Control of Novel Versus Practiced Movements in Frontal and Striatal Dysfunction.

Abstract:
The role of fronto-striatal circuits in visuomotor learning is still highly debated (Wise, 1996). Some have suggested that populations with fronto-striatal dysfunctions may show difficulties with visuomotor acquisition of mirror-inverted movements. We have recently shown that patients with frontal lesions are impaired in the first trials of mirror-inverted movements (Richer et al., 1999). This study aimed at comparing the performance of patients with frontal excisions, early Huntingtons disease and Tourette syndrome to matched control groups, on mirror-inverted reaching movements on a graphics tablet. In non-inverted reaching movements, all groups showed normal precision measured by trajectory length. When feedback was mirror-inverted, controls produced movements containing large trajectory corrections in the first trials and improved dramatically after 32 trials. Frontal patients showed poor corrections in the first phase of adaptation but reached normal level after 32 practice trials, indicating problems with the attentional control of novel movements. Tourette patients showed normal trajectories in the first phase of adaptation but their precision did not further improve after 20 trials, suggesting normal control of novel movements but difficulties in late acquisition. HD patients showed problems before and after practice, suggesting that they combine problems of frontal and Tourette patients. These results indicate a dissociation between the visuomotor processes affected in these populations. This dissociation suggests that different components of fronto-striatal circuitry may be involved in different phases of visuomotor learning.