Dear Caitlin,
> I have read over many of the posts here about how to execute time derivative models, but I have a few further questions.
>
> 1. My understanding of finding activity for the time derivative is that this activity reflects regions where the peak of the hrf occurs approximately 1 sec either before or after the canonical hrf (depending on whether the contrast weight is positive or negative). Is that an appropriate interpretation?
Sort of. The combination of the canonical hrf and the temporal
derivative can model a wide variety of shapes of hrfs (see Figure 1 in
Calhoun et al, 2004). Some of these fall in the category of a
time-shifted canonical hrf, but others less so. So the safest
interpretation is probably just that there is a non-canonical shape to
your hrf, with further interpretation depending on actually looking at
your data.
> 2. If we use a time derivative model, what does one usually report? Just activity associated with the time derivative? Activity associated with the hrf as well?
It is difficult to interpret the temporal derivative term in the
absence of knowing something about the canonical hrf as well, so it's
good to report both. And in fact, if you are interested in latency
differences, you will probably need to account for both of these terms
in your analysis. The following papers may be of some help:
Calhoun VD, Stevens MC, Pearlson GD, Kiehl KA (2004) fMRI analysis
with the general linear model: removal of latency-induced amplitude
bias by incorporation of hemodynamic derivative terms. NeuroImage
22:252-257.
Henson RNA, Price CJ, Rugg MD, Turner R, Friston KJ (2002) Detecting
latency differences in event-related BOLD responses: Application to
words versus nonwords and initial versus repeated face presentations.
NeuroImage 15:83-97.
Steffener J, Tabert M, Reubemn A, Stern Y (2010) Investigating
hemodynamic response variability at the group level using basis
functions. NeuroImage 49:2113-2122.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Center for Cognitive Neuroscience and
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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