Signs of source solutions with fixed orientation

✉️ Mailing List Archive (read-only)
1

Hi MNE community,

I was working on a source localization method, and I observed this sign
issue when I did a simple simulation.
I simulated a waveform for an ROI in the left auditory cortex
("left_aud_dipoles.png"), with a fixed orientation (normal direction), but
using MNE, some of the waveforms of the dipoles in that ROI had flipped
signs. (see "truth_left_aud.png" and "mne_left_aud.png", near time 0 ms).
I learned that this is common because of the sulci or gyri structure.
But is this also common in simulations? Is it inevitable?
If this is true, when I measure the error of the source solution, should
I flip the signs or take the absolute values before comparing with the
truth?

Thanks a lot.
Best,

Ying
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mail.nmr.mgh.harvard.edu/pipermail/mne_analysis/attachments/20140815/a92bd562/attachment.html
-------------- next part --------------
A non-text attachment was scrubbed...
Name: truth_left_aud.png
Type: image/png
Size: 50457 bytes
Desc: not available
Url : http://mail.nmr.mgh.harvard.edu/pipermail/mne_analysis/attachments/20140815/a92bd562/attachment.png
-------------- next part --------------
A non-text attachment was scrubbed...
Name: mne_left_aud.png
Type: image/png
Size: 168327 bytes
Desc: not available
Url : http://mail.nmr.mgh.harvard.edu/pipermail/mne_analysis/attachments/20140815/a92bd562/attachment-0001.png
-------------- next part --------------
A non-text attachment was scrubbed...
Name: left_aud_dipoles.png
Type: image/png
Size: 116638 bytes
Desc: not available
Url : http://mail.nmr.mgh.harvard.edu/pipermail/mne_analysis/attachments/20140815/a92bd562/attachment-0002.png

2

hi Ying,

I was working on a source localization method, and I observed this sign
issue when I did a simple simulation.
I simulated a waveform for an ROI in the left auditory cortex
("left_aud_dipoles.png"), with a fixed orientation (normal direction), but
using MNE, some of the waveforms of the dipoles in that ROI had flipped
signs. (see "truth_left_aud.png" and "mne_left_aud.png", near time 0 ms).
I learned that this is common because of the sulci or gyri structure.
But is this also common in simulations? Is it inevitable?

it's hard to avoid but if you use a sparse solver that will try to "pick"
the right side of the sulcus it is possible.

see for example the red dipole in the estimates of:

http://martinos.org/mne/stable/auto_examples/inverse/plot_mixed_norm_L21_inverse.html#example-inverse-plot-mixed-norm-l21-inverse-py

on the right hemisphere the auditory response is explained with one
red dipole on the planum temporale.

If this is true, when I measure the error of the source solution, should I
flip the signs or take the absolute values before comparing with the truth?

There are many different error metrics for inverse methods (L2, peaks
distances, time series correlations, earth-movers distance, F1-score,
ROC etc.). You should always use sign flip trick to average signed
estimates.

HTH
Alex