----- Original Message -----
From: Krish <[log in to unmask]>
To: SPM MAILBASE <[log in to unmask]>
Sent: Wednesday, January 19, 2000 1:26 AM
Subject: fMRI: Perform global scaling normalisation, or not?
> Hi everyone,
>
> In a recent posting
> (http://www.mailbase.ac.uk/lists/spm/1999-11/0152.html) Jesper Andersson
> mentioned that global intensity normalisation could under certain
> cirumstances introduce artefactual activations.
>
> This worried me and I have analysed one of my recent datasets with and
> without global scaling normalisation. You can see the results at:
>
> http://www.liv.ac.uk/mariarc/scaletest.html
>
> Basically there were quite marked changes in both positive and
> negatively correlated activity. Global intensity normalisation seemed to
> reduce the amount of positive activity and enhance the amount of
> negative.
>
> So my
> questions are:
>
> 1) Is this a widely known fact? Personally, I hadnt really considered
> that global normalisation could introduce artefacts until Jesper
> mentioned it. Once you think it through it's fairly obvious (imagine a
> brain in which half the brain is showning postively correlated activity.
> After global scaling, the other half will be negatively correlated).
Dear Krish,
Over the past few years there have been a number of discussions on the
help line regarding this isssue. Is it widely known fact? Well, a good
reviewer and/or editor will point it out to you. The classic citation in
this area is Aguirre, Zarahn, & D'Esposito (1998). The inferential impact of
global signal covariates in functional neuroimaging analyses. Neuroimage, 8,
302-306.
>
> 2) Does everybody use global intensity normalisation?
I would have to echo Stephen Smith's response that I believe that most
groups, in particular those using SPM, do perform global intensity
normalization. In groups that are aware of this issue, usually they report
the degree of correlation of the global signal with the experimental
paradigm, as recommended by Aguirre et al.
>
> 3) The problem is greatest when large areas of the brain are either
> positively or negatively activated, such that, presumably, the volume
> mean becomes significantly (anti) correlated with the effect of
> interest. Does this mean that it is more desirable to have an
> experimental designs with a fairly "high-level" baseline condition, so
> that the total amount of activity in the brain is minimised, apart from
> a few small clusters?
>
Again, I agree with Stephen Smith, preferably, one should be able to control
for this issue in the statistical analyses, not the experimental paradigm.
I believe that future versions of SPM are planned that implement Jesper's
technique (Andersson, (1997). How to estimate global activity independent of
changes in local activity. Neuroimage, 60, 237-244).
I have discussed this issue with a number of colleagues off the spm line and
would be keen to know if other groups have any other comments on this issue.
Best regards,
Kent
______________________________
Kent A. Kiehl
2255 Wesbrook Mall
Departments of Psychology and Psychiatry
University of British Columbia
Vancouver, B.C., Canada, V6T 2A1
http://www.psychiatry.ubc.ca/sz/nilab/personnel/kiehl/
Office 604-822-0777; Lab 604-822-7128; Fax: 604-822-7756
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