Distributed sensory coding by cerebellar complex spikes in units of cortical segments

Summary Sensory processing is essential for motor control. Climbing fibers from the inferior olive transmit sensory signals to Purkinje cells, but how the signals are represented in the cerebellar cortex remains elusive. We examined the olivocerebellar organization of the mouse brain by optically measuring complex spikes (CSs) evoked by climbing fiber inputs over the entire dorsal surface of the cerebellum. We discovered that the surface was divided into approximately 200 segments each composed of ∼100 Purkinje cells that fired CSs synchronously. Our in vivo imaging of evoked responses revealed that whereas stimulation of four limb muscles individually similar global CS responses across nearly all segments, the timing and location of a stimulus were derived by Bayesian inference from coordinated activation and inactivation of multiple segments on a single trial basis. Our findings suggest that the cerebellum performs segment-based distributed population coding by assembling probabilistic sensory representation in individual segments.