The neurobiology of choice : responsibility of the superior colliculus for making pursuit and saccade target selections

This dissertation is organized into a single chapter describing work that has contributed to our understanding of the role of the Superior Colliculus (SC) in the selection of visual stimuli as object-based goals for pursuit and saccade gaze orientations. The chapter is organized as a complete research report with its own abstract, introduction, results and discussion sections. In addition, the chapter is flanked by the general dissertation introduction and conclusion sections, which place the work into a more global perspective. By applying microstimulation that was below the threshold for evoking a saccade to the intermediate layers of the SC while monkeys performed a visual discrimination and 2- alternative forced choice task, we were able to bias the monkeys' selection of targets in favor of those stimuli that appeared on contralateral side of the screen. Specifically, selection was biased toward targets that appeared within a location of the screen represented by the population of the SC in the vicinity of our stimulating electrode. Of greatest significance was the fact that microstimulation biased target selection even when the resulting eye movement did not go toward the stimulated zone but instead traveled ipsiversive to the site of the electrode (as was the case during smooth pursuit trials). This finding suggests that the SC is responsible for selecting visual stimuli as targets for gaze orientation, regardless of the exact trajectory required to foveate a target. This replaces the long-held notion of the Superior Colliculus as a saccade machine with a more modern view of this brainstem structure as having a key role in the visuomotor decision network