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Thanks, Tim. I'll find avwmaths and try it out. Yes, I do have a sensible reason for doing it. We're simply trying to define an ROI within the visual cortex for a subsequent quantitative analysis and we won't be drawing any statistical inferences about NOT B! If that were permissible, we'd all be in Nature every week ;-) Andy >Hi - you want to be a little bit careful with this. > >A feature of null hypothesis testing is that "NOT B" isn't very >meaningful. That is, we are not able to say that a voxel does not activate >to stimulus B, we can only say that if this voxel were not activating, >then we wouldn't be all that surprised to see this data - it is really a >very weak statement, and you certainly won't be able to perform null >hypothesis style inference on a mask which is formed as the intersection >of "A" with "NOT B" . > >Hence, there isn't an option for this in Feat > > >Having said that - if you have some sensible reason for performing this >intersection then you can do it with avwmaths. > >something like > >avwmaths B -bin -mul -1 -add 1 -mas A A_NOT_B > >should give you a mask which defines the intersection of A with NOT B >which you can use for further analysis. > >Cheers > >T > > > >On Tue, 29 Jun 2004, Andy Smith wrote: > >> Is any kind of contrast masking possible in FEAT other than a >> standard exclusive mask? >> >> I have defined two contrasts from the same experiment and I want to >> identify all voxels that are significantly activated in contrast A AND NOT >> contrast B. It has to be one-way, i.e. I don't want to include voxels that >> are active in B and not A. An exclusive mask gives voxels active in both >> but not in either alone, so that's no good. >> >> Any ideas on how to do this would be much appreciated. (It shouldn't >> be too hard to write a script to do it, but there's no point in re-inventing >> the wheel.) >> >> Thanks, >> Andy Smith >> >> PS. I'm turning to you guys because SPM won't do it, so here's a >> chance for you to gain a convert! >> > >-- >------------------------------------------------------------------------------- >Tim Behrens >Centre for Functional MRI of the Brain >The John Radcliffe Hospital >Headley Way Oxford OX3 9DU >Oxford University >Work 01865 222782 >Mobile 07980 884537 >-------------------------------------------------------------------------------