Thank you for your patience Anderson. Below are a just responses marked by >>>
In this context C1 tests if significance is > 0 adjusting for age; C2 and C3 indicate if the covariate age is positively on negatively related to FA. As such corrp_tstat2 is not a mirror opposite of corrp_tstat1 Correct?
Precisely. The -1, however, doesn't require a design.mat and design.con. Using just -1, the two contrasts are mirrored, but yep, you're right, in this case it's different.
>>>Yes, but I recall as well from a previous email that with a covariate(s) added (age here), the option -1 together with the options -d for the design and -t for the contrast should be used, so that a sign-flip test is made.
As your previous email made lucid, given a set of significant L> R coordinates, the same set of coordinates could be reported as R < L in a between group test.
Yes, but only with the same contrast reversed. Otherwise, they will be different.
However, in the single sample tbss_sym test the cluster reports (method :
http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/Randomise/UserGuide#Getting_Cluster_and_Peak_Information_from_Randomise_Output) for the L>R and R>L show different sets of significant coordinates. For example, here are the first 2 lines of L>G and R>L cluster reports for the expert sample; I have added the JHU-ICBM info associated with the coordinates in fslview.
L>R
Cluster Index Voxels MAX MAX X (mm) MAX Y (mm) MAX Z (mm) L>R, JHU-ICBM Intensity: 1-p
14 323 11.2 -30 41 -3 5% IFOF; 3% ATR 0.9628
13 40 9.42 -29 30 24 3% UN; 3% IFOF; 3% ATR 0.9687
R> L
14 3529 17.8 -31 -43 28 32% SLF; 11% SLF (temporal part) 0.998
13 434 9.23 -49 -35 -11 21% SLF (temporal part); 21% SLF; 3% ILF; 3% IFOF 0.9707
So while a given set of L> R coordinates could be reported also as R < L, the cluster reports (and the images associated with cluster report significant coordinates corrp_tstat images in fslview) indicate different main tracts, or aspects of main tracts, have significantly different FA in the left and right hemispheres. Is this a reasonable conclusion?
L>R and R>L will only be the same if the contrasts are reversed. If the contrasts (and of course, EVs) are the same, but one image is the negative of the other (as in L>R vs. R>L), they won't be the same.
Actually, to avoid confusion, I'd suggest that you delete one of these images (either R>L or L>R) because they are really just the same as the other, just with opposite signs. You can probably avoid a good deal of confusion by getting rid of one of them.
Single group, tbss_sym
Group EV1 EV2
1 1 -1
1 1 27
1 1 -4
1 1 -2
1 1 -4
1 1 -5
1 1 -3
1 1 -5
1 1 -3
Contrasts
group mean 1 0
pos eff age 0 1
neg eff age 0 -1
This looks good. I'd only delete one of the two 4D files you are using, as I mentioned above, and add a contrast [-1 0].
>>> I did remove the second 4D file (that was the oringial input file multiplied by -1) and added the [-1 0 ] contrast, which became tstat2 (RgL), and the results were, again identical to the original R>L (the L>R input multiplied by -1). The contrasts for the alternative version with the added contrast [-1 0] are below. I see that it does reduce confusion this way, but the R>L is quit different from the L>R. Redoing the cluster report resulted in the same pattern as the original R>L (the L>R input multiplied by -1) shown above. It seems then, that the R>L does differ from L>R
L>R mean 1 0
R>L mean -1 0
pos eff age 0 1
neg eff age 0 -1