Hi Antoni,
When you say ‘see results’, I’m guessing you mean fwe_pvalue < 0.05?.
One explaination for not observing significant fixels in the FC analysis is due to variance. In general, there is larger variation across subjects in brain morphology than aFD. This is particularly true in certain structures like the CST and splenium.
No, it’s multiplicative FDC = FC x FD. So both FD and FC contribute to the effect size of FDC.
One way to get a feel for what is going on is to look at the absolute effect size with respect to the control mean. We have documentation on how to compute this in the upcoming release (see the fixel-based analysis section). If you have been using the soon-to-be-legacy *.msf format to store fixels, you can convert them to the new directory-based format using fixelconvert (also available in the tag_0.3.16 branch. If you want to install this, be sure to either do a fresh clone and install, or run ./build clean && ./configure after checking out the branch)
Finally, if you have significantly different FDC fixels, in the same region as FD, but with a reduced effect or spatial extent, then perhaps not having a significant result in FC makes sense.
Cheers,
Dave