[Eeglablist] 0.01 Hz highpass filter isn't running

[Lena Marreel]

Hi Metin,

I recently went down a similar route on highpass filters.
Some practical points I found out, EEGlab FIR filters (the new default)
don't offer a filter order high enough to be able to handle those low
frequency cut offs. An alternative is to use IIR butterworth filters
instead.
I tend to just report the filter order, which informs about the role off of
your filter in the frequency domain. I recommend ERPlab (
https://urldefense.com/v3/__https://erpinfo.org/erplab__;!!Mih3wA!AVaLZlRIT0nEkiq_O4CriQT7S1krYWhGT8mL_WWktZN5mdeaVnC6QAePhj76drsQnSyZZUJh5uZxGVdyYyLvt_ExzQ0$ ) as a useful tool to play around and see how
different parameters and filters interact with your data. The ERPlab
interface has some useful plots to show you this. ERPlab becomes
an extension of your EEGlab GUI and has functions that build on EEGlab
functions.
I don't know if pop_eegfiltnew() offers an IIR setting, but here is an
alternative ERPlab filter I use
EEG = pop_basicfilter( EEG, 1:64, 'Filter', 'highpass', 'Design', 'butter',
'Cutoff', 0.01, 'Order', 4, 'RemoveDC', 'on' );

I too noticed the filters running slow. Though, admitingly, we (lab
colleagues and I) consider it something we cannot fix ourselves and just
accepted it as 'the way things are'. The best cause we could guess at was
related to how EEGlab calls in the function over and over taking a lot of
memory.

Hope this helps!

Best,
Lena

On Wed, Feb 7, 2024 at 8:26 PM Metin Özyağcılar via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> Dear EEGLAB community,
>
> I have recently tried to change the settings for my highpass filter that I
> used to pre-process my data. I was using a 0.05Hz highpass filter, now
> trying out 0.01 Hz, and I am using the function pop_eegfiltnew(). However,
> when I tried to run the function, even after 10-20minutes I see no
> progress.
>
> After doing some research and reading the manuals online, I’ve seen a
> recommendation that for FIR filters a lower limit of about 0.1 is
> recommendable and limits lower than that are not. I’ve additionally learned
> that the lower and higher limits used in this function do not directly
> correspond to the “cut-off” (e.g using 0.05, the “cut-off frequency” is set
> to 0.025). All of these led me to have some questions:
>
> 1) Is there any way to still run 0.01 highpass filter with pop_eegfiltnew()
> and if so how?
> 2) If I chose to run a 0.05 Hz highpass filter instead, would it be
> appropriate to still set the lower limit as 0.05 but also report the
> outcome of the function when writing the paper (i.e. transition band width:
> 0.05 Hz / passband edge(s): 0.05 Hz / cutoff frequency(ies) (-6 dB): 0.025
> Hz / filtering the data; zero-phase, non-causal)? I feel not so confident
> playing around with the parameters and the default settings of the
> function.
> 3) On an unrelated note: the filters are run very slow, even the 0.05Hz.
> Does anyone else maybe have the same issue and if so how are you dealing
> with it? I updated MATLAB, EEGLAB, even my operating system to the latest
> version + freed up my storge but nothing has changed.
>
> Any feedback would be highly appreciated. Thank you!
>
> Best,
> Metin
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