Motor sequencing with left hemisphere damage. Are some cognitive deficits specific to limb apraxia?

Sixteen left-hemisphere stroke patients, who were apraxic or nonapraxic, and 17 control subjects performed sequences of hand postures that varied in the number of different postures (repetitive and heterogeneous) and sequence length (one to five). Performance of the left hand (ipsilateral to stroke) was compared with a control group using the left hand. All stroke patients had slower reaction times and were slower to execute single hand postures, but the apraxic group was not slower than the nonapraxic group. Both the apraxic and the nonapraxic groups had similar problems scheduling or timing motor programs for both sequence types such that inter-response times were more affected by sequence length than the control group. However, only the apraxic group showed abnormalities in preprogramming heterogeneous sequences. The apraxic group also made more errors and had longer movement times (MTs) than for the other groups, but only for heterogeneous sequences containing more than three hand postures. The nonapraxic group did not show slower MTs or greater errors, regardless of the type or the length of sequences. These results suggested deficits in encoding, generating single movements and in scheduling or timing a series of actions which generally attributable to left hemisphere damage. However, abnormalities in temporal organization processes prior to and during movement were specific to apraxia. The dissociation between the two stroke groups on some but not all aspects of sequencing has implications for different cognitive mechanisms supporting motor sequencing.