SYNCHRONIZATION OF FAST (30-40 HZ) SPONTANEOUS OSCILLATIONS IN INTRATHALAMIC AND THALAMOCORTICAL NETWORKS

The synchronization of fast (mainly 30 to 40 Hz) oscillations in intra thalamic and thalamocortical (TC) networks of cat was studied under ke tamine-xylazine anesthesia and in behaving animals by means of field p otential, extra- and intracellular recordings from multiple sites in t he thalamic reticular (RE) nucleus, dorsal (sensory, motor, and intral aminar) thalamic nuclei, and related neocortical areas. Far from being restricted to tonically activated behavioral states, the fast oscilla tions also appeared during resting sleep and deep anesthesia, when the y occurred over the depolarizing component of the slow (<1 Hz) oscilla tion and were suppressed during the prolonged hyperpolarizations of RE , TC, and cortical neurons. The synchronization of fast rhythms among different thalamic foci was robust. Fast rhythmic cortical waves and s ubthreshold depolarizing potentials in TC neurons were highly coherent ; however, the synchronization of the fast oscillation required record ings from reciprocally related neocortical and thalamic foci, as ident ified by monosynaptic responses in both directions. The short-range sp atial confinement of coherent fast rhythms contrasted with the large-s cale synchronization of low-frequency sleep rhythms. Transient fast rh ythms, appearing over the depolarizing envelope of the slow sleep osci llation, became sustained when brain activation was elicited by stimul ation of mesopontine cholinergic nuclei or during brain-active behavio ral states in chronic experiments. These data demonstrate that fast rh ythms are part of the background electrical activity of the brain and that desynchronization, used to designate brain-active states, is an e rroneous term inasmuch as the fast oscillations are synchronized not o nly in intracortical but also in intrathalamic and TC networks.