Our study has shown that resting (eyes closed) EEG power is heritable in the Plains Indian dataset: 0.4–0.6 for alpha, 0.3 to 0.4 for theta and 0.3 for beta. Heritability is approximately the same at all scalp locations. We also demonstrated that EEG power is stable, at least over a two year period. Our results are consistent with earlier studies 
and demonstrate the stability of EEG phenotypes.
We performed multipoint linkage analysis with dense marker coverage over all autosomes: nearly 4000 unlinked SNPs at an average distance of 1 cM. The convergence of linkage peaks on chr 5 for alpha and beta EEG power produced LOD scores (3.53 and 3.48 respectively) that exceed the threshold LOD score (3.3) proposed by Lander and Kruglyak 
for genome-wide significance in fine-mapped linkage scans of human pedigrees. Within this convergence of peaks there was also a suggestive linkage finding for theta EEG power (LOD
2.2). Our convergent findings on chr 5 suggest a common genetic origin for alpha, beta and theta EEG power. Indeed, we expected that a substantial portion of the genetic variance would be attributable to a common source since we have shown that the genetic correlation between alpha, beta and theta power is high: approximately 0.75. This finding is supported by earlier work 
. Further analysis revealed that CRH-BP
, located at the apex of the convergent linkage peaks, was significantly associated with alpha power in two independent populations. In addition there was a non-significant trend for association in the same direction for theta and beta power. Although the findings were not as strong in the U.S. Caucasians as in the Plains Indians, the effect sizes were very similar (1.7 and 1.8, respectively) but the sample sizes were very different (188 and 310, respectively) suggesting that the U.S. Caucasian study was under-powered.
The aim of this study was to identify candidate genes for addiction and anxiety but also candidate genes that might be implicated in the generation of the EEG itself. Transmembrane currents originating in parietal/occipital regions for alpha and the hippocampus and limbic system for theta, are responsible for the magnitude of the recorded field. The rhythm generator for alpha originates in the thalamus 
and in the hippocampus for theta 
although the thalamus can act as an independent pacemaker for both alpha and theta rhythms 
. Cholinergic and GABAergic afferents contribute to hippocampal theta activity 
. The beta rhythm is thought to be mediated by GABAA
. Therefore it seems likely that numerous genes, other than CRH-BP,
may have an influence on the EEG.
In our study, CRH-BP
variation showed the same direction of relationship to alpha power in two independent populations. Moreover, CRH-BP
variation was significantly associated with AUD in the U.S. Caucasians and anxiety disorders in the Plains Indians. CRH-BP
codes for a high affinity binding protein for corticotrophin releasing hormone (CRH), the primary mediator of the mammalian neuroendocrine and behavioral response to stress. In humans, CRH-BP is widely distributed throughout the body and is found in several brain regions including the cerebral cortex, the hippocampus, amygdala, lateral septal nucleus and a variety of midbrain structures 
. Although its function in the brain is largely unknown, CRH-BP is thought to modulate CRH activity because a large proportion (65–90%) of total CRH is complexed with CRH-BP and is therefore unavailable for receptor activation 
. Centrally administered CRH increases anxiety-like behavior in rats across a wide range of paradigms 
. Although CRH-BP deficient mice have normal baseline and stress-exposed HPA axis function they show increased anxiety-like behavior indicating an important role for CRH-BP in the CNS extra-hypothalamic stress response system 
. CRH-BP located in the ventral tegmental area (VTA) has been shown to modulate the effects of CRH on stress-induced relapse to drug abuse 
. Thus CRH-BP
is a logical candidate gene for anxiety and addiction.
Although the relationship of CRH-BP to EEG is unknown, a relationship of CRH to EEG has been established in animal models, including mechanism of effect. CRH is known to increase the firing rate of LC neurons and to influence global forebrain EEG activity 
. LC activation influences the firing properties of thalamic neurons that are implicated in the generation of the alpha rhythm 
and induces fronto-cortical and hippocampal EEG changes in rats 
. A logical possibility for the influence of CRH-BP
on the EEG might involve CRH binding in the locus coeruleus (LC) and VTA (although CRH-BP
mRNA has not been detected in rat LC and it is not known if it is present in human LC 
). CRH-BP may be required for CRH to potentiate NMDAR-mediated excitatory postsynaptic currents in the VTA that herald the switch from regular firing to burst firing in dopamine neurons (41). The VTA sends dopamine containing projections to the LC that influence LC neuronal activation 
. One could therefore speculate that VTA CRH-BP
variation might influence the resting EEG via the VTA dopamine–LC–thalamic neuronal oscillator pathway.
