A new theory of cerebellar function: movement control through phase-independent recognition of identities between time-based neural informational symbols.

A new theory designed to explain the functions of the cerebellum in the control of movement and the unique anatomy of that structure is presented. The heart of this proposed explanation is that the cerebellum generates increased numbers of outputs from particular Purkinje cells whenever the same pattern of pulses in time is presented both to its mossy fiber and its climbing fiber input systems. The postulated function of the unusual anatomy of the cerebellum is to permit it instantly to recognize and respond to identical patterns presented through these two channels regardless of the phase differences between these two signal sources, where phase differences are defined as differences in times of arrival of patterns of inputs from these two sources. The first means putatively used involves the summation of pulses comprising a given pattern of inputs simultaneously at many different Purkinje cells by virtue of the different speeds of conduction of the parallel fiber axons of granule cells. The second means is the addition of an input from the climbing fiber system that, together with the simultaneous parallel fiber inputs, leads to a discharge of particular Purkinje cells, which discharge temporarily increases the size of EPSP's generated by the parallel fiber synapses involved in the cell's discharge. This specific synaptic potentiation, in turn, makes it possible for the cell to respond by generating closely consecutive additional discharges provided that the same patterns of discharge are presented both to the climbing fiber system and the mossy fiber system. This happens because later pulses in identical patterns will arrive simultaneously at previously facilitated synapses via parallel fibers and at synapses of the climbing fibers, thereby causing additional spatial summations and discharges to occur. According to this explanation the patterns that are compared in the above manner are symbols (patterns of pulses) produced by sensors of current positions and symbols derived from memory and also representing these same positions. When patterns from these two sources are identical, the multiple outputs of specific Purkinje cells inhibit an automatic feedback loop and thereby indirectly cause the attenuation and arrest of movement. The evolution of these concepts resulted in very specific "predictions" of particular connections involving the olive, pons, red nucleus, dentate, thalamus, and sensory-motor cortex. All of these predictions were found to be consistent with evidence in the literature. Points of difference between this theory and all prior ones are discussed as are several critical tests of its validity, and the putative evolution of cerebellar structure and function.(ABSTRACT TRUNCATED AT 400 WORDS)