TABLE 1. Overview of the utility of selected candidate electrophysiological intermediate phenotypes in ADHD.

This table summarises the findings following a systematic review of the literature, described in this paper. Databases Pubmed, Ovid MEDLINE, PsycINFO and EMBASE were searched using combinations of the key words EEG, ERP, electrophysiology, heritability, twin, family, endophenotype, genetic, ADHD within each selected domain. The computer search was supplemented with bibliographic cross-referencing.

Intermediate phenotype	Association with ADHD	Heritability	Familial/genetic overlap with ADHD	Genetic associations with ADHD risk variants
EEG power	↑ theta ↓beta ↑theta-beta ratio	High	High familial correlations for theta and beta in multiply affected families (not unaffected relatives)	SLC6A3 DRD4
	Partially inconsistent	Consistent	Limited	Consistent
EEG power: very low frequency fluctuations	↓power at rest ↓rest-task attenuation	Unknown	Unknown	Unknown
	Limited
EEG: coherence and connectivity	↑ inter- and intrahemispheric coherence	Moderate	↑ alpha asymmetry in first-degree relatives indicating familial risk factor	COMT Serotonin 1A receptor
	Partially inconsistent	Partially inconsistent	Limited	Limited
ERP: Inhibitory and attentional processing	↓ go-P3 amplitude ↓ no-go-P3 amplitude ↓ cue-P3 ↓ CNV ↓ no-go-anteriorisation ↓ N2	Moderate	↓ P3 not familial or associated with familial risk for broader externalising conditions ↓ no-go-P3, cue-P3 and CNV demonstrate familial association with ADHD	SLC6A3 DRD2 DRD4 COMT TPH2
	Partially inconsistent	Consistent	Partially inconsistent	Consistent
ERP: Performance monitoring	↓ error negativity ↓ error positivity ↓ N2	Moderate	↓ error negativity ↓ error positivity ↓ N2 demonstrated in non-affected first-degree relatives	SLC6A3 DRD4 COMT 5-HTTLPR
	Inconsistent	Limited	Limited	Consistent

Key: Unknown: Genetically sensitive designs are required to confirm utility.

Inconsistent: Multiple parameters/studies within the domain demonstrate non-replication. Confounding effects of sample and paradigm differences and presence of subtypes and comorbid conditions must be explored.

Partially inconsistent: A small number of parameters/studies within the domain demonstrate non-replication. Confounding effects of sample and paradigm differences and presence of subtypes and comorbid conditions must be explored.

Limited: Promising consistency but further replication is required due to a limited number of studies.

Consistent: To date fulfils this criterion for a potential intermediate phenotype of ADHD but further work in ADHD samples is required.