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-zo ne by a linear-weighted summation of eye acceleration, velocity, and positi on during optokinetic response (OKR). The present study investigated the SS rates during combined optokinetic and vestibular stimuli of the cells reco rded in the previous paper. During the sinusoidal vestibuloocular reflex (V OR) in the light (VORL) and in the dark (VORD) the firing modulation was sm all. During VOR suppression (VORS) by head and visual-pattern rotation in t he same direction, the modulation was deep, with the peak coinciding roughl y with peak ipsiversive head velocity. During VOR enhancement (VORE), the m odulation was deep, with the peak coinciding roughly with peak contraversiv e head velocity. If we interpret these data in relation to eye and head mov ements, the cells in the cat were comparable to the horizontal-gaze-velocit y Purkinje cells in the monkey that encode a linear summation of eye and he ad velocity signals, Alternatively, if we interpret the data on the basis o f the inverse dynamics theory, the SS rates during VORL, VORS, and VORE wer e well-fitted by the OKR components of the movements (subtraction of VORD f rom VORL, VORS, and VORE eye movements, respectively), but not by the whole movements, using the coefficients calculated during OKR. It is concluded t hat the data are interpretable by both theories when the VOR gain (eye move ment/head movement) is close to 1 and the firing is dominated by eye veloci ty information.