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
>
>
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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)
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