Amygdala, hippocampus and discriminative fear conditioning to context.

Various measures of fear have been shown to condition to a fearful context with different acquisition rates (Antoniadis EA, McDonald RJ. Fear conditioning to context expressed by multiple measures of fear in the rat, Behav Brain Res 1999;101(1):1-14). Freezing, locomotion, urination and preference are 'fast' measures of fear in that they discriminatively condition to context after a single training session, while ultrasonic vocalizations and defecation are 'slow' measures of fear given that they condition following three training sessions. In the present experiment we sought to assess the contribution of the amygdala and the hippocampus in this form of learning. Existing views differ on the degree of involvement of each memory structure. This discord probably emerges from the common use of non-discriminative paradigms and the assessment of a single measure of fear. With the use of a discriminative paradigm and the assessment of multiple measures of fear, results indicate that the amygdala is a memory structure that selectively mediates the conditioning of heart rate, and the hippocampus selectively mediates the conditioning of defecation and body temperature. The conditioning of preference, locomotion, freezing and ultrasonic vocalizations, necessitate the participation of both memory structures while the conditioning of urination does not seem to require the participation of either the hippocampus or the amygdala. The proposed view ascribes an equal role in fear conditioning to both the amygdala and the hippocampus.