Effects of olivo-ponto-cerebellar atrophy (OPCA) on finger interaction and coordination

Objectives: We investigated changes in finger interaction and coordination in patients with olivo-ponto-cerebellar atrophy (OPCA) using the recently developed approach to motor synergies based on the principle of motor abundance. Methods: OPCA patients and control subjects performed sets of maximal and submaximal force production tasks by the fingers of each of the hands. Indices of multi-finger synergies were quantified within the framework of the uncontrolled manifold hypothesis. Results: The patients showed lower maximal forces, higher indices of finger interdependence (enslaving), and lower indices of multi-finger synergies stabilizing total force in four-finger tasks. In addition, the patients showed an impaired ability to adjust synergies in preparation to a quick action (small and delayed anticipatory synergy adjustments). The synergy indices showed significant correlations with the clinical scores (both UPDRS total motor scores and ataxia related sub-scores). The observed changes in the indices of finger interaction and coordination were qualitatively similar to those reported earlier for patients with Parkinson's disease; however, the magnitude of the changes was much higher in the OPCA group. Conclusions: These findings fit the hypotheses on the role of the cerebellum in assembling motor synergies and in the feed-forward control of action. They suggest that the synergy index measured in artificial, constrained laboratory tasks may be predictive of more general changes in motor behavior. Significance: The results suggest that studies of multi-digit synergies may be particularly sensitive to subcortical disorders and may provide a much-needed tool for quantitative assessment of impaired coordination in such patients.