It is tempting to conclude that refining an endophenotype may help to identify specific variants. The P300 clearly represents activity from different neural sources that is partially overlapping in time, which is projected to the surface of the scalp. Its amplitude is determined to a significant degree by activity in specific frequency ranges, especially delta and theta (Karakas, Erzengin, & Basar, 2000; Kolev, Demiralp, Yordanova, Ademoglu, & Isoglu-Alkaç, 1997). One might think that time-frequency representations of P300-related activity yield candidate endophenotypes that are more fundamental in some way than the P300 and thus more sensitive to genetic effects (i.e., associated with larger effects). We therefore conducted a follow-up investigation in the same sample to examine power and inter-trial phase locking, a measure of consistency of the brain response across trials, of delta and theta activity in the P300 window (Malone, McGue, & Iacono, 2016). Although we obtained one genome-wide significant association, it has no obvious connection with brain activity, and unless replicated, it cannot be considered meaningful. Thus, decomposing P300 into simpler time-frequency components did not produce a fundamentally different
