[FieldTrip] computing threshold for PLV

Subramaniam Iyer eeguser at hotmail.com
Sat May 18 21:42:05 CEST 2013

Dear FT Experts
Thank you for your inputs. I should add here that I have continuous EEG data (so effectively single trial). The way I am computing PLV is by dividing my data into windows of 2 seconds each and computing PLV for each window and then I finally average the PLV values of all the windows.
So when we talk of shuffling trials, in my case should i consider each of 2 second window as a trial ? Sorry if this sounds stupid as I am new to this kind of analysis.

From: smoratti at psi.ucm.es
Date: Sat, 18 May 2013 20:11:11 +0200
To: fieldtrip at science.ru.nl
Subject: Re: [FieldTrip] computing threshold for PLV

Dear Sheraz,
You might consult the following article: Lachaux, J. P. et al. 1999. Measuring Phase Synchrony in Brain Signal. Human Brain Mapping 8: 194-208. In principle you have to create surrogate data that represent the null hypothesis that there is no phase locking between your electrodes, sources, etc. Then you create hundreds or thousands of surrogate data sets and calculate each time you PLV. That way you create a distribution of your PLV values under the Null hypothesis. Then you check in what percentile your observed PLV lies (e. g. if its beyond the 95% percentile you would reject the Null hypothesis).
Hope that helps,

________________________________________________________Stephan Moratti, PhD

see also: http://web.me.com/smoratti/

Universidad Complutense de MadridFacultad de PsicologíaDepartamento de Psicología Básica ICampus de Somosaguas28223 Pozuelo de Alarcón (Madrid)Spain
Center for Biomedical TechnologyLaboratory for Cognitive and Computational NeuroscienceParque Científico y Tecnológico de la Universidad Politecnica de Madrid
Campus Montegancedo28223 Pozuelo de Alarcón (Madrid)Spain

email: smoratti at psi.ucm.es
Tel.:    +34 679219982

El 18/05/2013, a las 18:21, Subramaniam Iyer escribió:Dear FT Experts,

I have calculated PLV for a set of EEG data from 5 different patients. Now I want to convert the PLV matrix of each patient into a binary matrix. For this I need a threshold PLV value below which I can assume the phase locking is zero.
My question is, how do I compute this threshold. I know hard thresholding is one option ( for ex setting 0.1 or 0.2 as threshold), but I guess it is not a very good option.

Can somebody suggest a better and robust (statistical) way of determining the threshold ? _______________________________________________
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