[FieldTrip] Correspondence channels-coils in HCP files

[François Tadel]

Hello,

I wrote a FieldTrip>Brainstorm import function based on Elekta files, 
and I was using the .tra matrix to get the correspondence between the 
channels and the coils:
https://github.com/brainstorm-tools/brainstorm3/blob/master/toolbox/io/in_data_fieldtrip.m#L107

I based this code on your description of the .tra field: "NxM matrix 
with the weight of each coil into each channel":
http://www.fieldtriptoolbox.org/faq/how_are_electrodes_magnetometers_or_gradiometers_described

This doesn't work with the processed 4D recordings from the HCP dataset 
because the .tra matrix is dense.
Is this because the 4D compensation matrix is integrated in this .tra 
matrix? Or because there is some additional PCA/ICA projection done 
during the preprocessing?
How do I recover the sparse .tra information that simply contains the 
weights channels-coils from the HCP processed files?

Thanks,
François

-- 
François Tadel, MSc
MEG / McConnell Brain Imaging Center / MNI / McGill University
3801 rue University, Montreal, QC H3A2B4, Canada