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Thomas Stephan wrote: > 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. At the first level, if you say no to proportional scaling (ie. scaling each voxel in a scan with the global mean for that scan), then SPM defaults to proportional scaling over the session ie. normalising voxels in scans with the global mean for all scans in that session. So SPM will have done subject-specific global scaling at the first level. There is no need to do it again at the second level. Best wishes, Will. > > > 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 > > -- William D. Penny Wellcome Department of Cognitive Neurology University College London 12 Queen Square London WC1N 3BG Tel: 020 7833 7478 FAX: 020 7813 1420 Email: [log in to unmask] URL: http://www.fil.ion.ucl.ac.uk/~wpenny/