Effects of reversible pharmacological shutdown of cerebellar flocculus on the memory of long-term horizontal vestibulo-ocular reflex adaptation in monkeys

The adaptation of the horizontal vestibulo-ocular reflex (HVOR) provides an experimental model for motor learning. Two studies, using cats and mice, respectively, have recently suggested pharmacologically that the memory of adaptation is located multiply in the cerebellum and brainstem. Here, we examined the effects of acute cerebellar flocculus shutdown on the adaptation in four monkeys. Two hours of 0.11 Hz-10° turntable oscillation while viewing a stationary checked-patterned screen through the left–right reversing prism decreased the HVOR gains by 0.16, and 3 days of prism wearing combined with 2 h of daily turntable oscillation decreased the HVOR gains by 0.27. Injections of lidocaine into bilateral flocculi did not affect the nonadapted HVOR gains, but depressed the visual suppression of the HVOR. They recovered the HVOR gains decreased by 2 h of training, but very little affected the HVOR gains decreased by previous 2 days of training. Injections of control Ringer's solution did not affect the gains adapted by 2 h or 3 days of training. These results are consistent with the previous studies, and suggest that the memory trace of adaptation of the HVOR initially resides in the flocculus but later resides, presumably, in the vestibular nuclei in the monkey.