Lateralized motor control mechanisms underlie lateralized motor deficits in early Parkinson's Disease

Parkinson's Disease (PD) motor features typically begin unilaterally, but it remains unclear whether dominant versus non-dominant side motor symptoms differentially affect lateralized aspects of fine motor control.

We recruited 16 right-handed participants with mild, asymmetric PD (8 primarily left-side affected, 8 primarily right-side affected) and 10 age-matched controls to complete a bimanual tablet task in which one hand draws a stylus to targets (leveraging trajectory control mechanisms) while the other hand simultaneously stabilizes a stylus over a stationary target (leveraging stabilizing control mechanisms). The styli are connected via a resistance band to mimic internal forces that arise during asymmetric, bimanual activities of daily living.

Both PD groups demonstrated preserved trajectory control in their dominant hand relative to controls ( p > .072), but significantly impaired stabilizing control in their non-dominant hand ( p < .022), with the largest deficits observed in the left-side PD group. Trajectory control performance significantly correlated with an overall measure of motor severity ( r (14) = .69; p = .004), while stabilizing control did not ( p = .56).

These findings suggest that left- and right-side motor symptoms in early PD produce distinct patterns of functional impairment (rather than simple contralateral analogs) that may be related to underlying lateralized motor control mechanisms. This work highlights a potential gap in standard clinical motor assessments, which may not adequately capture non-dominant side stabilizing deficits in early PD.