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Dear Jan, Your hunch is correct. These strange looking contrasts that are created automatically look strange because they have been operated upon by SPMs estimate of the error covariance (nonsphericity). I would ignore them and specify my own contrasts. An identity matrix that spans the whole design matrix may not be valid as eg. the column of 1's may be perfectly collinear with combinations of other columns. You could instead specify different contrasts that span the variables of interest eg. the F-contrast [1 -1 0; 0 1 -1] for a 1 x 3 ANOVA. Best, Will. Jan Gläscher wrote: > Dear list, > > I posted this message about a week ago, but got no answer. I wonder if > any of the SPM wizards is willing to comment on this issue. > > Thanks a lot. > Jan > > > ----- Forwarded message from Jan Gläscher <[log in to unmask]> ----- > > From: Jan Gläscher <[log in to unmask]> > To: [log in to unmask] > Subject: confused about effects of interest contrast weights > > Dear Will, Karl and others, > > I have encountered a contrast weight matrix in the standard 'effects of > interest' F-contrast at the 2nd level (different ANOVAs) that I cannot > explain. > > I had expected an identity matrix as contrast weights, however the > actual values in SPM.xCon(1).c are actually quite dissimilar. In > addition, the same scaling effect of the contrasts weights occurs when > I specify an F-contrast via 'columns for reduced design' in the contrast > manager. In fact, when I try so specify an identity matrix > [eye(size(SPM.xX.X,2))] then this is not a valid contrast. > > Could somebody explain this scaling of contrast weights to me? Does it > have to do with the non-sphericity correction? > > Thanks a lot for your insights, > Jan > > -- William D. Penny Wellcome Department of Imaging Neuroscience University College London 12 Queen Square London WC1N 3BG Tel: 020 7833 7475 FAX: 020 7813 1420 Email: [log in to unmask] URL: http://www.fil.ion.ucl.ac.uk/~wpenny/