[FieldTrip] A puzzle: Coherence on planar gradiometers using virtual channel as seed

[Stephen Whitmarsh]

Hi there,

I was wondering if anyone could provide some advice on the specifics of
calculating coherence on planar gradiometers (Elekta Neuromag system). In
short I would like to calculate coherence between an average of several
sensor based on the topography of the power (a seed virtual channel if you
want), and all the MEG sensors.

I would rather deal with combined gradiometers when in the spectral domain,
given that those give me a clear and consistent topography within and
between subjects. However, I can imagine combining gradiometers at
different stages of the analysis:

1) Combine gradiometers on the timecourse data before spectral analysis.
2) Combine gradiometers after the spectral analysis, but before the
coherence analysis.
3) Combine gradiometers after the coherence analysis.

Option 2 has the advantage of staying in the same topographical
representation in the spectral power estimates (to determine sensors) as
well as in the coherence, making it easier to compare topographies of power
and coherence. However, this does not allow me to use a 'pre-computed'
average, i.e. virtual channel, of several *combined *gradiometers as the
seed, since those are specified already in spectral estimate
(ft_freqanalysis using fourier or fft).

A solution for this might be to determine the seed based on combined
gradiometers, then 'cut up' those channel labels into their original
gradiometer labels, and use those(uncombined) gradiometers as the seeds in
ft_freqanalysis. After the spectral analysis I can then first combine the
gradiometers, then average the different seed channels, before and
calculate the coherence with those as the seed.

Does this sound valid to you people? I would like to check with you people
if there are any concerns doing this. There might also be a simpler way
doing it...

As far as I am aware, there is no FieldTrip tutorial or FAQ where coherence
analysis is applied to Neuromag data, so I hope this might also help other
people.

Regards,
Stephen

p.s. I am probably using the word 'seed' wrong here - feel free to update
my nomenclature :-)
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