Dear Stephan,
> Hi Kalina, your page has been very helpful in filling in some gaps in
> my SPM knowledge. I was so happy to happen upon it while checking out
> the Gabrieli page. In particular, the global scaling discussion
> helped to clarify the issue beyond what I got from the Neuroimage
> note by G. Aguirre. Still, I have a question regarding global
> scaling. If you do not use global scaling at the 1st level of a
> random effects analysis, should you also not use it at the second
> level? I suppose my question boils down to: is it valid to perform an
> entire analysis, either fixed or random, with no correction for
> subject differences in overall mean signal? What artifacts does one
> risk?
To my understanding, there are 2 types of scaling:
1) Global Scaling - which is done if 'global scaling' is selected. This
means the voxel values in every V-file get divided by the mean of all
voxels for this V-file. This scaling can be problematic if the global mean
correlates with the task; it can introduce changes in the signal.
2) Grand Mean Scaling - which is done if 'global scaling' is NOT
selected. During this scaling, the voxel values in every V-file get
divided by the mean of the global means for all V-files within a
session. This type of scaling does not introduce changes in the signal.
So, even if you don't select 'global scaling', there will be a
correction for subject differences in overall mean signal, via the Grand
Mean Scaling (I believe, but other may correct me on this).
> Thanks for your help. I think the answer would be of interest to the
> SPM mail list.
I hope this is helping rather than confusing things further,
Kalina
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Kalina Christoff Email: [log in to unmask]
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