RELATIONSHIP BETWEEN POSTERIOR THALAMIC NUCLEUS UNIT-ACTIVITY AND PARIETAL CORTICAL RHYTHMS (BETA) IN THE WAKING CAT

Cat behavioural states of attentive fixation on a target are associate d with episodes of electrocortical rhythms at 40 Hz (''beta rhythms'') in the parietal cortex. Previous field potential studies indicate tha t the nucleus posterior pars medialis of the thalamus displays this pa rticular rhythmic activity. We investigated single units of the nucleu s posterior pars medialis and its surrounding nuclei to assess their p articipation in the cortical beta rhythms. Only a small proportion of thalamic cells underwent changes in their firing pattern during beta e pisodes. ''Beta-related cells'' were localized in the nucleus posterio r pars medialis or its immediate vicinity; no such beta-related cells were found in other regions of the lateral thalamus. Some beta-related cells showed a one-spike to one-wave relationship (''homorhythmicity' '), while others displayed a prolonged decrease or a suppression of th eir firing throughout each beta episode (''pause cells''). For compari son, neurons in the same thalamic area were also recorded during sleep episodes with slow waves and spindles: there was no correlation betwe en spindles and cell firing. Thus, the nucleus posterior pars medialis thalamic nucleus contains cells whose firing is correlated with the b eta rhythms. No such correlation was found with sleep spindles.