We have two questions:
- In our 3dLME model (see below) we have 2 parameters : Age * Time (2 groups of Age and 3 Time moments). From this model we are interested in the main effect of Time and in the interaction of Age * Time. Both groups contain 15 subjects.
When writing the results in our manuscript, co-authors comment on the fact that a certain glt-test is a post-hoc test and should only be performed on significant brain regions in the main effect. Of course it is possible to perform a two-stage approach, and include the outcome of the main effect as a mask for the glt-tests. However, this feels rather artificial to us because normally, glt-tests are already specified in the same model, similar to for instance SPM.
What is your opinion on this matter, and how should we reply/tackle this question?
- Related to this: When doing the posthoc test at whole brain, we see in AFNI that the resulting F-map of the main effect compared to a Z-map from a corresponding glt test, shows much smaller regions in the F-test (at the same p-value). We find this somewhat counterintuitive. Is this related to the way we should perform post-hoc testing (question 1) or could this be due to a more conservative way the F-test is calculated?
Many thanks in advance,
Jan-Bernard and Kelly
Model specification: Please note, we deleted some nonrelevant glts in this thread.
3dLME -prefix 2LVL_AgeByTime
-gltLabel 1 ‘(YPostVsPre)Vs(OPostVsPre)’ -gltCode 1 'Group : 1young -1old Session : -1pre 1post’
-gltLabel 5 ‘(YRetVsPost)Vs(ORetVsPost)’ -gltCode 5 'Group : 1young -1old Session : -1post 1retention’
-gltLabel 13 ‘YoungPreVsPost’ -gltCode 13 'Group : 1young Session : 1pre -1post’
-gltLabel 15 ‘OldPreVsPost’ -gltCode 15 ‘Group : 1old Session : 1pre -1post’
–gltLabel 18 ‘YoungRetVsPost’ -gltCode 18 'Group : 1young Session : 1retention -1post’
-gltLabel 20 ‘OldRetVsPost’ -gltCode 20 ‘Group : 1old Session : 1retention -1*post’
Subj Group Session InputFile
O1 old pre /data/local_dir3/motorlearning/MixedEffects/input_AgeByTime/OlderAdults/O1/Results_afterART/con_0004.nii
O1 old post /data/local_dir3/motorlearning/MixedEffects/input_AgeByTime/OlderAdults/O1/Results_afterART/con_0006.nii
O1 old retention /data/local_dir3/motorlearning/MixedEffects/input_AgeByTime/OlderAdults/O1/Results_afterART/con_0008.nii