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Steve, thank you for the clear explanation. cheers martin On Sun, Oct 12, 2008 at 9:38 AM, Steve Smith <[log in to unmask]> wrote: > Hi, > > The main reason volume-wise intensity normalisation is generally > discouraged is that it isn't a particularly sensible way of fixing most > known 'problems'; > > - If your activation is strong then IN will be influenced by the activation > and you will get a) reduced activation in real areas and b) induced > artefactual 'negative activation' elsewhere. > > - If you have strong structured noise (e.g. RSNs) then IN is an > over-simplistic way of removing such problems (compared e.g. with ICA-based > cleanup). > > - However, if you have some nasty yet dumb artefact, such as your scanner > has applied some arbitrary scaling to each timepoint separately, the IN is > probably better than doing nothing. > > Cheers. > > > > > > > On 12 Oct 2008, at 09:24, Martin Monti wrote: > > Hi, I've run a set of first level FEAT analyses, with and without >> intensity normalization (mainly just curiosity..). Comparing the results, at >> times they match fairly well, other times however the results are quite >> wildly different. I'm not sure what to make of it. Can I get a little more >> info on the point? Are the differences likely to reflect noise/movements or >> actual activation? And, sorry for going more basic, I know it is "generally >> discouraged" so when/why would it be of use? >> >> cheers >> >> martin >> >> >> > > --------------------------------------------------------------------------- > 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<http://www.fmrib.ox.ac.uk/%7Esteve> > --------------------------------------------------------------------------- >