There are several limitations of previous quantitative EEG studies of BD. One is the reliance on measures of EEG power over a frequency band as the primary unit of analysis. In addition, correlation and coherence among electrode sites are limited to a single time period and may be heavily influenced by volume conduction effects (Nunez et al., 1997). Recently, the measure of synchronization likelihood (SL) has been used to capture the spatio-temporal interactions among electrodes. SL is a novel signal analysis technique that is appropriate for characterizing both linear and nonlinear interdependencies between time series (Stam and van Dijk, 2002) as measured by EEG (Stam et al., 2002, 2003a,b). Hence, it expresses the synchronization of oscillatory activity of each EEG electrode to the other electrodes by measuring the extent to which each pair of electrodes share self-similarity, independent of frequency. When averaged over time, SL produces an index of global synchronization strength with the requisite high temporal resolution to detect rapid fluctuations in synchronization and desynchronization (Montez et al., 2006). Our previous study demonstrated that SL in the resting EEG can detect the fast changing and topologically coherent fluctuations in functional networks in healthy adults (Betzel et al., 2012).
