INVOLVEMENT OF SUBCORTICAL AND CORTICAL AFFERENTS TO THE LATERAL NUCLEUS OF THE AMYGDALA IN FEAR CONDITIONING MEASURED WITH FEAR-POTENTIATED STARTLE IN RATS TRAINED CONCURRENTLY WITH AUDITORY AND VISUAL CONDITIONED-STIMULI

The goal of this work was to test the involvement, in fear conditionin g, of afferents to the lateral nucleus of the amygdala originating fro m the auditory thalamus, auditory cortex, and perirhinal area. The aco ustic startle reflex was used as the behavioral index of conditioning because it is reliably enhanced in the presence of a conditioned stimu lus (CS) previously paired with a footshock. Auditory and visual CSs w ere used to assess the modality specificity of lesions to the above br ain areas. Pre- or posttraining lesions of the entire auditory thalamu s including the ventral, dorsal, and medial divisions of the medial ge niculate body, the posterior intralaminar nucleus, and the supragenicu late nucleus, completely blocked fear-potentiated startle to the audit ory CS, but had no effect on fear-potentiated startle to the visual CS . Posttraining lesions mostly restricted to the ventral and dorsal div isions of the medial geniculate body specifically disrupted fear-poten tiated startle to the auditory CS. However, retraining in rats sustain ing ventral and dorsal medial geniculate body lesions led to reliable fear-potentiated startle to the auditory CS. Posttraining lesions most ly restricted to the medial division of the medial geniculate body, po sterior intralaminar, and suprageniculate nuclei did not disrupt fear- potentiated startle. These results indicate that the auditory thalamus is specifically involved, through either its direct or indirect amygd aloid afferents, in fear conditioning to auditory CSs. Pre- or posttra ining lesions mostly restricted to the primary auditory cortex did not reliably attenuate fear-potentiated startle to the auditory or visual CSs. Extensive posttraining lesions of the perirhinal area (including secondary auditory cortices), but not its rostral aspect alone, block ed fear-potentiated startle to both CSs. However, reliable potentiated startle was observed to both CSs following similarly extensive pretra ining lesions of the perirhinal area. Because post- but not pretrainin g lesions of the perirhinal area blocked fear-potentiated startle nons pecifically, at least with regard to auditory and visual CSs, the resu lts are consistent with the involvement of the perirhinal area in gene ral memory functions such as information storage or retrieval. Alterna tively, the secondary auditory and/or perirhinal cortices might functi on as multimodal sensory relays to the amygdala.