Motor dynamics encoding in cat cerebellar flocculus middle zone during optokinetic eye movements

We investigated the relationship between eye movement and simple-spike (SS) frequency of Purkinje cells in the cerebellar flocculus middle zone during the optokinetic response (OKR) in alert cats. The OKR was elicited by a se quence of a constant-speed visual pattern movement in one direction for 1 s and then in the opposite direction for 1 s. Quick-phase-free trials were s elected. Sixty-six cells had direction-selective complex spike (CS) activit y that was modulated during horizontal (preferring contraversive) but not v ertical stimuli. The SS activity was modulated during horizontal OKR, prefe rring ipsiversive stimuli. Forty-one cells had well-modulated activity and were suitable for the regression model. In these cells, an inverse dynamics approach was applied, and the time course of the SS rate was reconstructed with mean coefficient of determination 0.76, by a linear weighted superpos ition of the eye acceleration (mean coefficient, 0.056 spikes/s per deg/s(2 )), velocity (5.10 spikes/s per deg/s), position (-2.40 spikes/s per deg), and constant (mean 34.3 spikes/s) terms, using a time delay (mean 11 ms) fr om the unit response to the eye response. The velocity and acceleration ter ms contributed to the increase in the reconstructed SS rates during ipsilat eral movements, whereas the position term contributed during contralateral movements. The standard regression coefficient analyses revealed that the c ontribution of the velocity term (mean coefficient 0.81) was predominant ov er the acceleration (0.03) and position (-0.17) terms. Forward selection an alysis revealed three cell types: Velocity-Position-Acceleration type (n = 27): velocity, position, and acceleration terms are significant (P < 0.05); Velocity-Position type (n = 12): velocity and position terms are significa nt; and Velocity-Acceleration type (n = 2): velocity and acceleration terms are significant. Using the set of coefficients obtained by regression of t he response to a 5 deg/s stimulus velocity, the SS rates during higher (10, 20, and 40 deg/s) stimulus velocities were successfully reconstructed, sug gesting generality of the model. The eye-position information encoded in th e SS firing during the OKR was relative but not absolute in the sense that the magnitude of the position shift from the initial eye position (0 deg/s velocity) contributed to firing rate changes, but the initial eye position did not. It is concluded that 1) the SS firing frequency in the cat middle zone encodes the velocity and acceleration information for counteracting th e viscosity and inertia forces respectively, during short-duration horizont al OKR and 2) the apparent position information encoded in the SS firing is not appropriate for counteracting the elastic force during the OKR.