We questioned whether increased theta-gamma coupling was indeed due to enhanced organization of gamma frequency activity by theta or if this was spurious coupling due to independent changes in the theta and gamma range that were each phase-locked to the pip. . To rule out this possibility, we performed two analyses. First, we compared our results to a shift predictor obtained by shifting the gamma power relative to the theta phase by × seconds (where × is an integer value between 1 and 30) so that pip onsets were still aligned. The shift predictor had no strong patterns of theta-gamma coupling (Figure 2B) and thus a significantly weaker strength of theta-gamma phase coupling than the CS+ (Figure 2C; p<.05/21, sign-rank). Second, pip-evoked responses (0-300 ms from pip onset) were removed and theta-gamma coupling calculated without these segments; this did not diminish the strength of coupling for either gamma range (data not shown). Thus, the enhanced theta-gamma coupling during the CS+ is not an artifact of pip structure, but instead suggests a fundamental role for fear-related theta oscillations in organizing high frequency neuronal activity that outlasts the pip.
