Re: [Marsbar-users] Percent signal change question
Status: Beta
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matthewbrett
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From: Thomas S. <tst...@ne...> - 2007-07-10 10:14:54
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Hi, just to give a more intuitive explanation of the problem: The Marsbar-FAQ tells me that the percent signal change is expressed in relation to the mean signal of the ROI. But: When I look at all 4 conditions of the experiment, the mean of those 4 conditions signal changes should be the same as the mean signal of the ROI, because there are no other periods in this experiment. So how can they all be negative? I think I still missed a detail with that? Best regards Thomas Dr. Thomas Stephan Abteilung fuer funktionelle, strukturelle und molekulare Bildgebung Zentrum fuer Sensomotorik Klinikum Grosshadern der Universitaet Muenchen Marchioninistr. 23 81377 Muenchen tst...@ne... +49 089 / 7095-4819 (Fon) +49 089 / 7095-4801 (Fax) Thomas Stephan schrieb: > Hi, > > I used marsbar to extract percent signal change values for all 4 > conditions of a block design experiment. > The experiment included 4 conditions, and all of them have been > modelled, because there is no real > baseline condition in this experiment. ROIS have been defined as > spheres around the maxima of the > RFX-activation clusters. Percent signal change values have been > extracted for each subject and each condition. > These values have been averaged to give a group-percent-signal-change > estimate for each experimental condition. > For some of the ROIs, all of these four values are negative, for other > ROIs all of them are positive. > Now the question: > As I read in the marsbar documentation, all the values are scaled to > the mean of the roi. > How can it be that the values do not sum to zero? The 4 conditions > model the whole period of the experiment, > so I thought that they should sum to zero. > > Best regards > Thomas Stephan > |