LLN, le 24/01/07
Dear Lena,
Here is what I suggest:
(1) compute all your 1-level contrasts (e.g., [activation-baseline])
for each subject individually,
(2) port the corresponding con_* files to 2-level analyses using:
- 1-sample t-tests with a contrast set at [1] for each group separately;
- 2-sample t-tests with your two groups and a contrast set at [1 -1]
(or [-1 1] depending on what you're looking at).
May I also suggest to be careful with the "direction" of your
subtractions, both for the group effect ([patients-controls] or
[controls-patients]) and the task effect ([activation-baseline] or
[baseline-activation]). Especially for the latter, be sure that you
really do expect some areas to be more activated in the baseline than
in the "activation" task...
Hope this helps,
Mauro.
>Dear SPM:ers,
>
>I have a dataset with two groups, patients and controls. These have two
>conditions, baseline and activation. It's probably simple, but I seem to
>have a problem choosing the correct SPM design type for this study. I
>want to look at several things:
>
>* Compare controls, baseline vs. activation
>* Compare patients, baseline vs. activation
>* Compare baseline, controls vs. patients
>* Compare activation, controls vs. patients
>
>I thought "multi-group: conditions & covariates" would be the correct
>one for all of these, but SPM won't let me put "0" to exclude columns
>when I specify the contrast. Do I have to choose a separate design for
>all four of these analyses? Please help.
>
>Thank you!
>
>Best regards,
>Lena Edström
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Mauro PESENTI
Research Associate, National Fund for Scientific Research (Belgium)
Unite de Neurosciences Cognitives
Departement de Psychologie
Universite Catholique de Louvain
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