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?
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.
> 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.
> But isn't this the same as adding a dispersion derivative, which
> convolve a longer HRF automatically for RTs and capture that signal
> the same way as a variable epoch approach?
> Best regards.