Dear Paul,
> I have been using the multiple regression in SPM8 on some PET data. I want
> to compare two models (linear and exponential). I want to look at separate
> contrasts for the linear and the exponential fits. However when I put both
> of them in the plots do not come out looking exactly like a linear or
> exponential function, they have some strange bends in them. I think this
> has something to do with orthogonalizing the basis functions. Does anyone
> have any suggestions or know the location of the MATLAB code that implements
> the multiple regression? Can the orthogonslization be turned off?
The standard multiple regression shouldn't include any automatic
orthogonalization. However, if you're including both the linear and
exponential (quadratic?) predictors in the same model and they aren't
independent, then the parameter estimates will be reflecting the
independent contribution of each regressor, which may explain your
results.
I think the key thing would be to double-check the design matrix (X,
i.e. SPM.xX.X) and verify that your predictors look as expected, and
that the full model is giving a reasonable fit. If you actually want
to compare the linear vs. exponential fit objectively, then you may
want to construct separate models and look at the residuals, AIC/BIC,
or something similar.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Center for Cognitive Neuroscience and
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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