Control of multidigit grasping in Parkinson's disease: effect of object property predictability.

We examined the extent to which subjects with Parkinson's disease (PD) modulate normal fingertip forces during five-digit grasping based on the object's center of mass (CM). We also tested the effect of trial-to-trial predictability of CM location on the distribution of all fingertip forces relative to thumb force. Ten right-handed subjects with PD (OFF and ON medication) and 10 healthy age-matched control subjects participated. Subjects lifted a manipulandum that measured normal forces exerted by each digit. The CM location was changed from trial-to-trial either in an unpredictable (random) or predictable (blocked) order. Discriminant analysis and information theory were used to quantify the extent to which force-sharing patterns could be discriminated as a function of CM location. All subjects modulated fingertip normal forces as a function of CM location regardless of its predictability, although larger forces were employed when its location was unpredictable. However, in controls, normal force modulation of individual fingers to the object's CM location occurred over a greater range of forces when the CM location was predictable than when it was unpredictable. In contrast, subjects with PD exhibited a similar force modulation to CM location regardless of its predictability. There was a clearer discrimination of force-sharing patterns when the CM location was predictable for controls but not for subjects with PD OFF medication. Medication improved the time course of normal force modulation to CM location. These results indicate that subjects with PD maintained the ability to modulate individual fingertip forces to the object's physical properties. Nevertheless, subjects with PD did not benefit from the a priori knowledge of object CM location to the same extent as controls. These findings support the notion that PD affects the ability to use anticipatory control mechanisms.