Hi Nadia
> Intuitively, I would set the contrast like this:
> (DAT group1/ DAT group2/ Alcohol use group1/ Alcohol use group2)
> 1/-1/1/-1
> I assumed that this would give me the activity in areas that are both related to
> the activity in the striatum in group 1 and to their alcohol use.
> (and -1/1/-1/1 for group2).
so here you ask is the activity in my voxel more explained by the
activity in the striatum + alcohol use gp 2 than in group 1
note that you are pooling together striatum + alcohol and therefore it
doesn't tell you what the effect of alcohol is
> But someone else suggested:
> 0/0/1/-1
> To me it seemed as if this contrast would only look at the effect of alcohol use
> and not take into account the activity of the seed striatum.
> But maybe I misunderstand what the first two zero's mean.
this is a more usual way to think of testing covariates, ie is the
activity in a voxel more relate to alcohol use in gp 2 than gp1 while
you controlled for the correlation with the striatum which is correct
BUT you want to know if th regions already identified are influenced by
alcohol use .. so I think the best approach is to use masking and/or
conjunctions with [-1 1 0 0] = which voxels show difference between
groups removing the effect of alcohol use, [0 0 -1 1] = which voxels
show difference between alcohol use removing the effect DAT gp,
conjunction null [-1 1 0 0] U [0 0 -1 1] for voxel showing both effects
(independently) and maybe also [0 0 -1 1] masked inclusively by [-1 1 0
0] for voxels showing differences related to alcohol while also being
different between groups.
Good luck
Cyril
--
Dr Cyril Pernet,
SBIRC fMRI Manager
Lecturer in cognitive neuroimaging
SFC Brain Imaging Research Center
Division of Clinical Neurosciences
University of Edinburgh
Western General Hospital
Crewe Road
Edinburgh
EH4 2XU
Scotland, UK
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tel: +44(0)1315373661
http://www.sbirc.ed.ac.uk/cyril
http://www.sinapse.ac.uk/
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.
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