Dear Jon,
You might also consider using the 'parametric modulation' option to explore
time-dependent effects by creating boxcar regressors that are modulated by
an exponential (or other) function of time. This is slightly more
complicated to set up, but has the potential merit of greater flexibility in
characterising the nature of any time-dependent effect.
Richard also said:
> One could imagine a condition by time
> interaction occurring non-specifically, perhaps even because of the
> physics of the scanner.
Although I agree with Richard's general points, I'm not sure about this
specific point. Overall fMRI signal in all conditions may wander over time
for a variety of experimentally uninteresting reasons. For example, low
frequency drifts in signal are common to all conditions and usually removed.
But relative differences in signal between two conditions that change over
time ('condition-by-time interactions') have by their nature
condition-specific causes. I agree these may be trivial or uninteresting,
but in Richard's example I think would have to be some condition-specific
scanner physics that affected the active condition but not the rest
condition (or vice-versa) in a time-dependent way.
best wishes,
Geraint
--
Dr. Geraint Rees
Wellcome Advanced Fellow, Lecturer,
Division of Biology 139-74, Institute of Neurology,
California Institute of Technology, University College London,
Pasadena CA 91125 London WC1N 3BG
voice 626-395-2880 020-7833-7472
fax 626-796-8876 020-7813-1420
web http://www.klab.caltech.edu/~geraint
--
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