[FieldTrip] wPLI analysis pipeline

[Wanze Xie]

Hi Daniel,
One solution is to segment your five trials into 1s or 2s epochs if they
are long enough.  Re the negative values: if you used the debiased WPLI
method just ignore the negative values; in contrast, if you used the WPLI
method you may calculate the absoluate value of the WPLI values, i.e.,
abs(WPLI).

Best,
Wanze

Schoffelen, J.M. (Jan Mathijs) <jan.schoffelen at donders.ru.nl> 于2019年6月13日周四
下午2:45写道：

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