There is no evidence that any of the CRH-BP
SNPs are functional. Using HapMap data it can be seen that CRH-BP
is buffered from adjacent genes by several haplotype blocks indicating that a functional locus is likely to reside within CRH-BP
or its environs. One mechanism for functional differences may be a second CRH-BP
isoform that has been identified in brain. CRH-BP is highly conserved across vertebrates and in humans encodes a 322 amino acid protein that contains 5 disulphide bonds that are essential for CRH binding (). This change in peptide sequence might affect protein folding and stability that might alter CRH binding affinity. Indeed, a C-terminus truncated peptide that shows conformational changes has been detected in the plasma of patients with inflammatory disease 
. Additionally there are regions of high conservation located within both the 3′-UTR and the introns of this novel 3′-region of the gene which may influence alternative exon usage or mRNA folding (). The cross-species conservation is suggestive of function and may explain why we found significant associations with anxiety, alcoholism and alpha EEG power within a haplotype block that extends from intron 5 beyond the previously identified end of the gene and which encompasses the region that contains the exons encoding this novel isoform ().
CRH-BP Peptide Sequence Conservation.
A COGA study found significant linkage of bipolar derivations of beta EEG power to a region of chr 4 (64.7–75.5 cM) that includes the GABAA
receptor complex (65.7–66.1 cM) (Figure S1
. Using the same dataset, another study found significant linkage peaks for beta on chrs 1, 4, 5 and 15 
. One early study using only 95 markers found linkage for low voltage alpha EEG on chromosome 20q and noted evidence for genetic heterogeneity 
. We did not replicate this finding made in German Caucasians. Our study also identified suggestive linkage peaks for EEG power on chr 4 but for theta and alpha and not beta, and at 40 cM–48 cM; i.e. nowhere near the GABAA
receptor complex. Moreover COGA's chr 5 linkage peak at 233 cM was nowhere near ours, nor did we identify linkage peaks on chrs 1 and 15. Also, COGA found no evidence for linkage for alpha or theta EEG power. Reasons for a lack of comparability between our and the COGA study may include differences in montage (we used the more common monopolar montage), derivation of EEG spectral power and dataset structure. Interestingly, a recent linkage study of nicotine addiction in an African American sample using pairwise non-parametric analysis found a chr 5 linkage peak with a LOD score of 3.04 at 95.4 cM (where alpha power in our study had a LOD score of 3.27). Multipoint non-parametric analysis identified a linkage peak with a LOD score of 2.31 at 103.5 cM (where alpha power in our study had a LOD score of 2.2) 
We recorded the EEG from only six electrodes because ours was an offsite study in more than 300 participants and the acquisition of quality data had to be balanced against study time. These six electrodes were selected to focus on alpha power nevertheless we found linkage peaks for beta and theta power. Although the strongest chr 5 linkage peak for alpha was at O1 the strongest association with CRH-BP was at PZ where EEG power was maximal in Plains Indians. In the US Caucasians EEG power was maximal at O1/O2 and was associated with CRH-BP variation. Finally, no corrections were made for multiple testing because EEG power at all leads was highly correlated, moreover our results were replicated in an independent dataset.
In conclusion, we and others have shown that resting EEG power is heritable and as discussed above, numerous genes are likely to contribute to variation in EEG power. So far, only one published study 
has identified a gene (GABRA2
) that may influence EEG power. Our linkage scan identified CRH-BP
as a strong candidate gene and we subsequently showed that CRH-BP
is associated with alpha power, AUD and anxiety disorders in two ethnically diverse populations. Further work needs to be done to identify functional loci for GABRA2
. Here, the use of a large family in an American Indian population isolate may have been important to identify linkage for a trait which may show high genetic heterogeneity. In terms of cross population validation, it is therefore of interest that our results were replicable in Caucasians.