Amplitude envelope correlation detects coupling among incoherent brain signals

Cognitive processing involves γ-activation over broad cortical regions. Phase coupling of these activities has rarely been reported for areas far apart. Other forms of coupling are generally not detected by conventional measures. Here, we use amplitude envelope correlation (AEC), which can detect signal coupling without phase coherence, even among different frequencies. We apply it to subdural recordings from humans performing a visual delayed match-to-sample task and systematically compare it with spectral amplitude and coherence. The different measures often show divergent results. In particular, AEC reveals γ-coupling completely missed by coherence. We argue that coherence and AEC are adapted to different cortical mechanisms of short- and long-range interactions, respectively.