The role of the flocculus in vestibular compensation after hemilabyrinthectomy.

Unilateral lesions of the cerebellar flocculus were made in two groups of cats chronically implanted for eye-movement recording. In the first group (3 cats), the floccular lesion preceded by 40-70 days a unilateral labyrinthectomy on the contralateral side. In the second group (5 cats), the serial order of the two lesions was reverted, the unilateral flocculectomy following unilateral labyrinthectomy by about 60 days in 2 animals and by 16-27 months in the other 3. The effects of the unilateral labyrinthectomy on the vestibulo-ocular reflex (spontaneous nystagmus and asymmetrical responses) were extensively tested by using natural vestibular stimulations. It was found that recovery from these effects was severely delayed in animals from the first group (flocculectomy prior to labyrinthectomy), although in animals from the second group the flocculectomy secondary to the labyrinthectomy only produced a transient asymmetry of vestibulo-ocular responses. It is concluded that the flocculus is required for initiating (not for maintaining), the compensatory process following peripheral lesions of the vestibular system.