Many different kinematic and kinetic signals have been proposed as possible variables under the control of the primary motor cortex. Despite the presence of direct projections to motor neurons, muscle activation has received less attention as a controlled variable. Furthermore, although it is well known that descending fibers project to multiple motor pools, an objective, quantitative study of the relation between neuronal modulation and the activity of groups of muscles has not previously been reported. We have recorded the discharge of 310 neurons located in the primary motor cortex of two monkeys, along with the activity of a variety of arm and hand muscles. Data were recorded while the monkey reached to and pressed a series of illuminated buttons. The similarity of a given neuron's discharge with respect to each muscle was determined by calculating the linear cross-correlation between its discharge rate and each rectified, filtered electromyogram. A "functional linkage vector" was then constructed, which expressed the similarity of that neuron's discharge to the entire set of muscles. We discovered discrete groups of functional linkage vectors within the high order muscle space for both monkeys which corresponded to functional properties of the neurons measured by other methods. Several of these groups appeared to represent a functional synergy of muscles, such as those required to extend the limb, press a button, or open the hand in preparation for the press. When the dimensionality of this space was reduced by a principal components analysis, the originally identified clusters of neurons remained well separated. These results are consistent with the hypothesis that the discharge of individual neurons in the primary motor cortex encodes the activity of a relatively small number of functionally relevant groups of muscles. It will be important to determine whether these results will also apply to more complex behavior, and to what extent these functional muscle synergy representations remain fixed across behaviors.

Individually optimized tcMEP monitoring and localized transcranial activation in the brain might be possible through FEM modeling.

Simultaneous single neuron and local field potential (LFP) recordings were made in arm-related areas of the cerebellar nuclei (CN) and primary motor cortex (M1) of two monkeys during a reaching and button pressing task. Microstimulation of focal sites in CN caused short latency (median = 3.0 ms) increases in discharge in 25% of 210 M1 neurons. Suppressive effects were less common (13%) and observed at longer latencies (median = 9.9 ms). Stimulation in CN also caused reciprocal facilitation and suppression in averages of antagonist muscle electromyograms (EMGs). The latency of these effects was approximately 8-11 ms. In contrast to the selectivity of unit and EMG effects, stimulation-evoked changes in LFP occurred over a broad range of sites. There were no significant short-latency effects detected in cross-correlation histograms between single neurons in CN and M1. However, CN spike-triggered averages of M1 LFPs were observed in a few cases (10% of 126 cases). In one-half of these, there were effects both before and after the CN spikes, which may reflect causal effects from M1 to CN, as well as from CN to M1. Overall, these results demonstrate a spatially specific, short latency, primarily excitatory pathway from CN to M1. The relatively rare effects at the single neuron level may have resulted from the difficulty in achieving optimal alignment between cerebellar and cerebral sites because of the specificity of these connections.

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