Sensorimotor integration in the inferior olive of larval zebrafish

Abstract

The ability to learn and refine movements is crucial for adapting to an ever-changing environment. The inferior olive (IO) is believed to play key roles in motor learning, timing motor output and detecting mismatches between predicted and actual sensory feedback. The classical theory of cerebellar processing posits that the IO sends error signals to Purkinje cells, guiding motor output and updating internal models. Yet the exact role of the IO during motor adaptation and learning remains uncertain. To address these questions, we designed and tested a novel system to quantify mechanosensory-based motor transformations in zebrafish larvae, enabling simultaneous brain imaging. Using two-photon calcium imaging of IO neurons expressing GCaMP6f, we presented visual, mechanosensory, and vestibular stimuli to fish while recording fictive behaviour using electrophysiological tail recordings. By regressing individual neurons to these sensory variables, we found they were directionally selective and velocity sensitive to sensory input and visual stimuli revealed a clear spatial organisation in the IO. Whilst many neurons also correlated with motor output, few displayed multimodal responses to stimuli (2.4 - 5.4%). Co-presenting visual and flow stimuli suppressed neuronal responses to visual input. To examine IO activity during motor adaptation we utilised a virtual reality behavioural assay in which paralysed fish interact fictively with a visual scene and adjust their motor output to changes in feedback strength. Sensory and motor cells as well as cells responsive to self-generated visual feedback were identified. Close examination revealed motor cells were tightly locked to the timing and amplitude of bouts. Reafferent cells notably increased activity during significant forward displacements, suggesting that the IO compares external and reafferent sensory signals to monitor adaptation. To understand how the IO may generate sensory errors, we presented stereotyped optic flow with random deviant backward-moving visual stimuli which revealed that fish delayed swim onsets in response to deviant stimuli, a response dependent on the IO. Direction-selective neurons displayed robust responses to deviant stimuli, which were locked to the velocity of the stimulus. Despite the fish's motion increasing the velocity of the backward stimulus, the neuronal response remained unchanged, highlighting that neural representations of sensory prediction errors encode the external sensory environment rather than being influenced by the fish's movement. Together these findings suggest that the cerebellum encodes an internal model of the external world.