[Mne_analysis] Source current directions

[Ghuman, Avniel]

Hi Ying,

The orientations follow the cortical surface. Thus, the activity from a dipole on one side of a sulcus or gyrus will show the opposite sign on the other side of the sulcus or gyrus (because the euclidean orientation of the dipole does not change, but its orientation relative to the cortical surface did).

Best wishes,
Avniel

From: Ying Yang <yingyan1 at andrew.cmu.edu<mailto:yingyan1 at andrew.cmu.edu>>
Reply-To: Discussion and support forum for the users of MNE Software <mne_analysis at nmr.mgh.harvard.edu<mailto:mne_analysis at nmr.mgh.harvard.edu>>
Date: Wednesday, April 9, 2014 11:59 AM
To: "mne_analysis at nmr.mgh.harvard.edu<mailto:mne_analysis at nmr.mgh.harvard.edu>" <mne_analysis at nmr.mgh.harvard.edu<mailto:mne_analysis at nmr.mgh.harvard.edu>>
Subject: [Mne_analysis] Source current directions

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