Dear Dr. Gaser, Dear List, at the moment I am unsure about whether correcting for total brain volume (TIV) in a VBM analysis of unmodulated gray matter segments (density) does make sense or not. On Dr. Gaser's homepage --- I have the feeling --- there are two slightly opposite statements: statement (a) (please refer below) clearly states that TIV should not be used as a nuisacne factor with unmodulated data. On the other hand, I read statement (b) (please see below) as correcting unmodulated data for TIV gives me relative density after correcting for TIV, which seems to make more sense to me. Which procedure is the more valid one, or for which kind of question would I use which procedure respectively? Thanks for any help in advance. Best regards, Lucas Eggert ****************** STATEMENT (a): http://dbm.neuro.uni-jena.de/vbm/vbm2-for-spm2/calculate-raw-volumes/ "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." STATEMENT (b): http://dbm.neuro.uni-jena.de/vbm/segmentation/modulation/ Interpretation If we follow the commonly used terms "volume" for modulated data and "density" (or concentration) for unmodulated data and concentrate on GM there are many possible ways to correct or not correct for different brain size: *No modulation:* *Correction * *Interpretation* nothing relative density globals "localised" relative density after correcting for total GM or TIV (multiplicative effects) AnCova "localised" relative density that can not be explained by total GM or TIV (additive effects) -- Lucas Eggert, M.Sc. Institute of Cognitive Science University of Osnabrueck Albrechtstrasse 28 D-49076 Osnabrueck Germany Phone: +49-541-969-44-28 Website: http://www.cogsci.uni-osnabrueck.de/~leggert/