CORRELATION OF PRIMATE RED NUCLEUS DISCHARGE WITH MUSCLE-ACTIVITY DURING FREE-FORM ARM MOVEMENTS

1. We recorded from 239 neurons located in the magnocellular division of the red nucleus of four alert macaque monkeys. At the same time, we recorded electromyographic (EMG) signals from as many as twenty elect rodes chronically implanted on muscles of the shoulder, arm, forearm a nd hand. We recorded EMG signals for periods ranging from several mont hs to a year. 2. The monkeys were trained to perform three free-form f ood retrieval tasks, each of which activated all of the recorded muscl es and most of the neurons. The 'prehension' task required simply that the monkey grasp a piece of food from a fixed point in space. The 'ba rrier' task required the monkey to reach around a small barrier to obt ain the food, and the 'Kluver' task required that food be removed from small holes. During the prehension task, we found approximately equal numbers of neurons that were strongly active while the hand was being moved toward the target (70% of units), and while the food was being grasped (60%). Relatively few units were active as the hand was return ed to the mouth (15%). 3. Data files of 1-2 min duration were collecte d while the monkey performed a single behavioural task. Whenever possi ble, we recorded files for all three tasks from each neuron. For each file we calculated long time-span analog cross-correlations (+/-1.28 s ) between instantaneous neuronal firing rate and each of the full-wave rectified, low-pass filtered EMG signals. We used the peak correlatio n and the time of the peak as two summary measures of the functional r elation between modulation of neuronal activity and EMG. 4. The magnit ude of the strongest correlations was between 0.4 and 0.5 (normalized to a perfect correlation of +/-1.0). Distal muscles were the most freq uently correlated, and extensors were more frequently correlated than flexors. For all monkeys, the lags for well correlated muscles were di stributed broadly about a uni-modal value near 0 ms. Eighty five per c ent of the correlations larger than or equal to 0.25 had peaks between -150 and 200 ms. 5. The activity of each neuron was represented in a muscle co-ordinate system by an n-dimensional 'functional linkage vect or', each element of which was the peak correlation with one of n musc les. The vector for any given neuron points in a particular direction in muscle space, depending on the similarity between the activity of t he neuron and the activity of each muscle. Sequential files recorded f rom a given neuron during a single behaviour yielded very similar link age vectors, whereas the vectors for different neurons pointed in quit e different directions in muscle space. The variation in linkage vecto rs for any given cell tested repeatedly during different behaviours wa s intermediate between that of the same cell-same task condition and t he different cell-same task condition. Therefore, the correlation betw een the activity of the magnocellular red nucleus (RNm) neurons and li mb muscles is not completely invariant for different tasks, but it var ies less than for different RNm neurons. 6. A distinct advantage of th e cross-correlation method is its suitability for use with EMG signals , which are otherwise difficult to analyse quantitatively, particularl y during free-form movements. By studying EMG, we were able to make me aningful comparisons between quite wide ranging types of behaviour. Be cause there was essentially no subjective judgement required on the pa rt of the experimenter, the method was both objective and efficient. T he resulting functional linkage vectors provide information that is im portant for understanding how sets of neurons and muscles are used in a variety of natural behaviours.