On Tue, 8 Feb 2005 19:19:48 +0100, Karsten Specht
<[log in to unmask]> wrote:
>Dear Hyo Jong,
>
>> I have a basic question about global scaling and grand mean.
>> I searched archives and found similar questions asked without
>> clear answers to them.
>>
>> Should I set 'Global scaling' option during the first level
>> analysis, if I plan to apply results to second level analysis
>> for group comparison later?
>> The scale of baseline from each subject are not same. Thus,
>> I am confused whether the first level analyses should have
>> the global scaling option or spm2 handles this matter by default.
>> Also, what is purpose of setting 'grand mean' to a specific value?
>
>I would recommend to skip the scaling on the first level, and skip it
definitely on the second level.
>Your session/subject regressor, assuming that you don’t have scaling
differences within one session, captures the different baselines on the
first level.
Session-specific regressors capture the baseline, but they don't capture
scaling differences between the sessions. That's why grand mean scaling
is necessary.
>The scaling on the second level is only useful, if your are going to
analyses other images than con_images; MRI-Perfusion images or PET images,
for example. Here, it is necessary, that the global mean is the same
across subjects.
>In case of a 'classical' second level analysis, you are analysing the con-
images, which are already appropriately scaled, i.e. positive values are
reflecting a signal increase, negative values a decrease and the value
itself represents somewhat like the strength of the effect.
>
>Good luck,
>
>Karsten
>
>--
>-------------------------------------------------------------
>Karsten Specht, PhD
>
>Department of Biological and Medical Psychology
>University of Bergen
>Jonas Lies vei 91
>5009 Bergen
>Norway
>Tel.: +47-555-86279
>Fax: +47-555-89872
>[log in to unmask]
>http://fmri.uib.no/
|