Imaginary coherence (ImC) was used to analyze FC. ImC, one of the most widely used methods for measuring FC in neuroscience research, can determine the degree of phase coherence between two signals measured at different electrode sites (Nolte et al., 2004; Hassan et al., 2014). The advantage of ImC is that it reduces the volume conduction problem, in that the brain activity at one source can be measured at multiple electrodes. Thus, ImC allows for observation of the actual brain activity, except for artifacts caused by volume conduction (Nolte et al., 2004). In this study, resting EEG activity was filtered by theta, lower-alpha, and upper alpha bands, and then ImC values were calculated using the following equations. First, if the Fourier transform values of two electrodes i and j for frequency f are xi(f) and xj(f), respectively, the cross-spectrum is defined as follows:
