For statistical comparisons, the electrode sites were grouped into six scalp regions, and six representative electrodes from each region were included in the analysis; all six electrodes were included from temporal regions (Figure 2). This is similar to the method used by Kamarajan et al. (2005a). The obtained amplitude and latency values of N2 peaks were analyzed using a linear mixed-effect model of SAS Proc Mixed Procedure (SAS 9.1, SAS Institute Inc., NC, USA). The mixed-effects model included group (controls, alcoholics), condition (Go/NoGo), region (frontal, central, parietal, occipital, left temporal, and right temporal), electrode (6 representative electrode sites nested within each region) age, age2 and their interactions as fixed effects, where condition (Go/NoGo) and electrode coordinates (x, y, z) were treated as repeated measures. To determine direct (Kronecker) product structures based on distance between electrodes (i.e., x, y, z), a spatial anisotropic exponential (EXPA) matrix was used to model within subject covariance structure of the data. A scatter plot of the N2 peak amplitude and latency values with age showed a curvilinear trend, which was consistent with the observations of
