The spatio-temporal filtering hypothesis of the cerebellar cortex: evidence from VSD imaging

The functional mechanisms of the cerebellar cortex are still object of debate and it is not fully clear how mossy fiber inputs are transformed in the granular layer and retransmitted to the molecular layer and Purkinje cells. Here the spatio-temporal properties of granular-to-molecular layer transmission in response to mossy fiber bursts of different frequencies have been investigated using voltage-sensitive dye imaging. The granular layer was optimally excited above ~50 Hz and the molecular layer responded above ~100 Hz with a steep gain curve. The high-pass filtering properties depended on GABA and NMDA receptors: NMDA receptors determined mossy fiber – granular layer frequency-dependence, while GABA receptors determined granular to molecular layer frequency-dependence. Moreover, GABA receptors reduced granular layer gain through a dynamic mechanism (-103%) rather than tonic inhibition (+17%). These results indicate that the mossy fiber pathway favors bursts-burst transmission, which is dynamically controlled by the local circuitry in a frequency dependent manner.