Sensorimotor electroencephalogram rhythmic activity: a functional gate mechanism.

Neurophysiological evidence for a negative feedback loop in somatosensory thalamic nuclei has provided a basis for understanding the origins of rhythmic activity in the electroencephalogram (EEG) of sensorimotor cortex. Studies reviewed here suggest that state-specific activation of this mechanism is responsible both for a variable pattern of rhythmic frequencies during wakefulness and the spindle burst pattern during sleep. It is proposed further that these rhythms index immobility and a common set of physiological changes in sensorimotor function, including reduced excitability in both afferent and efferent pathways. It is hypothesized, therefore, that the closing of a thalamic "inhibitory gate" is indicated by the occurrence of these rhythms. While a greater resolution of specific relationships is necessary, the study of these EEG patterns clearly provides a measure of complex physiological events in the nervous system, and can be also useful in identifying functional abnormalities.