MOTOR COORDINATES 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-de gree-of-freedom manipulanda. This task allowed us to study discrete mo vements about most of the joints of the arm. Single-unit, kinematic an d electromyographic (EMG) signals from ten to twenty muscles of the up per 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 calc ulated between RNm discharge rate and the kinematic variables, positio n and velocity, and between RNm and each of the EMG signals. Statistic ally significant cross-correlation peaks were found in 24% of the posi tion 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 n eurones with strong functional relations to one or more movements and/ or muscles. These could then be analysed in detail, on a trial-by-tria l basis. 3. The similarity between the dynamics of EMG and velocity si gnals of many highly practised movements makes it difficult to determi ne which might be the more likely target of RNm control. Therefore, we sought exceptions to this pattern, in order to distinguish between th ese two possible modes of control. For example, at the end of a moveme nt, muscles occasionally remained active as velocity approached zero. Small corrective movements were often accompanied by a disproportionat ely large EMG. During these periods, RNm activity usually followed the time course of one or more of the EMG signals as opposed to the veloc ity 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 resembl e the velocity signals unless the latter were passed through a rectifi er. 4. The results support the hypothesis that the red nucleus generat es motor commands in a muscle-based co-ordinate system. Covariation be tween RNm discharge and velocity may result indirectly from correlatio ns between muscle activation and movement. We discuss how the cerebell ar cortex might convert the distributed representation of target posit ion, known to be present in the posterior parietal cortex, directly in to dynamic, muscle-based commands in the rubro-cortico-cerebellar limb premotor network.