First Articulation of the Concept of a Reticular Activating System

In 1949, building upon the work of Berger, Bremer, and others, Moruzzi and Magoun (1949), noted that electrical stimulation of the basal diencephalon and anterior midbrain resulted in physiological and behavioral activation in cats. This led to the hypothesis of the RAS as a group of structures and pathways necessary for "waking up," including the mental activity of dreaming during sleep. Moruzzi and Magoun noted Berger's observation that the transition from sleep to wakefulness correlated with a change in the EEG from high-voltage slow waves to lower voltage fast activity (alpha blockade). These EEG changes occurred with any afferent stimulation that produced increasing alertness. Several earlier investigators had stimulated various sites in the ventral dien-cephalon, midbrain (including periaqueductal gray (PAG)) and pons, leading to cortical activation, but until the articulation of the RAS concept, there was no integrated theory for how this transformation occurred, either physiologically or anatomically. Moruzzi and Magoun extended and integrated multiple findings around parameters of cortical activation and alertness and in so doing repudiated earlier assumptions that alpha blockade resulted from afferent stimulation directly to the cerebral cortex. In addition, Moruzzi and Magoun noted that when an activation pattern was induced in the cortex, the pattern was not constrained to the sensory cortex of appropriate modality, and the corresponding area of the sensory cortex was not the first to be activated. "Whether somatic, auditory, or, to a lesser extent, visual stimulation was employed, when an arousal reaction was evoked, it appeared simultaneously in all parts of the cortex, and often continued for considerable periods in it after afferent stimulation had ceased" (Moruzzi and Magoun, 1949, p. 469). Moruzzi and Magoun, through stimulating the RAS but avoiding any sensory afferents, achieved cortical activation. Data on brain lesioning studies from Lindsley et al. (1950), as well as their own barbiturate studies, led Moruzzi and Magoun to conclude that arousal begins with the RAS:

The conception of sleep as a functional deafferentation of the cerebrum is not opposed by this evidence if the term "deafferentation" is broadened to include interruption of the ascending influence of the brain stem reticular activating system, the contribution of which to wakefulness now seems more important than that conducted to the cortex over classical sensory paths. (1949, p. 471)

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