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We addressed this problem in the in-press paper I mentioned earlier. One simple thing that can be done is to analyse both motion-corrected images and non motion-corrected images and compare the results. Real activations should typically be more significant after motion correction, while artifacts will be less significant - so you have a way of identifying and excluding probable artifacts. You can then include MPs as covariates and see if that further increases significance. Tom ----- Original Message ----- From: Anja Ischebeck <[log in to unmask]> Date: Friday, October 7, 2005 8:51 am Subject: Re: [SPM] Antw: Re: [SPM] motion parameters as covariates > Hi Daniel, > > to my experience motion heavily correlated with the paradigm > cannot be addressed in a satisfactory way. > > Motion artifacts typically appear as activation at the brain boundary, > on the border of ventricles and the eyes. > > If one calculates both models (with and without mps) > the result is that entering the mps can remove nearly all > of the original signal (e.g. activation in language areas > such as broca). Also, motion artifacts are not completely > removed. I could live with this if motion artifacts would be > removed more than signal but this only holds the more > motion is absent or uncorrelated with signal. > > Therefore, entering the mps into the model in the case > of significant motion correlated to the paradigm does > not make much sense. > The only other option is, of course, to discard the data > and not to do such an experiment again. > > Anja > > > > > (areas of strong brightness contrast) > > Dr. Anja Ischebeck > Innsbruck Medical University > Clinical Department of Neurology > Anichstrasse 35 > A-6020 Innsbruck - Austria > tel.: +43 (0) 512 504 23661 > > >>> "Daniel H. Mathalon" <[log in to unmask]> 07.10.2005 14:55 > >>> > Anja, > Thanks for your reply. Yes, I understand that the first level > models > will change when motion parameters are added, particularly when > motion correlates with events of interest. However, I'm not sure > whether there is good solution in this case. Removal of variance > associated with motion may also remove task-related brain > activation, > yet ommission of motion parameters from the model raises the > possibilty that activations are actually caused by motion-related > artifacts. > > Dan > > > > > >Hi Dan, > > > >you are right, of course, with regard to the degrees of freedom. > >Degrees of freedom are only relevant for fixed effects analysis. > >Only the con images are entered into the random effects model so > >the error term is calculated differently (over all con images instead > >of over all scans) and the degrees of freedom are determined by the > >number of con-images alone. > > > >However, this does not mean that entering motion parameters or not > >does not affect also the random effect analysis. It does, on the > >level of fitting the model (getting the con-values). > > > >The random effects analysis will be affected if entering > >additional regressors also changes the fit of the model for the > >predictors > >of interest. This is not very likely for motion perfectly > uncorrelated > >to the > >predictors of interest. > > > >The case is very different, however, for motion correlated > >with your paradigm. Consider the extreme case, (e.g. a patient > overtly > > > >naming picture in the scanner alternating with rest periods, testing > >for task vs. rest or only task) > >In this case motion caused by articulation is highly likely and > >will be correlated with the predictor of interest (onset of > >task-blocks). > >If you enter mps into this model, statistics will suffer, because > >mps will regress out (too) much of the signal (task). This will then > >affect the random effects analyis. > > > >I hope this helps, > >Best, > > > >Anja > > > > > > > > > >Dr. Anja Ischebeck > >Innsbruck Medical University > >Clinical Department of Neurology > >Anichstrasse 35 > >A-6020 Innsbruck - Austria > >tel.: +43 (0) 512 504 23661 > > > >>>> "Daniel H. Mathalon" <[log in to unmask]> 06.10.2005 15:37 > >>>> > >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 > >Pager: (203) 867-7756 > > > -- > 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 > Pager: (203) 867-7756 >