[FieldTrip] How this Warning: output time-bins artrial 30, frequency 4 (7.00 Hz) affect data? Differences in power spectrum and

[Maité Crespo García]

 Hi Helena,
The warning probably means that the time points you are specifying in cfg.toi do not fully match those present in your data. Try giving a real time interval extracted from your variable time inside stim. 

You should get an output of time-frequency bins centered at each toi you specify. However, some time-frequency bins at the edges are missing due to the "boundary effects". I recommend you to check the time frequency tutorial Time-frequency analysis using Hanning window, multitapers and wavelets - FieldTrip toolbox

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Time-frequency analysis using Hanning window, multitapers and wavelets -...


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Best wishes,Maite



   En viernes, 23 de octubre de 2020 9:31:25 GMT+1, Helena Pereira <hri.pereira at campus.fct.unl.pt> escribió:  
 
 Dear FieldTrip community, 

I am a beginner using fieldtrip, so I know I am missing something in this domain:
I am making a time-frequency analysis of EEG data with wavelets. However, I am getting the following warning:

Warning: output time-bins artrial 30, frequency 4 (7.00 Hz)Warning: output time-bins are different from input time-bins
Using this parameters:    % Wavelet power
    cfg = []
    cfg.output = 'pow';
    cfg.channel = [1:64];
    cfg.keeptrials='no';
    cfg.method = 'wavelet';
    cfg.width = 7;
    cfg.toi = -2.0:0.01:2.0;
    cfg.foi = 4:1:20;
    p_stim = ft_freqanalysis(cfg,stim);
I can mitigate the warning by changing the cfg.toi from [ -2.0:0.01:2.0] to  [ -2.0:0.5:2.0] 
However, the plot of the spectrum changes a lot, as I expected as the time window changes from intervals from 0.01 to 0.5:With the warning:

Without the warning:


I know I am missing something regarding signal analysis... Why the differences in the changing the step of the interval leads to the warning?

Can you help me? I am not sure if the warning can influence the final results. 

Best regards, 
Helena Pereira

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