Hi Douglas, 

Thank you for your reply. 

To follow up, I am setting up GLM analysis with 1 categorical variable and 1 continuous variable (age X weight). My fsgd looks like this:

GroupDescriptorFile 1
Title Cortical Thickness Interaction between Age and Weight
Class class1 plus blue
Class class2 circle green
Variable           Demean Age X Demean Weight
Input 001 Class1 500
Input 002 Class2 433
...

I want to test for an interaction between age and weight while controlling for group. Instead of putting-in age and weight separately, I multiply them to test for interaction. Can you tell me if following contrast vector will correctly test for this? 

0 0 0.5 0.5

Thank you for your time, 

--
David S. Lee
Associate Research Specialist
Center for Healthy Minds
University of Wisconsin - Madison
(608) 890-1115