Dear Doug and Freesurfers,
I'm running a group cortical thickness analysis covaried by age, and we're curious if there is a way to correlate thickness and age within groups and then look at whether or not there is a difference in the strength of the thickness/age relationship between groups. Originally the contrast file we used was the group-x-age.mtx (0 0 1 -1) looking at whether or not there was a difference between the group age slopes, but we then realized we're not interested in whether the slopes of the relationships are different, but rather if the strength of the cortical thickness/age relationship differs between groups.
I ran mri_glmfit with an fsgd file that was set up as follows: GroupDescriptorFile 1 Title OSGM Class Group1 Class Group2 Variables Age Input subject1 Group1 30 Input subject2 Group2 40
I've taken a look at the wiki (http://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G1V) and can't seem to find a contrast file that does what we're looking for. Any advice on what sort of contrast file we should use, or whether we need to run a different analysis than glmfit, would be greatly appreciated! Thanks!
Best, Alex