Correlation and spectral analysis of relations between single unit discharge and muscle activities

Correlation and spectral analysis was used to study functional linkages between single-cell discharge in the magnocellular red nucleus and the electromyographic activity of several limb muscles. Long sequences of unit discharge and EMG activity were recorded while feline subjects performed a food retrieval task. Unit discharge and muscle activity were patterned in bursts that corresponded to different phases of the task. There was sufficient variability in the parameters of these bursts to regard the signals as pseudorandom variables, thus facilitating a correlation analysis. Cross-correlation functions computed between unit discharge and each muscle EMG served to characterize the strength and reliability of linkages between a single unit and various limb muscles. Auto-correlation and auto-spectral density functions provided summary measures of the temporal and frequency characteristics of the signals. Power in the signals was concentrated in a behaviorally relevant range (0.2-8 Hz). Coherence functions showed peaks that indicated which frequency components were well correlated with unit discharge. Two-sided impulse responses served to characterize the dynamic properties of the linkages. These methods are shown to be valuable in characterizing noncausal, as well as causal, linkages through multisynaptic pathways in the nervous system.