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Sure - the easy thing is to use a more robust summary statistic that is not so affected by outliers - for example, just use the median values from the Featquery output instead of the mean. Steve. On 13 May 2007, at 06:45, Xiaoyun Liang wrote: > Hi Steve, > Thank you very much! Yes, the average psc might be being driven by > negative outliers caused for example at brain/CSF boundaries.But I > selected ROI from group level activated map, the voxels of no interest > should be excluded. And I wonder if any methods is available for > solving > such problem. > Many thanks! > > > > Xiaoyun > > > > > > > > > > > > > > On Sat, 12 May 2007 07:01:36 +0100, Steve Smith <[log in to unmask]> > wrote: > >> Hi - if psc in a single voxel is negative, it is because the effect >> (contrast of parameter estimates) is negative - for example, you are >> getting less BOLD response at rest than during task. >> >> However, if psc is negative when averaged over a region of interest, >> it does not necessarily mean that the effect is also negative on >> average: the average psc might be being driven by negative outliers >> caused for example at brain/CSF boundaries. >> >> Cheers. >> >> >> On 12 May 2007, at 05:32, Xiaoyun Liang wrote: >> >>> Hi, >>> When I computed the psc, I found that the mean psc is negative >>> in some >>> ROIs, such as FFA, with the stimulus of face. However, I think it >>> should >>> be positive. Can anyone give me a reason? Thanks a lot! >>> >>> >>> >>> >>> Xiaoyun >> >> >> --------------------------------------------------------------------- >> --- >> --- >> 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 >> --------------------------------------------------------------------- >> --- >> --- >> ===================================================================== >> ==== ------------------------------------------------------------------------ --- 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 ------------------------------------------------------------------------ ---