Psychophysiological evidence for impaired reward anticipation in Parkinson's disease.

OBJECTIVE: According to a widely held view, mesencephalic dopamine neurons mediate feedback-based learning by broadcasting an error signal that indexes the difference between anticipated and actual response-contingent reward. The present experiment tested whether impaired learning of a probabilistic classification task by individuals with Parkinson's disease (PD) is associated with abnormal reward expectation.
METHODS: The stimulus-preceding negativity (SPN), a brain potential known to reflect anticipation of motivationally significant events, was compared for blocks in which subjects anticipated high or low monetary rewards and punishments.
RESULTS: The SPN was reduced in amplitude in patients relative to controls in the high monetary incentive condition. Furthermore, whereas the SPN varied in size as a function of cue complexity for control subjects, it did not for patients.
CONCLUSIONS: These results suggest that anticipatory processes within cortical portions of the reward system are impaired by PD. SIGNIFICANCE: These data support previous claims that the SPN offers an electrophysiological index of activity within cortical portions of the reward pathway, and that reinforcement-based learning is impaired in PD.