Differential fear conditioning induces reciprocal changes in the sensory responses of lateral amygdala neurons to the CS+ and CS-

In classical fear conditioning, a neutral sensory stimulus (CS) acquires th e ability to elicit fear responses after pairing to a noxious unconditioned stimulus (US). As amygdala lesions prevent the acquisition of fear respons es and the lateral amygdaloid (LA) nucleus is the main input station of the amygdala for auditory efferents, the effect of auditory fear conditioning on the sensory responsiveness of LA neurons has been examined. Although con ditioning was shown to increase CS-evoked Lh responses, the specificity of the changes in responsiveness was not tested. Because conditioning might in duce nonspecific increases in LA responses to auditory afferents, we re-exa mined this issue in conscious, head-restrained cars using a differential co nditioning paradigm where only one of two tones (CS+ but not CS-) was paire d to the US. Differential conditioning increased unit and field responses t o the CS+, whereas responses to the CS- decreased. Such changes have never been observed in the amygdala except in cases where the CS- had been paired to the US before and fear responses not extinguished. This suggests that f ear conditioning is not only accompanied by potentiation of amygdalopetal p athways conveying the CS+ but also by the depression of sensory inputs unpa ired to noxious stimuli.