On-beam synchrony in the cerebellum as the mechanism for the timing and coordination of movement.

In trained reaching rats, we recorded simple spikes of pairs of Purkinje cells that, with respect to each other, were either aligned on a beam of shared parallel fibers or instead were located off beam. Rates of simple spike firing in both on-beam and off-beam Purkinje cell pairs commonly showed great variety in depth of modulation during reaching behavior. But with respect to timing, on-beam Purkinje cell pairs had simple spikes that were tightly time-locked to each other (either delayed or simultaneous) and to movement, despite the variability in rate. By contrast, off-beam Purkinje cell pairs had simple spikes that were not time-locked to each other, neither delayed nor simultaneous. We discuss the implications of these observations for the cerebellar role in timing and coordinating movement.