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Dear Laura,
The way you set up your GLM and DCM depends on the question you want to ask. What would you like to find out from the DCM?
Let me give you an idea of what’s typical. I’ll assume you expect the feature to increase or decrease coupling between certain regions,
when a picture is being displayed.
1. For extracting ROIs, you would create a GLM with two columns: picture and picture+feature (plus any nuisance regressors). You should
not model the fixation baseline in your GLM. Extract your ROIs from this GLM. Ensure you have an effects of interest F-contrast defined, and use this when you extract ROIs.
2. Create a new GLM for use in DCM. This will have two columns called ‘stimulus’ and ‘feature’. The stimulus column contains the onsets
of all picture trials, with or without the extra feature. The feature column only models trials with the extra feature. Now, construct a DCM with stimulus as driving input. Include ‘feature’ as a modulatory input, to test which connections are strengthened
or weakened in the presence of the feature.
Best,
Peter
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of Laura Enax
Sent: 10 June 2015 14:21
To: [log in to unmask]
Subject: [SPM] DCM - define the GLM
Dear SPM experts,
I am currently struggling with the very first step in DCM - the GLM specification for my DCM analysis. As I read in several tutorials and also in the
SPM manual, the GLM needs to be set up a little bit differently, compared to standard analyses.
I have three conditions: subjects see a low-level baseline (fixation cross), a picture, or a picture with an additional feature. I am interested in how
the third condition (i.e., picture with an additional feature) influences the connectivity between several regions.
If I understand correctly, one has to have a factor that varies the driving input (i.e., this would be picture vs. fixation ?) and one factor that varies
the modulating input (i.e., this would be picture with feature vs. picture?).
Now I am not sure how to set this up in the GLM, which I need for further analyses. Is the driving input the fixation, or a combination between fixation
and picture, or even between all of them? Is the modulating input only the picture with the feature, or both, the picture with the feature and the normal picture? If I need both pictures as modulators, would I have to test all combinations (i.e., test if picture
modulates connectivity beween x and y, AND test if both modulate connectivity between x and y AND test if picture with feature modulates connectivity between x and y ? Or would I always specify that both (together) should modulate the connectivity beween region
x and y?
Sorry if I am unclear, feel free to ask questions.
Thanks a lot
Best
Laura