Limb-sequencing deficits after left but not right hemisphere damage.

The performance of right and left hemisphere stroke patients was compared to normal control groups on a task where subjects alternately hit two targets which varied in size from 0.5 to 6.5 cm. The stroke patients used the arm ipsilateral to damage, and the control groups used the same arm as their respective stroke group. Lesion size and location were similar for the two stroke groups. No deficits were found for the right hemisphere stroke group. The left stroke group's tapping speed was not slower at the smallest target, but became progressively slower relative to the control group's as target size increased. Variability in tapping speed increased as target size increased for all except the left stroke group. While the entire left stroke group was as accurate as their controls, the apraxic, but not nonapraxic, patients made more errors on smaller targets only. Two explanations for these findings both emphasize the left hemisphere's special role in motor programming; one focuses upon its dominance for movements which are independent of sensory feedback and the other emphasizes its specialization for processing rapid temporal information.