Abstract
People with Parkinson’s disease show impairments in their ability to use memory information to guide choices of action when faced with perceptual uncertainty. Changes in the inhibitory output of the basal ganglia underlies motor symptoms in Parkinson’s disease. The superior colliculus, a brainstem target of the basal ganglia, is known to play a role in aspects of attention and decision-making. Therefore, we asked whether changes in the level of inhibition in the superior colliculus altered the ability of monkeys to make perceptual decisions. Trained monkeys performed a two-choice perceptual decision-making task in which they reported the perceived orientation of a dynamic Glass pattern, before and after unilateral, reversible, inactivation of the superior colliculus. We found that unilateral SC inactivation produced significant decision biases and changes in reaction times consistent with a causal role for the primate superior colliculus in perceptual decision-making. Fitting signal detection theory and sequential sampling models to the data showed that superior colliculus inactivation produced a decrease in the relative evidence for contralateral decisions, as if adding a constant offset to a time-varying evidence signal for the ipsilateral choice. The results provide causal evidence for an embodied cognition model of perceptual decision-making and provide compelling evidence that the superior colliculus of primates (a brainstem structure) plays a causal role in how evidence is computed for decisions-a process usually attributed to the forebrain.