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I should have mentioned: Mean RTs to the three stimulus types were just under 2000 ms, so roughly speaking the ISIs were in the range of 4.5 to 7.5 s (mean=5.5 s). On Mon, Feb 4, 2013 at 9:05 AM, Bob Spunt <[log in to unmask]> wrote: > Dear experts, > > I am writing to see if anyone can share their thoughts on my analytical > approach. In an event-related design, participants made manual responses to > three stimulus types: A, B, and C (stimulus onset asynchrony varied from 6.5 > to 9.5 s, mean of 7.5 s). At response, stimuli were replaced by a fixation > cross until the onset of the next stimulus. My research question regards > these interstimulus intervals (ISIs). Namely, is variation in BOLD activity > before stimulus onset (i.e., during the ISI) associated with response time > (RT) to stimulus onset? I want to estimate this association for each of the > three stimulus types. > > I am currently using a parametric modulation analysis, with three regressors > modeling the response to each of the three stimulus types (variable epoch, > duration = RT) and three regressors modeling the response to the ISI > preceding each of the three stimulus types (impulse function). (I should > briefly note that trials with outlier RTs are removed and modeled in a junk > regressor.) I chose to model the response-to-ISI as an impulse function to > minimize multicollinearity in the design matrix. Finally, I included three > additional regressors modeling variability in the response to the ISI as a > function of the following parameters (all de-meaned): > > 1. RT to the next stimulus (the regressor of interest) > 2. Total duration of the ISI > 3. A binary variable indexing whether or not the prior stimulus was of the > same type as the next stimulus > > With this approach, the models estimate and I am actually getting reasonable > results. However, given that I have not found any precedent for this > approach in the literature, I am writing to see what others think. If you > have any thoughts, I would be grateful to hear them. > > Cheers, > Bob >