Synchrony among rhythmical facial tremor, neocortical 'alpha' waves, and thalamic non-sensory neuronal bursts in intact awake rats.

A fine (approx. 9 c/sec) tremor of the jaw and/or vibrissae was observed in normal rats while they were standing still and not showing gross bodily movement. The tremor was distinctly different in frequency, intensity, and behavioral context, from movements involved in gnawing, tooth chattering, or exploratory sniffing. Individual tremor movements (recorded as EMG) occurred in synchrony with individual bursts of multiunit activity (MUA) recorded in the ventrobasal complex of the thalamus and with individual 'spikes' in the cortical (frontal-occipital) EEG. Single trains of this rhythmical activity often lasted more than a minute. The phase relationships between EMG and MUA differed among individuals, but tended to remain consistent within each individual. Movement artifacts were ruled out since (1) the moments of occurrence of individual tremor movements and MUA bursts were interdigitated rather than simultaneous, and (2) during high amplitude EMG bursts accompanying sni ffing (associated with EEG theta rhythm), tooth chattering, eating or licking, no corresponding activity in the MUA was observed. We also ruled out the possibility that the neural activity was generated by reafference, for (1) during vigorous non-tremor sniffing movements of the vibrissae, or chattering or chewing movements of the jaws, the rhythmical MUA was absent (although the units did discharge if the vibrissae contacted an obstacle or were brushed by the experimenter), (2) rhythmical MUA often continued both during brief pauses in the motor tremor, and in its absence, and (3) injection of Xylocaine s.c. into the face abolished sensory responses of the thalamic units, but the rhythmical MUA persisted. We discuss evidence which suggests that (1) the rhythmical cortical EEG waves are the equivalent in the rat of the alpha (mu) rhythm, and (2) the existence of parallels between alpha-tremor and Parkinsonian tremor in terms of their mechanisms and functions.