Dear Elizabeth,
I would simply measure your response times and put the real stimulus durations in your model, instead of letting a Taylor series approximations such as temporal/dispersion derivatives estimate your varying response. I think series expansions are only useful when you don't now the variability in HRF duration/timing, and want to catch it. Since you do know the real stimulus durations (by recording reactions) and hence the expected changes in HRF shape, you could use that a priory knowledge to create your GLM instead of adding derivatives. Or perhaps I am missing the point here?
Good luck,
Bas
-----Oorspronkelijk bericht-----
Van: SPM (Statistical Parametric Mapping)
[mailto:[log in to unmask]]Namens Elizabeth Chua
Verzonden: vrijdag 1 april 2005 22:56
Aan: [log in to unmask]
Onderwerp: [SPM] variable durations and dispersion derivatives
Hello SPMers-
I'm using a self-paced event-related design where the stimulus offsets are controlled by the
subjects responses. There are differences in reaction times between different conditions which
are typically on the order of 1-2 seconds, but occasionally as large as 3 seconds. There is also
varability with a condition for the stimulus duration, typically on the order of 1 second.
My understanding is that modeling the durations models different "heights" for the hemodynamic
response. Is this correct? If so, does this work within a condition, between conditions, or both? Is
this recommended?
Also, my reading of the dispersion derivative is that this accounts for differences in the width/
length of the hemodynamic response. Is this something I should use with my reaction time
differences? or will it remove differences between conditions that might be meaningful?
Thank you in advance,
Elizabeth
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