ABOUT CONSCIOUSNESS

<< Consciousness >> is the feeling that each individual has of his own existence (self-awareness) and also the state which permits him to perceive his environment (consciousness and wakefulness). These two types of consciousness, apparently quite different, are in fact the two converging stems of the perceptions that the nervous system provides to the organism. To explain this better, it is necessary to refer to the evolution of the nervous system organization among the species and to imagine what perception the nervous system provides at each stage of this evolution, or in each species harbouring a nervous system corresponding to a given stage. At the beginning, the role of the nervous system is to protect the organism and only by an automatic and reflex organization. It corresponds to an unconscious alertness which permits to perceive the environment; it is the prefiguration of the future consciousness. Self-awareness seems to appear with the centrencephalic and limbic structures which are the highest part of the nervous system in the inferior mammals and the central part in man. At this stage, acquisition memory and affectivity with its different manifestations (emotions, mood, pain...) are established. This kind of consciousness is difficult to imagine since it is only sensorial and affective. Mental consciousness appears with the mental activity in the cerebral hemispheres. It is more than likely that it exists in the superior mammals though it is impossible to imagine what it corresponds to. It reaches the highest development in the man in whose the frontal lobes have the largest extent. Speech is the most striking figure of this development. At each stage of the evolution, consciousness expresses the nervous system activity, centered on a perception, the level of which corresponds to the improvements occurring at each stage and superadded to the previous ones. Hence it is obvious that consciousness has no specific center or pathway. It relies upon structures that participate in triggering and regulating the nervous activity and that utilize the particular centers of perception and analysis. These structures are: the reticular formation and the diffuse thalamic system. This approach of consciousness allows an analysis of its different pathological and physiological aspects : a) coma is produced by lesions of the reticular formation which extends all along the central part of the brain stem; the severity of coma is related to the level of lesions: the more caudal is the lesion, the more severe is the lesion and the worse is the prognosis; b) sleep is a state of unconsciousness or of decreased consciousness, which divides in two phases: the slow waves sleep and the paradoxical sleep (sleep with rapid eye movements). These two phases correspond to two different levels of unconsciousness, related to a decreased activity at two different levels in the brain secondary to the reduced activity of two different parts of the reticular formation: a) cerebral hemispheres and rostral part of the reticular formation for the slow wave sleep, as witnessed by the EEG reduced cortical activity; b) spinal cord and caudal part of the reticular formation for the paradoxical sleep; cortical activity persists but muscular tonus is lost. This study about consciousness led to questions about << attention >> which is a state of << hyperconsciousness >>. We know almost nothing about its physiological mechanism. It obviously takes an important part in cognitive functions and some neurological and psychiatric diseases might stem from its malfunction.