The performance of patients with lesions involving the basal ganglia (BG) was compared to that of patients with prefrontal (PFC) lesions, thalamic (TH) lesions, and age-matched controls in order to examine the specific role of the BG within the frontal-subcortical circuits (FSCC) in task switching. All the BG patients and none of the other participants showed a marked increase in error rate in incongruent trials where correct responses depended upon the choice of the correct task rule. Some BG patients erred in failing to switch tasks and others failed despite their attempt to switch tasks. Additionally, reaction time results indicate abnormal response repetition effects among the BG patients; failure in benefiting from advance task information among all the patients; and increased task mixing costs following PFC lesions. The authors conclude that although the frontal-subcortical circuits jointly determine some behaviors (such as benefiting from preparation), the BG play a unique role within the FSCC in action selection and/or the inhibition of irrelevant information.

Regions within the frontal and parietal cortex have been implicated as important neural correlates for cognitive control during conflict resolution. Despite the extensive reciprocal connectivity between the cerebellum and these putatively critical cortical areas, a role for the cerebellum in conflict resolution has never been identified. We used a task-switching paradigm that separates processes related to task-set switching and the management of response conflict independent of motor processing. Eleven patients with chronic, focal lesions to the cerebellum and 11 healthy controls were compared. Patients were slower and less accurate in conditions involving conflict resolution. In the absence of response conflict, however, tasks-witching abilities were not impaired in our patients. The cerebellum may play an important role in coordinating with other areas of cortex to modulate active response states. These results are the first demonstration of impaired conflict resolution following cerebellar lesions in the presence of an intact prefrontal cortex.