Dear SPM group,
Using SPM 99b we tried to study correlations between an anatomic variable
named A and the amount of activation during two conditions T and R (we have
only two conditions and only one covariate)
We have n subjects and we entered the scans as followed :
T1 R1 T2 R2 ......Tn Rn
the covariate was :
A1 -A1 A2 - A2 ......An - An
We computed two contrasts in the result section.
The first was the activation T - R (weight 1 -1 0) and was named TR, the
second was the positive correlation between A and (T - R) (weight 0 0 1) and
was named corrA.
We then tried to get the correlation coefficient and the graphic plot
corresponding to each maximum displayed on the correlation map corrA using
the “Plot” function and then the“Fitted and adjusted response” option.
The question we have is about the next step which is to choose the
appropriate “contrast”.
In our case, we have two possibilities which are to either select TR or
corrA.
We tried both and obtained slightly different results. We don't know which
one is the one we are looking for.
We are guessing the appropriate solution is to select the file TR
corresponding to the difference between T and R rather than the one
corresponding to the correlation map corrA but we are not sure of it and
would also like to understand better how the software works in both cases.
Thank you for your help.
Fabrice Crivello, PhD
Groupe d'Imagerie Neurofonctionnelle
UPRES-EA 2127 Université de Caen & CEA-DSV-LRC 13V
GIP Cyceron, Bld Henri Becquerel, BP 5229
14074 Caen Cedex, France
Tél. : 00 33 (0)2 31 47 02 92
Fax : 00 33 (0)2 21 47 02 22
e-mail : [log in to unmask]
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