In a probabilistic classification task, subjects predict a category outcome (vanilla or chocolate ice cream) based on features on a Mr. Potatohead doll (moustache, hat, bowtie or glasses). (A) Training was either feedback-based, as in prior studies, or observational, involving no feedback. (B) Controls showed similar levels of learning under both conditions; individuals with basal ganglia disruption due to Parkinson’s disease (PD) were impaired at feedback-based learning, but not at observational learning (Shohamy et al., 2004b).