Probabilistic classification and the basal ganglia
A supervised learning task with a probabilistic relationship between cues and correct response was shown to be impaired in Parkinson’s patients, but not in hippocampal amnesiacs.
This shows a double dissociation between declarative memory and probabilistic classification (i.e. there are two separate learning systems), and shows that the basal ganglia are somehow involved in probabalistic classification.
See NeuroWiki:ProbabilisticClassificationLearning for details and links.