Predictive and reactive finger force control during catching in cerebellar degeneration.

We investigated how patients with cerebellar degeneration control fingertip forces to resist a perturbation imposed on a handheld load. Patients and healthy sex- and age-matched control subjects held an instrumented receptacle between the index finger and thumb. A weight was dropped into the receptacle either unexpectedly from the experimenter's hand with the subject being blindfolded or expectedly from the subject's opposite hand. This paradigm allowed us to study predictive and reactive modes of finger force control. Patients generated an overshoot of grip force, irrespective of whether the weight was dropped expectedly or unexpectedly. When the weight was dropped from the experimenter's hand, grip force lagged behind the load perturbation at impact in patients and controls. When the weight was dropped expectedly from the subject's opposite hand, healthy subjects started to increase grip force prior to the release of the weight. This observation is indicative for a predictive mode of force control. In contrast, the grip force profile of cerebellar patients was not processed in anticipation of the time of impact when the weight was dropped from the opposite hand. Our data suggest involvement of cerebellar circuits in a predictive, but less in a reactive, mode of fingertip force control during manipulative behavior.