I recently start to work on PPI analysis. I met a problem that PPI.Y has only NaN value. After searching the SPM archives, I know the problem coming from extracting VOI data from multi-sessions. After removing all zeros columns in the xY.X0, I got some reasonable value in the PPI.Y. My question is what the rest columns of matrix xY.X0 are besides the column with all 1s.

In my study (6 runs), I defined 13 conditions and used 6 motion covariates. The orignical xY.X0 has 13 columns. The first column is all 1s (I extract VOI data from the 1st run). The next five columns are all 0s (I think those are from other runs). What are the rest of 7 columns? They are not motion covariates. Even though it comes from motion covariates, it should have 6 columns, right? but there are 7 columns. In the spm_regions.m, xY.X0 is defined as [whitened] confounds (including drift terms). What does it mean? and how to get those values? Thanks!

Best wish!

Weidong Cai

Biopsychology

Department of Psyhcology

State University of New York at Stony Brook