I hope this e-mail finds you well. I am doing a VBM analysis. I used a two-sample t-test and TIV as a nuisance factor and when I use the grand mean scaling in the design section, the results are significant. However, when I do not use it, there are no significant results in the same contrast.
he yellow highlighted one is the one that when I choose, it gives me significant results in contrast (1 -1) but when I select no, it becomes non-significant.
How is it different from this one? When I use this one, it gives significant results in the opposite contrast (-1 1) of only two voxels!!!
My Question is
1- why does this happen? Can I proceed using it? When should I use it?
Are there other resources to understand this matter as I could not find one that explains scaling by subject???
Thank you in advance