Decision-making in the primate brain : formation, location, and causal manipulation

Interaction within the environment relies on the ability to accumulate sensory evidence in favor of a decision. Despite the paramount importance of decision-making to survival, the neural instantiations and computational principles governing the process have remained elusive. In this thesis I consider how sensory evidence is accumulated to guide decisions, and where in the primate brain this process takes place. I report the results of three main experiments. In the first, I test whether sensory evidence is accumulated differentially for motion in the frontoparallel plane (i.e. 2D motion; left/right) compared to motion through depth (i.e. 3D motion; towards/away). I show that integration of 3D motion is different than 2D and likely relies on a mechanism that is distinct. In the second experiment, I test an influential theory in cognitive neuroscience: that neurons in the monkey lateral intraparietal (LIP) cortex accumulate sensory information in favor of a decision communicated by an eye-movement. I found that despite strong correlations between LIP responses and decisions, reversible inactivation of neurons in LIP had no measurable impact on decision-making performance. More generally, I show that decision-related activity does not necessarily play a causal role in choices. In the final experiment, I test whether the process of making a decision stands to influence functions that are decision irrelevant. I found that causally manipulating the amount of sensory evidence available to human observers influenced decision-irrelevant oculomotor commands, suggesting that even during non- oculomotor decisions, oculomotor regions of the brain are recruited. Taken together, the experimental findings reported motivate new ideas about evidence accumulation and advance our understanding of the decision-making process in the primate brain.