Dear FSL crew,
I have a question regarding the interpretation of the results of a TBSS analysis.
I compare two groups (each n= 20) regarding their FA values, while controlling for age. Additionally, I want to explore the effect of a third variable on the FA values. I am using randomise with TFCE.
The design.mat file is set up as follows:
/NumWaves 4
/NumPoints 40
/Matrix
1 0 6.78 -0.2
1 0 4.25 -.01
1 0 -3.78 0.09
...
0 1 -3.23 0.21
0 1 1.56 -0.23
0 1 -5.05 0.56
...
while the design.con looks like:
/NumWaves 4
/NumPoints 2
/Matrix
1 -1 0 1
-1 0 0 1
The 3rd collumn is age and the 4th collumn is the "effector" variable (both demeaned).
The two resulting -coorp images show different significant clusters. I have a question regarding the interpretation of these: What exactly do the significant voxels/cluster mean, given the GLM setup? I would understand that e.g. tstat1 means "Results of TBSS voxelwise comparisons of FA values while accounting for the effect of X, corrected for age. Significant voxels in tstat1 indicate higher positive relationship between FA value and X for group 1 than group 2", vice versa in tstat2.
The critical part where I struggle to wrap my head around is "Significant voxels in tstat1 indicate higher positive relationship between FA value and X for group 1 than group 2", since I'm not enterely sure if this really includes a comparison between groups (such as a t-test), while also accounting for the effector and controlling for age. Or is tstat1 "only" the relationship of the effector with the FA values while controlling for age, without a comparison between the groups?
I would really appreciate any advise on this issue. Thank you very much in advance!
Best,
Johannes
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