[Eeglablist] question about filtering

[Karolina Broś]

Dear Andreas,

thank you very much! This is really helpful :)

I will apply the other filter setting and make sure I have two copies of
the file!

Best wishes,
Karolina




Pozdrawiam serdecznie
dr Karolina Broś

Institute of Applied Linguistics
University of Warsaw
ul. Dobra 55
00-312 Warszawa
*www.ils.uw.edu.pl <http://www.ils.uw.edu.pl>*

*www.karolinabros.eu <http://www.karolinabros.eu>*

2018-06-15 12:32 GMT+02:00 Andreas Widmann <widmann at uni-leipzig.de>:

> Hi Karolina,
>
> > Am I right to assume that to get a 1Hz high-pass filter I should enter a
> 2Hz edge?
> >
> > EEG = pop_eegfiltnew(EEG, [],2,[],true,[],1);
> Yes,  with default filter order this will give you a 1 Hz cutoff. 2 Hz
> passband edge is, however, rather extreme; so this is not necessarily what
> I would recommend for your purpose. You can have a 1 Hz cutoff highpass
> combined with a narrower transition band if you manually set filter order.
> For example with 1 Hz transition band width, your stopband edge would be at
> 0.5 Hz and the passband edge at 1.5 Hz. You compute filter order for this
> filter as
>
> df = 1; % requested transition band width
> fs = 500; % sampling frequency
> order = 3.3 / (df / fs) % round to next even integer, here 1650
>
> EEG = pop_eegfiltnew(EEG, [],1.5,1650,true,[],1);
>
> will then give you a 1 Hz highpass with 1.5 Hz passband edge. See here if
> you want to read more on the underlying concepts:
> http://home.uni-leipzig.de/biocog/eprints/widmann_
> a2015jneuroscimeth250_34.pdf
>
> > Also, only theoretically, is it possible to high-pass filter data twice?
> Would that distort the data and disable ICA?
> >
> > Let's say we have a dataset filtered with a cutoff frequency of 0.1Hz
> but then we want to run ICA at 1Hz because it's the recommended
> threshold... Is it possible to take the 0.1 dataset and apply a more robust
> filter to it?
> Technically this is possible. The 0-0.2 Hz band will then be attenuated
> more strongly. Given the 1.5 Hz passband edge filter I suggested, the
> differences will be minimal as this band is completely in the stopband of
> the new filter. Given your 2 Hz passband edge filter there will be small
> differences which I would not expect to be problematic. However, in
> general, one should also consider that the convoluted interval is longer
> (the sum of both filter orders) possibly spreading potential artifacts
> unnecessarily wide. As computation time for the higher highpass cutoff is
> considerably shorter due to lower filter order, personally I usually prefer
> to have to two parallel (on separate copies of the data) rather than
> consecutive filtering steps.
>
> Hope this helps,
> Andreas
>
> > Am 14.06.2018 um 17:41 schrieb Karolina Broś <k.bros at uw.edu.pl>:
> >
> > Dear all,
> >
> > I am preprocessing my data and I want to run ICA for artifact rejection
> on 1Hz-filtered data and then apply weights to 0.1Hz-filtered data...
> >
> > From what I understand, the EEGLAB eegfiltnew function uses passband
> edges which do not exactly correspond to cutoff frequencies.
> >
> > Am I right to assume that to get a 1Hz high-pass filter I should enter a
> 2Hz edge?
> >
> > EEG = pop_eegfiltnew(EEG, [],2,[],true,[],1);
> >
> > and to get 0.1 I should enter 0.2Hz?
> >
> > EEG = pop_eegfiltnew(EEG, [],0.2,[],true,[],1);
> >
> >
> > Also, only theoretically, is it possible to high-pass filter data twice?
> Would that distort the data and disable ICA?
> >
> > Let's say we have a dataset filtered with a cutoff frequency of 0.1Hz
> but then we want to run ICA at 1Hz because it's the recommended
> threshold... Is it possible to take the 0.1 dataset and apply a more robust
> filter to it?
> >
> > I would really appreciate your feedback!
> >
> > Karolina
> >
> >
> >
> > Institute of Applied Linguistics
> > University of Warsaw
> > ul. Dobra 55
> > 00-312 Warszawa
> > www.ils.uw.edu.pl
> >
> > www.karolinabros.eu
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