[FieldTrip] wPLI analysis pipeline

[daniel.strahnen at uni-ulm.de]

Dear Fieldtrippers,

 

I am a doctoral student and my research fields are local field potentials
(LFPs). 

Therefore I wanted to calculate the weighted phase lag index which is
implemented in the Fieldtrip toolbox.

The data I want to analyse has the following structure:

data = 

 

  struct with fields:

 

      label: {7×1 cell}

       time: {1×5 cell}

      trial: {1×5 cell}

    fsample: 1000                  % Seven channels and five trials

 

The code I produced for the wPLI is the following:

%% Freqanalysis to obtain the cross spectral density

cfg_freq = [];

cfg_freq.method = 'mtmfft';

cfg_freq.output = 'powandcsd';

cfg_freq.channel = {'CH2' 'CH4'};

cfg_freq.keeptrials = 'yes'; % do not return an average of all trials for
subsequent wpli analysis

cfg_freq.taper = 'dpss';

cfg_freq.tapsmofrq = 1;

cfg_freq.foilim = [0 48];

freq_data = ft_freqanalysis(cfg_freq, data);

%% WPLI analysis

cfg_conn = [];

cfg_conn.method = 'wpli_debiased';

wpli = ft_connectivityanalysis(cfg_conn, freq_data);

wpli = ft_checkdata(wpli, 'cmbrepresentation', 'full','datatype','freq');

 

 

I now have two questions:

1.	Why are there negative results? I already saw this question in a
previous discussion but I’m still not sure about the hundred percent correct
answer.

 

2.	The results I get look rather strange at the first glance because
there are a lot of consecutive One-values which seems unrealistic to me.
Do you think my analysis has a mistake or the data is corrupted?

 

Thank you very much for your help in advance and best regards

 

Daniel

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https://doi.org/10.1371/journal.pcbi.1002202