Saccade Suppression by Electrical Microstimulation in Monkey Caudate Nucleus

It has been suggested that the caudate nucleus, the input stage of the basal ganglia, facilitates and suppresses saccade initiation based on its anatomical characteristics. Although the involvement of the caudate nucleus in saccade facilitation has been shown previously, it is still unclear whether the caudate nucleus is also involved in saccade suppression. Here, we revealed the direct involvement of the caudate nucleus in saccade suppression by electrical microstimulation in behaving monkeys. We delivered microstimulation to the caudate nucleus while monkeys performed the prosaccade (look toward a peripheral visual stimulus) and antisaccade (look away from the stimulus) paradigm. The reaction times of contralateral saccades were prolonged on both prosaccade and antisaccade trials. The suppression effects on reaction times were stronger on prosaccade trials compared with antisaccade trials. The analysis of reaction time distributions using the linear approach to threshold with ergodic rate model (LATER model) revealed that microstimulation prolonged reaction times by reducing the rate of rise to the threshold for saccade initiation. Microstimulation also worsened correct performance rates for contralateral saccades. The same microstimulation prolonged and/or shortened the reaction times of ipsilateral saccades, although the effects were not as consistent as those on contralateral saccades. We conclude that caudate signals are sufficient to suppress contralateral saccades and influence saccadic decision by controlling contralateral and ipsilateral saccade commands at the same time.