Dear Thomas,
it's not clear to me why you would evern want to do global signal normalization
for fMRI data. If you are including a high-pass filter in your design then
things like low-frequency drifts will be taken care of on a voxel-by-voxel
basis, and if you include motion parameters as regressors then you will also
get rid of any intensity artifacts related to motion. Perhaps someone else
could comment on why one would want to do global normalization for fMRI - we
never do it these days and we get perfectly reasonable results.
cheers,
russ
Thomas Stephan wrote:
> Dear List,
>
> to deal with the problems regarding correlations between global brain signal
> and the task in
> fMRI experiments we use the tool rd_taskcorr provided by Kalina Christoff to
> compute
> the significance of those correlations.
> If we have very simple experiments with an ABABAB design, we can just skip
> global scaling
> if we detect a significant correlation.
> Now we analyze data of an experiment with 3 different stimulation conditions
> and corresponding
> three different rest conditions. What should be done if there is a
> correlation between global signal
> and e.g. Rest3, but no significant correlation for Stim1-3 and Rest1-2 ?
> Should we compute some contrasts with global scaling and other contrasts
> without ?
>
> Thanks for your opinions
> Thomas Stephan
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> Thomas Stephan Email: [log in to unmask]
> Klinikum Grosshadern
> Neurologisches Forschungshaus
> Marchioninistr. 23 Fon: +49 089 / 7095-4819
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> ======================================================================
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Russell A. Poldrack, Ph. D.
MGH-NMR Center
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