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Hi Luna
DCM.U.idx is used by the “specify group” batch GUI when replicating a template DCM over subjects. It allows the GUI to know which experimental conditions to import from the subjects’ SPMs. The first
column in DCM.U.idx is the condition number (within the Matlab structure SPM.Sess.U in your SPM.mat), and the second column is the regressor number (1 for the main regressor, 2 for the first parametric regressor, 3 for the second parametric regressor etc).
It will only have an impact on your results if you are using the “specify group” batch to generate your subject-specific DCMs from a template. Is that the case here?
Best
Peter
From: [log in to unmask] <[log in to unmask]>
Sent: 17 March 2024 08:50
To: Zeidman, Peter <[log in to unmask]>
Cc: [log in to unmask]
Subject: RE: [SPM] What is the DCM.U.idx parameter for?
Hi Peter,
Thank you!
Initially, I thought I understood the logic behind the numbers in the DCM.U.idx generated by the DCM GUI. The matrix looked like this:
DCM.U.idx=[1 1
2 1
3 1
4 1]
I interpreted it as follows: the first column ranging from 1 to 4 corresponds to the four experimental conditions, and the second column consists entirely of ones.
Based on this understanding, I defined all DCMs in my code using the same idx matrix. Namely, I applied this matrix to DCM.U.idx in all subjects' DCM files.
However, I later discovered that the DCM GUI generates the DCM.U.idx for another DCM (including parametric modulation regressors) in this format:
DCM.U.idx=[1 1
1 2
1 3
1 4]
This discrepancy has left me uncertain whether my approach of defining the same matrix for all subjects' DCM.U.idx was incorrect.
Would defining the DCM.U.idx matrix incorrectly in the code have a significant impact on the results?
Cheers, Luna
12 Mar 2024, 18:07 by
[log in to unmask]:
Hi Luna
This is nothing to worry about. If I remember correctly, I added that field to help with specifying DCMs using the batch editor. Its omission won’t alter your results.
Best
Peter
From: SPM (Statistical Parametric Mapping) <[log in to unmask]> On Behalf Of Luna Sato
Sent: 06 March 2024 03:23
To: [log in to unmask]
Subject: [SPM] What is the DCM.U.idx parameter for?
⚠ Caution: External sender
Hi experts,
When checking DCM results, I found certain subjects' DCM fields include the parameter DCM.U.idx, while others don't. I suspect this variation might be due to different SPM versions used.
I'm wondering about the significance of DCM.U.idx. Can I combine subjects with and without this parameter in group analysis? Or should I consider redoing some DCM analyses?
Best regards,
Luna