where f1and f2 are two frequencies whose ratio is given by the integers n and m, i.e., n f1 = m f2, and θi(f, t) denotes the instantaneous phase signal. A n:m coupling between two signals occurs when the mean resultant length of Φijn,m is non-vanishing (Tass et al., 1998). This can be evaluated, e.g., through the n:m phase-locking-value (PLVn:m) which is defined as
