Hi Kauser,

That bandpass limit should take out another 140+ DoF. This suggests that you are not using afni_proc.py for the analysis. It would include the regressors, censoring and bandpassing in one step. It is typical that when band passing is done separately, it is not done correctly.
Would you show your 3dTproject command?

Regarding the white matter PCA regression, making a big difference does suggest there is BOLD signal coming from the white matter PCs (or ANATICOR), which would corrupt the results (in an unknown way).

But one basic problem is the voxel size, as Paul mentions. With such big voxels, a proper erosion is almost impossible, since it would often leave no voxels for the resgression. This includes the CSF, and notably, will vary quite a bit across subjects.

The mask you show does not look eroded to me. If that were already eroded by 2 voxels, then the original WM mask would include the dilation of this one by 2 voxels, and that would go way into the grey matter.

Tissue-based regression begs for smaller voxels.

- rick