[Mne_analysis] Combine planar gradiometers after TFR

[Geoff Brookshire]

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Hi Alex,

Thanks for your response! I agree that it would make sense to combine
planar gradiometers by taking their sum, not the RMS, but I don’t see how I
can do that. Is there a built-in function to combine planar grads on TFRs?

Thanks,
Geoff

On Sat, Feb 1, 2020 at 10:17 AM alexandre.gramfort at inria.fr <
alexandre.gramfort at inria.fr> wrote:

>         External Email - Use Caution
>
> hi Geoff,
>
> if your TFR are contain power you have already only positive values. You
> sum
> gradiometers you can have cancelations of the effect due to polarity? does
> it
> answer your question? Now this suggests that if TFR is already power yes
> summing
> is enough
>
> Alex
>
> On Fri, Jan 31, 2020 at 3:09 PM Geoff Brookshire
> <G.Brookshire at bham.ac.uk> wrote:
> >
> >         External Email - Use Caution
> >
> > Hi all,
> >
> > I'm currently running a time-frequency analysis, and I'm having some
> trouble with power.plot_topomap. I'm trying to plot the difference between
> two conditions, but the plotting function only shows positive values when I
> try to plot gradiometers. According to the documentation, this function
> combines them using RMS:
> > ch_type : 'mag' | 'grad' | 'planar1' | 'planar2' | 'eeg' | None
> >     The channel type to plot. For 'grad', the gradiometers are
> >     collected in pairs and the RMS for each pair is plotted.
> >
> > My understanding, however, is that this is not the correct behavior for
> TFRs. Since TFRs no longer include information about the direction of the
> dipole, wouldn't you want to combine planar gradiometers by taking their
> sum? Taking the RMS after computing TFRs causes me to lose the direction of
> the effect. This site explains a similar analysis in Fieldtrip:
> http://www.fieldtriptoolbox.org/example/combineplanar_pipelineorder/
> >
> > Is there a way to combine planar gradiometers by taking the sum of each
> channel instead of the RMS? This would be analogous to the Fieldtrip
> function ft_combineplanar.
> >
> > Alternatively, is there a different way to plot the difference between
> two TFRs over gradiometers, preserving decreases in power?
> >
> > Thanks!
> > geoff
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