Thanks for the quick response! So if I wanted to use the Calhoun 2004 approach, I should be able to use the Steffener 2010 correction to address the violation of the assumption that the regressors were standardized and generate a new beta.nii.gz file where the primary beta values have been replaced with the Calhoun 2004 measure. Can I assume the three regressors are more or less orthogonal? I got non-zero numbers when I tried to test the assumption in the Xtmp.X variable
sum(X(1:207,4).*X(1:207,5))
but not hugely non-zero so maybe just rounding errors?
On 07/09/2013 04:11 PM, Joseph Dien wrote:
Hi,
I have a question about how mkcontrast-sess works. I ran an
analysis using the mkanalysis-sess option spmhrf 2 so there are three
regressors for each predictor, the primary, the latency, and the
dispersion. When specifying the contrast weights for mkcontrast-sess,
the documentation indicates that they are specified in terms of the
conditions as numbered in the paradigm file, not the individual
regressors. Furthermore there only appears to be one contrast value
output for each contrast, not three.
How are the three regressors being handled? I can think of several
scenarios:
1) the contrast weights are not actually in terms of conditions (the
documentation is incorrect), they are actually in terms of the
regressors (so contrasting conditions 1 and 2 could be specified as -a
1 -a 2 -a 3 -c 4 -c 5 -c 6).
2) the latency and dispersion regressors are being ignored (a common
practice). The contrast weights should therefore be specified as -a 1
-c 2.
This is what happens. If you want to use the derivatives, then you need
to spec -setwdelay. When you run the command, it will prompt you for 3
values to use. If you spec 1 0 0, then it will be the same as the
default. If you want to test only the first derivative, then you would
spec 0 1 0. Note that the 3rd regressor is the 2nd derivative wrt time,
not the first derivative wrt the dispersion parameter. You cannot get
the Calhoun 2004 value using a contrast (it is non-linear).
doug
3) The Calhoun et al (2004) approach is being used to combine the
three regressors into a "derivative boost" amplitude
measure. The contrast weights should therefore be specified as -a 1 -c 2.
Thanks for any help you can give me!
Joe
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Joseph Dien,
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University of Maryland
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Phone: 202-297-8117
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Joseph Dien,
Senior Research Scientist
University of Maryland
Phone: 202-297-8117