Russ, Stephen and Paul,
thank you for your help.
For the single subject analysis I still do not know what to do with a data
set where
2 out of 6 covariates correlate with the global signal. I should not apply
the SPM
built in method of global scaling because of the 2 that correlate, so I
suggest
it would be best to omit global scaling for this subject at all. (?)
For random effects analysis: Is grand mean scaling done implicitly when SPM
generates
contrast images ? I remember there was a note on the list that one should
not use
global or grand mean scaling at the second level, if you work on contrast
images.
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
======================================================================
----- Original Message -----
From: "Paul Laurienti" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, June 27, 2001 3:47 PM
Subject: Re: global signal
> Thomas and Russ,
>
> Global normalization is an interesting topic. For single subject analyses,
it is
> possible to eliminate global normalization. However, if you are analyzing
a group,
> some type of normalization must be used. For fixed effects analyses, you
have to
> use global scaling. For random effects analyses you have other options
such as
> grand mean scaling.
>
> However, there are cases when global normalization is required. One
example would
> be when the global signal changes with the task. With our global
normalization,
> such a data set would show whole brain activation. Unfortunately, region
> activations can influence the global calculation and global normalization
can alter
> the significance of "real" activations. There is currently no perfect
solution, but
> Jesper Andersson has been working on this issue. He recently published a
paper
> addressing global changes in PET (Andersson et. al, Neuroimage 13,
1193-1206, 2001)
> and previously suggested a method (Andersson et. al Neuroimage 6, 237-244,
1997)
> using a masking technique. A comparison of techniques was presented at
OHBM by
> Maria Gavrilescu et. al (NeuroImage 13(6) S122, 2001).
>
> So, in short, there are conditions where global normalization is very
important but
> it remains debatable as to which method should be used.
>
> Paul Laurienti
>
> 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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