Theta-fast gamma coupling occurred most strongly when the instantaneous frequency of theta was 6 Hz (Figure 2D), consistent with the peak theta frequency observed during aversive tone presentations (Figure 2A). This 6 Hz coupling peak was absent during pretone. In contrast, slow gamma oscillations were maximally coupled with theta in the 9-10 Hz range, suggesting that slow gamma is relatively insensitive to the fear-evoked 6 Hz oscillation (Figure 2E). There was also significantly greater enhancement in theta-fast gamma coupling from pretone to the CS+ (Figure 2E, right; p=.014, sign-rank), compared to that seen for slow gamma, although both increases were significantly different from pretone (fast gamma, p=1.57 × 10−5; slow gamma, p=.011). For both of these reasons, we focused on the fast gamma oscillation as it relates to fear learning for further analysis.
