Selected epochs were converted to ASCII files for further analysis with DIGEEGXP software, developed in our department (CJS). Five selected epochs per patient were filtered in the broad frequency band (0,5–48 Hz). The phase lag index (PLI) was used to calculate functional connectivity of the selected epochs [23]. The PLI measures the asymmetry of the distribution of instantaneous phase differences between different EEG signals. The PLI rules out volume conduction as a confounding factor, since the presence of a consistent, nonzero phase lag between two time series cannot be explained by volume conduction alone. The PLI is therefore less effected by common sources, while performing at least as well as the synchronization likelihood (SL) [24] in detecting true synchronization. The PLI ranges between 0 and 1. A PLI of more than 0 indicates phase locking to a certain extent, whereas a PLI value of 0 indicates no coupling or coupling with a phase difference centered around 0±π radians. For a detailed description of PLI calculation, see Stam [23]. The PLI was calculated between all channels, resulting in a 20 by
