Dear Silke,
> when correcting for total intracranial volume (TIV), should I treat TIV as a
> Covariate or as Global Value for the Global Calculation (as suggested in the
> VBM Tutorial). I did both analyses (TIV as Covariate or as Global Value) and
> the results are different. TIV as Covariate seems to increase the
> statistical power to detect group differences compared to TIV as Global
> Value.
I don't know if there is a "correct" answer. Barnes et al. (2010)
suggest including it as a covariate, which is what I have usually
done. We talk about the issue of covariate vs. global correction in
the context of total gray matter in a recent neuroimage paper—this is
a slightly different emphasis but may be of some use in interpreting
the two sets of results:
Barnes J, Ridgway GR, Bartlett J, Henley SMD, Lehmann M, Hobbs N,
Clarkson MJ, MacManus DG, Ourselin S, Fox NC (2010) Head size, age and
gender adjustment in MRI studies: a necessary nuisance? NeuroImage
53:1244-1255.
Peelle JE, Cusack R, Henson RNA (2012) Adjusting for global effects in
voxel-based morphometry: Gray matter decline in normal aging.
NeuroImage 60:1503-1516.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Center for Cognitive Neuroscience and
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
|