Behavioral and EEG changes following chronic brain stem lesions in the cat.

Abstract The effect upon behavior and the EEG of chronic lesions in the rostral brain stem has been studied in a series of cats. Of lesions in sensory paths, in the periaqueductal grey, or in the midbrain tegmentum and basal diencephalon, only the latter, in a position to interrupt the ascending reticular activating system, were followed by chronic somnolence and EEG synchrony. Activation of the synchronized EEG by somatic and auditory stimulation was still possible either after the reticular activating system or after long sensory paths had been interrupted immediately behind the thalamus. Collateral sensory excitation of the reticular activating system in the lower brain stem, and the direct arrival of afferent impulses at some site above the midbrain, thus serve equally well to induce EEG arousal, but only in the former instance does prolonged wakefulness follow. Poverty of behavior after lesions of the reticular activating system suggests that its influences may be directed both cephalically to activate the EEG and caudally to facilitate motor activity, in maintaining the waking state.