Dear Rik,
Thanks for those useful threads.
Just to clarify: If I do want to co-vary out common RT effects, would my
original suggestion (a) be an appropriate alternative to the parametric
model approach in the thread you referred to?
Cheers,
Tobias.
Tobias Egner, PhD
fMRI Research Center
Columbia University
Neurological Institute, Box 108
710 West 168th Street
New York, NY 10032
Tel: (+1) 212 342 0121
Fax: (+1) 212 342 0855
Email: [log in to unmask]
> -----Original Message-----
> From: Rik Henson [mailto:[log in to unmask]]
> Sent: Tuesday, February 15, 2005 11:47 AM
> To: Tobias Egner; [log in to unmask]
> Subject: Re: [SPM] RT covariate/variable event duration
>
>
> > A couple of related questions concerning the modeling/control for
> reaction
> > time differences between conditions in an event-related design. If I
> want to
> > include RT as a covariate to control for common RT effects across 4
> > conditions (rather than use condition-specific parametric modulations),
> but
> > not turn my analysis into a parametric analysis (such as in Will Penny's
> > example:
> > http://www.fil.ion.ucl.ac.uk/~wpenny/datasets/face-rep/SPM2/README-
> SPM2.txt)
> > , could I do the following:
> >
> >
> >
> > a) create a model where I only have one even-type (all events), and
> > employ RTs as a parametric modulation of that regressor; then extract
> the RT
> > regressor from the design matrix and enter it as a user-specified
> regressor
> > in my original model?
> >
> >
> >
> > b) How about directly modeling each event as having the duration of
> its
> > associated RT (i.e. event durations would vary between ca. 500-1000ms,
> > rather than being modeled as stick functions)? Is there a strong a
> priori
> > reason not to analyze data in this way? And finally, would these 2
> analyses
> > possibly produce the same results?
>
>
> Tobias -
>
> For your question b, see:
>
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind04&L=spm&P=R555203&I=-1
>
> For your question a, for which I don't see the rationale unless you are
> simply
> wondering how to covary RT effects that are common across conditions
> (rather
> than separately for each condition), this can be done with parametric
> modulations
> as described on this thread:
>
> http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind04&L=spm&P=R199168&I=-1
>
> Rik
>
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>
> DR RICHARD HENSON
>
> MRC Cognition & Brain Sciences Unit
> 15 Chaucer Road
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>
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