Dear Maria,
>I had posted a question earlier regarding fractional signal change, and now
>realize that this is actually the % signal change. I understand that for my
>fMRI data using SPM99 I can, Plot > fitted and adjusted responses > choose my
>contast > plot against scan or time > this gives me a graph of % signal change
>over time.
>
>Previous posting have stated that I would need to choose "proportional scaling"
>What does this mean? Is this choosing 'scaling' or 'none' for Global
>Normalisation?
For fMRI, SPM (by default) scales each image within a session with
100/(mean of the GMIs of this session). For a single session, this does
not have any effect on the parameter estimation or inference, etc. It
is just a global scaling of the whole session data. For multiple sessions,
this grand mean session scaling has the advantegeous effect that each
session is scaled to the same grand mean of 100. Note that this is done
by default, i.e. if you click 'none' when asked for proportional
scaling. You can instead apply proportional scaling, which scales each
image by 100/(mean of its GMI).
There is a very good description of how the % signal change is
determined via SPM given by Stefan Kiebel in the SPM archives
(http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0011&L=spm&P=R14245)
Hope that helps,
Arshad
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Dr. Arshad Zaman
Magnetic Resonance & Image Analysis Research Centre
University of Liverpool
Liverpool
L69 3BX
Tel + 44 (0) 151 794 5645
Fax + 44 (0) 151 794 5635
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