The contributions of three key limbic system structures-the amygdala, the hippocampus, and the septum-to human and animal behavior are analyzed, focusing primarily on learning and motivational variables. The amygdala is assumed to be part of a stimulus-analyzing system involved in the processing of reinforcers and salient cues. This system contributes to the formation of conditioned reinforcers and, thus, to the ability of arbitrary cues to control species specific behavior. The hippocampus is assumed to be part of a different stimulus-analyzing system which processes nonsalient stimuli. It habituates all stimuli not associated with reinforcement, this process occurring on any nonreinforced presentation. However, those nonsalient stimuli followed by reinforcement are registered as secondary cues. These cues are devoid of conditioned reinforcing properties. The septum is viewed as a coordinating structure that handles the flow of information between these two systems. The principles of operation of these structures are based on neurological and behavioral data from both animals and humans.

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