clusternorm vs clustere

[Sebastian Markett]

Hi everyone,

Can anyone point me to the difference between
swe_clusternorm_Tstat_lpFWE-WB_c012.dscalar.nii and swe_clustere_Tstat_lpFWE-WB_c01.dscalar.nii outputs?

While running identical non-parametric analyses on different constrast images I noticed that I sometimes get either (but never both) images for a given analysis. The images seem to carry similar information.
Clusternorm images are not listed in the manual. How come I sometimes get those instead of *clustere* output?

Thanks a lot!
Sebastian

[Thomas Nichols]

Dear Sebastian,

I'm so sorry that our documentation has kept up with the very latest changes we made.  

As part of CIFTI inference we wanted to be able to provide cluster size inferences for 'brain ordinates' data, consisting of the surface and subcortical volumes.  However, the null distribution for surface clusters and volume clusters will be totally different.  We tried some simple transformations (e.g. square root for surface, cube root for volume) but they didn't work well, and so we instead use a Box-Cox transformation on the null distribution of surface and volume clusters separately.  Once we have Box-Cox transformed the cluster sizes, we use the median and the half-quartile (Q3-Q2) of the Box-Cox-transformed values to shift and scale into approximate Z scores.  What was once "k_E" (cluster extent) is now "k_Z" (normalised cluster extent).

Hence, if you look at the Results page carefully, you should see "Wild Bootstrap norm. ext. thresh. k_{Z} > ..." indicating that the transformation has happened, and then you'll get  swe_clusternorm* files.  Except....

Sometimes, when there are very few clusters or they are mostly just tiny, of size 1 or 2, the half-quartile can be 0 and the transformation breaks down.  In that case, we throw up our hands and don't do any Box-Cox standardisation and resort to just good old element- and voxel-counting to measure cluster size.  Then you'll get "Wild Bootstrap extent threshold k > ..." and swe_clustere* files.

Sorry this isn't in the documentation!  I'll make an issue to get this fixed.

Does this all make sense?

-Tom

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[Sebastian Markett]

Dear Tom,

thanks for your explanation! This makes a lot of sense. I had a hunch that it was something like this. Good to know that the two files are comparable and that I can use either of them.

Cheers

Sebastian