> 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?
I think it would work fine - though I'm still unsure why you want
to do it this way - you could model all your events as the same
"SPM type", but then code not only RT as a parametric modulation,
but also code, as extra modulations (using 1's and -1's as dummy
variables), which event corresponds to which "true type" (condition).
Rik
>> -----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
>>
>> ---------------------------------------------------------
>>
>> DR RICHARD HENSON
>>
>> MRC Cognition & Brain Sciences Unit
>> 15 Chaucer Road
>> Cambridge, CB2 2EF
>> England
>>
>> EMAIL: [log in to unmask]
>> URL: http://www.mrc-cbu.cam.ac.uk/~rik.henson
>>
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>>
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>
>
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