Dear Paul, dear Edward
> I was wondering if anyone has used reaction times as a parametric
> modulator? I understand how to do this but I am having problems
> interpreting the results. I have a hypothesis that when subjects have
> decreased reaction times (in my paradigm), specific brain regions will
> be activated. Is parametric modulation the most appropriate tool to
> explore this issue?
It is. You are asking the question whether there is a linear relationship
between RT and brain activity. A possible reason for a correlation could
be that the more activity a certain set of brain regions show, the faster
the response ( and vice versa). You can also test for well behaved
nonlinear relationships, e.g. a quadratic component.
I agree with Edward that efMRI is the appropriate modality to do this.
There is however a caveat in event related fMRI. Consider a simple visual
RT task. Most likely the onset for the modelled response in SPM is the
visual task. The first problem is that you will always see motor activity,
which of course can be correlated to RT. Another problem is RT itself: The
onset of the motor event is determined by the RT. I.e. different RTs lead
to different motor response latencies. If you are using a HRF model only
those differences in latency can (falsely) appear as a difference in
amplitude. In this case a temporal derivative can help and it would be
interesting to see which areas show a parametric modulation of the
derivative regressors (different latency, most likely motor associated)
and which show a true (early) amplitude modulation.
-Christian
--
Dr. Christian Buechel
Neurologische Universitaetsklinik, Haus B
Universitaets-Krankenhaus Eppendorf
Martinistr. 52
D-20246 Hamburg
Germany
Tel.: +49-40-42803-4726
Fax.: +49-40-42803-5086
email:[log in to unmask]
www.uke.uni-hamburg.de/kliniken/neurologie/pages/mitarbeiter/buechel_c.htm
www.uke.uni-hamburg.de/kliniken/neurologie/pages/forschung/cnl_index.htm
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