In summary, our study used two approaches to help localise genetic variants influencing information processing speed: association with single SNPs and association within significant biological pathways. Our speed measures shared common variance, and this overlap was demonstrated for most of the top association signals in the SNP analysis, where at least one, and up to four, other variables were nominally associated. Only one SNP reached suggestive association levels for multiple traits (i.e., 2- and 8-choice RT), suggesting that pleiotropic genes have different strengths of association with correlated traits. Further, it might be that much smaller gene effects (and not detectable in our study at genome-wide level) influence pleiotropy among speed traits and, conversely, genes with larger effect influence the unique genetic variance also known to influence speed measures. Five of the candidate genes identified from the biological pathways analysis of the speed factor were implicated in the pathophysiology of AD. Several genes (e.g., APOE, APP) predisposing to AD have been associated with variation in normal cognition, and there is some indication that the effect of these genes may be stronger
