I think I found an answer here (Andrade, A., Paradis, A., Rouquette, S., & Poline, J. (1999). Ambiguous Results in Functional Neuroimaging Data Analysis Due to Covariate Correlation. NeuroImage, 10(4), 483-486. doi: 10.1006/nimg.1999.0479)

Which was pointed to by Poline et al in Chapter 9 of this book (Friston, K. J., Ashburner, J. T., Kiebel, S. J., Nichols, T. E., & Penny, W. D. (2006). Statistical Parametric Mapping: The Analysis of Functional Brain Images (1er ed.). Academic Press.)

Good luck.
Max

2009/12/17 Maximilien Chaumon <[log in to unmask]>
Dear Paul,

I'm running into exactly the same kind of question than what you asked here. Did anyone help you on this?
I would greatly appreciate if you could let me know how it all went... or if you found any good reads on the topic.

Thanks a lot,
Max


2009/11/25 Paul Smeets <[log in to unmask]>
Dear all,

I would greatly appreciate anybody's thoughts on the following, if only some reassurance:
I have a stimulus with 2 parametric modulators, INT and PNS (both subjective ratings). I am primarily interested in the modulating effect of INT independent of that of PNS. However, INT and PNS tend to covary. If I understand correctly, if I use PNS as the first modulator and INT as the second, then testing for modulation by INT will give me the unique effect of INT, i.e. the thing I am interested in. Conversely, if  I use INT as the first modulator and PNS as the 2nd, then testing for PNS will give me the unique effect of PNS.
The question is, what does testing for the first modulator tell me in these 2 cases?
Is there any other/better way to disentangle the effects of two modulators which strongly covary?

Thanks for any thoughts/advice on this,

Paul
 
-- 
~~~~ 

Paul A.M. Smeets, PhD
Image Sciences Institute
University Medical Center Utrecht
Heidelberglaan 100, Rm Q0S.459
3584 CX Utrecht, The Netherlands
Tel: +31 88 75 53172

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