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

Stephen Whitmarsh stephen.whitmarsh at gmail.com
Wed Jan 18 13:47:54 CET 2017

Hi there,

Oops, no that does not fly either.


On 18 January 2017 at 12:43, Stephen Whitmarsh <stephen.whitmarsh at gmail.com>

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