readiness potential. Anticipatory saccades were preceded by an increased frontal and parietal negativity. Prior to express saccades, a frontal negativity was observed, which started 135 ms after the disappearance of the fixation point. It is assumed that the frontal negativity prior to express saccades corresponds to the fixation-disengagement discharge described in the frontal eye field of monkeys. Therefore, we hypothesize that fast regular saccades are the result of an increased readiness signal, while express saccades are the result of specific preparatory processes.