plot_topomap appears to invert the sign of the data.

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1
  • MNE version: 1.0.3
  • operating system: Windows 11

Hello, everyone. I am processing power spectrum density with MEG data. I have noticed that the mne.viz.plot_topomap provide the same image even when I invert the sign of the data.

The below is a sample and the obtained image. “epochs” is an Epochs instance derived from a .fif file.

info = epochs.pick_types(meg='grad').info
psds_orig = np.linspace(-0.5, 0.5, 204)
psds_inv = psds_orig*-1.0
fig, ax = plt.subplots(2,2)
vmin, vmax = -0.3, 0.3
mne.viz.plot_topomap(psds_orig, info, axes=ax[0,0],cmap='jet', vmin=vmin, vmax=vmax);ax[0,0].set_title('original')
mne.viz.plot_topomap(psds_inv, info, axes=ax[0,1],cmap='jet', vmin=vmin, vmax=vmax);ax[0,1].set_title('inverted')
ax[1,0].plot(psds_orig);ax[1,0].set_title('original data');ax[1,0].set_xlabel('grad channel')
ax[1,1].plot(psds_inv);ax[1,1].set_title('inverted data');ax[1,1].set_xlabel('grad channel')

topomap_sample

I expect that when the sign of the data is inverted, the color of the topomap would also be inverted. However, the two data provide the same image.

I am confused with the situation and need help to get around the problem.

Thank you

Katsuya Ogata

2

There are two planar gradiometers at each location in the topoplot. The topoplot will combine the signal at these two gradiometers by computing sqrt(sens1 ** 2 + sens2 ** 2). Hence, any information about the sign (rather, the signal direction) will be lost in this type of plot. Try plotting the planar1 and planar2 sensors independently (epochs.pick_types(meg='planar1'))

3

Thank you very much for your reply. Now I understand the situation.

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