[FieldTrip] data segmenting and frequency analysis

Richard Bethlehem rb643 at medschl.cam.ac.uk
Wed May 20 12:34:32 CEST 2015

Dear Fieldtrippers,

Being somewhat new to using fieldtrip I was hoping someone would be able to help me sort out some parcellation and data-handling issues. In short: I'm working with resting state EEG data, recorded with a 64-channel BioSemi system and would like to do some graph based analyses on this. The data is 4 minutes (eyes-open, eyes-closed 1-minute epochs) and ideally I would like to split these two 1 minute eyes-closed parts into 30*4s epochs. After this I would like to run a wavelet decomposition for the frequency analysis and obtain the adjacency matrix based on weighted phase lag index.

Thus, my questions are:
1. How to handle parcellating data? Shoud I use a)"toi" in ft_freqanalysis as a sliding window or b) split at trial definition?
If a: how does freqanalysis handle multiple trials? I currently use this:
    cfg.method = 'wavelet';
    cfg.output = 'powandcsd';
    cfg.channel = 1:64;
    cfg.foilim = [0 70];
    cfg.toi = [5 10 15 20 25 30 35 40 45 50 55]
    [freq_data] = ft_freqanalysis(cfg, data_iccleaned) % run over ICA cleaned data

Which does give me 11 fragments, but I am not sure how the two 1-minute epochs/trials are handled in this or if they are adequately handles in the subsequent ft_connectivityanalysis?

If b: how can you split trial in different epochs of a certain length and how does ft_freqanalysis handle multiple trials?

2. How to deal with faulty or noisy channels? If there are some noisy channels, do you remove these? Removing channels does not seem ideal as it would mean having to deal with uneven graphs (graphs with different numbers of nodes). Leaving them in might bias the wpli estimation?

3. Can you set the interval length for ft_freqanalysis (and if so, how)? It currently takes 0.02Hz increments, is that standard?

4. Is it necessary to tell ft_connectivityanalysis to use mutliple trials/segments? I currently get the message saying it needs more than 1 trial, even though ft_freqanalysis returns an x*x*11 matrix (and I'm assuming the 11 are the windows set for the wavelet decomposition). I am just using this:

    cfg.method = 'wpli';
    wpli_data = ft_connectivityanalysis(cfg, freq_data);

5. Is it possible to automatically obtain the wpli adjacency matrix per frequency band?
    Is there a fieldtrip function to average over a range of frequencies (and/or timepoints/trils) from the ft_connectivity analysis output?
    Is there a fieldtrip function to convert individal wpli's back to a complete channel*channel matrix as opposed to the 1D vector is seems to return (that lists each individual wpli)?

Any help would be much appreciated!


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