Abstract:
We present a new theory of cerebellar function that is based on synchronization, delayed reverberation, and time windows for triggering spikes. We show that granule cells admit mossy fiber activity to the parallel fibers only if the Golgi cells are firing synchronously and if the spikes arrive within short and well-defined time windows. The time window mechanism organizes neuronal activity in discrete `time slices' that can be used to discern meaningful information from background noise. In particular, Purkinje cell activity can trigger rebound spikes in deep cerebellar nuclei cells which project via brain stem nuclei and mossy fibers back to the cerebellar cortex. Using a detailed model of deep cerebellar nuclei cells, we demonstrate that the delayed firing of rebound spikes is a robust mechanism so as to insure that the reverberated activity re-arrives in the mossy fibers just during the granule cell time window. Large network simulations reveal that a form of synaptic plasticity at the parallel fiber/Purkinje cell synapses that relies on the timing of parallel and climbing fiber spikes allows the system to learn, store, and recall spatio-temporal patterns of spike activity. In addition to the relevance of spatio-temporal pattern learning for motor-control and timed-response tasks, the present theory sheds new light on several experimental observations such as the high degree of climbing fiber spike synchrony, the width of micro zones, and developmental changes in inhibitory postsynaptic currents of granule cells.
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