Dear SPM users,
I have a 2x2 factorial design where I would like to run an DCM.
Briefly, it is a simple attentional task where subjects need to find a target amoung distractors.
In half of trials the target location is cued (others the cue is uninformative), and in half of trials a salient distractor appears.
So we have 4 conditions:
- Cue - with Salient Distractor
- Cue - without Salient Distractor
- No Cue - with Salient Distractor
- No Cue - without Salient Distractor
I create a GLM specifically for my DCM analysis where I have 3 regressors (+ nuisance variables):
- regressor 1: onsets of cue display (including cue and uncued trials) > use as driving input for the DCM (matrix C)
- regressor 2: onsets of trials with Cue (the one which is informative) > use as modulatory input for the DCM (matrix B1)
- regressor 3: onsets of trials with Salient Distractor > use as modulatory input for the DCM (matrix B2)
I extracted the time series of my 3 regions of interest (adjusted with F-contrast modelling my 3 regressor of interest)
QUESTION 1: Is my GLM and time-series extraction correct?
Then I specified a full DCM : full A matrix, full B matrices, and matrix D = [1 0 0; 1 0 0; 1 0 0; ].
QUESTION 2: Then I saw in the PEB tutorial that I could try two options: automatic search of nested models, or specified myself some reduced models to test.
If I understand correctly, if I don't have any covariate to add in the model and I do not want to compare between groups, but I just want to find the best model for all my subjects, I can simply use a design matrix M=ones(N,1) ?
QUESTION 3: If I use the automatic search, can I consider that the parameters exhibiting a high posterior probability are important to model my task effects?
QUESTION 4: How can I evaluate the interaction effect between my factors? Do I need to add a 4th regressor in my GLM that specified the onsets of trials where both Cue and Salient Distractor appear?
Many thanks in advance for any help!
Carole
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