[FieldTrip] Questions on Functional Connectivity results using debiased wPLI

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

Hi Sergio,
> 
> 1) Regardless of whether I use my full time window of interest (0-1.5 s) or whether I estimate wPLI for two separate shorter time windows (0-.7 s and .7-1.5 s), the results are always identical to one another. Is this normal behaviour in wPLI analyses or is there something potentially wrong with how I've implemented it? 

Yes, you made a mistake in the (assumptions underlying) the implementation: the cfg.toi that you supply before ft_freqanalysis is not going to have any effect if you use ‘mtmfft’ as a method, since this method just provides a single estimate of the spectrum using the whole ’trial’ for the estimation.

> 2) I notice that wPLI values are way lower for low-frequencies (4-7hz) than for higher-frequencies (8-12hz and 15-30hz). While this could be a real effect associated with my task, the fact that this result is consistent across different conditions makes me wonder whether I've done something wrong in the ft_freqanalysis configuration. 

I think that this is just a property of the data.

Best wishes,
Jan-Mathijs


> 
> Thanks a lot in advance for any help anyone can provide! 
> 
> -----------
> 
> cfg                 = [ ];
> data               = ft_preprocessing(cfg,rwdata);
>         
> %resample data
> cfg                      = [ ];
> cfg.resamplefs   = 250;
> data                    = ft_resampledata(cfg, data);
> 
> data =
>   struct with fields:
> 
>       fsample: 250
>       trial:   {1×117 cell}
>       time:  {1×117 cell}
>       label: {100×1 cell}
>       cfg:    [1×1 struct]
>         
> % next, call the ft_freqanalysis function in our source data
> cfg                     = [ ];
> cfg.output          = 'fourier';
> cfg.method        = 'mtmfft';
> cfg.taper            = 'dpss';
> cfg.foi                 = linspace(4,30,30);
> cfg.tapsmofrq     = 4;
> cfg.toi                 = 0:.02:1.5;   % try 0:.02:.7 and .7:.02:1.5
> tf_data                = ft_freqanalysis(cfg,data);
> 
> tf_data = 
>   struct with fields:
> 
>             label:     {100×1 cell}
>             dimord: 'rpttap_chan_freq'
>             freq:       [1×30 double]
>             fourierspctrm: [4563×100×30 double]
>             cumsumcnt: [117×1 double]
>             cumtapcnt:   [117×1 double]
>             cfg:               [1×1 struct]
>         
> % estimate wPLI values
> cfg                    = [ ];
> cfg.method       = 'wpli_debiased'; 
> connect_data   = ft_connectivityanalysis(cfg,tf_data);
> 
> connect_data = 
>   struct with fields:
> 
>                label:     {100×1 cell}
>                dimord:  'chan_chan_freq'
>                wpli_debiasedspctrm: [100×100×30 double]
>                freq:  [1×30 double]
>                cfg:   [1×1 struct]
> 
> Best,
> 
> Sergio.
> 
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