Dear Neil,
> I have the worry that by smoothing the data in an FMRI analysis, that if
the
> activation that is present in the brain covers a v small area the
smoothing
> process may wipe it out. Why would smoothing the data be a good idea -
If in fMRI the (unknown) activation only covers an area of one voxel,
you do indeed better without smoothing. You would then go in the results
section for a peak height test to identify these activations.
However, one of the underlying assumptions is that the BOLD effect
smears any activation on the cortical sheet to a range of some
millimeters such that your most interesting activations in the grey
matter usually cover an area larger than one voxel.
> since there is an option in the stat tests to apply a threshold, and
perform
> some sort of gaussian smoothing.
Yes, in the results section there is a thresholding, but you cannot
perform a Gaussian smoothing at this stage. The result section (in spm99
after pressing the volume button) just reports the estimated smoothness
of the statistical map under the null hypothesis.
Best wishes, Stefan
--
Stefan Kiebel
Functional Imaging Laboratory
Wellcome Dept. of Cognitive Neurology
12 Queen Square
WC1N 3BG London, UK
Tel.: +44-171-833-7478
FAX : -813-1420
email: [log in to unmask]
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