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Select only MEG channels and exclude bad channels
Create events if there is no event in the data
Create epochs based on the events
Compute for each epoch its mean amplitude across all electrodes and times
Compute the mean of all the mean amplitudes of each epoch
Compute the standard error of that mean
SNR = result step 5. / result step 6.
Compute SNR on a subset of channels
Select only magnetometers or gradiometers