Activity of midbrain reticular formation and neocortex during the progression of human non-rapid eye movement sleep

To clarify the neural correlates and brain activity during the progression of human non-rapid eye movement (NREM) sleep, we examined the absolute regi onal cerebral blood flow (rCBF) during light and deep NREM sleep and during wakefulness in normal humans using positron emission tomography with (H2O) -O-15. Relative changes in rCBF during light and deep NREM sleep in compari son to the rCBF during wakefulness were also analyzed. During light NREM sl eep, the rCBF in the midbrain, in contrast to that in the pons and thalamic nuclei, did not decrease when compared to that during wakefulness, whereas rCBF decreased in the left medial frontal gyrus, left inferior frontal gyr us, and left inferior parietal gyrus of the neocortex. During deep NREM sle ep, the rCBF in the midbrain tegmentum decreased, and there was a marked an d bilateral decrease in the rCBF in all neocortical regions except for the perirolandic areas and the occipital lobe. There have been three groups of brain structures, each representing one type of deactivation during the pro gression of NREM sleep. The activity of the midbrain reticular formation is maintained during light NREM sleep and therefore represents a key distingu ishing characteristic between light and deep NREM sleep. Selective deactiva tion of heteromodal association cortices, including those related to langua ge, occurs with increasingly deep NREM sleep, which supports the recent the ory that sleep is not a global, but it is a local process of the brain.