Dear Paul,
> My study has 3 stimulation conditions a, b and c. I have constructed the
> con***.img files and an using them for the analysis. I have 2 questions:
>
> 1. Is it critical that I model my rest period when analyzing the
> individual data?
No, it isn't critical. You don't in general need to model rest explicitly as
a separate condition. There's been some discussion of this recently on the
list, which you may be able to track down in the list archive if you're
interested at http://www.mailbase.ac.uk/lists/spm
> 2. what test would you recommend I use for the second level analysis if
> I want to determine the areas of activation for each condition (a, b, c)
> and the areas activated during c but not a or b?
A one-sample t-test should do the trick for your first question. This will
test the null hypothesis that the sample mean is zero (i.e. no difference
between condition X and implicitly modelled rest).
Your second question unfortunately can't be addressed statistically. You
cannot assert the null hypothesis and identify areas are NOT significantly
activated by conditions a or b. This is a statistical issue, rather than an
SPM limitation! At a descriptive level, within SPM you could use exclusive
masking of condition c activations by conditions a & b. However I stress
that the resulting SPM should be treated purely in a descriptive fashion,
and this is not a statistically valid way to approach the data!
Best wishes,
Geraint
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Dr. Geraint Rees
Wellcome Advanced Fellow, Lecturer,
Mailstop 139-74, Institute of Neurology,
California Institute of Technology, University College London,
Pasadena, 12 Queen Square,
CA 91125 London WC1N 3BG
voice (626) 395-2880
fax (626) 796-8876
web http://www.klab.caltech.edu/~geraint
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