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