It's a common practice to include the total brain volume as a regressor in a VBM analysis, but I think it is rather unusual in an fMRI data analysis though. You can interpret your results as the difference in the fMRI activation between the carriers and non-carriers, ADJUSTED for the total brain volume. This makes sense if you strongly believe that the BOLD activation is stronger among people with a large brain (or vice versa) and you want to account for that.
As you can imagine, the total brain volume is a global measure of the brain size. If you suspect that the allele is causing local atrophies in some specific areas and that such atrophies are causing the difference in the fMRI activation, then you might want to do a VBM analysis to compare any structural differences between the two groups.
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Niels Petter Bj
Sent: Thursday, June 26, 2008 8:50 AM
To: [log in to unmask]
Subject: [SPM] Brain volume as a regressor in fMRI analysis
We have done an fMRI analysis in spm using a multiple regression model.
We have used genotype (or strictly speaking, presence/absense of one
allele) as a (binary) regressor and total brain volume (from FreeSurfer)
as a second regressor. When we include this brain volume regressor and
set the contrast vector to 1 0 (1 = genotype) we find large activation
clusters. In contrast to this, we find no activation clusters at all if
we use a simple regression model with only genotype and no volume regressor.
We also find no activation if we use a two sample t-test approach with
the genotypes as two groups.
So, the question is:
How to interpret this difference in BOLD activiation when using/not
using total brain volume as a regressor? And does it really
make sense to use total brain volume as a regressor at all?
Niels Petter Bjørre Sigvartsen, PhD Student
University of Oslo, Department of Psychiatry
Ullevål University Hospital
Tel. +47 23027331