Dear Abraham and Darren,
Darren is right, you can use different designs for extracting VOI's and later building a DCM models. Say you have treatment during a task. Than, hypothetically, you could make an GLM without the treatment to extract your VOI's and create another GLM ("dummy") design which includes the treatment as an input to specify your DCM models. If you search for DCM literture you will find such examples. Also, you often might use different contrasts to extract different VOI's (look at Klaas 10 simple rules paper).
I am sure that Klaas will have good advices about where to be careful in making this "dummy" design.
Greetings
Branislava
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