Effects of unilateral flocculus lesions on vestibulo-ocular responses in the cat

Unilateral lesions of the cerebellar flocculus were performed in three chronically-implanted adult cats. Following the lesion a spontaneous nystagmus was observed in the dark, with the fast phase directed to the lesioned side. Vestibulo-ocular responses in the dark became asymmetrical. Responses to velocity steps exciting the labyrinth ipsilateral to the lesion were strongly increased. A decrease, although less marked, was observed in the opposite direction. Responses to sinusoidal oscillations in the dark were also asymmetric with respect to both the cumulative eye displacement during rotation in the two directions and the interval between two consecutive reversals of eye movement. These differences were greater at the lower tested frequencies (0.01 HZ) than at the higher (0.1 HZ). Spontaneous nystagmus disappeared in about 10 days and a complete symmetry of the vestibulo-ocular responses was restored in about 3 weeks. It is concluded that a unilateral lesion of the flocculus leads to two separate, but interacting, effects upon vestibulo-ocular responses.