Hi Benjamin,
The model that you need is the interaction one that you noted:
http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Two_Groups_with_continuous_covari
ate_interaction
Why are you saying that it isn't correct?
You can't look for an interaction between groups unless your model
includes separate Behavioral EVs for each group.
cheers,
-MH
--
Michael Harms, Ph.D.
-----------------------------------------------------------
Conte Center for the Neuroscience of Mental Disorders
Washington University School of Medicine
Department of Psychiatry, Box 8134
660 South Euclid Ave. Tel: 314-747-6173
St. Louis, MO 63110 Email: [log in to unmask]
On 7/29/13 1:03 PM, "Benjamin Philip" <[log in to unmask]> wrote:
>I'm having trouble organizing a FEAT third-level analysis correctly. I
>have 2 groups, and a behavioral measurement for each participant. What I
>really want to see is "Behavior-correlated activity in patients" minus
>"Behavior-correlated activity in controls". (P*b) - (C*b).
>
>I tried the interaction model suggested at
>http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM#Two_Groups_with_continuous_covar
>iate_interaction but it's not addressing the correct question.
>Theoretically I say this because we're looking for a subtraction, not an
>interaction. More convincingly/clear-to-express, it's empirically the
>wrong model, because it produces results wildly different from what we
>see comparing (P*b) and (C*b) by eye.
>
>I've also tried a contrast of [1 -1 1] on the EVs "Patient, Control,
>Behavior" - i.e. like this oversimplified version:
>Pat Ctl Bhvr
> 1 0 .4
> 1 0 -.2
> 0 1 -.3
> 0 1 .1
>...If I do that, [1 -1 1] looks pleasingly different from [-1 1 1], but
>something is deeply wrong with this modeling: [1 -1 1] and [1 -1 -1] and
>even [1 -1 0] look nearly identical. The same areas can't be both
>behavior-correlated and behavior-anticorrelated, so I'm doing something
>wrong. [1 1 -1] and [1 1 1] also look near-identical, but at least [0 0
>1] and [0 0 -1] show different areas.
>
>Any suggestions for how to implement (P*b) - (C*b)?
>
>Thanks,
>
>-Benjamin Philip
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