Hi Michael,
> I had a quick question about smoothing when doing VBM. I've read on
> Christian Gaser's VBM website
> (http://dbm.neuro.uni-jena.de/vbm/segmentation/) down at the bottom of
> the page, that as a rule of thumb, smoothing should be 70% lower than
> what your target smoothness is (given you left the default cut off for
> spatial normalization at 25mm). So if for example, my target smoothness
> was 12mm FWHM, then I should smooth my modulated gray matter images at
> around 8-9mm.
This only applies if you use MODULATED images as the modulation step
will increase smoothness by virtue of the fact that the spatial
normalization parameters (and hence, the Jacobian determinants of its
matrix) are not calculated for every voxel. Therefore, for all those
voxels "in between", interpolation is needed. If you check a j-image
representing the Jacobian, you will be suprised of how few features you
will be able to make out... therefore, the idea is that if you smooth
unmodulated data by 12mm, you should smooth modulated data by about 9mm
to get comparable results (I just ralized that I am repeating
Christian's words here but there are worse sins :)
> Is there anything out there published I can use to cite
> in a paper for this "rule of thumb" or is this some esoteric thing that
> people who use SPM have always done and just never bother to report it
> in papers? I ask, because some of our reviewers want to know where is
> the precedence for smoothing using this rule of thumb.
Oh well, you could write that in order to account for the additional
smoothness induced by the modulation step and in order to make the
results comparable with earlier studies investigating gray matter
density and due to the matched filter theorem, suggesting a width of
about 12mm for cortical regions of interest, the final smoothing was
chosen etc...
... theoretically, one could assess the smoothness of the Jacobian and
calculate its impact, and perhaps Christian knows if this has been done
(which would be the reference you're looking for :)
Best,
Marko
--
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Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473
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