Cortico-basal ganglia pathways are essential for the recall of well-established visuomotor associations.

Recent human neuroimaging studies, supported by lesion studies with nonhuman primates, have suggested that learning arbitrary associations between sensory cues and behavioural responses requires interactions between the infero-temporal, prefrontal and premotor cortices. We directly tested the hypothesis suggested from our neuroimaging experiments that functional links between the basal ganglia and premotor cortex are involved in the process via which task performance becomes automatic. We made unilateral excitotoxic lesions, centred on the internal pallidum, in four macaques previously given extensive experience on the associations between nonspatial visual cues and movements of a joystick. The basal ganglia lesion was later combined with a premotor cortical lesion in the opposite hemisphere so as to interrupt the connections between them. Three of the animals were subsequently found to be impaired in relearning pre-operatively acquired associations; they eventually succeeded but made three-times as many errors. A fourth animal was unimpaired but its premotor cortex lesion was later found to be incomplete. Response times were only marginally increased and the learning of novel associations appeared relatively unaffected by these lesions. As a control, the effects of a unilateral premotor cortex lesion were assessed with two additional animals but this lesion did not result in a relearning impairment. We therefore suggest that when visuomotor associations have become well established through over-training, performance depends on connections between the basal ganglia and premotor cortex.

The role of the cerebellum in preparing responses to predictable sensory events.

Despite numerous studies on the effects of lesions of the mammalian cerebellum on coordination, adaptation and learning, the precise nature of this structure's contribution to motor control remains controversial. This paper reviews the results of a series of behavioural studies with monkeys trained to make rapid, accurate sequences of responses to visual targets. The effects of discrete cerebellar lesions on the performance of these animals is discussed in the light of recent theories about how the cerebellum might be concerned with learning to anticipate certain kinds of sensory events. Additional studies are considered that advocate sensory prediction as a fundamental cerebellar function that could contribute to many of the behavioural processes with which the cerebellum has been implicated. In particular, it is demonstrated how such information could be employed in the augmentation of motor learning by the formation of expectations about the sensory feedback arising from movements and interactions with the environment. Whilst it is argued that the cerebellum may not be unique in being able to perform such functions, comparative anatomical studies suggest that it may operate with an unequalled degree of temporal precision. Such precision forms the signature of skilled motor acts.