Hi Vania,

I'll try to answer both of your posts. To address your last forum post (and maybe this one, too), it's not clear what you mean by "raw" vs. "processed" images. I'm not familiar with the OASIS dataset, but you should not be using raw DICOM images. SPM can only handle NIFTI (.nii) or image and header (.img / .hdr) file types. I use dcm2niix to convert DICOM files to NIFTI format but you can also use SPM's DICOM conversion tool (though it's slower than dcm2nii). 

Using SPM's segmentation tool should give you six new volumes. The second of those (the c2 image) should be your White Matter volume. This youtube video should be helpful to familiarize yourself with segmentation in SPM: https://www.youtube.com/watch?v=_SSzN9KL_qw

If this does not address the issue, and you're still getting an error message, it would be helpful if you could copy and paste the error from the matlab window into your next forum post. 

Good luck!