I would point you to this excellent write up in Andy's brain blog
http://andysbrainblog.blogspot.ca/2014/08/parametric-modulation-with-spm-why.html
PMs are orthogonal to the main basis functions, and to each other. A side effect of this is that they are orthogonalized in order. So, the first PM is orthogonal to the main HRF basis set. All subsequent PMs are orthogonal to all preceding PMs, meaning they account for variance in the data not accounted for the earlier PMs.
If you put in attention first, it is in no way controlled for difficulty. If you want to remove difficulty effects from the data prior to considering he attention regressors, you need to put difficultly first. Mind you, this gives the difficulty regressors the first crack at the data, and it may take some of the attention related variance away (especially if they are correlated).
Hope that helps,
Colin Hawco, PhD
Neuranalysis Consulting
Neuroimaging analysis and consultation
www.neuranalysis.com
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-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Mike
Sent: September-07-16 10:35 PM
To: [log in to unmask]
Subject: [SPM] Two parametric modulator
Hi,
My regressor of interest is a working memory period. If I added one parametric modulator (trial-by-trial attention level) and used contrast 1 for this modulator, the regions I identified are those whose activity is parametrically modulated by attention during working memory. If I added two parametric modulators for this regressor, attention level and difficulty score that subject reported in each trial, and used contrast 1 0, can I say I am identifying attention-modulated regions regressing out the potential confounding effect of task difficulty?
Thanks
Mike
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