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For my understanding, in SPM would a variable epoch model be implemented by using the respective RTs as durations for single events, instead of 0s? Or is there more to it? Esther Jason Steffener wrote: > The variable epoch model uses the RT from each trial; therefore, it is > able to capture trial specific variance. The impulse with HRF + > derivatives may capture some of the variance due to RTs but it > essentially takes the average RT over all trials for this condition. And > as Chris points out there may be some RTs where the impulse model can in > no way accuratly account for. > > I also feel that the HRF + derivatives should be used to capture > hemodynamic variations and not neural variations. Otherwise you make it > very difficult to tease about which is which. > > Jason. > > On Tue, Mar 17, 2009 at 6:19 PM, Chris Watson > <[log in to unmask] > <mailto:[log in to unmask]>> wrote: > > I think it would depend on the shape of your HRF. The variable epoch > model has boxcars that are as long as the RT,. If you used an > impulse model, convolved with the canonical hemodynamic response, > even adding the dispersion derivative might not capture the signal > for long RT's (as the shape of the HRF in the variable epoch model > will be quite different from the canonical). E.g. in one of our > tasks, we see RT's of up to 7000ms. I don't think an impulse model > even with both derivatives would do nearly as well as an epoch model. > > > Dorian P. wrote: > > Dear all, > > Thinking about a previous discussion on the list, we said that > reaction time effects are better captured by a variable epoch > durations, which adapts to reaction time length. > In a couple of papers was shown that a variable epoch aproach is > better than parametric modulations. > > http://www.sciencedirect.com/science/article/B6WNP-4T77G33-4/2/cc5ef4a8e9fbff5b4a99bd5f05663bf9 > http://www.columbia.edu/cu/psychology/tor/Posters/grinband_HBM06.pdf > > But isn't this the same as adding a dispersion derivative, which > would > convolve a longer HRF automatically for RTs and capture that signal > the same way as a variable epoch approach? > > Best regards. > Dorian. > > >