Dopaminergic Vulnerability in Parkinson Disease: The Cost of Humans' Habitual Performance.

Humans can simultaneously combine automatic/habitual and voluntary/goal-directed aspects of behavioral control. Habitual routines permit us to perform well practiced task-components with minimal or no voluntary attention. Evidence from animal and human investigations indicates that dopaminergic neurons in lateral substantia nigra, which innervate the sensorimotor striatum, are engaged during the acquisition and performance of automatized skills and habits. Typically, in Parkinson disease (PD), there is a differential loss of dopamine, which occurs earliest and most severely in the caudal sensorimotor striatum, a subdivision of the striatum implicated in habitual control. We suggest that frequent reliance on habitual performance may be a critical functional stressor, which, when combined with other more general risk factors, could explain the selective neurodegeneration of the nigrostriatal motor projection in PD.