Synchronous climbing fiber activity enables instructive signaling for cerebellar learning through modulation of disinhibitory circuits.

Motor learning relies on signals that instruct adaptive plasticity following errors. In the cerebellum, climbing fibers (CFs) provide these instructions to Purkinje cells (PCs). Yet CFs fire continuously, even without errors, requiring molecular layer interneuron (MLI) inhibition of PCs to counteract CF excitation and prevent maladaptive plasticity. Here, to identify how this regulatory inhibition is contextually suppressed to selectively permit error-driven learning in mice, we combined connectomics, functional recordings, computational modeling and behavioral manipulations. We discovered that CFs target not only PCs but also a specific MLI subtype that inhibits PC-targeting MLIs, creating serial disinhibition. These disinhibitory MLIs integrate multiple CFs, causing increased activation with CF synchrony. This stronger disinhibitory drive allows larger CF-evoked calcium responses in PCs. Disruption of MLI-to-MLI inhibition prevents CF-instructed motor learning, confirming the necessity of this disinhibitory pathway. Therefore, population synchrony selectively enables CF-driven plasticity through disinhibitory network interactions, demonstrating that instructive signaling is a product of circuit-level processing.