Just to expand a bit on the last point, which is to model the same subject as multiple sessions (instead of as one long session). If the subject moved significantly, then SPM realignment will leave a residual motion effect in the data from interpoloation (Groontoonk, 2000) and field non-uniformity (Friston, 1996). When the subject moves only between sessions, then the residual motion effect will essentially be constant within-session. If the data are modeled as multiple sessions, SPM will find a different mean value for each session, and that will remove the residual motion effect. Thus, the usual SPM procedure of model each session separately, realign each session, then co-register the sessions, and normalize, is robust against movements between sessions.
-Paul
----- Original Message -----
From: "Marko Wilke" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, December 1, 2010 11:33:46 PM
Subject: Re: [SPM] Question about realign
Hello,
> I conducted a study that contains 3 independent runs. But what
> frustrated me were 6 of 16 of my participants’ head motion between the
> runs after realign appeared > 3mm (they rested for about 1 min between
> each run), but within each runs, the result is fine.
That should not frustrate you too much, as it is both not so bad and
taken into account. Motion during sessions is much worse than motion
between sessions.
> So, I was wondering whether I can pre-process my data separately as 3
> independent runs then converge data together when doing modeling?
No, you should not, as you would then not have the functionals in the
same space (each session would remain being shifted from the previous
run). For exactly this reason, spm brings all runs into one space (the
first image of the first session, usually) when doing realign. This
allows you to normalize all images usng the same set of normalization
parameters later-on, for example.
> Is there any particular reason that we should pre-process our data in
> one session even if we have several runs?
You should not provide it as one session but as several sessions of the
same subjects. SPM will then know what to do. You could, conceivably,
realign them independently from each other and then coregister them all
to the same target, but due to the above, this is not necessary (and
likely not more exact either, although I haven't explored this
systematically).
Hope this helps,
Marko
--
____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn
____________________________________________________
--
Paul K. Mazaika, PhD.
Center for Interdisciplinary Brain Sciences Research
Stanford University School of Medicine
Office: (650)724-6646 Cell: (650)799-8319
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