[FieldTrip] wPLI analysis pipeline

[Schoffelen, J.M. (Jan Mathijs)]

Hi Daniel,

If you only have 5 trials in your input data, the estimate itself will be enormously biased, and possibly by consequence is the bias correction ‘overshooting’.

Best wishes,
Jan-Mathijs


On 13 Jun 2019, at 19:10, daniel.strahnen at uni-ulm.de<mailto:daniel.strahnen at uni-ulm.de> wrote:

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.



  1.  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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