> 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?
This definitely makes sense. Note that the thing you are interested in is not the duration of each word per se, but the duration of the neural activity associated with processing each word. The assumption is that a different response time reflects a difference in the duration of the underlying neural activity preceding that response. So this seems like an excellent situation in which to employ a parametric modulator.
> 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 don't have practical experience here, so maybe others could comment. I would start with a second order polynomial expansion. If the quadratic term ends up not explaining very much, I suspect it won't hurt you too much, either—you lose degrees of freedom at the single subject level, but not at a group level. You can then look at this empirically and decide whether the second order expansion was helpful.
Hope this helps!
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
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3400 Spruce Street
Philadelphia, PA 19104