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Subject:

Re: fMRI models

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Date:

Mon, 28 Feb 2000 18:54:58 -0800

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 Dear Jean-Pierre, > In the 'fMRI models' specifications, we have defined 2 sessions and 2 > conditions per session. However, in the 'Results' specifications, to define > the contrasts, we must indicate 6 values, and not 4 values as expected. > Therefore, we have provided the following contrasts: > > A1-B1 and B1-A1, so 1 -1 0 0 0 0 and -1 1 0 0 0 0, respectively > A2-B2 and B2-A2, so 0 0 1 -1 0 0 and 0 0 -1 1 0 0, respectively > A1-A2 and A2-A1, so 1 0 -1 0 0 0 and -1 0 1 0 0 0, respectively > B1-B2 and B2-B1, so 0 1 0 -1 0 0 0 and 0 -1 0 1 0 0 0, respectively > > Our first question is: what is the meaning of the last two values ? These are the session-specific constant terms, modelling the mean level of activity in each of the two sessions. These used to be in the 'covariates of no interest' partition of the design matrix in SPM96. > Second, we also want to compare, at least in a first time, the different > conditions with the rest condition. How must we define the contrasts to > compare, for instance: > > A1-R > B1-R > A2-R > B2-R > A1A2-R > B1B2-R You can use T contrasts of [1 0 0 0...], [-1 0 0 0...], [0 1 0 0...] and so on for the activations and deactivations associated with each condition in each session relative to baseline. best wishes, Geraint ------------------------------------------------------------------ Dr. Geraint Rees Wellcome Advanced Fellow, Lecturer, Mailstop 139-74, Institute of Neurology, California Institute of Technology, University College London, Pasadena, 12 Queen Square, CA 91125 London WC1N 3BG voice (626) 395-2880 fax (626) 796-8876 web http://www.klab.caltech.edu/~geraint ------------------------------------------------------------------ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%