Nonmotor Functions of the Cerebellum: An Introduction

Abstract

The concept of nonmotor functions of the cerebellum (little cerebrum) 1 is an intriguing proposal that has garnered little attention in the past but has become a relatively recent focal point for neuroscience investigators. The preponderance of anatomic and clinical evidence supporting the traditional view of the cerebellum as a motor mechanism has understandably overwhelmed the sporadic reports of behavioral and intellectual dysfunction associated with cerebellar pathology during the 19th and most of the 20th century.2-7 Developments during the past 25 years, coinciding with the development of functional MR imaging, however, have greatly increased our awareness and understanding of cerebellar cognitive functions. Neuroimaging in conjunction with anatomic and clinical investigations is helping to cultivate new ways of thinking about cerebellar organization and function. This vignette will introduce some of these major observations of nonmotor cerebellar functions from a neuroradiologic perspective.The correlation of cerebellar function and morphology has been a long-standing mystery that raises interesting questions. Although the cerebellum accounts for only about 10% of the mass of the brain, why does it contain as many neurons as all the rest of the central nervous system combined?1 Does this imply the existence of underappreciated functions beyond modulation of motor activities? Why did the lateral aspect of the human cerebellum, the cerebellar hemispheres, undergo enormous enlargement during the course of human evolution?8 Furthermore, why did this lateral cerebellar growth seem to parallel the evolutionary enlargement of the prefrontal cortex? 9,10 Functional MR imaging combined with clinical observations provide a novel backdrop to frame the answers.More recent concepts of cerebellar organization provide a more complete picture of the cerebellum. The traditional morphologic description of the cerebellum subdivides it into lobes, lobules, and zones (Fig 1). The phylogenetic description categorizes the cerebellum into the vestibulocerebellum (archicerebellum), spinocerebellum (paleocerebellum), and cerebrocerebellum (neocerebellum) (Fig 2).11 The oldest portion, the vestibulocerebellum, primarily receives input fibers from the vestibular system. The phylogenetically intermediate component, the spinocerebellum, receives fiber input directly from the spinal cord. The newest part, the cerebrocerebellum, receives input from many different areas of the cerebral cortex. Interesting observations from fMRI studies, however, offer a different type of cerebellar organization based on function. Mapping of cognitive functions shows a lateral cerebellar distribution, while the sensorimotor cerebellar functions are more medially located (Fig 3). 12This medial-to-lateral functional gradient also applies to the group of 3 deep cerebellar nuclei (fastigial; interposed, consisting of the globose and emboliform nuclei; and dentate). The cognitive functions of the cerebellum, therefore, are primarily distributed in the lateral...

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