The fate of spontaneous synchronous rhythms on the cerebrocerebellar loop.

How does the cerebellum participate in neocortical rhythms? Neocortical signals destined for the cerebellum are integrated in the pontine nuclei (PN) with cerebellar output signals via a direct, reciprocal feedback loop with the cerebellar nuclei (CN). The present study investigated the fate of two spontaneously occurring rhythms in rat neocortex under ketamine anesthesia-slow wave activity at around 1 Hz and gamma oscillations-within this pontonuclear feedback loop. Coordinated oscillatory neuronal activity was studied using simultaneous multineuron recordings in primary motor cortex (M1), PN, and lateral CN. It was revealed that slow burst firing-known in neocortex as "up and down states"-is readily conveyed within the pontonuclear feedback loop and thus engages the entire cerebropontocerebellothalamic loop. In contrast, gamma band synchronous oscillations reached only the PN under the present experimental conditions. Surprisingly, many CN single units were actually found to oscillate in the gamma range, but they completely failed to synchronize with other units in either CN or PN. These results show firstly that slow concerted activity can readily engage the entire cerebrocerebellar loop. Secondly, they raise the possibility that fast gamma oscillations may be incompatible with cerebellar processing and get blocked out. Future studies in behaving animals are needed to answer the question whether signals coded in gamma band frequency are converted to another carrier code using the feedback control exerted by the pontonuclear loop.