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)?<br>
<br>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&#39;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)?<br>
<br>Thanks!<br clear="all"><br>-- <br><div>Jenessa S. Price, M.A.<br>Graduate Student, Clinical Psychology and Neuropsychology</div>
<div>University of Cincinnati</div>
<div>Campus Representative, American Psychological Association of Graduate Students (APAGS)</div><br>