I repost two questions which I posted 2 days ago which haven't received any responses.
I know these are simple questions but I would really appreciate any kind of help:
Dear SPM experts,
I have two questions regarding the usage of a parametric modulator in a semantic priming experiment:
1. Does it make sense to use reaction time as a parametric modulator of a target word which follows a priming word and of which subjects have to decide as quickly as possible if the target word is related to the priming word or not?
In the book on SPM by Karl Friston et al. I read in chapter 15:
..."So the choice of model depends on the assumptions about the duration of neuronal activity in the particular region of interest. If this is unknown, trials whose durations vary over a few seconds (as with typical RTs) are probably best modelled with two regressors: one modelling events, and a second modelling a parametric modulation of the response, by the RT on each trial."
In my case the length of the events (presentation of target word) is always the same and there-fore does not depend upon the RTs. Does it still make sense to use RT as a parametric modu-lator of the target word?
2. If it does make sense than the second question is: Which order of polynomial expansion should I use? I would’ve guessed that RT’s require a second order polynomial expansion but I’m quite unsure about this!
I would appreciate any kind of help!