We analyzed log-transformed power in each of the four frequency bands as well as total power at Cz. Bipolar leads act as a spatial high pass filter and minimize contributions from far sources, which makes our two bipolar leads particularly useful for measuring alpha activity arising from occipital-parietal areas. Log-transformed alpha power estimates and peak alpha frequency estimates from the bipolar leads were highly correlated (r = .94 for both). We therefore averaged them into a single measure of alpha power and alpha peak frequency, respectively. There were 3,966 subjects with occipital-parietal data and 3,948 with data from Cz. We used as covariates in all analyses generation (parent or adolescent twin), gender, and chronological age. We also included a dummy variable representing recording system (Biosemi or Grass) and the 10 PCs from EIGENSTRAT as covariates (cf. Iacono et al., 2014).
