We expected that enhanced theta power and theta-gamma coupling during fear should be accompanied by an increase in the strength of gamma oscillations, as has been reported in the auditory cortex (Headley and Weinberger, 2013). Contrary to our expectation, however, BLA fast gamma power was lower during the CS+ than the CS− in discriminators (Figure 3A-B; n=14). In generalizers, there was no significant difference (Figure 3B; n=9). This relationship between relative fear and gamma power also held true when considering discrimination on a continuous basis: the greater the discrimination between CS+ vs CS−, the greater the difference in fast gamma power (Figure 3C; r=0.48, p<.05). The power change was specific to the fast gamma oscillation, as there was no fear-related difference in power in the slow gamma range (Figure S4A; p>.05, sign-rank). There was also no difference in multi-unit firing rate (Figure S4B; p>.05, sign-rank) or power in higher frequency spectral components (150-800 Hz; p>.05, sign-rank), which followed firing rate changes closely (Figure S5C-D), suggesting that the change in fast gamma power did not reflect spike contamination. These data raise the possibility that activity in the fast gamma range may reflect a novel safety-related signature in the amygdala.
