The circuit properties of the cerebellar cortex revealed by Voltage-Sensitive Dye (VSD) imaging

Although the cellular physiology of cerebellar neurons has made remarkable progress, the analysis of circuit functional properties is still incomplete. In order to asses theoretical predictions, we have performed VSD imaging measurements addressing three main issues. 1) VSD imaging was used to measure combinatorial responses in the cerebellum granular layer showing "combined excitation" and "combined inhibition". These depended on whether the response were enhanced or reduced, lasted for tens of ms and were regulated by synaptic inhibition. This is the first demonstration of the presence of combinatorial operations in the cerebellum. 2) VSD imaging was used to measure responses to bursts at different frequencies. Transmission through the mossy fiber–granular layer-molecular layer pathway was frequency-dependent generating a cascade of two high-pass filters regulated by NMDA and GABA-A receptors. 3) VSD imaging was used to investigate patterns transmission from the granular to molecular layer. High-frequency bursts were enhanced along vertical transmission lines but not along parallel fibers, suggesting that they could be specialized to convey low-frequency signals throughout the cerebellum. These results support the hypothesis that the mossy fiber input of the cerebellar cortex implements a complex spatio-temporal filter, in which local computations (and potentially long-term synaptic plasticity) can differentially redistribute activation among the neuronal elements.