[Mne_analysis] Negative Weighted Phase Lag Index Values

[Andrea Brovelli]

Suggestion: in Fieldtrip, the WPLI is implemented in its signed version, 
it is not the eq.8 of Vinck' paper, which is bounded [0, 1].

The estimator is (line 73 in ft_connectivity_wpli.m 
<https://github.com/fieldtrip/fieldtrip/blob/master/connectivity/ft_connectivity_wpli.m> 
):

E(Im(X))/E(|Im(X)|)

Maybe the MNE code got inspiration from it.

Andrea


Le 14-Jun-17 à 10:48 PM, Eric Larson a écrit :
>
>     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.
>
>
> From what I recall the debiasing can indeed produce (generally small) 
> negative values, so that at least I would expect. From Vinck et al., 2011:
>
>     If the WPLI exceeds the PLI, then the debiased WPLI-square
>     estimator will be negatively biased for small sample sizes.
>
>
> The WPLI, however, doesn't have this characteristic, and from what 
> I've seen in a brief look at the MNE code, I'm not sure where it could 
> come from (based on where we use abs()). So a minimal example would 
> help us track it down.
>
> Eric
>
>
>
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