This study presents the first direct investigation of the hypothesis that dopamine depletion of the dorsal striatum in mild Parkinson disease leads to impaired stimulus–response habit formation, thereby rendering behavior slow and effortful. However, using an instrumental conflict task, we show that patients are able to rely on direct stimulus–response associations when a goal-directed strategy causes response conflict, suggesting that habit formation is not impaired. If anything our results suggest a disease severity–dependent deficit in goal-directed behavior. These results are discussed in the context of Parkinson disease and the neurobiology of habitual and goal-directed behavior.

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