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Two-photon imaging of a spatial pattern population code in the cerebellum

Simon R Schultz

Department of Bioengineering, Imperial College London, Uk

Climbing-fiber-evoked complex spikes in cerebellar Purkinje cells tend to be locally synchronized. What is the role of this synchrony in the encoding and processing of sensory information within the olivocerebellar cortical circuit? We used in vivo two-photon calcium imaging, together with an information-theoretic data analysis technique, to investigate this question. Our results support previous findings that spontaneous synchrony falls off over hundreds of microns mediolaterally. The synchrony we observed was anaesthetic-dependent, and was increased following administration of Harmaline, in accord with an olivary source. We were able to elicit calcium transients in cerebellar region Crus IIa by peri-oral stimulation, observing that a periodic train of airpuff stimuli resulted in entrainment of calcium transients. Periodic sensory stimulation also resulted in a transient modulation of synchrony following stimulus onset. Conjecturing that this stimulus-dependent synchrony might be the signature of a spatial pattern population code, we analysed the information content of the complex spike population code, as reflected by calcium events, about stimulus timing. We found that spatial patterns of calcium events from up to seven cells provided on average 44% more information than was available by counting the number of events across the pool. The olivocerebellar feedback circuit thus may act to correct sensory error on a signal represented by, rather than single-neuron responses, the spatial pattern of activity in a local pool of climbing fibers.