Dear SPMers,
I have a couple of questions regarding setting up a design matrix to use in a DCM analysis. I have a 2x2 factorial design and for the DCM analysis I would like to include the two main effects, the interaction, all conditions and each level of the first factor separately.
Main Effect A Main Effect B Int ALL A1 A2
FA-L1,FB-L1 1 1 1 1 1 0
FA-L1,FB-L2 1 -1 -1 1 1 0
FA-L2,FB-L1 -1 1 -1 1 0 1
FA-L2,FB-L2 -1 -1 1 1 0 1
To set up these contrasts in the first-level fMRI model specification I created a single condition that included all trials then created a Parametric Modulation for each effect with an array of 1s, 0s and -1s for values. My questions are:
1. Is this the correct way to go about achieving the desired design matrix?
2. This specification shows under parameter estimability that some of the betas are not uniquely specified. Is this an issue for DCM? And if so, how might I set up the model differently to avoid this but still have all the desired effects to use as modulatory effects in DCM?
Any help or suggestions would be much appreciated.
Sincerely,
Ashley Safford
Ashley Safford
Neuroscience PhD. Candidate
George Mason University
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http://archlab.gmu.edu/people/ahamlin2/
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