where df and F denote an incremental step in the frequency domain and the frequency band of interest, respectively. MPSI solely detects the directionality of phase-lagged coupling; a positive value of MPSI indicates that the vector-signal I leads the vector-signal J, while a negative value indicates the opposite. Moreover, similarly to MIM, MPSI is invariant under invertible and static linear transformations, and thus it is independent on rotations of the physical coordinate system of MEG source space (Basti et al., 2018). In the case of two univariate time series, MPSI coincides with PSI, apart from a normalization factor.
