Representations of Subsecond Duration-Based Timing by Complex Spike Synchrony in Cerebellar Purkinje Neurons

The cerebellum contributes to associative motor learning and sensorimotor coordination in part by tracking subsecond time intervals between behaviorally relevant events, raising the question of how duration, or absolute time, is encoded. Here, we investigated whether information about duration is present in Purkinje cell complex spikes during repetitive sensory stimuli. Crus I Purkinje cells expressing the fast calcium indicator GCaMP8f were imaged at high speed (250 fps), allowing detection of complex spike-associated calcium signals from hundreds of Purkinje cell dendrites simultaneously, with 4 ms temporal resolution, in awake head-fixed mice of both sexes. Air puffs were applied to the whisker pad in stimulus trains that varied in the mean and variance of interstimulus intervals (ISIs, 100-900 ms). In responsive cells, the mean probability of complex spike firing increased approximately fivefold ∼35 ms post-puff, primarily owing to well-timed spiking after the stimulus rather than an increase in spike rate. The maximal response probability, and in some cells also the response latency, varied linearly with ISI. The values of both variables were consistent for each ISI, regardless of the attributes of the stimulus train, suggesting that they carried information about absolute, rather than relative, durations between stimulus pairs. Because each puff evoked only one or zero complex spikes per Purkinje cell, the dependence of spike probability on ISI emerged as a trial-by-trial dependence of the degree of synchronous firing on ISI, suggesting that subsecond absolute timing of somatosensory signals may be represented by complex spike synchrony across populations of Purkinje cells.