Perceptual decisions arise after considering the available sensory evidence. When sensory information is unreliable, a good strategy is to rely on previous experience in similar situations to guide decisions. What brain areas are involved in how people combine past experience and sensory information to make perceptual decisions is unknown. We developed a novel, perceptual decision-making task and manipulated the statistics of the sensory stimuli presented to people with Parkinson’s disease (PD), a neurodegenerative disease involving the basal ganglia and healthy participants to determine the influence of past experience on decision-making. We discovered that people with PD are impaired at combining previously learned information with current sensory information to guide decisions. We modeled the results using the drift-diffusion model (DDM) and found that the impairment corresponds to a failure in adjusting the amount of sensory evidence needed to make a decision. These results led us to explore how decision thresholds are established in the brain. To understand this, trained monkeys performed different decision-making tasks, involving detection as well as discrimination. Applying a signal detection theory analysis as well as computational modeling of behavioral data using the DDM, we found strong evidence that the superior colliculus, a brainstem structure that received strong modulatory input from the basal ganglia, plays a causal role in perceptual decision making by regulating how evidence is computed for form decisions.