The orienting response evoked by the appearance of asalient stimulus is modulated by arousal; however, neural underpinningsfor the interplay between orienting and arousal are notwell understood. The superior colliculus (SC), causally involvedin multiple components of the orienting response includinggaze and attention shifts, receives not only multisensory andcognitive inputs but also arousal-regulated inputs from variouscortical and subcortical structures. To investigate the impact ofmoment-by-moment fluctuations in arousal on orientingsaccade responses, we used microstimulation of the monkeySC to trigger saccade responses, and we used pupil size andvelocity to index the level of arousal at stimulation onset becausethese measures correlate with changes in brain states and locuscoeruleus activity. Saccades induced by SC microstimulationcorrelated with prestimulation pupil velocity, with higher pupilvelocities on trials without evoked saccades than with evokedsaccades. In contrast, prestimulation absolute pupil size didnot correlate with saccade behavior. Moreover, pupil velocitycorrelated with evoked saccade latency and metrics. Together,our results demonstrated that small fluctuations in arousal,indexed by pupil velocity, can modulate the saccade responseevoked by SC microstimulation in awake behaving monkeys.
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