[FieldTrip] problem with combing high_pass filter and log transform

[John Begnoche]

Hello,


I am plotting outcomes from a frequency analysis of resting state sensor space EEG data, for the analysis I use the code below, and then plot with singleplotER, if I first high pass filter my data at 1Hz during a prepocessing step, my plotting shows no activity until 35Hz then it spikes to a plateau for the rest of he higher frequencies, after a log transform I get a sine wave like shape. With no HP filter the data look like normal power spectrum data, why is the HP filter changing my data in this way?


    cfg = [];
    cfg.output = 'pow' ;
    cfg.method = 'mtmfft';
    cfg.pad = 'nextpow2';
    cfg.taper  = 'hanning';
    cfg.foilim = [3 50]; %change for appropriate range of frequencies
%     cfg.tapsmofrq = [2] ;
    cfg.keeptrials = 'no';
    %EEG_freq_segmented{i_run} = ft_freqanalysis(cfg, data_segmented_no_artifacts);
    %EEG_freq_segmented{i_run} = ft_freqanalysis(cfg, data_segmented);
  % freqfourier{i_run} = ft_freqanalysis(cfg, data_segmented_preproc_ICArm_no_artifacts);
    freqfourier= ft_freqanalysis(cfg, data_segmented_no_artifacts);
     %%%%%%log transform%%%%%%%%%%%%%
    cfg           = [];
    cfg.parameter = 'powspctrm';
    cfg.operation = 'log10';
    freqfourier    = ft_math(cfg, freqfourier);

Thank you,
Patrick


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