PRIMATE RED NUCLEUS DISCHARGE ENCODES THE DYNAMICS OF LIMB MUSCLE-ACTIVITY

We studied the dynamical relationship between magnocellular red nucleu s (RNm) discharge and electromyographic (EMG) activity of 10-15 limb m uscles in two monkeys during voluntary limb movement. Recordings were made from 158 neurons during two different kinds of limb movement task s. One was a tracking task in which the subjects were required to acqu ire targets displayed on an oscilloscope by rotating one of six differ ent single degree of freedom manipulanda. During this task, we recorde d the angular position of the manipulandum. The monkeys also were trai ned in several free-form food-retrieval tasks that were much less cons trained mechanically. There was generally significantly greater neuron al discharge during the free-form tasks than during the tracking task. During both types of tasks, cross-correlation and impulse response fu nctions calculated between RNm and EMG were predominantly pulse-shaped , indicating that the dynamics of the RNm discharge were very similar to those of the muscle activity. There was no evidence during either t ask for a substantial dynamical transformation (e.g., integration) bet ween the two signals as had been previously suggested. In only 15% of the cases, did these correlations have step or pulse-step dynamics. Th ere was a relatively broad, unimodal distribution of lag times between RNm and EMG, based on the time of occurrence of the peak correlation. During tracking, the mode of this distribution was similar to 50 ms, with 80% of the lags falling between -100 and 200 ms. During the free- form task, the mode was between 0 and 20 ms, with 65% of the lags betw een -100 and 200 ms. A positive lag indicates that RNm discharge prece ded EMC. The shape and timing of both the cross-correlation and the im pulse response functions were consistent with a model in which many RN m neurons contribute mutually correlated signals which are simply summ ed within the spinal cord to produce a muscle activation signal.