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Hi - if I understand you correctly, the answer would probably be that it is dangerous to include an empty EV in a contrast that you care about, as this will likely have an unpredictable result (in theory the PE associated with an empty EV could be anything...) so you should probably go ahead with merging EVs 2 and 3 and treat them together. Cheers. On 14 Dec 2007, at 16:59, Ben Levy wrote: > I'm trying to understand some puzzling results from my FSL analyses > and, > specifically, I want to know more about how FSL handles empty EV files > (where there are no events of specific condition during that run). > In my > analysis, I am sorting trials into different conditions based upon the > trial-to-trial subjective report of the subject. So a subject > performs a > task and then depending on their rating, that trial gets sorted into > Condition 1, 2, or 3. We were originally interested in regions that > were > parametrically active across the the 3 conditions, but many subjects > (more > than half) never used the 3 response. So we have decided just to > look for > regions that are more active in Condition 2 and 3 than in Condition > 1. My > first approach at doing this was to code each condition in a > separate EV > file and then combine across them in a contrast later (so the contrast > would be -2 1 1 for conditions 1<2&3). Since then I have changed to an > approach where I just include the 2s and 3s together in the same EV > file > with no differentiation between them. My problem is that these two > approaches yield very different patterns of activity. To be > specific, the > first approach (modelling 2s and 3s separately and then combining > together > later) yields much more activity then when I model 2s and 3s together. > Looking at the difference between these activation maps it appears > that > the extra regions correspond to voxels that are less active on 1s than > baseline. Is it possible that all those empty EVs for condition 3 are > being treated as baseline (i.e., a bunch of consistent 0 values are > being > added in)? So my original attempt at a 2&3>1 map was actually > regions that > were more active during a mixture of baseline and activity during > conditions 2 & 3. Does this make sense? If this isn't what is going > on, > can anyone explain to me what is? I think the key here is what FSL > does > when it has an empty EV. > > Any advice would be greatly appreciated. > > Ben > --------------------------------------------------------------------------- Stephen M. Smith, Professor of Biomedical Engineering Associate Director, Oxford University FMRIB Centre FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK +44 (0) 1865 222726 (fax 222717) [log in to unmask] http://www.fmrib.ox.ac.uk/~steve ---------------------------------------------------------------------------