I’ve computed a VBM analysis using the DARTEL-toolbox to investigate GM density differences in SPM8. The aim of the study was to investigate differences in GM density in two groups of patients. I set up the analysis according to the SPM8 manual (chapter 26 Dartel tools) using unmodulated GM images of the patients.
Do I have to incorporate the individual total intracranial volume in the second-level between-group comparison as a covariate or does the analysis already account for this nuisance variable? So is it possible to analyze the date using an unpaired t-test or do I have to use the total intracranial volume as a covariate in the second-level analysis?
Thanks and kind regards