Jill,

I am definitely not an expert on motion correction. A FMRI physicist might be a better source for this. I will just comment a little bit on the last two questions in your original message:

> 4) what are peoples thoughts on including motion parameters as
> regressors of no interest? i know that some include motion
> parameters as regressors of no interest for each subject as a matter of
> course, while others include them only for subjects who have
> particularly problematic motion, or where it seems to improve
> statistical results. would those of you who do so be willing to post
> a brief message about the pros and cons of your method of choice?

My view is this:

- motion correction for all subjects

Pros: (1) well-balanced analysis for all subjects, especially for group analysis; (2) same degrees of freedom for all subjects

Cons: (1) violates the principle of minimism - wasting some regressors for some unnecessary corrections; (2) lose degrees of freedom for those "good" subjects.

- motion correction only for those problematic subjects

Pros and cons are just the flip-side of the case of doing correction for all subjects.

Personally I would do motion correction only for those problematic subjects and include only those problematic motion parameters. But again, this is just my personal preference.

> 4a) what are your thoughts about using just some, but not all motion
> parameters as regressors of interest? i.e. should it be all or none?
> can one include just translation, or just rotation? or include just
> one axis if is it looks problematic?

Personally I would check first the output from 3dvolreg, and pinpoint those troubling motions. Then I would only include those troubling ones as regressors of no interest. Pros and cons are similar by the same token as above.

Hope this helps,
Gang