State-dependent fluctuations of low-frequency rhythms in corticothalamic networks.

We have studied the variations in the degree of correlated firing within the low-frequency sleep rhythms (< 15 Hz) between cortical, thalamic reticular and thalamocortical neurons during changes in the amplitude and frequency of brain electrical activity in anaesthetized cats. Extracellular discharges of neuronal groups of two to five physiologically identified cortical and thalamic units were recorded simultaneously with independent microelectrodes. The firing patterns and the temporal correlation between spike-trains were evaluated by auto- and crosscorrelograms. Although the animals were under deep anaesthesia, additional doses of the same or different anaesthetics were able to alter the electroencephalographic pattern, inducing waves with higher amplitude. Similar transitions occurred spontaneously. We found that the presence of rhythmic behaviour in cells of corticothalamic networks, as well as their degree of correlated firing, was extremely sensitive to even slight alterations in the state of the electroencephalogram. Cells belonging to the same functional system, but located distantly, became highly synchronized upon the increased amplitude of brain waves. Thus, an electroencephalogram characterized by slow waves corresponds to a state of rhythmic and correlated firing among cortical and thalamic neurons. The highly coherent activity during sleep patterns transcends the borders which limit the functioning during the waking brain.