Differential effect of right and left basal ganglionic infarctions on procedural learning

Objective: To assess the effect of ischemic infarctions affecting the basal ganglia (BG) region on a series of procedural learning tasks. Background: The basal ganglia hypothesis of procedural learning is a matter of debate. As most of the relevant research so far is based on examination of patients suffering from Parkinson disease, this inconsistency might reflect either lesion heterogeneity existing in this pathologic group or the heterogeneity of the procedural learning tasks. Method: Twelve patients with lesions confined to the right (BGr), 10 to the left (BG1) BG region, and 15 matched controls participated in the study. Three procedural learning tasks were used: Tower of Hanoi Puzzle, Mirror Reading, and Porteus Mazes. Declarative memory and general intelligence were also tested. Results: Verbal declarative memory was impaired in the BG1 group. For each procedural learning task, baseline performance and learning rate were analyzed. Tower of Hanoi Puzzle: Baseline performance of the BG1 group was impaired compared with the other groups. The BGr group was the only group that did not improve over learning trials. MR: Baseline performance of the BGr group was impaired compared with the other groups. The groups' learning rate did not differ significantly. Porteus Mazes: Baseline performance of both patient groups was impaired compared with that of the control group. Learning rate over repetitive trials of the same maze was impaired in the BGr group. However, the transfer of procedural learning to a newly exposed maze was impaired in the BG1 group. Conclusions: First, right and left basal ganglia play different roles in different procedural learning tasks. Second, procedural learning is not a unitary capacity subserved by any single neural mechanism.