Dear Feng,
the reasons for changing the modulation for CAT12 were best described here:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=SPM;c95da4bf.1707
If you have used TIV as nuisance parameter for the “affine+non-linear” modulation and skipped this step for the “non-linear only” modulation the results should not differ too much. However, if your TIV is correlating with you parameter of interest then this will affect the analysis because you will not only remove variance explained by TIV but also by your parameter of interest. Please check the orthogonality of your design and alternatively use TIV as global scaling. This is described in the CAT12 manual (page 21). Furthermore, I would recommend to rather use CAT12 for ongoing studies.
Best,
Christian
>Dear VBM/CAT experts,
>
>I have conducted VBM analysis using VBM8 toolbox on my structural imaging data. Specifically, I performed modulate procedure twice: one used the option “non-linear only” and the other is “affine+non-linear” and then use the total TIV as nuisance parameter in the statistical model. However, when I finish the group comparison, I had a weird result. There were big differences among these two T maps (I mean, after multiple comparison correction, the survived regions are different, and the overlapped regions are small). Actually, because both of these two ways represent the “relative gray matter volume”, I had expected that the results “at least similar”. Is my result reasonable?
>
>In addition, I found in the VBM8 toolbox, it recommends the non-linear only option to remove the confounding effects of different brain sizes. In contrast, in CAT12, it recommends the option “affine+non-linear” and uses the TIV as nuisance parameter. I know “non-linear only” performs multiplicative effects correction, but “affine+non-linear” performs additive effects correction. The idea behind the option “non-linear only” is that scaling of affine normalisation is indeed a multiplicative (gain) effect and we rather apply this correction to our data and not to our statistical model (http://www.neuro.uni-jena.de/vbm/segmentation/modulation/). Why this option is not recommends in CAT12?
>
>Here, I am just wondering which option is better or which one is more biological meaningful to represent the relative gray matter volume? Why my results are different and which method I should use in the future?
>
>Any help would be greatly appreciated!
>
>Best,
>
>Feng
|