If I have the following FSGD with 4 classes and 3 variables, and I'd like to run an interaction of ROIa x ROIb controlling for the third covariate (eTIV), would I use the following contrast?

[0 0 0 0 0.25 -0.25 -0.25 0.25 0.25 -0.25 -0.25 0.25 0 0 0 0]

For the interaction of ROIa x ROIb x class + eTIV.

Thank you so much in advance for your help!

All the best,

Fred