Motor Adaptation Deficits in Ideomotor Apraxia.

OBJECTIVES: The cardinal motor deficits seen in ideomotor limb apraxia are thought to arise from damage to internal representations for actions developed through learning and experience. However, whether apraxic patients learn to develop new representations with training is not well understood. We studied the capacity of apraxic patients for motor adaptation, a process associated with the development of a new internal representation of the relationship between movements and their sensory effects. METHODS: Thirteen healthy adults and 23 patients with left hemisphere stroke (12 apraxic, 11 nonapraxic) adapted to a 30-degree visuomotor rotation. RESULTS: While healthy and nonapraxic participants successfully adapted, apraxics did not. Rather, they showed a rapid decrease in error early but no further improvement thereafter, suggesting a deficit in the slow, but not the fast component of a dual-process model of adaptation. The magnitude of this late learning deficit was predicted by the degree of apraxia, and was correlated with the volume of damage in parietal cortex. Apraxics also demonstrated an initial after-effect similar to the other groups likely reflecting the early learning, but this after-effect was not sustained and performance returned to baseline levels more rapidly, consistent with a disrupted slow learning process. CONCLUSIONS: These findings suggest that the early phase of learning may be intact in apraxia, but this leads to the development of a fragile representation that is rapidly forgotten. The association between this deficit and left parietal damage points to a key role for this region in learning to form stable internal representations. (JINS, 2017, 23, 139-149).

Frontal and parietal cortex contributions to action modification.

Successful achievement of task goals depends critically on the ability to adjust ongoing actions in response to environmental changes. The neural substrates underlying action modification have been a topic of great controversy: both, posterior parietal cortex and frontal regions, particularly prefrontal cortex have been previously identified as crucial in this regard, with most studies arguing in favor of one or the other. We aimed to address this controversy and understand whether frontal and parietal regions might play distinct roles during action modification. We tested ipsilesional arm performance of 27 stroke patients with focal lesions to frontal or parietal regions of the left or right cerebral hemisphere, and left or right arm performance of 18 healthy subjects on the classic double-step task in which a target is unpredictably displaced to a new location, requiring modification of the ongoing action. Only right hemisphere frontal lesions adversely impacted the timing of initiation of the modified response, while only left hemisphere parietal lesions impaired the accuracy of the modified action. Patients with right frontal lesions tended to complete the ongoing action to the initially displayed baseline target and initiated the new movement after a significant delay. In contrast, patients with left parietal damage did not accurately reach the new target location, but compared to the other groups, initiated the new action during an earlier phase of motion, before their baseline action was complete. Our findings thus suggest distinct, hemisphere specific contributions of frontal and parietal regions to action modification, and bring together, for the first time, disparate sets of prior findings about its underlying neural substrates.