Hello Roberto,
I would briefly like to comment on the judgement of total grey
matter or total intracranial volume, used as regressors in a
VBM analysis, as being not legitimate. Also, this is not
a general opinion to the best of my knowledge in the circle
of researchers using VBM.
This terminology seems too harsh and may not exactly reflect,
that with both analysis, and with the third version (no
global covariate) you test entirely different phenomena.
If you test MODULATED GM maps that have been generated by SPM5
or SPM8, then the voxelwise GM values naturally also contains
global effects. Naturally, the global GM or TIV are *strong*
predictors of the local volume. If you do not correct for this
effect, you simply DENSITIZE your analysis wrt your main effect.
(e. g. SNPs, patients versus controls etc.), unless you match your
group for head size. Usually not practiced.
Difference between total GM or total TIV as covariate:
Several replies related to that have been circulated, so most has been
said here I think.
If you include total GM as covariate, you correct the local group
differences for global differences. What remains, is the local
differences that go beyond the effect of a global effect. It is
recommended, of course, that you report the main effect on the globals.
When there is a clear correlation between global and local effects
such as in AD, then you might wipe out the local effects. Still,
it needs to be clearly emphasized, that there is nothing wrong
or illegitimate about such an analysis.
Including TIV, as phrased by Jonathan, corrects for the more static,
non age-dependent body size/cranial vault co-correlation. Can be
interesting and useful for certain developmental/aging questions.
Not to include any covariate, as explained can be done, but there is
an unspecific increase of local variance in your GM data that is just
not useful.
Practically, we used the GM, WM and CSF maps to generate a TIV
value, but applied an "intracranial mask" on CSF to get rid of
critical areas. This mask is e. g. shown in a small paper on acromegaly
(where TIV itself changes), Sievers et al., in Pituitary 2009.
(I did not take into account now the better estimation of CSF and
extracranial tissue compartments with SPM8 segmentation).
best wishes,
Philipp
Max Planck Institute of Psychiatry
NMR Research Group
Kraepelinstr. 2-10
80804 Munich
Mail: [log in to unmask]
Phone: 0049-89-30622-413
|