Kim,
looks like nobody replied as yet, so I'll take a stab at this. Note,
however, that it is slightly unusual and not without dangers to post
reviewer's comments, as the reviewer may well be among the list
subscribers. In any case, two comments:
> *1) The authors should state why they selected an [arbitrary] threshold
> of 0.2, and if the sensitivity of their results varied as a function of
> the threshold chosen.*
>
> As far as I know, ‘0.2’ threshold is most commonly used in prior
> published VBM studies, and therefore, I used absolute threshold masking
> of ‘0.2’ without any suspicion.
>
> Is this threshold of 0.2 arbitrary? Is there a good reference supporting
> the use of 0.2 threshold? Please give me an advice to cope with this.
It is, in a way, arbitrary, but that does not automatically make it a
bad choice. If you select all your smoothed normalized gray matter
images with the following code snippet
% === start ===
V = spm_vol(spm_select);
vols = spm_read_vols(V);
[h,b] = hist(reshape(vols,prod(size(vols)),1),254);
figure; plot(b,h);
% === end ===
this will give you an idea of the average histogram of your data. Note
that the bulk of values will be close to either 1 or 0, particularly
when using vbm8's advanced features such as hMRF. This lower threshold
serves to exclude spurious results where patients and controls differ by
minute amounts in (essentially) noise, so I think it is well-justified
to use it (which is probably because it is also pretty common).
> *2) In reporting their VBM results the authors should include the
> estimated FWHM smoothness (not the same as the smoothness applied during
> preprocessing) and the resel count.*
Along the lines of smoothness, I would be more concerned with
non-stationarity than with the resulting smoothness. I do not know the
specifics of your design and methods, but a simple, global extent-based
approach to correcting for multiple comparisons is questionable for VBM
studies. There are approaches for addressing this (search for
"non-stationarity" in the list's archives).
Hope this helps, and a happy new year to all who celebrate it tomorrow,
Marko
--
____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn/
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