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Dear Jon, > The data were acquired with a block design, 15 seconds OFF, > 15 seconds on, etc, etc. Did the method Richard proposed require > some re-ordering of the data? Basically I'm not too sure about > how to specify the required model. You don't need to reorder the data. What I think Richard was suggesting was that instead of specifying two conditions in a boxcar design (active and rest), you instead specify four conditions (active_early, active_late, rest_early, rest_late). Take the onsets for _early from blocks acquired in the first half of scanning, _late from the second half. Then you can use a the contrast he suggested to test for a difference between activation and rest that is greater during the first half, compared to the second half, of your scanning run. I was suggesting a related but different approach; specify just one condition, but then use the 'parametric modulation' button to create a modulation of that boxcar by some function of time. I'd be happy to help you set this up but it may be easier done off-list. > Also, considering modification of BOLD signal due to scanner > 'variation' - does SPM detrend the time course data? If so > wouldn't a time dependent reduction in BOLD signal be physiological > rather than artifactual? That was what Richard & I were debating I think. I was arguing that artefactual differences due to the scanner would affect each condition equally, so could not account for differences between active and rest that varied over time (activation x condition interaction). Richard was arguing (correct me Richard if I am misrepresenting your argument !!), on the other hand, that he could envisage sources of artefactual variability that could differentially affect 'active' conditions (in essence affecting the gain of the BOLD response to neural activity, rather than just adding a constant offset). So Richard wanted (ideally) to identify not just an activation-by-time interaction, but also show that this putative correlate of learning could be reset by a psychological manipulation at the end of the session (thus ruling out artefactual causes). Hope this helps, best wishes, Geraint -- Dr. Geraint Rees Wellcome Advanced Fellow, Lecturer, Division of Biology 139-74, Institute of Neurology, California Institute of Technology, University College London, Pasadena CA 91125 London WC1N 3BG voice 626-395-2880 020-7833-7472 fax 626-796-8876 020-7813-1420 web http://www.klab.caltech.edu/~geraint -- %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%