Well - there is still a problem (in FMRI data) of random correlations of
the global timecourse with the model, causing a lateral shift in (for
example) the histogram of t or Z values (even for resting data). This is a
problem....however, we don't recommend doing global intensity
normalisation, because there is currently no method which is sophisticated
enough to do the job properly...
Stephen M. Smith
Head of Image Analysis, FMRIB
Oxford University Centre for Functional MRI of the Brain
John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
On Wed, 27 Jun 2001, Russ Poldrack wrote:
> 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
> > ======================================================================
> > Thomas Stephan Email: [log in to unmask]
> > Klinikum Grosshadern
> > Neurologisches Forschungshaus
> > Marchioninistr. 23 Fon: +49 089 / 7095-4819
> > 81377 Muenchen Fax: +49 089 / 7095-4801
> > ======================================================================
>
> --
> Russell A. Poldrack, Ph. D.
> MGH-NMR Center
> Building 149, 13th St.
> Charlestown, MA 02129
>
> Phone: 617-726-4060
> FAX: 617-726-7422
> Email: [log in to unmask]
> Web Page: http://www.nmr.mgh.harvard.edu/~poldrack
>
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