Influences of motor instructions on the reaction times of saccadic eye movements.

When a gap period is inserted between the fixation point extinction and the target presentation, the distribution of saccadic reaction times has two distinct peaks: one at 150-250 ms (ordinary saccades) and another at approximately 100 ms (express saccades). The distribution of saccadic reaction times can be explained by the linear approach to threshold with ergodic rate (LATER) model, in which the value of a decision signal increases linearly from a start level to initiate a saccade when the signal value reaches a threshold. We hypothesized that a gap period and/or an instruction signal can modulate the parameters of the model to determine when a saccade is initiated. Two reciprobit plots of reaction times, one for ordinary and the other for express saccades, for a task with both a gap period and visuospatial instruction, were constrained by a common infinite-time intercept, although no such constraint was observed during task performance without a visuospatial instruction. We interpreted the results that either the threshold, the start level, or the rate of increase of the decision signal of the model was switched in a bistable manner by both the visuospatial instruction and a gap period, but not by the gap period alone.