[VoxBo] threshold calculator in IDL vv

[Joonkoo Park]

Hi everyone,

I want to clarify a couple of things about the threshold calculator in IDL
voxbo view. I often use the old vbview (from IDL) command because it is
sometimes more informative. The threshold calculator window has data entry
fields #vox, FWHM, k, eff df, and alpha. I believe that all these parameters
are used to calculate a critical t value.

As far as I understand, eff df is the temporal effective degrees of freedom
(is this correct?). If this is the case, then why is eff df the same for all
voxels when each voxel may have totally different time series structure of
BOLD signals? I am probably confused because I don't know the underlying
statistics behind it, but at least what I'd like to confirm is that eff df
is a single value across all voxels.

Another thing. My critical t value in the threshold calculator has the same
value no matter what number of voxels I put in the #vox data field (I did
press updata button). I assumed that it would deal with multiple comparison
issues here, but critical t value stays the same whether I'm looking at 1
voxel or 1000 voxels. Is this a bug? Then, the following question is what
method does it use to cope with the multiple comparison problem?

To illustrate, #vox=16150 FWHM=0 k=1 effdf=16.69 alpha=0.05 give me critical
t=6.851, which seems like a Bonferroni corrected t-value. However,
#vox=16150 FWHM=0 k=1 effdf=16.69 alpha=0.05 also give me the exact same
critical t=6.851.

Thanks in advance, as always!

Joon
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