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|Title:||THE ROLE OF THE CAUDATE NUCLEUS IN SACCADE SUPPRESSION AND FACILITATION FOR ANTISACCADES IN MONKEYS|
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|Authors/Affiliations:||1 Masayuki Watanabe*; 1 Douglas Munoz; |
1 Queen's University, Kingston, ON, Canada
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|Content:||Objectives: Suppression of automatic responses and facilitation of volitional actions are two of the essential functions to flexibly generate an action in response to a sensory event. The involvement of the frontal-basal ganglia (BG) pathways in these functions has been revealed by the antisaccade paradigm (look away from a visual stimulus). However, recent neuropsychological studies cast doubt on the role of the BG in antisaccade control. By directly recording neural activity from the caudate nucleus, a major input stage of the BG, in monkeys performing the antisaccade paradigm, we examined the role of the BG in antisaccades.|
Methods: After monkeys maintained their eyes on the central fixation point (FP), a visual stimulus was turned on at 15° left or right from the FP. Based on the color of the FP, the monkeys were required to make a saccade to the stimulus (prosaccade) or to the direction opposite the stimulus (antisaccade). The pro and antisaccade instructions, in addition to left and right stimulus locations, were randomly interleaved in the block of trials. We isolated single phasically active neurons in the caudate nucleus, presumably GABAergic projection neurons, and examined their saccade direction selectivity as well as correlation with behavioral performance.
Results: We identified neurons carrying signals facilitating contralateral saccade initiation (CNs). However, a subset of the CNs might have facilitated an erroneous saccade toward a contralateral stimulus when an ipsilateral antisaccade was required. We also identified another group of neurons whose activity was stronger in ipsilateral saccades than in contralateral saccades on antisaccade trials (INs). The activity of the CNs and the INs was higher on correct performance than when monkeys failed to suppress a saccade toward a visual stimulus on antisaccade trials. Furthermore, their activity was negatively correlated with saccade reaction times on trial-by-trial basis (higher activity followed by shorter reaction times).
Conclusion: We conclude that the caudate neurons carry saccade suppression and facilitation signals and are involved in volitional control over antisaccades. We hypothesize that the CNs facilitate contralateral saccades while INs suppress inappropriate saccade commands issued by a subset of CNs. Because CNs and INs are presumably projection neurons, the competitive interaction between CNs and INs might be carried out in structures downstream from the caudate nucleus.
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