[FieldTrip] Clarification on plotting WPLI

[Prasandhya Astagiri Yusuf]

Hi Peter,
sorry for the (very) late response, I guess you have already figured the
answer by now. Anyhow, in case someone has similar problem, I try to share
my experience here in the forum.

I recently computed the debiased WPLI and experienced the same problem. I
also found this negative value of debiased WPLI quite frequently asked in
this mailing list, even Mr. Martin Vinck himself has already responded to
it on June 2011.
So as stated in the paper (Vinck et al. 2011), PLI and WPLI result has the
range from 0 to 1. But the debiased WPLI could yields negative values as a
consequence of debiasing method. One of his latest paper (Phillips, 2014),
they  mentioned in the Materials and Methods that "the debiased WPLI
estimator (Vinck et al. 2011), ranging from zero (*negative values can
incidentally occur because of limited sampling*) to one (maximum
coherence)".
So in my opinion, we can neglect this negative values and convert them to
zeros. But surely, we should not take the absolute value from it.

Best,
Sandhy


Prasandhya A. Yusuf

*Medizinische Hochschule Hannover*

Experimentelle Otologie / OE 8891 /

VIANNA / M13

Feodor-Lynen-Str. 35

D-30625 Hannover



On Mon, Dec 21, 2015 at 8:39 PM, Peter Lyons <plyons at udel.edu> wrote:

> Hello Everyone,
>
> I have a question regarding the interpretation of negative values in the
> debiased wPLI. Although the reference page on the debiased wPLI notes that
> estimates are computed by squaring the wPLI values, I and others have
> noticed that some of the values in the debiased wPLI spectrum are negative.
> How should these negative values be interpreted in reference to 0?
>
> To view the results of the debiased wPLI connectivity analysis
> (ft_connectivity analysis with cfg.method = 'wpli_debiased'), I've been
> using the ft_topoplotER function. However, I'm concerned that the default
> scaling (zlim = 'maxmin') of the plot does not seem to register the
> negative values in the debiased wPLI spectrum. That is, the negative values
> are not being plotted. It is curious, however, that the ft_topoplotER
> does register the negative values in the *non-debiased *wPLI spectrum.
>
> Is there a reason why the ft_topoplotER function is treating the negative
> values from the debiased vs. non-debiased wPLI spectrums differently? Is
> there perhaps a more appropriate plotting function I should be using to
> view the debiased wPLI output?
>
> I would greatly appreciate any guidance in this.
>
> Thank you!
>
> Peter
>
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