The two-arousal hypothesis: Reticular formation and limbic system.

It is postulated that there are 2 major systems in the brain that maintain the ongoing behavior of the vertebrate organism. Arousal System 1 is related to the reticular activating system. It maintains the arousal of the organism, and provides the organization for responses. Arousal System II is related to the limbic system, and provides control of responses through incentive-related stimuli. The organization of these 2 mechanisms is postulated to be mutually inhibitory. It is shown how such an organization is of value in understanding reinforcement as part of a reciprocal relation between drive (Arousal System I) and incentive (Arousal System II). Memory is viewed as a consequence of this reciprocity. The present paper grew out of an attempt to understand the relation between two recent major findings in neurobiology. The first was described by Moruzzi and Magoun (1949) and concerned itself with a medial core primarily in midbrain and hindbrain whjch was capable of activating or arousing the cerebral cortex. This system has often been referred to as the Ascending Reticular Activating System (ARAS). The second finding concerned the discovery (Olds & Milner, 1954) of a series of points primarily within the forebrain which when stimulated prpduced effects similar to normal reward. The proposal offered here is that these two discoveries represent the description of two major brain mechanisms that are critical to the organism for the execution of appropriate behavior sequences. Because the two systems operate in such an intimate 1 The author wishes to thank Stanley A. Lorens and Benton J. Underwood for reading a preliminary draft of the manuscript, an4 tp acknowledge his debt to P. Milner, S. Glickman, and J. Olds. During the preparation of the manuscript the author was supported by United States Public Health Service contract MH11991. fashion, much confusion has occurred in the analysis of certain data related to these systems. It is the purpose of the paper to show that an analysis of brain function assuming two arousal systems may be of value in the future to help correct this confusion. A second purpose is to organize a sufficient amount of information to point out where empirical facts appear necessary, and to stress certain special issues which have arisen in considering the information relevant to the two-arousal hypothesis.