Synchronization of fast (30-40 Hz) spontaneous oscillations in intrathalamic and thalamocortical networks

The synchronization of fast (mainly 30 to 40 Hz) oscillations in intrathalamic and thalamocortical (TC) networks of cat was studied under ketamine-xylazine anesthesia and in behaving animals by means of field potential, extra- and intracellular recordings from multiple sites in the thalamic reticular (RE) nucleus, dorsal (sensory, motor, and intralaminar) thalamic nuclei, and related neocortical areas. Far from being restricted to tonically activated behavioral states, the fast oscillations also appeared during resting sleep and deep anesthesia, when they occurred over the depolarizing component of the slow (<1 Hz) oscillation and were suppressed during the prolonged hyper-polarizations of RE, TC, and cortical neurons. The synchronization of fast rhythms among different thalamic foci was robust. Fast rhythmic cortical waves and subthreshold depolarizing potentials in TC neurons were highly coherent; however, the synchronization of the fast oscillation required recordings from reciprocally related neocortical and thalamic foci, as identified by monosynaptic responses in both directions. The short-range spatial confinement of coherent fast rhythms contrasted with the large-scale synchronization of low-frequency sleep rhythms. Transient fast rhythms, appearing over the depolarizing envelope of the slow sleep oscillation, became sustained when brain activation was elicited by stimulation of mesopontine cholinergic nuclei or during brain-active behavioral states in chronic experiments. These data demonstrate that fast rhythms are part of the background electrical activity of the brain and that desynchronization, used to designate brain-active states, is an erroneous term inasmuch as the fast oscillations are synchronized not only in intracortical but also in intrathalamic and TC networks.