The cerebellum and cognition: cerebellar lesions impair sequence learning but not conditional visuomotor learning in monkeys

Claims that the cerebellum contributes to cognitive processing in humans ha ve arisen from both functional neuroimaging and patient studies. These clai ms challenge traditional theories of cerebellar function that ascribe motor functions to this structure. We trained monkeys to perform both a visuomot or conditional, associative learning task and a visually guided sequence ta sk, and studied the effects of bilateral excitotoxic lesions in the lateral cerebellar nuclei. In the first experiment three operated monkeys showed a small impairment in post-operative retention of a visuomotor associative t ask (A) but were then not impaired in learning a new task (B). However, the impairment on A could have been due to a problem in making the movements t hemselves. In a second experiment we therefore gave the three control anima ls a further pre-operative retest on both A and B and then tested after sur gery on retention of both tasks. Though again the animals showed motor prob lems on task A, they reached criterion, and at this stage could clearly mak e both movements satisfactorily. The critical test was then retention of ta sk B, and they were not impaired. In the final experiment (serial reaction time task) the monkeys response times on a repeating visuomotor sequence we re compared with those For a pseudo-random control sequence. After bilatera l nuclei lesions they were slow to execute the pre-operatively learned sequ ence but were still faster on this than on the control task. However, when they were then given a new repeating sequence to learn, they never performe d the sequence as quickly as they had on retention of the first sequence. W e conclude that the cerebellum is not essential for the learning or recall of stimulus response associations but that it is crucially involved in the process by which motor sequences become automatic with extended practice. ( C) 2000 Elsevier Science Ltd. All rights reserved.