Neurophysiological support of consciousness during waking and sleep.

The aim of this review was to try to establish the current neurophysiological knowledge capable of explaining the differences of mental functioning during the different stages of sleep and waking. The analysis focused on the cortical state. Waking is characterized by electrophysiological activities (low voltage and gamma range EEG field patterns, unitary activities) and cerebral blood flow reflecting an activated state. On the contrary, neurochemical influences are marked by inhibitory afferent processes since dopamine, noradrenaline, serotonin and histamine tend, for the most part, to inhibit cortical neurons by diffuse release at the level of varicosities. During slow wave sleep, all these brain stem influences sustaining the cortical state decrease and transiently disappear just prior to onset of REM sleep. During REM sleep, pontine and mesopontine ascending activating influences invade the cortex in their turn while neurochemical inhibitory influences disappear with the exception of dopaminergic ones. We hypothesize that the activating influences acting on the cortex allow the latter to function, just as petrol makes an engine run, but that the diffuse inhibitory influences somehow regulate cortex functioning. Therefore, it is understandable that, during waking, mental activity is reflective and rational, and that psychological content is less intense during slow wave sleep. During REM sleep, the activated and mostly disinhibited state might induce the characteristic dream activity which appear to be rather ill-considered and illogical. Persistent dopaminergic input combined with the absence of noradrenergic input may induce psychological activities somewhat similar to those related to psychotic syndromes. Deactivation of part of the prefrontal cortex could contribute to this unusual mental activity.