EEG Recording & Processing has been detailed previously19. Briefly, resting (eyes-closed) EEG was recorded for 4.25 min (256 seconds) at a rate of 256 samples/second; the entire time-series was analyzed by Fourier transform methods. EEG procedures were identical at all collection sites. This study examined eight EEG inter- and intra- hemispheric parietally coupled coherence phenotypes (depicted in Supplemental Figure 5; namely, theta (low theta: 3-5 Hz and high theta: 5-7 Hz) between interhemispheric parietal-occipital bipolar pairs (P3-O1—P4-O2), neighboring interhemispheric centro-parietal (C4-P3—C3-P3) bipolar pairs, and right and left intrahemispheric fronto-parietal (P8-P4—F8-F4, P7-P3—F7-F3) bipolar pairs. Because of differences in low (3-5 Hz) and high (5-7 Hz) theta bands suggested in this study (correlations ranging from 0.60-0.80), and by previous work in COGA34 and other studies 35-37, we examined low and high theta bands separately to reduce phenotypic heterogeneity. Data on the full or combined theta band (3-7 Hz) is presented in Supplemental Materials. Use of bipolar electrode pairs effectively results in a low resolution spatial filter that reduces volume conduction effects38. Further details regarding the EEG coherence data processing has been detailed previously5 and is provided in the Supplemental Material (EEG Coherence Recording & Data Reduction).
