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Dear Joanna, > > There is no automatic facility but the percent (of whole brain signal) > > activation of a voxel is easily calculated from the parameter estimates > > - the variable 'beta' in working memory following a plot. These values > > correspond to a VOI defined by the spatial smoothing kernel, centred > > on the selected voxel. > > > > I hope this helps - Karl > > > I would like to know exactly how "the percent (of whole brain signal) > activation of a voxel is easily calculated" from the numbers in the beta > images. When I followed the above instructions, I got the numbers .4097, > .3342, .4533., .1301 and 199.64. I realize the first four numbers > are parameter estimates corresponding to my four conditions. I do not > know what the last number is. Could someone give me an example equation > with these numbers that would give me the percent of whole brain signal > associated with the eg. third condition for this voxel? I think what Karl meant was that one can calculate the percent change in a voxel (with respect to the mean signal at that voxel) from the estimated parameters. To get the percent change, you still have to multiply the design matrix times the estimated parameter vector and divide the change in the interesting fitted effect by an estimate of the mean signal. In your case, the fifth element of the beta vector corresponds to the session effect, which can be seen as an estimate of the mean at that voxel. If you're interested in the percent signal change with respect to the global mean signal (as computed during the SPM-analysis), the units at a given plot of adjusted data are adimensional and are scaled to 100. This means you can interpret the observed change in a plot as percent with respect to the global mean signal. Stefan -- Stefan Kiebel Functional Imaging Laboratory Wellcome Dept. of Cognitive Neurology 12 Queen Square WC1N 3BG London, UK Tel.: +44-(0)20-7833-7478 FAX : -7813-1420 email: [log in to unmask] %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%