The first three PCs (PC1-PC3) computed from SNPRelate42 and genotype array, birth cohort (prior to 1930, 1930-1949, 1950-1969, and 1970 and after) were also included as covariates to reduce the risk of false-positives owing to population stratification. Established thresholds for genomewide significance (p< 5 x 10−8) were used. Genome-wide Complex Trait Analysis (GCTA)47 was used to determine SNP heritability of parietal coupled theta EEG coherences in the analytic sample. We performed MAGMA48 competitive gene-set analyses using the summary statistics from the GWAS of theta EEG coherence using FUMA v1.2.8 (Functional Mapping and Annotation of Genome-Wide Association Studies). The Genotype-Tissue Expression (GTEx v649) database was used to obtain gene expression levels in 10 brain tissues. In addition, we examined the most significantly associated variant for low theta EEG using HUGIn (Hi-C Unifying Genomic Interrogator50), assessing DNA-DNA interactions which can localize a GWAS finding to a specific gene that may be nearby or hundreds of kb away using data generated by Schmitt et al.51 Search parameters included the two available brain tissues (dorsolateral prefrontal cortex and hippocampus) in neural progenitor and embryonic stem cell lines.
