[FieldTrip] problem with ft_resampledata after ft_preprocessing (inc. Hilbert)

[Elisa Carrus]

Hi FT users,

I am having some problems with resampling (downsampling) the data.

Here is what I'm doing:

I preprocess the continuous data to obtain band-pass filtered signal in 
several frequencies (4-8,8-12,12-30, etc), also taking the abs value of 
the Hilbert transform. The absolute value of the Hilbert is ultimately 
what I need.

I have used the following code:
/
//cfg = [];//
//cfg.channel = 'MEG';//
//cfg.demean = 'yes';//
//cfg.detrend = 'yes';//
//cfg.lpfilter = 'yes';//
//cfg.hilbert = 'abs';//
//cfg.hpfilter = 'yes';//
//cfg.hilbert = 'abs';//
//cfg.lpfreq = 30;//
//cfg.hpfreq = 12;//
//cfg.dataset = '0061_syntask_02_raw_ssst_td.fif';//
//cfg.continuous = 'yes';//
//beta = ft_preprocessing(cfg);//
/
After this, I epoch the data as follows

/cfg = [];//
//cfg.trl = trl2; % this is a trial definition previously created//
//beta_ep = ft_redefinetrial(cfg,beta);/

and then I downsample as follows:

/cfg = [];//
//cfg.resamplefs = 500;//
//beta_epdw = ft_resampledata(cfg, beta_ep)/

I don't encounter any errors, but the strange thing is that the data 
looks fine up to before I downsample (i.e.g. if I plot one trial, it's 
all positive values, as I would expect from the abs of the Hilbert), but 
if I plot the downsampled data, the signal has both negative and 
positive values.

Is there something I'm missing?

Thanks a lot if you can help,

Elisa







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