A huge amount of physiological research shows the importance of the role of the cerebellum in motor control (Ito, 1984; and many others). It is natural for physiologists to try to understand what the cerebellum function is and when and which corrections or other modifications of cerebral cortical motor programs are introduced by the cerebellum. It seems to us that in recent decades the most interesting hypotheses on cerebellar performance have been proposed by the late David Marr in ‘A theory of cerebellar cortex’ (Marr, 1969). According to Marr, the main operation of the cerebellar circuitry is the switching on of proper motor commands by the current sensory input and the automatic adaptive acquisition of such cerebellar network capability. The location of the cerebellum – in the crest of almost all the ascending and descending nervous tracts – is definitely strategic for such a function. The unique combination of tens of thousands of granular cells (GrCs) and one climbing fibre (CF) at one Purkinje cell seems to be crucial for it.

The kernel of Marr's theory is composed by the postulates of the GrC–PC synaptic modification due to simultaneous excitation of the climbing fibre (CF) and parallel fibres (PF). In other words, Marr supposed that the Purkinje cell memorizes the afferent conditions in which it ought to be active.