 |
 |
Based on your description, I don't think you can model the interactions with that design matrix. The number of columns you need for the two categorical variables alone is the product of the number of levels in each (minus one if SPM automatically includes the constant regressor). If you also include a covariate like age, you need to multiply by an additional factor of two, if I recall correctly. The fact that your current design matrix (probably) models the main effects would indicate that your categorical variables each have two levels. If my claims above are correct, you'd need 2*2*2 = 8 columns in the design matrix (if the constant isn't modeled).