Dear Natalia,
> I am analyzing an fMRI-training study we performed on stroke patients using
> a motor task. During the task I recorded several parameters like for example
> the force applied by the patients to perform the task and the range of
> movement. The patients of the study were measured twice before the training
> and after the training. Since the task performance of the patients is better
> after the training, I would like to introduce the aforementioned parameters
> (mean force per scan and mean range of motion per scan), as regressors of no
> interest at single subject level. The aim is to ensure that the observed
> brain activation after the training is due to the training itself and not
> because the participants performed the task better. Is it correct to use
> such parameters (force, range of motion) as regressors of no interest or
> not?
In general, including some of these additional parameters as
regressors (of no interest) seems like a sensible thing to do.
However, it gets tricky if they differ on one of these parameters
across sessions that you are interested in comparing. If they
actually differ on a regressor, then you probably don't want to
include it. This is explained in more detail in the very helpful
Miller & Chapman paper:
Miller GA, Chapman JP (2001) Misunderstanding analysis of covariance.
Journal of Abnormal Psychology 110:40-48.
In which case, you will be left with pre/post training being
confounded with some of your other measures, and just have to live
with that being a possible confound. However, you may be able to get
some information by doing correlations across subjects—e.g., do
participants who perform the task better activate a brain region more
or less than participants who perform it worse. If not, that might
also suggest that the differences you see are not due to simple task
performance.
Hope this helps!
Best regards,
Jonathan
--
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104
USA
http://jonathanpeelle.net/
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