Thanks.
Is the orthogonalization method (Desjardins et al. 2001) implemented anywhere?
Best,
Chris
On 15 July 2011 23:05, Matthew Brett <[log in to unmask]> wrote:
> Hi Chris,
>
> On Fri, Jul 15, 2011 at 7:54 PM, Chris Filo Gorgolewski
> <[log in to unmask]> wrote:
>> Dear SPMers,
>> As far as I know SPM provides two methods of removing global signal
>> effects: scaling with scan (volume) mean or session mean. I managed to
>> find the code responsible for calculating those means and applying
>> corrections, but I was wondering if those means were later on used as
>> nuisance regressors (and if yes where is it in the code). I also heard
>> heard that SPM was departing from removing global signal due to
>> negative activation one can get. Therefore it would be great if
>> someone could shed some light on this issues.
>
> I've not paid close attention to this recently, but here's my understanding:
>
> Each scan i=1..N has a global value g_i, giving a vector g of globals, length N.
>
> The default in FMRI is to normalize the average global signal in the
> run to 100, that is, each voxel value in scan i is multiplied by g_i /
> mean(g) * 100 - for each session separately.
>
> The default in PET is to add the vector g as a covariate of no
> interest (ANCOVA adjustment).
>
> There was a phase were we were dividing each voxel value by g_i -
> proportional scaling. This was the thing generating most of the odd
> negative signal, and it's very rare to do that these days.
>
> I think that's right, please y'all do correct me if I'm wrong.
>
> Best,
>
> Matthew
>
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