[FieldTrip] problem with combing high_pass filter and log transform

[Eelke Spaak]

Hi Patrick,

Could you provide some more details? First, how are you applying the
high-pass filter? Also, could you give the code you use for plotting
and post the resulting figures?

Best,
Eelke

On Sat, 20 Apr 2019 at 04:53, John Begnoche <jbegnoche at sralab.org> wrote:
>
> 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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