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/