Dear FreeSurfer developers and experts,

If I have a binary factor (Group1: patients, Group2: controls) and a continuous variable (age), I know I can follow Contrast2 in the following wiki to test the interaction effect.

http://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G1V

Contrast2: [0  0  1  -1]  tests if Age slope in patients (Group1) > controls (Group2), and if Age slope_Group1 - Age slope_Group2 > 0, I can see red/yellow.

For example, cortical thickening is accelerated in patients (vs. controls).

I am wondering how I can test for whether cortical thinning is accelerated in patients (vs. controls)?

Here, the slope for Age would be negative. And if cortical thinning is accelerated in Group1 (vs. Group2), I should observe Age slope_Group1 < Age slope_Group2, that is, Age slope_Group1 - Age slope_Group2 < 0, thus, I should expect to see blue/cyan?

Best,

Daniel