You need both. You need to model all of your conditions and have unmodelled events. When people talk about unmodelled events, they are generally referring to the time when the subject is not doing anything (e.g. fixation cross, blank screen, etc.)

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and Harvard Medical School
Office: (773) 406-2464
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On Mon, Feb 28, 2011 at 9:46 PM, Melea M. <[log in to unmask]> wrote:

Hi,

I run a simple fMRI experiment with 3 conditions A/B/C, whereas B and C are usually followed by a response. If I want to compare condition A to B and C, how do I set up my contrast? Is it 2 -1 -1 or is it 1 -1 -1?
(The number of conditions for A is a bit higher than for each single condition B or C)

Moreover, do I also have to model the response phases after B and C in my design matrix? One researcher on a conference said that I need to define ALL my conditions, and another one said that I need to have unmodeled events, that SPM can compute a baseline.

What is the best way (and why)?

Thanks so much for your help,
Melea