The Neural Control of Visually Guided Eye Movements
D. Temporal Factors in Visually Guided Saccadic Eye Movements

1. The conditions under which express saccades can and cannot be generated.

a. How important is it to extinguish the fixation spot before the target is presented?

effect of gap time on distribution of saccadic latenciesFigure 30 shows the distribution of saccadic latencies using three different gap times, which is the time between the termination of the fixation spot and the appearance of the target. With a gap times of 100 milliseconds numerous express saccades occur (which form the first peak of the bimodal distribution). The frequency of express saccades falls off dramatically as the gap time is decreased. as shown in the second and third panels of Figure 30. When the fixation spot is not extinguised at all, express saccades become extremely rare. Thus extinguishing the fixation spot 100-200 milliseconds before the target onset is an important condition for express saccade generation.