The ability to recognize individuals is crucial for establishing stable social relationships, including membership in a group, friendship, pair bonds, social status, dominance hierarchies, territorial networks, and numerous other aspects of social behavior and organization. These results are the first to use a rodent model to characterize neural circuits involved in the recognition of equally well known individuals and the corresponding emotional responses to them. In experiment 2, temporary inactivation of the CA1 region of anterior dorsal hippocampus by microinfusion of lidocaine eliminated the avoidance of the familiar winner, but a saline control injection had no effect. Immunohistochemistry for c-Fos and Egr-1 implicate several areas of the brain in individual recognition, particularly the anterior piriform cortex, the CA1 and CA3 regions of anterior dorsal hippocampus, anterior and posterior dentate gyrus, and perirhinal cortex. Subjects tested with the familiar winner avoided this stimulus male, but subjects tested with the familiar, neutral male were attracted to him.
One day later, losers of fights were tested in a Y-maze for reactions to one of the two familiar males. In experiment 1, we use a new method for studying true individual recognition in which male hamsters first had different experiences with two stimulus males (exposures to one male across a wire-mesh barrier and fights with another male). Most laboratory studies of individual recognition in rodents have studied differential responses to familiar versus unfamiliar individuals rather than differential responses to equally well known individuals having different significance for the subject. The ability to recognize individuals is essential for many aspects of social interaction and social organization, yet we know relatively little about the neural mechanisms underlying this ability.
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