Functional connectivity between intra-cortical sources is computed using lagged phase synchronization, which is less susceptible to volume conduction artifacts [88]. As explained by Canuet et al. [83], lagged phase synchronization represents nonlinear functional connectivity between any two signals reflecting phase similarity or synchrony. This measure is calculated for each frequency band as a normalized Fourier transform. Further, this method does not have artifactual zero-lag contribution (noise) but contains only the physiological signals of interest, because the instantaneous component of the total connectivity which is considered to be the confounding factor of volume conduction is statistically partialled out in this method [83]. This lagged phase coherence between two brain regions reflects neuronal communication between these regions, reflecting coherent, synchronous oscillatory activity between the sources [59]. According to Pascual-Marqui [88], the instantaneous coherence is the real part of the complex valued coherency and the imaginary part of the complex valued coherency is more affected by instantaneous dependence, whereas the lagged coherence, which is employed in eLORETA, is devoid of the confounding effect of instantaneous dependence due to volume conduction and low spatial
