To control for the effects of normal aging, we derived a single age-corrected value for each subject for total power, evoked power, and PLF scores. First, total power PCA factor scores were regressed on age in the HC group (age range 12-37 years). Next, the resulting regression equation was used to derive age-specific predicted values that were subtracted from the observed values and divided by the standard error of regression, yielding age-corrected total power factor z-scores for subjects across all groups. The resulting age-corrected z-scores reflected deviations in standard units from the values expected for a normal healthy subject of a given age. This method has been used previously to correct for normal aging effects in structural MRI data (Pfefferbaum et al., 1992). The method is preferable to using age as a covariate in an ANCOVA because it removes normal aging effects while preserving any pathological aging effects present in the patient data. This procedure was repeated to acquire age-corrected evoked power factor z-scores and PLF factor z-scores for all subjects across groups.
