Synchronization of low-frequency rhythms in corticothalamic networks.

We have investigated the degree of synchronization between cortical, thalamic reticular and thalamocortical neurons of cats during low-frequency (< 15 Hz) sleep-like oscillations, as they appear under anaesthesia. We have also studied the effects exerted by cortical stimulation on the synchronization among thalamic units. Parallel experiments [Steriade et al. (1996) J. Neurosci. 16, 392-417] in this laboratory have demonstrated the similarity between the slow oscillation (< 1 Hz) under ketamine-xylazine anaesthesia and that occurring during the natural state of resting sleep. Spontaneous activity was recorded simultaneously, with independent microelectrodes, from groups of two to five physiologically identified neurons. The rhythmicity of spontaneous activity and the temporal relations between cellular discharges were statistically evaluated by auto- and crosscorrelation techniques. We have found no topography in the distribution of synchronization between thalamic reticular and thalamocortical cells. Only the slow, cortical-generated oscillation (< 1 Hz) displayed a stable frequency and correlation among groups of cortical and thalamic cells. The other two sleep oscillations (thalamic-generated spindles at 7-14 Hz and clock-like delta at 1-4 Hz) fluctuated in frequency and the degree of correlation between neurons varied. Cortical volleys entrained and synchronized thalamic cells, and triggered synchronized spindling in the thalamus. These results extend for large populations of cortical and thalamic neurons the phase relations found in intracellular recordings.