Impaired generalization of weight-related information during grasping in cerebellar degeneration.

When we repetitively lift an object, the balance between grip force normal to the object's surface and load force tangential to the object's surface is accurately programmed to match the physical object properties within a few lifts. Here, we ask if the accuracy of grip force scaling to object weight and the transfer of weight-related information from one hand to the other is impaired in cerebellar degeneration. Subjects with generalized cerebellar degenerative disorders were tested. Subjects first repeatedly lifted a constant weight with the dominant hand, followed by a series of lifts of the same weight with the opposite hand. The experiments were performed with a light and a heavy weight. Patients and controls scaled the grip force output differentially to different weight. The comparison of grip force scaling for the first and last lifts with a constant weight demonstrated that healthy subjects and cerebellar patients adjusted grip forces more accurately to a specific weight with increasing number of lifts performed at each hand. The ability to transfer weight-related information from one hand to the other was analyzed by comparing the last lift with a constant weight of the dominant hand with the first lift of the same weight performed by the opposite hand. Healthy subjects scaled the grip force output precisely to a given weight immediately after a change in hand, suggesting that they succeeded to transfer weight-related information in between both hemispheres. In contrast, cerebellar patients produced an inaccurate grip force overshoot when lifting a given weight with the opposite hand. Our data suggest that the cerebellum plays a major role for the generalization of weight-related information during object manipulation.