A file “vbm_volumes.txt” is saved in the actual directory with the absolute volumes of gray/white matter and CSF (in ml). If this file already exists the new values will be appended.
You can use this function to read raw volumes from this file. This file can be also opened with Excel (or gnumeric) as text-file with tabs as delimiters.
To get sure that the order in vbm_volumes.txt is according to the files that will be used in the design matrix you can order the volume values by selecting the respective files. This is helpful if the order in vbm_volumes.txt is different or the number of files differs.
You can use these variables either as nuisance in an AnCova model or as user-specified globals with the “global calculation” option. Depending on your hypothesis and/or your data you can just use gray matter (“gm”) or calculate the sum of gray/white matter with “gm+wm”. The use of raw volumes as nuisance or globals is only recommended for modulated data. These data are corrected for size changes due to spatial normalization and are thought to be in raw (un-normalized) space. In contrast, un-modulated data are yet corrected for differences in size due to spatial normalization to a reference brain and there is no need to correct for these differences again.