Dear SPM group:
I know there has been previous correspondence on this topic with
regard to calculating percent change and I wanted to check out my
understanding. I am interested in obtaining condition means for fmri data
on a voxel by voxel basis separately for each of a group of subjects. Each
subject was processed separately. We performed a blocked analysis where
there were 4 conditions (and also included the 4 temporal derivatives to
adjust for the possibility of minor timing errors). Since SPM
automatically includes the intercept there are then 9 columns in the design
matrix for extracting contrasts
(cond1, td1, cond2, td2, cond3, td3, cond4, td4, constant). I
think the answers to the following questions are "yes" but would appreciate
confirmation of this or guidance if I am missing something.
(1) Does the following contrast test for a condition 1 effect versus zero
(null hypothesis, no slope)?
cond1 td1 cond2 td2 cond3 td3 cond4 td4 constant
1 0 0 0 0 0 0 0
0
(2) Do the obtained contrast image values indicate the condition 1 mean,
but with the global scaling removed since the intercept was included in the
model?
(3) Is the sum of beta1 (ie, cond1) plus beta9 (ie, constant) the actual
mean for condition 1?
Best wishes and many thanks,
Andy
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Andrew J. Saykin, PsyD, ABPP
Associate Professor of Psychiatry & Radiology
Neuropsychology Program and Brain Imaging Laboratory
Department of Psychiatry - DHMC
Dartmouth Medical School
Lebanon, NH 03756
Tel. (603) 650-5824
Fax (603) 650-5842
email:[log in to unmask]
http://synapse.hitchcock.org
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