Subject: | | Re: Two parametric modulator |
From: | | Colin Hawco <[log in to unmask]> |
Reply-To: | | [log in to unmask][log in to unmask]> wrote:
> Dear everyone, > > I have excluded the first few volumes of an fMRI data series, but I would > like to account for trials that were presented during this period, in other > words, I would like to enter negative onsets. It seems that if they occur > "too early", SPM treats the onset as if it were not present at all. I found > this old message https://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=SPM; > 4da16ab5.0908 and http://www.fil.ion.ucl.ac.uk/spm/software/spm2/ > improvements.html > > "The option to perform a second-order or generalised convolution analysis > using Volterra kernels is now assumed to be the same for all sessions. You > can now also specify negative onset times up to 32 time bins (that default > to 1/16 inter-scan intervals)." > > In the meantime this seems to have been moved into spm_get_ons.m. I can > adjust the lines in which ton is defined accordingly, i.e. 6001 instead of > 33, but I'm not sure about two lines on sf, where 128 is added and not the > otherwise 32 default time bins in one line, and 32 again in the one with > comment % stimulus. I have also tried out a few values like 0, 32, 6000 and > had the impression that this results in identical predictors. > > Best > > Helmut >[log in to unmask] |
Date: | | Thu, 8 Sep 2016 09:43:25 -0400 |
Content-Type: | | text/plain |
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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
[log in to unmask]
-----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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