Dear Anja,
I am not sure why the degree of freedom costs associated with
entering MPs into the first level model matters in the context of
second level random effects analysis. If one is only passing betas
or con images to the second level, the costs of spending degrees of
freedom at the first level are not relevant. Wonder what you think
about this.
Dan
>Hi,
>
>just my two pennies (is that the right expression?)
>to the discussion:
>
>To my experience
>(I tried out different designs with or without MPs modelled for years:
>
>in this case the simple 6 parameter model - translation/rotation)
>it is not very easy to compare methods
>if one does not take into account the amount of motion in total
>and (very likely to be of importance:) the amount of motion
>correlated to the paradigm or event regressors.
>
>If there is nearly no motion, entering the Mps will conly cost
>degrees of freedom. On the other hand, if there is moderate
>motion, entering the 6 MPs (or the more complex models)
>will explain a lot of variance.
>However, the 6 parameter MP model has its limitaitons:
>With high T-field scanners or excessive (instead of moderate) motion
>as in the case of children or patients
>excess motion cannot be modelled satifactorily.
>At least my results were abysmally bad in these cases and I
>excluded the subjects in question - which is a darn pity.
>
>Excessive motion means (in a 3T field): movement of 0.5 mm
>between two consecutive scans (twiching) - this seems to
>be bad enough for statistics.
>
>I missed in Brett's abstract a mention of the degree of motion
>in the data used for his comparison.
>
>I would therefore like to hear of people who have experience with
>this (excessive motion or moderate motion and high field scanners).
>Maybe the mode complex models of motion are doing
>much better than the simple one in these special case.
>
>Best,
>
>Anja
>
>
>
>Dr. Anja Ischebeck
>Innsbruck Medical University
>Clinical Department of Neurology
>Anichstrasse 35
>A-6020 Innsbruck - Austria
>tel.: +43 (0) 512 504 23661
>
>>>> Matthew Brett <[log in to unmask]> 05.10.2005 17:45 >>>
>Hi,
>
>> I don't think it is the speed of the movement that increases the
>effect
>> of artefacts, it really is the displacement. The artefact is due to
>the
>> spin excitation history of the voxel, i.e. energy transmitted to a
>brain
>> region before thatt region was at its current voxel location (maybe
>you
>> should ask an MR physicist instead of reading this...).
>
>Just a question - my impression was that the current thinking is that
>spin-history is a rather minor factor in the motion-related variance.
>Is that true? Certainly there can be quite large effects from motion
>by distortion interactions - this is stuff Chloe Hutton and Jesper
>Andersson have worked on.
>
>I did a very tiny study of including movement parameters up to the 24
>regressor spin-history model (which will include the effects modeled
>by the difference of the parameters) and found, like Tom Johnstone,
>that only the movement parameters themselves seemed to be robustly
>useful:
>
>http://www.mrc-cbu.cam.ac.uk/~matthew/abstracts/Moves/moves.html
>
>The link points to my HBM2005 abstract.
>
>Best,
>
>Matthew
--
Daniel H. Mathalon, Ph.D., M.D.
Assistant Professor of Psychiatry
Yale University School of Medicine
Mail Address: Psychiatry Service 116a
VA Connecticut Healthcare System
950 Campbell Ave
West Haven, CT 06516
Fax: (203) 937-3886
Office Phone: (203) 932-5711, ext. 5539
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