Issues with defining contrast in mixed model

[Sara Pisani]

Dear SwE experts,

I hope this email finds you well.

This is the first time I am using the SwE toolboox and I have a quite basic question. I have a model with between-subject factor (i.e., groups), and a within-subject factor (i.e., visits), but I am having issues defining my contrasts. I have entered all images in this order: Group1Visit1, Group1Visit2, Group2Visit1, Group2Visit2. I then defined the groups and the visits with a vector following the order in which the scans were entered. However, I am not clear on how to define the contrast to examine difference in GMV from visit 1 to visit 2 between groups as my design matrix is as below and I cannot differentiate between the groups/visits entered. 

I would be very grateful if you could kindly assist with this query.

Many thanks in advance!

Best regards,
Sara