Responses to affective stimuli in monkeys with entorhinal or perirhinal cortex lesions.

Recent efforts to define the functions of the primate rhinal (entorhinal and perirhinal) cortical areas have focused on their interaction with the hippocampus in the mediation of normal memory. Less is known on the functional meaning of their strong connections to the amygdala, a key substrate for emotion. A previous study (Meunier and Bachevalier, 2002) showed evidence that complete rhinal ablations yield changes in monkeys' behavioral responses to affectively salient stimuli. Here, we studied monkeys with separate entorhinal or perirhinal ablations in the same paradigm, where responses were triggered by four stimuli: an unfamiliar human, a conspecific stimulus, a toy snake, and a familiar (generally rewarded) junk object. The two separate lesions produced similar changes, and each replicated the effects of complete rhinal lesions (i.e., attenuated affiliation and enhanced defense). Failure to modulate responses based on previous experience (i.e., memory difficulties) may explain these affective changes. This interpretation does not account, however, for the sparing of some memory-dependent modulations of defense, nor for the lack of correlation between the animals' affective changes and their own recognition memory performance. Alternatively, rhinal damage may introduce a negative bias in the risk assessment of affectively salient stimuli, a proposal more compatible with Gray and McNaughton's (2000) anxiety-centered view of medial temporal functions, than with prominent mnemonic/perceptual functional models of the hippocampal/rhinal duo. Reconciling the two perspectives may improve our understanding of rhinal functions.