A cognitive affective role for the cerebellum

Traditional neurological teaching suggests that, functionally, the cerebellum is devoted entirely to motor control. Accordingly, the cerebellum is proposed to be primarily concerned with co-ordination in skilled voluntary movement, as well as control of motor tone, posture and gait. In its motor regulatory role, the cerebellum is assumed to function as a comparator that adjusts motor outputs on the basis of afferent signals relating to planned motor action and reafference signals concerning excecuted motor acts. This motor view of cerebellar function has withstood numerous challenges over the past half century (Dow and Moruzzi, 1958). In the past decade, a growing body of empirical data, largely derived from functional neuroimaging studies, have implicated the cerebellum in diverse higher cognitive functions (Leineret al., 1995). What has been lacking is convincing documentation of cognitive deficits in patients with pathology restricted to the cerebellum. In this edition of Brain, Schmahmann and Sherman (1998) provide such neuropsychological and behavioural evidence. Their paper provides detailed clinical descriptions of prominent non-motor deficits in a carefully documented cohort of patients with cerebellar pathology. The authors conclude that the pattern of deficit in these patients is characteristic enough to suggest that it forms a recognizable syndrome, the Cerebellar Cognitive Affective Syndrome. Its defining features are disturbances in executive function, spatial cognition, language and emotional regulation of behaviour. Unlike previous clinical reports, where deficits were mainly those that detailed neuropsychological evaluation, the core deficits were evident enough to be elicited by standard bedside evaluation. Given the complex anatomy of the cerebellum, an obvious question is whether components of the syndrome are associated with pathology of distinct cerebellar regions. Unfortunately, the size of the series and the often diffuse nature of the associated pathology do not allow clear answers to this question. Nevertheless, the authors note a strong tendency for posterior lobe lesions to be associated with the core syndrome, while patients with vermis lesions had pronounced affective disturbance. A widely accepted principle of neural organization is the interdependence of structure and function. Within this framework, it is instructive to consider whether cognitive deficits in patients with cerebellar pathology can be accounted for in terms of its known anatomy. Two facts about cerebellar anatomy seem pertinent. First, in relation to its intrinsic structure, the cerebellum accounts for over half the brain’s population of neurons. More importantly, in terms of its likely cognitive functions, the cerebellum receives inputs