[FieldTrip] Band Limited Power Analysis in Fieldtrip?

[Chaitanya Srinivas]

Hi Ian,

If you want to approximate a chebyshev filter with butterworth, you
obviously need a higher order than the actual chebyshev uses. Some simple
calculations are involved on your data  which basically suggests even to
get a 3dB of stopband attenuation for the delta range, you would be needing
an order of 3 for a chebyshev-1 type filter. Similarly you will be needing
a 4th order butterworth filter.

By default, even the filedtrip uses 4th order for butterworth filter. So I
would suggest you to use a 4th order butterworth filter to approximate the
chebyshev filter you mentioned here.

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*Chaitanya Srinivas Lanka Wiss. Mitarbeiter
                                      *

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On Tue, Feb 25, 2014 at 11:25 AM, Ian M. Andolina <i.andolina at ucl.ac.uk>wrote:

>  Hi, I wanted to use Fieldtrip to recreate the methodology used in a
> paper by Leopold et al., 2003 <http://dx.doi.org/10.1093/cercor/13.4.422>— the relevant methods section is the following:
>
> *“The LFP signals were resampled to 1 kHz, and then band-pass filtered
> into seven frequency ranges using a second order, bi-directional,
> zero-phase Chebyshev type-1 filter. The frequency ranges, as well as their
> rough mapping onto classically defined electroencephalographic conventions,
> were the following: δ (1–4 Hz), θ (5–8 Hz), α (9–14 Hz), β (15–30 Hz), γL
> (30–50 Hz), γH (50–100 Hz), and γVH (100–150 Hz). The resulting
> band-limited signals were full-wave rectified by taking their absolute
> value. They were then resampled to 20 Hz after low-pass filtering with an
> eighth order Chebyshev type-1 filter (cutoff = 8 Hz).”*
>
> My LFP data is sampled at 1kHz, and I manually pad with data either side
> of the event triggered region of interest. Fieldtrip doesn’t have a chebyshev
> type-1 filter, but would a second order two-pass (aka zero phase?)
> butterworth be good enough considering the frequency bands used? I’m using
> the following cfg passed to ft_preprocessing for the initial bandpass
> filtering and rectification:
>
> freq = {[1 4], [5 8], [9 14], [15 30], [30 50], [50 100], [100 150]};for j = 1:length(freq)
>  cfg                        = [];
>  cfg.padding                = 0;
>  cfg.bpfilter            = 'yes';
>  cfg.bpfilttype            = 'but';
>  cfg.bpfreq                = freq{j};
>  cfg.bpfiltdir            = 'twopass';
>  cfg.bpfiltord            = 2;
>  cfg.bpinstabilityfix    = 'reduce';
>  cfg.rectify                = 'yes';
>  bp{j} = ft_preprocessing(cfg,ft);end
>
> As far as I can see in ft_preprocessing rectification occurs after
> filtering, so this is as specified by Leopold *et al.,*
>
> Then for downsampling/resampling:
>
> cfg                        = [];cfg.padding                = 0;cfg.lpfilter            = 'yes';cfg.lpfreq                = 8;cfg.lpfilttype            = 'but';cfg.lpfiltdir            = 'twopass';cfg.lpfiltord            = 8;cfg.lpinstabilityfix    = 'reduce';
> bp{j} = ft_preprocessing(cfg,bp{j});
> cfg                        = [];cfg.resample            = 'yes';cfg.resamplefs            = 20;cfg.detrend                = 'no';
> bp{j} = ft_resampledata(cfg,bp{j});
>
> Does this seem a correct translation of this method in Fieldtrip? Are
> there any caveats other than the filter differences?
>
> Thanks, Ian
>
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> fieldtrip at donders.ru.nl
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