COUNTERMANDING SACCADES IN MACAQUE

A countermanding paradigm was utilized to investigate the regulation o f saccade initiation. Two rhesus monkeys were instructed to generate a saccade to a peripheral target; however, on a fraction of trials afte r a delay, the monkeys were signaled to inhibit saccade initiation. Wi th short delays between the presentation of the target and the signal to inhibit saccade generation, monkeys withheld saccades to the periph eral target. As the delay of the stop signal increased, monkeys increa singly failed to withhold the saccade. The hypothesis that the generat ion of the saccade is determined by a race between a go and a stop pro cess provides three explicit means of estimating the covert latency of response to the stop signal. This latency, known as stop signal react ion time, was estimated to be on average 82 ms for both monkeys. Becau se the stop signal latency represents the time required to exert inhib itory control over saccade production, the countermanding paradigm wil l be useful for studying neural mechanisms that regulate saccade initi ation.