How to get the contrast of two contrasts by using TDT?

Dear Martin,

Hi, I’m using TDT toolbox to analyze my results. I’m totally new to MVPA, so it may be a silly question.
I have learned that I can get a contrast result by setting the label into 1 and -1 for two regressors, but what I really want is to get the contrast result of two contrasts. For example, I want to know (regressor 1 - 2)-(regressor 3 - 4). I can just define contrast (1 -1 -1 1) in SPM, but what should I do in TDT?

Thank you very much. Looking forward to your reply.

Hi Hu,

Not a silly question at all. If I understand correctly, you want to see if there is more information between conditions A vs. B than conditions C vs. D? In that case, you would run two classification analyses, one with conditions A vs. B, and another with conditions C vs. D. That would give you two accuracy maps (for searchlight) or two accuracies (for ROI). Then you can run the contrast you mentioned by computing the difference between both images, e.g. with SPMs imcalc. If you now want to run your stats at the group level, your input would be the difference image between the two accuracy images.

Hope that helps!

1 Like

Dear Martin,
Thanks for your prompt reply.
It’s exactly what I want to analyze.
Thank you so much! It helps a lot!