Dear list,
Apologies if this has been dealt with before, but I couldn't find a
specific reference in the archive.
I have been using a fairly standard fMRI preprocessing procedure, in
which inspection of the realignment parameter plots has typically
indicated subject movement over sessions of anywhere between 0-5 mm.
I have been including these parameters as confounds of no interest in
the model in order to reduce the likelihood of type I errors.
Recently, however, a colleague and I independently came upon the
bizarre situation where a single subject appears to have practically
zero movement across sessions apart from in one session, where he
apparently moved 50mm in one direction at the start of the session,
stayed there for 100 or so timepoints, and then moved 50mm back to
zero again at the end of the session.
Obviously, the subject is unlikely to have undertaken these
gymnastics without our noticing it. My question, therefore, is what
might have caused SPM to think there is this much movement, and
whether there is any way of using the data from this session. Can
the inclusion of the realignment params in the model correct for this
degree of apparent "motion"? Is there any other way of convincing
SPM that in fact this movement does not exist? I have tried
realigning the session without the first and last few timepoints, and
there is no difference. I could just leave out this session, but if
there is an alternative, that would be preferable.
Any suggestions gratefully received.
Thanks,
Jon.
--
Dr Jon Simons
Department of Psychology
Harvard University
Rm 860, William James Hall
33 Kirkland Street
Cambridge, MA 02138
USA
Tel: + 1-617-495-3856
Fax: + 1-617-496-3122
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