The countermanding task probes both monkeys' and humans' ability to control behavior by requiring them to withhold planned movements and estimates the time needed to cancel the movement. To succeed in this task, subjects must control their performance. This laboratory has used the countermanding task to study the frontal eye field (FEF)(Hanes et al., J.Neurophysiol. 79:817, 1998), the supplementary eye field (SEF)(Stuphorn et al, Nature 408:857, 2000) and the anterior cingulate cortex (ACC) (Stuphorn and Schall, SfN Abstract 26:404.5, 2000) of macaque monkeys. While neurons in the FEF generated signals sufficient to control saccade production, neurons in the SEF and ACC registered the occurrence of errors and reward. To test the hypothesis, that SEF is part of a supervisory executive control system, we determined the effects of microstimulation (300 ms train, biphasic, 0.2 ms pulsewidth, 333 Hz) on countermanding performance in macaque monkeys. For each stimulation site, we first determined the current thr...