frequencies (4-8 Hz; Figure 1C), consistent with theta evoked by fear recall, which peaks around 6 Hz (Figure 2A; Pape et al., 2005). Additionally, while slow gamma oscillations most often occurred on the trough or early ascending phase of the theta oscillation, fast gamma oscillations occurred closer to the peak or late ascending phase (Figure 1D). Notably, BLA spikes showed a different (though overlapping) pattern of phase-locking compared to gamma (Figure 1D, bottom histograms), suggesting that slow and fast gamma activity could be distinguished from spike-related transients that can contaminate high frequency signals (Ray and Maunsell, 2011). To further explore the nature of theta/gamma relationship, phase-phase coupling was quantified as previously described (Belluscio et al., 2012). Within the BLA, there was significant n:m phase-phase coupling of both slow and fast gamma oscillations to the theta oscillation (Figure S3A-C). This analysis predicts that 9 slow and 15 fast gamma cycles occurred per full theta cycle (Figure S3B), consistent with coupling of a ~55 Hz and ~90 Hz oscillation with a 6 Hz theta oscillation. This analysis strongly supports that both slow and fast gamma represent genuine oscillations, as it would be unlikely for non-oscillatory signals to exhibit phase-phase coupling patterns.
