Firing rates and patterns of output and nonoutput cells in cortical areas 5 and 7 of cat during the sleep-waking cycle

The rate and patterns of spontaneous discharge in output cells and putative interneurons of cortical areas 5 and 7 were analyzed in the behaving cat during wakefulness (W), synchronized (S) sleep, and desynchronized (D) sleep, with (D+) and without (D−) rapid eye movements (REMs). Output cells were antidromically identified following stimulation of the thalamic and pontine nuclei. Interneurons were inferred from their thalamically elicited synaptic responses consisting of high-frequency spike barrages. Non-parametric (rank) statistics were used for tests of significance. A method derived from the Wilcoxon rank tests was developed to permit some statistical testing between interspike interval histogram patterns. Rate analysis showed that output cells discharge in all states of the sleep-waking cycle at significantly higher rates than interneurons. Corticothalamic and corticopontine cells decreased firing rates from W to S sleep and reached maximal rates in both (D+ and D−) substates of D sleep. In contrast, interneurons increased firing rates from W to S sleep and had their highest rates in D sleep, selectively occurring during REM epochs. In output cells, pattern differences between states consisted essentially of: (a) more short and long interspike intervals in S sleep compared to W, which indicates an increase in the burst-silence pattern in S sleep; (b) more short, but less long, intervals in D sleep, compared to both W and S sleep, which results from tonic excitation during D sleep and indicates that the firing pattern in D sleep is not an extreme type of S sleep. Compared to output cells, interneurons in all states had more short and long intervals and a significantly higher probability of spike bursts. The REM-related firing of interneurons may be related to recent investigations suggesting the consolidation of memory traces during D sleep.