The histological simplicity and organization of the cerebellar cortex have fascinated neuroscientists for more than 150 years. These structural features made it possible for early anatomists to establish the basic connectivity among the different cellular elements of the cerebellum; and the same features facilitated the physiological work of J. Eccles, M. Ito, R. Llinas, and others in the 1960s that established the polarity and other aspects of the synaptic connections. The cerebellum thus became the first central nervous system structure in a vertebrate for which a wiring diagram could be drawn showing the morphology of the different elements, their connectivity, and their physiological interactions. This knowledge generated a great deal of excitement in the late 1960s and convinced many neuroscientists that a fundamental understanding of a major central nervous system structure was near at hand. It seemed that only a few years' work would be necessary to establish “what the cerebellum does and how it does it,” in the phrase of the time. The excitement and promise were reflected in the title of the 1967 book by J. Eccles, M. Ito, and J. Szentagothai that summarized the anatomical and physiological findings, The Cerebellum as a Neuronal Machine. The circuitry of the cerebellum and its promise still fascinate many neuroscientists, but a good functional understanding continues to elude us.

Fascination with the cerebellum was heightened by the addition of a second theme to that of circuitry in our conceptual approach to cerebellar function.