We studied the dynamical relationship between magnocellular red nucleus (RNm) discharge and electromyographic (EMG) activity of 10-15 limb muscles in two monkeys during voluntary limb movement. Recordings were made from 158 neurons during two different kinds of limb movement tasks. One was a tracking task in which the subjects were required to acquire targets displayed on an oscilloscope by rotating one of six different single degree of freedom manipulanda. During this task, we recorded the angular position of the manipulandum. The monkeys also were trained in several free-form food- retrieval tasks that were much less constrained mechanically. There was generally significantly greater neuronal discharge during the free-form tasks than during the tracking task. During both types of tasks, cross-correlation and impulse response functions 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 task for a substantial dynamical transformation (e.g., integration) between the two signals as had been previously suggested. In only 15% of the cases, did these correlations have step or pulse-step dynamics. There 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 ~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 between -100 and 200 ms. A positive lag indicates that RNm discharge preceded EMG. The shape and timing of both the cross-correlation and the impulse response functions were consistent with a model in which many RNm neurons contribute mutually correlated signals which are simply summed within the spinal cord to produce a muscle activation signal.
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