[Mne_analysis] Source current directions

[Ying Yang]

Hi all,

I am new to MEG and MNE. I have a question about the normal directions in a
surface source space.   Are the normal directions at  all source positions
aligned with the pial to white matter direction, i.e  the pyramidal cell
axons in the cortex, or do they flip signs depending on the curvature?

If they all point to the white matter,  I would expect that the recovered
source currents by MNE or dSPM to have the same signs of activation (all
positive or all negative). However, when trying the example

"Compute MNE-dSPM inverse solution on single epochs",
http://martinos.org/mne/stable/auto_examples/inverse/plot_compute_mne_inverse_epochs_in_label.html#compute-mne-dspm-inverse-solution-on-single-epochs

 the currents in the auditory cortex split into a postive group and a
negative group.
Does it mean that some of the source currents are along the axons and some
are in reversed direction?

Thanks a lot.

Best
Ying
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