Countermanding saccades in macaque.

A countermanding paradigm was utilized to investigate the regulation of saccade initiation. Two rhesus monkeys were instructed to generate a saccade to a peripheral target; however, on a fraction of trials after a delay, the monkeys were signaled to inhibit saccade initiation. With short delays between the presentation of the target and the signal to inhibit saccade generation, monkeys withheld saccades to the peripheral target. As the delay of the stop signal increased, monkeys increasingly failed to withhold the saccade. The hypothesis that the generation of the saccade is determined by a race between a go and a stop process provides three explicit means of estimating the covert latency of response to the stop signal. This latency, known as stop signal reaction time, was estimated to be on average 82 ms for both monkeys. Because the stop signal latency represents the time required to exert inhibitory control over saccade production, the countermanding paradigm will be useful for studying neural mechanisms that regulate saccade initiation.