hi,
i'd like to pose a question to the community in an effort to
understand the current thinking regarding acceptable levels of motion
in epi data what to do about it (i.e. guidelines/methods for deciding
which, if any, scans to omit from analysis; including motion
parameters as regressors of no interest, etc). of course, looking at
one's data is the best tool for assessing motion and its effects on
any given dataset. that said, i'd like feedback on more objective
methods, and welcome any thoughts anyone cares to offer on the topic.
i'll start with a few questions:

1) some suggest that intrascan movement is particularly difficult for
most registration algorithms to handle, and that
estimating the translational motion from one scan to the next is an appropriate method for
quantifying intrascan movement. someone suggested that any scan for
which translation exceeds 20% of voxel size should be removed from
analysis. i'm not sure where that number came from, but does this
sound like a reasonable method of objective evaluation?

2) are there any similar thoughts on what would be considered
unacceptable levels of rotation, in terms of degrees? is there a
similar calculation to that described above in #1 to quantifying it?

3) how should task-correlated motion be dealt with? is some degree
of correlation to be expected, given that in most paradigms, subjects
are required to make some kind of overt response?

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?

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?

hopefully i'm not alone in pondering these issues and your answers
will be useful to others as well. thanks in advance for any feedback
you can offer.