Sorry but couldn't understand the difference between neural and
Probably I should read more on the topic, because I thought dispersion
derivative was also trial specific. But I can imagine a model with
mixed properties, so that normal regressors are convolved with impulse
HRF functions (dur = 0), while RT regressors convolved with variable
duration HRFs (dur = RT). At the end shouldn't be difficult for SPM to
asses both regressors. They just get e beta value who tells how well
the HRF for that regressor explains variability. Am I correct on this?
2009/3/18 Jason Steffener <[log in to unmask]>:
> Yes, you have it right.
> If you currently have events modeled their durations are 0. With the
> variable epoch model the durations become the trial specific RTs. Just make
> sure you are consistent between whether you are specifying time in TRs or
> On Wed, Mar 18, 2009 at 12:29 PM, Esther Fujiwara <[log in to unmask]>
>> 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
>>> the same way as a variable epoch approach?
>>> Best regards.