Purkinje cell activity in the middle zone of the cerebellar flocculus during optokinetic and vestibular eye movement in cats.

Based on the inverse dynamics theory, a previous paper reconstructed simple-spike (SS) firing rates of Purkinje cells in the cat's flocculus middle-zone by a linear-weighted summation of eye acceleration, velocity, and position during optokinetic response (OKR). The present study investigated the SS rates during combined optokinetic and vestibular stimuli of the cells recorded in the previous paper. During the sinusoidal vestibuloocular reflex (VOR) in the light (VORL) and in the dark (VORD) the firing modulation was small. During VOR suppression (VORS) by head and visual-pattern rotation in the same direction, the modulation was deep, with the peak coinciding roughly with peak ipsiversive head velocity. During VOR enhancement (VORE), the modulation was deep, with the peak coinciding roughly with peak contraversive head velocity. If we interpret these data in relation to eye and head movements, the cells in the cat were comparable to the horizontal-gaze-velocity Purkinje cells in the monkey that encode a linear summation of eye and head velocity signals. Alternatively, if we interpret the data on the basis of the inverse dynamics theory, the SS rates during VORL, VORS, and VORE were well-fitted by the OKR components of the movements (subtraction of VORD from VORL, VORS, and VORE eye movements, respectively), but not by the whole movements, using the coefficients calculated during OKR. It is concluded that the data are interpretable by both theories when the VOR gain (eye movement/head movement) is close to 1 and the firing is dominated by eye velocity information.