[FieldTrip] Calculating single PLI value for a frequency band

Schoffelen, J.M. (Jan Mathijs) janmathijs.schoffelen at donders.ru.nl
Mon Apr 4 09:04:10 CEST 2022

Hi Avi,

PLI, like any other connectivity metric needs to be computed across some unit-of-observation in order for it to make sense.
In FieldTrip, the  UOs for connectivity analysis are always considered to be the trial dimension.

In other words, no there is no way to compute a PLI for an individual trial (although you could get a ‘proxy’ for the single trial PLI by computing a so-called pseudo value, which from the top of my head (but please look this up to be sure) is computed as: N*PLI(all) - (N-1)*PLI(loo), where PLI(avg) is the PLI computed across all repetitions, and PLI(loo) is the PLI computed using a leave-one-out strategy. You may also have a look at this paper: https://pubmed.ncbi.nlm.nih.gov/25917516/ or this one: https://pubmed.ncbi.nlm.nih.gov/17569862/

Best wishes,

On 3 Apr 2022, at 06:42, Avijit Chowdhury via fieldtrip <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>> wrote:

Dear all,

I am novice to EEG analysis and I am trying to calculate Phase Lag Index for my resting state data.

I have redefined the data as 2 sec trials and then run ft_frequencyanalysis with “powandcsd” as cfg.output:

cfg = [];
cfg.length = 2;
data_trials = ft_redefinetrial(cfg, data);

cfg              = [];
cfg.method = 'mtmfft';
cfg.taper = 'hanning';
cfg.foilim = [8 13]; % I want to look only at alpha band
cfg.keeptrials = 'yes';
cfg.pad='nextpow2' ;
freq = ft_freqanalysis(cfg, data_trials);
 cfg = [];
cfg.method    = 'wpli';
PLI= ft_connectivityanalysis(cfg, freq);

With the above script, I get an output of PLI with the dimension of trials x channel x channel x frequency.

Is there a way I can get one PLI index for each trial, for each channel combination, across the frequency band I specified. Essentially, how can I get PLI values with dimensions: trials x channel x channel

I am very new to this so any help is appreciated. Also, If I am trying to filter my results to only the alpha band (8-13hz), is what I am doing the correct way to achieve this?

Thank you,

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