Evaluating dopaminergic system contributions to cued pattern switching during bimanual coordination.

Switching between different coordinated movements has been shown to be slow, with delayed responses and even freezing deficits in individuals with Parkinson's disease (PD). While it is well accepted that the dopaminergic system responds to dopamine replacement to ameliorate overall slowness (bradykinesia) and other motor symptoms of PD, it is unknown whether the dopaminergic system can influence overall coordination between limbs and if this may be impacted by the availability of sensory feedback. In the current study, PD and healthy age-matched control participants performed a rhythmic coordination task that required a cued voluntary switch between movement patterns (in-phase and anti-phase). PD participants performed the task first after overnight withdrawal ('off'), and subsequently after administration ('on') of dopamine replacement. Coordinated movements were performed while paced by an auditory metronome in two sensory conditions: 'no vision' or 'normal vision'. Measures of voluntary switch time and delayed responses revealed that PD 'off' required significantly more time than healthy participants to switch between movement patterns. Interestingly, PD 'off' demonstrated disrupted coordination, as revealed by mean (accuracy) and standard deviation (stability) of absolute error of relative phase. Dopamine replacement improved the time needed to switch and amount of delayed responses in PD participants, but had no influence on coordination itself. It is concluded that although modulation of the dopaminergic system improves the slowness during switching, coordination deficits may be the result of secondary impairments (possibly attention-related) that cannot be improved with dopamine replacement.