Thanks a lot, Dr Nichols. The link helps.
I was wondering why can't one just globally scale each person's fMRI data
around a grand mean of 1000 prior to stats? Is there a way to do this? Is
the problem that spm's masking routine and /or way of counting voxels as
within brain sometimes flawed?
Sincerely,
Jeff
On Mon, 1 Dec 2003, Thomas E Nichols wrote:
> > In response to my question below, Russ wrote that data are generally
> > globally scaled to a mean of 100 on the beta0 image I think (the
> > baseline)? Is this true by default for spm2 data as I found several
> > subjects with within-brain average beta0's in the 300's across all
> > voxels?
>
> I've observed this too. For an explanation see
>
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0204&L=spm&P=R11588&I=-1
>
>
> For more on how to robustly estimate typical intensity with the mode
> see Appendix II in
>
> Wen-Lin Luo and Thomas E. Nichols,
> Diagnosis and exploration of massively univariate neuroimaging models.
> NeuroImage, Volume 19, Issue 3, July 2003, Pages 1014-1032
>
> -Tom
>
>
> -- Thomas Nichols -------------------- Department of Biostatistics
> http://www.sph.umich.edu/~nichols University of Michigan
> [log in to unmask] 1420 Washington Heights
> -------------------------------------- Ann Arbor, MI 48109-2029
>
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