Differential involvement of amygdala and hippocampus in responding to novel objects and contexts

Different medial temporal lobe structures are involved in memory for different types of novel cues and novel relationships among familiar cues. We measured the behavior of rats with amygdala or hippocampal damage, when confronted with novelty in an incidental learning paradigm. We examined both direct and indirect measures of memory. Following habituation to an environment, proximal objects or distal cues were manipulated in several ways. We found that rats with hippocampal damage exhibited a deficit on direct measures of memory, but performed normally on all indirect measures. Rats with amygdala damage exhibited a deficit on a direct measure, and performed normally on an indirect measure, of memory for proximal object identity. Thus, the hippocampus may be necessary for success on direct measures of memory for distal cues and proximal objects and the relationships among them. Likewise, the amygdala may be necessary for success on some direct measures of memory, such as memory for aspects of proximal object identity. Neither the amygdala nor the hippocampus functions as a generalized novelty detection system. To the extent that we tap implicit and explicit knowledge using this paradigm, we suggest that in the rat, the amygdala and hippocampal systems are necessary for at least some types of explicit knowledge.