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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