[Mne_analysis] Negative Weighted Phase Lag Index Values

[Cushing, Cody]

Hey all,

Hopefully this helps rather than compounds your problem, but this also occurs when computing the debiased squared wpli, where there should undoubtedly be no negative values (unless the debiasing does something strange that I'm not aware of), just FYI. Generally, any negative values are small just as Jeff sees.  However, out of a group of 57 subjects that I ran this on, I got a minimum value of -0.3954, which isn't all that small.  I only mention this in case it is some shared routine between the wpli and wpli2_debiased that is generating these negative values.  Hopefully the data you get from Jeff is enough to figure out what's going on, but I can supply more if need be.

Cheers,
Cody


________________________________
From: mne_analysis-bounces at nmr.mgh.harvard.edu [mne_analysis-bounces at nmr.mgh.harvard.edu] on behalf of Eric Larson [larson.eric.d at gmail.com]
Sent: Wednesday, June 14, 2017 3:49 PM
To: Discussion and support forum for the users of MNE Software
Subject: Re: [Mne_analysis] Negative Weighted Phase Lag Index Values

Jeff,

When you get a chance, could you try to share some data (as small as possible) and a small code snippet (using coherence-computing routines) that show this problem to the related MNE issue I opened<https://github.com/mne-tools/mne-python/issues/4319>?

Eric


On Wed, Jun 14, 2017 at 3:22 PM, Eric Larson <larson.eric.d at gmail.com<mailto:larson.eric.d at gmail.com>> wrote:
Fig1C shows the individual vectors and how they're weighted -- I was looking at Eq.8 which defines WPLI, and the paragraph that followed where they state that 0 ≤ Φ ≤ 1 holds (due to the absolute value operations in the numerator and denominator). But maybe there is more to it or you don't need to take the absolute value of the expectation in the numerator, in which case it could be -1 ≤ Φ ≤ 1, is that what you have in mind...?

Eric


On Wed, Jun 14, 2017 at 3:13 PM, Andrea Brovelli <andrea.brovelli at univ-amu.fr<mailto:andrea.brovelli at univ-amu.fr>> wrote:

Dear all,

I don't know the implementation in MNE, but, as far as I know, the WPLI ranges between between -1 and 1 (see Fig. 1C in Vinck, Neuroimage 2011). The same is for the imaginary coherence (Nolte, Clin Neurophysiol 2004).

The strength of phase-synchronization can be indexed by the magnitude (or squared value) of the WPLI.

bye

Andrea




Le 14-Jun-17 à 7:51 PM, Eric Larson a écrit :
Yes it looks like WPLI should be bounded by 0 and 1. Can you open an MNE issue so we can look into it further?

Eric


On Wed, Jun 14, 2017 at 1:28 PM, Stout, Jeffrey <jstout at mcw.edu<mailto:jstout at mcw.edu>> wrote:
We are running analyses on resting state MEG data and we are finding negative values in the weighted phase lag index (wpli) measure.  We have also run this data using the phase lag index (PLI) measure and do not see any values below zero.  I thought that both the PLI and WPLI should be defined from 0 to 1.  Although the negative values tend to be small, we are seeing this across many subjects.

wpli=np.load('connmat_wpli2_debiased13_30hz_run1.npy')

np.min(wpli), np.max(wpli)
Out[22]: (-0.0048496421426467845, 0.14954351856205442)     <<<<<<<<< WPLI minimum value below Zero

pli=np.load('connmat_pli13_30hz_run1.npy')

np.min(pli), np.max(pli)
Out[24]: (0.0, 0.21375066880684856) <<<<<<<<< PLI minimum value equals zero


Is this a correct finding?  Any insight would be helpful.


Thank you,
Jeff Stout

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