[Mne_analysis] Combine planar gradiometers after TFR

[Geoff Brookshire]

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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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