Context dependent discharge characteristics of saccade-related Purkinje cells in the cerebellar hemispheres of the monkey.

In the previous paper (Mano et al, 1991), we reported the discharge patterns of saccade-related Purkinje cells during visually guided saccade task, which were recorded from posterior cerebellar hemisphere, the Crus IIa. In the present study, we analysed these P-cell's simple spike activity during the spontaneous saccade in inter-trial intervals (ITI) of visually guided saccade task, comparing with the activity during the visually guided saccade. We found that the modulation of simple spike discharges during spontaneous saccade was weaker than the modulation during the visually triggered saccade. We recorded single unit discharges of Purkinje cells from cerebellar posterior hemisphere (Crus IIa) in awake Japanese monkeys (Macaca fuscata), trained to perform simple reaction time saccade task gazing at a small light rear-projected on to a tangent screen 54 cm in front of the monkey. Horizontal and vertical eye positions were measured by a corneal search coil method. Comparison of simple spike activity associated with spontaneous saccade during ITI to the activity during visually triggered saccade clarified that the discharge patterns of simple spikes are basically the same during both types of the saccades, but the amount of the phasic modulation (increase of decrease of discharge rate) were larger for all directions (up, down, left and right) during visually guided saccade than that during spontaneous saccade in all saccade-related Purkinje cells so far examined in two monkeys. The modulation, however, cannot be assumed to have been induced by the visual stimulus per se. Because, the maximum increase of simple spike discharge rate aligned at saccade onset is larger than that aligned at target jump. And, the half width of the change was wider when aligned at target light jump than when aligned at the onset of saccades, in all the four directions, indicating the changes of the firing rate were more time-locked to the onset of saccadic eye movements than to the triggering visual stimulus. The present findings suggest that the cerebellar hemisphere plays a more important role in the control of externally triggered voluntary eye movements than in the control of self-initiated, self-paced eye movements. We discussed these findings combining with previous findings on limb movement-related P-cells (Mano et al, 1980, 1986, 1989), from view point of the general role of the cerebellar hemisphere in the control of voluntary movements.