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:
- 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).
- 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
- 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
Joseph Dien, Senior Research Scientist University of Maryland
E-mail: jdien07@mac.com mailto:jdien07@mac.com Phone: 202-297-8117 http://joedien.com//
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