On Thu, 21 Feb 2008 18:25:16 -0800, Jeanette Mumford <[log in to unmask]>
wrote:
<snip>
>As for you second question, I haven't used SPM in a while, but I'm fairly
>sure that like FSL it grand mean scales the data, so that on average all
>subjects have approximately the same timeseries average (in FSL this is
>about 10000). This is necessary to make the higher level analysis valid
for
>precisely the reason I think you pointed out. So both the %-signal change
>and the betas are comparable across subjects.
By default, SPM also does grand mean scaling. In SPM, grand mean scaling
means: for each session for a given subject, scale the data by the mean
over all voxels over the entire session. ("Session" means a time series
when the scanner is running continuously.)
<snip>
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