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Dear Leonardo,I assume this is all the same data (the same 4D file used as input for randomise), and that for each of the 10 instances of TBSS, you use a similar design, changing only the single EV that you have in the design, which is the data derived from the ROIs from a different modality, is this correct? If yes, then the following should work:
1) Run randomise for all the 10 different instances of TBSS, making sure to use the same seed and the same number of permutations for all. Use the -N option to save the maximum statistic for each run. At the end you'll have 10 different text files, each one containing, on each line, the largest statistic observed for the respective permutation. If you run, say, 20k permutations, each of these files will contain a column of 20k values (one per line).
2) Use Matlab, Octave, R, Python, Excel, LibreOffice Calc, or your favourite tool to load these 10 files all at once, and assemble them as an array/table/spreadsheet, putting side-by-side the 10 files, i.e., the table should contain 10 columns (one per TBSS run) and 20k rows (one per permutation).
3) Once the table is assembled, take the maximum across rows. If you are using Matlab or Octave, use max(X,2), where X is the table. If you are using Excel or LibreOffice, in the 11th column (letter K), use a formula such as =MAX(A1:J1) and expand it until the last, =MAX(A20000:J20000).
4) Sort the values of this column with the maximum values. If there are no ties, locate the value that is in the 95 percentile. Write down this value: this is the threshold that you can apply (with fslmaths) to the unpermuted statistical images (all the 10 from the 10 separate TBSS runs) to correct for family-wise error rate (FWER).
From your description, TBSS data with EV derived from another modality, I believe these are all quantitative variables, with no ties, such that it is very unlikely to find ties in the permutation distribution. However, if you observe ties in the column with the maximum values, let me know and I send you a small Matlab function that deals with these cases.
Hope this helps!All the best,
Anderson2013/1/18 Leonardo Cerliani <[log in to unmask]>
dear FSL people,
I have a question related to how to correct the final threshold in case of performing several TBSS (actually randomise). It may sound strange, so let me explain:
Suppose you have some measures acquired from a different modality (not DWI) in 10 regions of interest, and you want to perform a correlation with FA values voxelwise, using randomise and then correcting with TFCE.
The question is: what is your best guess about how to correct the final significance threshold for the fact of having done 10 TBSS instead of one...?
I hope the question does make sense to you (it does to some reviewers...)
Thank you very much for your help!
leonardo
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Leonardo Cerliani, PhD
BCN Neuro-Imaging Center
University Medical Center Groningen
Antonius Deusinglaan 2 (room 131)
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Social Brain Lab
Netherlands Institute for Neuroscience
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