Sequential stimulation of rat cerebellar granular layer in vivo: Further evidence of a ‘tidal-wave’ timing mechanism in the cerebellum

Here we present evidence that the cerebellar cortex in vivo specifically responds to sequential input to the granular layer, the main input site of the cerebellar cortex. Ordered sequences of electrical stimuli were delivered through an array of stimulating electrodes in such a way, that an apparent movement of the stimulus was produced. The parallel fiber population responses to sequential stimuli ‘moving’ at 7 different velocities (0.1–0.7m/s) and in two different directions (towards and away from the recording site) were measured extracellularly in the molecular layer. Population responses were maximal when the stimulus moved towards the recording site at a velocity close to the conduction velocity of parallel fibers. Responses were significantly reduced when the stimulus velocity was higher or lower. We conclude that the characteristic geometrical arrangement of parallel fibers enables the cerebellum to specifically detect precise spatio-temporal activity patterns in the mossy fiber system. These findings confirm earlier observations made in vitro and shed new light on the functional interpretation of cerebellar anatomy. Together with recent findings suggesting that precise spatio-temporal activity patterns play a key role in information processing in the neocortex, the results reported here are particularly important concerning the information exchange between the strongly interconnected cerebellum and neocortex.