Phase-amplitude coupling. (A) A signal with within-area phase-amplitude coupling. (B ) A low-pass filtered version of signal A isolates the slow-wave envelope (f_p). (C ) A bandpass filtered version of signal A isolates the faster oscillation (f_A). Phase-amplitude coupling is evident in that the amplitude of f_A is largest during one phase of f_p, the trough. (D ) This can be clearly seen in the amplitude distribution, where the average amplitude of f_A is plotted as a function of the phase angle of f_p. These data demonstrate a large deviation from the uniform distribution (shown as the dotted line).