Hi Lee,
did you check the collinearity and multi-collinearity of your design? With
rapid ER-fMRI design there is a real chance that events from trial n+1
explain variance caused by events for trial n, especially when using Taylor
series expansions. That might explain the dramatically different findings, it
would suggest your design is sub-optimal.
Kind regards,
Bas
Op woensdag 26 oktober 2005 04:11, schreef Siobhan M. Hoscheidt:
> SPM Users,
> I would like some opinions regarding the optimal way to analyse a
> dataset. The study is a 2x2 event-related design, crossing memory type
> (semantic, episodic) with spatial content (spatial, nonspatial). Each
> trial includes a memory question that the subject reads and then responds
> to, lasting a total of 8 seconds. Subjects respond to the question with a
> button press, usually 3-4 secs after the onset of the trial. The time
> period after the button press, based on their rt for each trial, is
> specified as a separate "wait" condition and is not of interest. There is
> a separate control condition (reading a string of nonwords) that also
> requires a button press after about 3-4 secs.
>
> I'm currently analysing the data in spm99 as event-related, with 0 duration
> and no global scaling, and got some small but reliable regions of
> activation when comparing the various conditions to the control
> condition. I then re-analysed the data, also using 0 duration and no
> global scaling, but adding time and dispersion derivatives. Now when I do
> a t-test on the canonical HRF component, I get a very different pattern of
> activations that are much "messier".
>
> I'd appreciate hearing a) how people would deal with long trials (3-4
> secs). Would you specify 0 duration or use the rt as the specified
> duration for each trial? And b) Why does including the time and dispersion
> derivatives change the t-test results for the canonical HRF so
> dramatically? Does anyone have suggestions on the best way to analyse this
> dataset?
>
> Any guidance or advice on the matter would be greatly appreciated.
> Thanks,
> Siobhan Hoscheidt
> Lee Ryan
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Dr. S.F.W. Neggers
dept. of Psychonomics,Helmholtz Institute
Utrecht University
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