Hi all, I got no response the first time around so I thought I would
repost. I think this is an important question for anyone interested in
calculating percent signal change.
Dear SPM experts-
I am attempting to calculate signal intensity changes at a given voxel
using the corresponding beta coefficients and contrast weight vectors.
In doing so, I discovered that an old analysis (I think with SPM99b)
yielded effect sizes (beta coefficients) which are different than a
repeat analysis (with SPM99) with exactly the same data and model (at
least everything in the model appears to be identical unless I am
missing something very subtle.). This is the case except for the
constant beta- which is identical across analyses. {I am running t-tests
with 2 conditions modelled as events and looking at [-1 1] and [1 -1]
contrasts}. I think my problem relates to a previous message that Karl
addressed, but did not resolve:
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0003&L=spm&P=R5027&m=2612
Note that I have carefully checked that I am tracking the same voxel and
using the same scaling (global) between analyses. Furthermore, the
resulting SPMs, Beta.imgs, and models appear indistiguishable between
analyses.
What might cause such a discrepancy in resulting beta coefficients
(excepting the constant)?
Any insight on this will be appreciated. I am currently very
distrusting of my % signal change calculations because of this.
Thank you,
Michael
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