Facilitation réticulaire des mécanismes visuels corticaux

Abstract

1. 1. A reticular or sensory arousing stimulation produces an important increase of the amplitude and a shortening of the latencies of all the positive and negative components of the cortical visual evoked by an electric shock in the optic chiasma. 2. 2. The increase in amplitude is the consequence of facilitatory effects acting differentially at the thalamic and the successive intracortical stages of the visual pathway. The classical curves of facilitation suggest the phenomenon, but to prove the differential facilitations, a graphical method of analysis has been elaborated. This methode eliminates the errors introduced by the measure of the absolute amplitude of the cortical components of the visual response and takes into account the part of the increase resulting from thalamic facilitation. 3. 3. The simultaneous recordings of the EEG and of the cortical evoked visual potential have made it possible to determine the levels and variations of vigilance for which this facilitation appears; the most favorable conditions are the crossing from sleep to wakefulness. A detailed study of the first 300 msec. following an arousing stimulation (corresponding to the classical Pavlov's orientating reaction) has shown very drastic facilitations during this period and especially when the arousing stimulus occurs in a sleepy preparation. 4. 4. A comparative study of specific facilitation (introduced by diffuse light) and non-specific facilitation (reticular stimulation) has been performed. The graphical analysis has shown that the facilitation by diffuse light is mainly cortical and not specially thalamic as originally proposed by Chang. The specific and non-specific facilitations can potentiate each other. 5. 5. In a discussion, it has been proposed that reticular ascending influences can produce this facilitations through three kinds of mechanisms: local subliminal depolarisations of the soma and basal dendrites, depolarisations of apical dendrites acting back on their soma by catecholamine effects and finally the effects of the spontaneous activity (created by the same ascending influences) on these depolarisations. This built-up subliminal excitatory state may have many other consequences besides the facilitation described in this paper. 6. 6. From a detailed study of the correlations between facilitation and electro-cortical activity, it appears that these two phenomena occur generally simultaneously. However their discussion can be observed in certain conditions, or artificially introduced; a facilitation without concomitant cortical activation is observed after slight cortical depression (i.e., introduced by slight anesthesia); an activation without facilitation is observed when intense cortical arousal takes place in an already awake preparation; prolonged activation also suppresses the facilitation. In consequence, no direct primary correlations can exist between sensory phenomena (perception) and brain waves. If there are correlations, they can only be secondary, due to the actions of brain waves on the facilitatory mechanisms. Between the two main consequences produced at the cortical level by the reticular ascending influences, the maintenance of a state of subliminal excitation facilitating the cortical mechanisms is the essential factor for the cortex in a waking state while the spontaneous activity of the cortex is only secondary in importance, or may even be lacking.