Dopaminergic Modulation of a Fast Visuomotor Pathway in Parkinson's Disease
Abstract
Parkinson’s disease (PD) is associated with reduced dopaminergic (DA) input to the dorsal striatum (DS). This study investigated the role of DA in modulating automatic, stimulus-driven reactions by assessing contextual control of stimulus-locked responses (SLRs) in 10 PD patients off and on DA medication. The SLR is the rapid recruitment of limb muscles that drives the arm towards suddenly appearing stimuli. Participants reached away from (anti-reach) or towards (pro-reach) a target on a screen, depending on instruction appearing 500 or 1000ms before target appearance. Modulation of SLRs was assessed by comparing SLR magnitude on anti- and pro-reach trials using surface electrodes. We predicted patients would exhibit less control of the SLR while off medication, especially with only 500ms of instruction. Patients modulated the SLR less with 500ms of instruction, but there was no effect of medication state, suggesting modulation of the SLR is independent of DA input to the DS.