Other measures of spatial connectivity can provide additional information about spatial synchronization of EEG oscillations. For example, synchronization likelihood (SL) provides a measure of the statistical relationships between two time series that is sensitive to both linear and non-linear interdependencies of the signal and unbiased after narrow band filtering (Stam, 2005). It is presumed that this measure reflects functional coupling between spatially distributed oscillating neuronal assemblies. Using a large sample of adult twins and extended twin families, Posthuma et al. (2005) found that SL is moderately to highly heritable (33–70%) especially in the alpha frequency range (8–13 Hz). Other studies by the same group (Smit et al., 2010; Smit et al., 2008) extended these findings to graph-theory based measures derived from synchronization likelihood. Two global network parameters, clustering coefficient (the proportion of interconnected local nodes) and average path length (average number of steps from a network node to other nodes) showed substantial heritabilities (38 and 46% on the average, respectively). Thus, the extent to which individual brain dynamics approaches the properties of the “small-world” network, i.e. combining modularity and global information integration, appears to be influenced by genetic factors.
