I have two groups and two time points and I would like to do a correlation analysis between a variable (cognition) and the change score in cortical thickness between the two time points (time2 minus time1). 

I would like to see if the correlation is different among each group, if there is an interaction with the group.

I have followed the paired t test instructions (MailScanner has detected a possible fraud attempt from "secure-web.cisco.com" claiming to be https://surfer.nmr.mgh.harvard.edu/fswiki/PairedAnalysis), but I am not sure about the contrast(s) I have to use. 

The paired-diff.fsgd file (second fsgd file) is the following:

GroupDescriptorFile 1

Title OSGM

Class Group1

Class Group2

Variables cognition

Input subject1pair Group1 0.546

...

How should it be the contrasts files to see the correlation of the variable cognition with the change score in thickness taking into account the groups? And should I use DODS or DOSS?

Using DODS:

0 0 0.5 0.5

 or 

0 0 1 -1 

I don't understand the difference between both contrasts. I think I should use the one of "0 0 1 -1" to see whether the cognition variable correlates with change in thickness differently according to the group. Is this correct?

Using DOSS:

0 0 1