Porjesz et al. (2002b) reported the results of linkage disequilibrium mapping of ERP phenotypes, including visual oddball P3 in a sample predominantly consisting of families with dense, multigenerational alcoholism. Of 351 highly polymorphic microsatellite markers tested for linkage separately with each of the EEG electrodes, none reached the nominal significance threshold (LOD score of 3), but several markers approached that value and were therefore considered “suggestive” of linkage; most notably, two markers on the chromosome 2 showed a trend to linkage with P3 amplitude at P4 and C4 locations. In addition to P3, the same study examined the N1 component obtained in the same VP3 paradigm and found significant linkage on chromosome 16. Although these results are very encouraging, drastic differences in the linkage results for neighboring electrodes (which typically show very high correlations with respect to P3 amplitude) raise important methodological question of whether P3 component at a given scalp location might reflect relatively independent processes influenced by distinct genetic factors. This issue should be addressed in future studies using e.g. independent component analysis and/or source localization in order to delineate genetic influences on dissociable neural processes contributing to the scalp-recorded P3 potential.
