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Hi Ed, that's the beauty of mixed-effects modelling. Your analysis looks OK - can't see anything wrong in the way you've set it up - problem is your extremely low number of degrees of freedom - that will reduce z-scores dramatically and also cause the higher-level estimates of the random-effects contribution to be very poorly conditioned (read: not very accurate). You might want to think about not modelling the between-scan variation and just treat it as a fixed-effects problem. cheers christian On 16 Apr 2004, at 02:57, Ed Vessel wrote: > Hi folks - > > I have a simple experiment with a block type design (16 second blocks) > in > which I show four types of images plus periods of fixation, 4 times in > a > scan. I then do another scan of the same conditions, with a different > block > ordering. > > I analyzed each scan individually using four EV's and creating a > contrast for > each image type, plus some contrasts I am interested in. I got a lot > of very > high z values in my contrasts (like as high as 13) which were > consistent in > both scans, so I thought that combining the scans would give me a good > boost > in power and reduction in noise. > > However, when I combine the two runs in a higher level analysis (by > selecting > both lower level feat directories as inputs and selecting all my > contrasts), > the resulting maps are consistent with what I saw before, but the max z > values are now down around 2-2.5! This is true both for the contrasts > of > single EV's (i.e. 1 for one EV and 0 for the rest in the low level > analysis) > and for contrasts between EV's (i.e. 1 -1, or in one case, 3 -1 -1 -1). > > Am I combining these two runs incorrectly? Or is it expected for the > z values > to go down so drastically? here is my higher level GLM setup (its > super > simple): > > Number of EV's: 1 > Number of groups: 1 > > Group EV1 > Inp 1 1 1 > Inp 2 1 1 > > Contrasts 1 > EV1 > C1 Combine 1 > > This is, of course, run on each of the lower level contrasts. I had > expected > an "average" of my two runs, which is basically what I get, but with > much > reduced z scores. Should I be including an EV for "run" or something > to soak > of variance between the runs? > > thanks, > Ed > > -- > Ed Vessel > New York University [log in to unmask] > Center for Neural Science > 4 Washington Place, Suite 809 > http://cns.nyu.edu/~vessel > New York, NY 10003 > (212) 998-3908 > -- Christian F. Beckmann Oxford University Centre for Functional Magnetic Resonance Imaging of the Brain, John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK Email: [log in to unmask] - http://www.fmrib.ox.ac.uk/~beckmann/ Phone: +44(0)1865 222782 Fax: +44(0)1865 222717