Role of cerebellum in adaptive modification of reflex blinks.

We investigated the involvement of the cerebellar cortex in the adaptive modification of corneal reflex blinks and the regulation of normal trigeminal reflex blinks in rats. The ansiform Crus I region contained blink-related Purkinje cells that exhibited a complex spike 20.4 msec after a corneal stimulus and a burst of simple spike activity correlated with the termination of orbicularis oculi activity. This occurrence of the complex spike correlated with trigeminal sensory information associated with the blink-evoking stimulus, and the burst of simple spike activity correlated with sensory feedback about the occurrence of a blink. Inactivation of the inferior olive with lidocaine prevented all complex and significantly reduced simple spike modulation of blink-related Purkinje cells, but did not alter orbicularis oculi activity evoked by corneal stimulation. In contrast, both acute and chronic lesions of the cerebellar cortex containing blink-related Purkinje cells blocked adaptive increases in orbicularis oculi activity of the lid ipsilateral but not contralateral to the lesion. These data are consistent with the hypothesis that the cerebellum is part of a trigeminal reflex blink circuit. Changes in trigeminal signals produce modifications of the cerebellar cortex, which in turn, reinforce or stabilize modifications of brainstem blink circuits. When the trigeminal system does not attempt to alter the magnitude of trigeminal reflex blinks, cerebellar input has little or no effect on reflex blinks.