Granule cells reorient cortical trajectories to separate contexts.

Cerebellar granule cells separate contexts not by expanding cortical representations but by rotating their low-dimensional trajectories apart between two skills sharing temporal structure, while premotor L5PT activity patterns generalized. Imaging both nodes in mice during parallel learning showed GrC remapping was coherent and strongest in expert animals. The work assigns cortex the role of invariant dynamic primitives and the cerebellum the reconfiguration for context-specific output.