I'm examining differences in cortical thickness in a sample of drug users and non-users. From the literature, it appears that the 2 covariates I should use for the GLM analysis will be gender and age. Other researchers have examined other possible covariates (like age of first use, # past year uses, IQ, etc.) using correlations between those variables and significant clusters that differed between groups. Is this the best way to go about translating a typical regression model into the Freesurfer statistical package (especially if I'm interested in dose-dependent relationships)?
Also, how do you suggest controlling for total brain volume? Other researchers have performed univariate analyses between groups on TBV controlling for age and gender, then examined mean cortical thickness, but I'm not quite sure what the steps are for this. Could someone help me identify the best way to control for TBV (i.e., is there a way to control from the first GLM analysis, or is this a possible covariate to enter in to the analysis)?
Thanks!