Encoding of movement dynamics by Purkinje cell simple spike activity during fast arm movements under resistive and assistive force fields

It is controversial whether simple-spike activity of cerebellar Purkinje cells during arm movements encodes movement kinematics like velocity or dynamics like muscle activities. To examine this issue, we trained monkeys to flex or extend the elbow by 45 in 400 ms under resistive and assistive force fields but without altering kinematics. During the task movements after training, simplespike discharges were recorded in the intermediate part of the cerebellum in lobules V-VI, and electromyographic activity was recorded from arm muscles. Velocity profiles (kinematics) in the two force fields were almost identical to each other, whereas not only the electromyographic activities (dynamics) but also simple-spike activities in many Purkinje cells differed distinctly depending on the type of force field. Simple-spike activities encoded much larger mutual information with the type of force field than that with the residual small difference in the height of peak velocity. The difference in simple-spike activities averaged over the recorded Purkinje-cells increased similar to 40 ms before the appearance of the difference in electromyographic activities between the two force fields, suggesting that the difference of simple-spike activities could be the origin of the difference of muscle activities. Simple-spike activity of many Purkinje cells correlated with electromyographic activity with a lead of similar to 80 ms, and these neurons had little overlap with another group of neurons the simple-spike activity of which correlated with velocity profiles. These results show that simple- spike activity of at least a group of Purkinje cells in the intermediate part of cerebellar lobules V-VI encodes movement dynamics.