Inter-limb coupling in bimanual rhythmic coordination in Parkinson's disease

Recently, it has been shown that rhythmic inter-limb coordination is disturbed in patients with Parkinson's disease (PD). The present study aims to investigate whether this coordination deficit is primarily the result of an impaired coupling, related to hypoactivation of the supplementary motor area (SMA), or primarily the indirect result of an asymmetrical distribution of PD symptoms over the left and right limbs (a peripheral process). Thirty PD patients and 30 matched control participants tapped with the index fingers anti-phase and left and right leading gallop patterns in four visual feedback conditions. Symmetrically affected participants performed significantly worse than asymmetrically affected and control participants in the gallop patterns. This result suggested that the central deficit has a stronger effect on inter-limb coupling in PD than the neuromuscular and biomechanical asymmetry between the limbs. Detailed analysis of inter-tap intervals (variability and correlation) suggested that this deficit leads to a compensatory asymmetrical inter-limb coupling in the primarily right-affected patient group, and under specific circumstances also in the primarily left-affected patient group. The difference in coordination strategy between left- and right-affected patients suggested that pre-morbid hand preference is an important structural constraint on the coupling strategies available to the participants.