Motor co-ordinates in primate red nucleus: preferential relation to muscle activation versus kinematic variables

1. Magnocellular red nucleus (RNm) neurones (n = 158) were recorded from two macaque monkeys during a tracking task using one of six single-degree-of-freedom manipulanda. This task allowed us to study discrete movements about most of the joints of the arm. Single-unit, kinematic and electromyographic (EMG) signals from ten to twenty muscles of the upper limb were collected for approximately 2 min while the monkey used a given manipulandum. Movements about different joints were studied by switching among manipulanda. 2. Cross-correlation functions were calculated between RNm discharge rate and the kinematic variables, position and velocity, and between RNm and each of the EMG signals. Statistically significant cross-correlation peaks were found in 24% of the position correlations, 22% of the velocity correlations and 32% of the EMG correlations. The highest correlations were for EMG, reaching above 0.60. The peak correlation provided an effective means of identifying neurones with strong functional relations to one or more movements and/or muscles. These could then be analysed in detail, on a trial-by-trial basis. 3. The similarity between the dynamics of EMG and velocity signals of many highly practised movements makes it difficult to determine which might be the more likely target of RNm control. Therefore, we sought exceptions to this pattern, in order to distinguish between these two possible modes of control. For example, at the end of a movement, muscles occasionally remained active as velocity approached zero. Small corrective movements were often accompanied by a disproportionately large EMG. During these periods, RNm activity usually followed the time course of one or more of the EMG signals as opposed to the velocity signal. In the majority of cases, RNm responses were bidirectional, less frequently unidirectional and rarely reciprocal. These patterns were similar to the patterns of muscle activity. They did not resemble the velocity signals unless the latter were passed through a rectifier. 4. The results support the hypothesis that the red nucleus generates motor commands in a muscle-based co-ordinate system. Covariation between RNm discharge and velocity may result indirectly from correlations between muscle activation and movement. We discuss how the cerebellar cortex might convert the distributed representation of target position, known to be present in the posterior parietal cortex, directly into dynamic, muscle-based commands in the rubro-cortico-cerebellar limb premotor network.