Molecular layer interneurons form an interconnected network in the cerebellar cortex that controls the activity of Purkinje cells. Even though much is known about the properties of these interneurons in isolated preparations, their activity pattern in vivo remains largely unknown. The present project proposes to investigate the functional status of the interneurons in awake, head-restrained mice, by performing two photon calcium imaging of the cerebellum with genetically-encoded calcium indicator GCaMP3. Imaging speed will be improved by implementing random access multi point scanning with acousto-optic deflectors. Once the basic activity pattern will be known, effect of sensory stimulation (tactile stimulation or eye puff) and rhythmic behavior (licking behavior) will be explored. In a second step of the project, calcium imaging of the interneurons will be combined with whole-cell recordings of postsynaptic Purkinje cells. In this manner the effects of interneuron network activity on the overall cerebellar circuit will be assessed. The results will be evaluated with the help of a collaborating modeling team. This study will shed light on the functional role of molecular layer interneurons, and more generally, on the way by which interneurons regulate the activity of brain network. It will also provide data that will help evaluating consequences of defects of gabaergic transmission in cerebellar pathologies. The applicant will benefit from the multidisciplinary training provided by this project.