Martijn -
I think this WIKIpage should help:
http://imaging.mrc-cbu.cam.ac.uk/imaging/ParametricModulations
Happy new year
Rik
> Hello Rik Henson and/or other SPM'rs that have time and knowledge to share an answer,
>
> I want to control for (between condition) reaction time in a 2x2 factorial design.
> I collapsed factors as parametric modulations, and add rt as paramatric modulator to
> control for global rt effects according to:
>
> https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind06&L=SPM&P=R750063
>
> I can create a regressor for all trials (factor A + B), and add each factor as a parametric
> modulator by:
>
> 1) using 2 modulations
> 1) factor A: A1 as [1 1 1 1 ...] and A2 as [-1 -1 -1 -1 ...]
> 2) factor B: B1 as [1 1 1 1 ...] and B2 as [-1 -1 -1 -1 ...]
>
> or
>
> 2) using 4 modulations
> 1) A1 as [ 1 1 0 0 0 0 0 0]
> 2) A2 as [ 0 0 1 1 0 0 0 0]
> 3) B1 as [ 0 0 0 0 1 1 0 0]
> 4) B2 as [ 0 0 0 0 0 0 1 1]
>
>
>
> I prefer the second option because I want to take effects of each seperate condition (A1,
> A2, B1, B2) to a second level factorial design.
>
> Obviously, this won't work with option 1. However, using option 2 gives some problems in
> the SPM design: one of the parametric modulators are zeros (i.e. black in the design).
> I guess this is due to the redundancy Rik is talking about in the thread mentioned above.
> I tried 3 param mod's (N-1), but I'm not sure which contrasts to use to capture the effects
> for each condition: Maybe [0 1 0 0 0], [0 0 1 0 0 ], [0 0 0 1 0 ] and [1 -1/4 -1/4 -1/4 0] ?
> These effects doesn't look quite the same as in the original categorical design though.
>
> In short: Can somebody help me to get beta's for each seperate condition (A1, A2, B1,
> B2) in a parametrical design, reflecting the same effects as if I used a categorical design
> with A1, A2, B1, B2 as seperate regressors?
>
> Thanks,
>
> Martijn
>
--
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DR RICHARD HENSON
MRC Cognition & Brain Sciences Unit
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