One check is to see whether the mean of the last beta in the design (the constant term) over the SPM-generated head mask is exactly 100. The ArtRepair toolbox includes a percent signal change function that includes this calculation. I often see a mean that is well over 100.
Best regards,
-Paul
----- Original Message -----
From: "Darren Gitelman" <[log in to unmask]>
To: [log in to unmask]
Sent: Monday, June 1, 2009 10:47:30 AM GMT -08:00 US/Canada Pacific
Subject: Re: [SPM] Grand mean scalin to 100
Manish
I believe the average global signal is scaled to 100.
-----
Darren Gitelman
On Mon, Jun 1, 2009 at 12:01 PM, Manish Dalwani <[log in to unmask]> wrote:
> Hi SPM'ers,
>
> If I have a contrast Condition A - Condition B (note condition B is not a
> resting state baseline), and SPM scales the baseline to 100 as per mean
> global intensity (that includes gray and white matter).
>
> Does it scale the condition B as baseline to 100 or does it first compare
> each condition to an implicit baseline and then compare the 2 conditions.
> (So let’s say condition A could be 101 and condition B could be 102).
>
> If the latter is true then where does it get it's implicit baseline from?
>
> Regards,
> Manish Dalwani
> Sr. PRA
> Dept. of Psychiatry
> Univ of CO Denver
>
>
--
Paul K. Mazaika, PhD.
Center for Interdisciplinary Brain Sciences Research
Stanford University School of Medicine
Office: (650)724-6646 Cell: (650)799-8319
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