Stability of bimanual coordination in Parkinson's disease and cognitive modulation of intention.

The functional integrity of the bimanual neuro-motor system of Parkinson's disease (PD) subjects (stageII) compared to controls (2 x n = 16) was evaluated by measures of coordination stability of tapping in in-phase, anti-phase, and 90 degrees-phase. Recently, intentional influence was modeled as an additive attractor function on the intrinsic dynamics, resulting in predictions tested by Scholz and Kelso (1990). In this study, the intentional influence was modulated by attaching cognitive meaning to the rhythmical pattern, which was expected to enhance the stability of coordination and, if effective, might be profitable to PD patients. Half of the PD subjects significantly lacked stability. They were less stable than controls, lost coordination at lower frequencies, and needed more time to switch between phase patterns. The reduction of stability was reflected in the progression of the disease. Cognitive meaning reduced variability of the single-hands but not of relative phase, and no effect on switching time was found. The results suggest a weaker coupling strength between the limbs in PD patients lacking stability.