[FieldTrip] How to apply ROI mask to grandavg

[Sonja Suntrup-Krüger]

Dear all,

I want to compare task-related brain activation in a ROI  in two groups of subjects. I performed LCMV beamformer analysis on single subject data and, after normalization, created a grandaverage of each group, which I need to compare statistically within a defined ROI. 
Since the option to create a ROI in ft_sourcestatistis is unfortunately no longer available since 2015, I had the following idea to create my ROI:
By using ft_volumelookup I created a binary mask from an anatomical atlas indicating my ROI in standard space. Now I would like to apply this mask on my grandaverage data. In specific, I want to multiply my mask values (zeros and ones) with the "pow" values in my grandavg.
This would set all the power values outside my ROI to zero. With the result from this procedure I can then calculate my statistc.

My binary mask has the dimensions: x,y,z
     mask [181x217x181 logical]
     coordsys: 'spm'

In my grandavg, the dimension "dim" is the same, but the power values ("pow") and their positions ("pos") are listed as a single long  vector for each subject:
     pow: [7109137x6 double]  (because there are 6 datasets in the grandavg)
     pos: [7109137x3 double]
     dim: [181 217 181]
     coordsys: 'spm'

My question is: How does the "x,y,z"-data structure of "dim" relate to the "single long vector"-data structure of "pow"/"pos"? If I had this information, I could transform my mask into the same structure and apply it as explained above.

Your help is greatly appreciated.
Thank you very much!
Sonja Suntrup-Krüger

-- 
Sonja Suntrup-Krueger, MD
Assistant Professor of Neurology
Institute for Biomagnetism and Biosignalanalysis
University of Muenster, Germany