Lesions of the amygdala do not affect the enhancement of the acoustic startle response by background noise.

The acoustic startle response (ASR) is enhanced in the presence of loud background noise. We examined whether or not this increase of response strength is mediated by the amygdala, which is known to be involved in various phenomena of enhancement of the ASR. To achieve this aim, we tested whether or not amygdaloid lesions with the excitotoxin N-methyl-D-aspartate (NMDA) would abolish the enhancement of the ASR by background noise in 13 Wistar rats. Loss of foot-shock sensitization in these rats, as well as histological evaluation, proved the successful destruction of the amygdala. However, the enhancement by background noise of the ASR, which was observed in sham-operated controls, was not affected in amygdala-lesioned rats. Therefore, we conclude that the background noise facilitation does not involve emotional components that are mediated by the amygdala. On the basis of these findings, we differentiate between the startle-enhancing effect of background noise and the amygdala-mediated effect of foot shocks on the ASR.