In HPC and cortex, lower frequency activity organizes higher frequency oscillations. Given the prominent fear-evoked theta oscillation in the BLA, we investigated whether the observed BLA gamma oscillations were coupled to theta. We examined both phase-amplitude coupling, where gamma power changes differentially with theta phase, and phase-phase coupling, where a fixed number of gamma cycles occur per theta cycle (Belluscio et al., 2012; Lisman and Buzsáki, 2008). Phase-amplitude coupling was examined using a comodugram to determine the extent by which high frequency (30-150 Hz) power was modulated by low frequency (0-30 Hz) phase (Tort et al., 2009), which we quantified with the mean resultant length (MRL), a measure of circular unimodality (higher MRL indicates that power peaks more reliably at a particular phase). We found that both slow and fast gamma bands were strongly coupled with oscillations in low theta 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
