Brain Activity and Levels of Consciousness

Résumé

There is a general introduction followed by an account of the principal methods of assessing human brain activity.

A most important technique uses radio-Xenon in quantifying and mapping the cerebral circulation. The findings of this method have been confirmed by radio-oxygens studies on patients. There is general agreement between these recent observations and older findings on metabolism and blood flow for the whole brain.

Electroencephalography also provides valuable data on the levels of cerebral activity.

There is firstly the classical work on the reticular activating system that via a relay in the non-specific thalamus induces cerebral activation as indicated by desynchronized EEGs. Two other pathways to the cerebral cortex are described because of their postulated implication in sleep: serotonergic that depresses cortical cells and noradrenergic that tends to cause arousal.

The most important and ubiquitous cause of unconsciousness is sleep, yet despite intensive studies the relationship of cerebral activity to sleep is enigmatic. It has long been known that oxygen consumption and blood flow are not reduced in sleep and may even be increased in the phases of deep sleep called paradoxical sleep. There is an account of the EEGs at various levels of sleep and of the spontaneous activity of cortical neurones during stages of sleep.

There is good evidence that sleep is induced and maintained by the serotonergic system, but it seems likely that the noradrenergic system is concerned in the phases of deeper sleep with the associated high levels of neuronal activity. But complex neuronal systems have to be envisaged to account for all the phenomena of sleep regulation on a diurnal cycle. There is brief reference to the pathological disorders: narcolepsy and hypersomnia. The relation of dreams to the rapid eye movements (REM) and the associated EEG is described and hallucinations are briefly considered.

If sufficiently generalized, convulsions result in loss of consciousness. The opposite state of greatly depressed neuronal activity also results in the unconsciousness of coma or of surgical anaesthesia.

On the hypothesis of dualist-interactionism, unconsciousness can plausibly be explained by the low level of cerebral activity in coma and anaesthesia and by the high driven level in convulsions. In these situations the liaison between the self-conscious mind and the spatiotemporal patterns of modular activity would be deteriorated or would fail altogether, hence the unconsciousness. However, the unconsciousness of sleep cannot so simply be explained. Possibly it is the changed temporal pattern of neuronal activity that is responsible. Dreams occur when there are changes in that pattern. The necessity for sleep remains an enigma. Moruzzi suggests it is a time for plastic reconstruction of the cortex after the intense activity of the waking day.