[Eeglablist] 0.01 Hz highpass filter isn't running

[Arnaud Delorme]

I agree with Lena,

The default EEGLAB filtering function cannot design a filter below 0.1 Hz. For this, you must use the ERPLab filter (install ERPLAB using the EEGLAB extension manager and then filter the data using ERPLAB).
However, some amplifiers already filter the data above this frequency (see, for example, https://urldefense.com/v3/__https://www.biosemi.com/faq/adjust_filter_activeone.htm__;!!Mih3wA!GPUYeDtZAm-OsFDbFXz-XEK1cGyCFv1roHWpn-6vN4Kz4f1mtY2F091zvcMjpbJnDL-S3b1281eJRAKi783RhjT3$ ).
In general, at least one study shows that filtering above 0.5 might be preferable for ERP analysis https://urldefense.com/v3/__https://www.nature.com/articles/s41598-023-27528-0__;!!Mih3wA!GPUYeDtZAm-OsFDbFXz-XEK1cGyCFv1roHWpn-6vN4Kz4f1mtY2F091zvcMjpbJnDL-S3b1281eJRAKi7-IZ8Naf$  (and that baseline subtraction cannot compensate for potential drifts in the baseline if you perform high pass filtering at a very low frequency such as 0.01 Hz).

Arno

> On Feb 7, 2024, at 1:18 PM, Lena Marreel via eeglablist <eeglablist at sccn.ucsd.edu> wrote:
> 
> 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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