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Options  Subscribe or Unsubscribe   Log In   Get Password Subject: Re: 2 groups-2conditions analysis

From:  Date: Thu, 27 Jan 2000 10:31:49 +0000 (GMT)

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 ```  | I ran SPM99 "Multi-group:conditions and covariates" comparing the diffences | between 2 conditions in 2 different groups as described in | http://www.mailbase.ac.uk/lists/spm/1999-06/0097.html | The study is attempting to identify hemisheric asymmetries in group 1 and group | 2. For each subject in both groups their normal scan is condition 1, and a scan | flipped about the x-axis is used for condition 2. Therefore, I have come up | with the following: | | group 1: differences between flipped vs nonflipped | VERSUS | group2: differences between flipped and nonflipped | | The results of contrast [1 -1] and [-1 1] appear to be the mirror image of each | other about the x-axis. Is this correct? Am I using the correct contrasts to | look at the differences between group 1 and 2 for flipped vs not flipped? How | does one interpret the contrasts?     Assume that the difference images are flipped-unflipped, then the left side of the image shows left-right, and the right side shows right-left. What appears on the right of the difference image is the negative of what appears on the left. Let the difference on the left be called (l-r)_1 and (l-r)_2 for the two groups, so the corresponding pixel on the right side of the images are -(l-r)_1 and -(l-r)_2.   For the l.h.s, the first contrast is therefore: (l-r)_1 - (l-r)_2 The second l.h.s contrast gives: - (l-r)_1 + (l-r)_2 The first contrast on the r.h.s shows: (-(l-r)_1) - (-(l-r)_2) = -(l-r)_1 + (l-r)_2 and the second on the r.h.s shows: -(-(l-r)_1) + (-(l-r)_2) = (l-r)_1 - (l-r)_2   As you can see, the first contrast on the l.h.s is the same as the second contrast on the r.h.s. Similarly the first contrast on the r.h.s is the same as the second on the l.h.s.     In effect, this analysis is a random effects model, since all the variance is estimated between subject.   I hope this reply has clarified rather than confused. All the best, -John         ```