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.
>
>
>
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