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

Stephen Whitmarsh stephen.whitmarsh at gmail.com
Wed Jan 18 12:43:44 CET 2017

Hi there,

It seems I might be able to solve it using the fourier method in
ft_freqanalysis, after which I can select and combine gradiometers and feed
those into ft_connectivityanalysis.

Thanks for your time,

On 18 January 2017 at 11:57, Stephen Whitmarsh <stephen.whitmarsh at gmail.com>

> 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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