Effects of accuracy constraints on reach-to-grasp movements in cerebellar patients

Reach-to-grasp movements of patients with pathology restricted to the cereb ellum were compared with those of normal controls. Two types of paradigms w ith different accuracy constraints were used to examine whether cerebellar impairment disrupts the stereotypic relationship between arm transport and grip aperture and whether the variability of this relationship is altered w hen greater accuracy is required. The movements were made to either a verti cal dowel or to a cross bar of a small cross. All subjects were asked to re ach for either target at a fast but comfortable speed, grasp the object bet ween the index finger and thumb, and lift it a short distance off the table . In terms of the relationship between arm transport and grip aperture, the control subjects showed a high consistency in grip aperture and wrist velo city profiles from trial to trial for movements to both the dowel and the c ross. The relationship between the maximum velocity of the wrist and the ti me at which grip aperture was maximal during the reach was highly consisten t throughout the experiment. In contrast, the time of maximum grip aperture and maximum wrist velocity of the cerebellar patients was quite variable f rom trial to trial, and the relationship of these measurements also varied considerably. These abnormalities were present regardless of the accuracy r equirement. In addition, the cerebellar patients required a significantly l onger time to grasp and lift the objects than the control subjects. Further more, the patients exhibited a greater grip aperture during reach than the controls. These data indicate that the cerebellum contributes substantially to the coordination of movements required to perform reach-to-grasp moveme nts. Specifically, the cerebellum is critical for executing this behavior w ith a consistent, well-timed relationship between the transport and grasp c omponents. This contribution is apparent even when accuracy demands are min imal.