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 <
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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.
