Dear Tavis,
>> I am trying to use SPM 99 to do a group analysis of some fMRI data
>> concerning cigarette smoking. Subjects are initially at rest in the
>> scanner (e.g. for 15 images), and then begin inhaling smoke (eg. for 85
>> images). We would like to know which areas of the brain increase their
>> activity as nicotine blood levels rise. Since we don't yet have a
>> catheter in place to directly measure blood levels, we will approximate it
>> as a linear increase from the time of first inhalation.
>>
>> How should I ask SPM to do a linear regression of this kind?
To add to Jesper's answer; would it be better to use something other than a
linear function to model the changing blood levels?
It's a long time since my clinical pharmacology training, but I seem to
remember (correct me if I'm wrong) that if you start an intravenous
infusion, plasma levels rise exponentially towards a steady state determined
by the infusion (inhalation) rate and rate constant of elimination i.e.
[nicotine] proportional to Kinhale*(1 - e^-Kelim.time)
This would suggest that it would be better to model the nicotine blood
levels with a user defined regressor that has this form i.e. (1 -
e^-scan_number) rather than just the scan number.
Best wishes,
Geraint
---------------------------------------------
Dr. Geraint Rees
Wellcome Advanced Fellow,
California Institute of Technology,
Pasadena,
CA 91125
voice (626) 396-2880
fax (626) 796-8876
web http://www.klab.caltech.edu/~geraint
---------------------------------------------
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|