Hi Poornima,
> I have searched through the archives for info on acceptable threshold for movement in our analysis, but with no luck.
This is because no-one really knows for sure. There is a rule of thumb
that "motion exceeding one voxel size" may be bad, but then again,
cannot see that from the realignment parameters as you will have to
combine translations and rotations. See the toolbox by Paul Mazaika for
an approach that does that for you. There also likely is no fixed number
as I guess it would depend on your data, paradigm, sequence...
> Can any one suggest if we use the realignment parameters as covariates in 1st level, what is the maximum movement (translation, rotation....) that is removed by these steps?
This question I believe cannot be answered, as you do not remove motion,
but the additional variance associated with it. See Friston et al.,
1996, or Johnstone et al., 2006, for some hints.
> For example, if a participant has moved say 8mm and 12deg in one block, can we include the participant in the analysis, assuming that using motion regressors in 1st level has taken care of this?
I am used to working with children, but 8 mm and 12 degrees is massive
motion even in my book. I would not expect the realignment parameters to
"take care" of this.
Cheers,
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
____________________________________________________
|