Dear FreeSurfer Experts,

In the example here: http://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf1G2V, it describes the testing of the main effects of two continuous variables.

In order to test the interaction effect, I think we can form an interaction term like age x weight as the third covariate, that is,

Regressor1: All ones. Codes intercept/mean for Main
Regressor2: age for each subject. Codes age slope for Main
Regressor3: weight for each subject. Codes weight slope for Main

Regressor4: age x weight for each subject. Codes age x weight interaction effect

and then specify a contrast [0 0 0 1]?

Is that the way to go?

Best,

Daniel