[Eeglablist] Problem understanding my EEG recorder filters

Tue Dec 3 13:02:38 PST 2024

Hi Brian,

I believe some dry active electrode systems implement hardware (analog) filtering between the active electrode and the amplifier, to increase SNR before applying the gain (to avoid amplifying the noise as well as the signal). This is because active systems (e.g., dry active wearable systems) have poorer impedance and SNR than passive systems using conductive gel. 

Otherwise, it is probably for visualization only. 

Simply apply the default EEGLAB filters after loading your data in Tools > Filter the data > Lower edge: 1 > Upper Edge: 40 
(or whatever cutoff frequencies you want).

or via command line for bandpass 1-40 Hz:
EEG = pop_eegfiltnew(EEG,'locutoff',1,'hicutoff',40);

And follow the EEGLAB tutorial for further data cleaning (re-referencing, removing bad channels, removing artifacts, ICA, etc.). All these steps are made easy in EEGLAB. 
https://urldefense.com/v3/__https://eeglab.org/tutorials/__;!!Mih3wA!GLKYT_SRNRzT3vdXmoRGSSmFuzre4WNO7Vn7uTZeW_JfQ_UWYBcJgVFtu0dZGCHk4D-hJwutf1AE4m5uHO6KptvvIQ$ 

Cedric

On Monday, December 2nd, 2024 at 3:15 PM, Enrico Fratto via eeglablist <eeglablist at sccn.ucsd.edu> wrote:

> Thank you, Brian, again.
> This is so kind of you!
> 
> 
> Enrico
> 
> On Tue, Dec 3, 2024, 00:09 Brian Harvey brian.harvey at biogen.com wrote:
> 
> > Hi Enrico,
> > 
> > Happy to help, I am not as ative of a contributor but in recent years have
> > gained some experience that may be of value...
> > 
> > The only rig I know that is DC coupled, requiring a reference to be chosen
> > at load and high pass filtering is Biosemi. Lifelines Neuro has a
> > relatively new ambulatory EEG amplifier (T4A) that is DC coupled and saves
> > in a BDF (24 bit cousin of the 16 bit EDF) and also needs to be filtered to
> > remove this DC component.
> > 
> > For what its worth I think you might consider a bandpass filter in the
> > range of interest and use the EEGLAB filtering tool (suggest changing the
> > default to FIR) and not overly concern yourself with the hardware amplifier
> > design; I assume you would be able to phone a technician from the
> > manufacturer to get the specs.
> > 
> > Cheers,
> > Brian
> > 
> > ------------------------------
> > From: Enrico Fratto frattoe at gmail.com
> > Sent: Monday, December 2, 2024 3:40 PM
> > To: Brian Harvey brian.harvey at biogen.com
> > Cc: eeglablist at sccn.ucsd.edu eeglablist at sccn.ucsd.edu
> > Subject: Re: [Eeglablist] Problem understanding my EEG recorder filters
> > 
> > EXTERNAL SENDER
> > 
> > Cannot be more grateful for your answer.
> > So you think that no filter at all is applied prior to storage? It
> > happened to me to read that, albeit wide, some machines might still apply
> > some filters, e.g. the HF anti aliasing and a LF excluding ultra slow drift
> > (unless the recording is DC) and this are preliminary ti fhe successive
> > software filters.
> > 
> > For instance, in many papers I have read that the recording is made
> > between 0.01 and 100 Hz at the hardware level.
> > 
> > Have you ever heard anything about?
> > 
> > Thank you so much again!
> > Hope to help as well in the future
> > 
> > Enrico Fratto, MD
> > Institute of Neurology,
> > Department of Medical and Surgical sciences
> > Università Magna Graecia,
> > Catanzaro
> > 
> > On Mon, Dec 2, 2024, 21:17 Brian Harvey brian.harvey at biogen.com wrote:
> > 
> > Hi Enrico,
> > 
> > In my experience amplifier filter settings at acquisition (as well as any
> > reference montages) are for visualization purposes only and the time series
> > is recorded to disk without any modifications (unless software offers this
> > capability in its settings). Therefore, I am not surprised to hear that the
> > EDF appears to be full bandwidth and without notch filter(s).
> > 
> > When pre-processing the EEGs I would first re-reference the data (if
> > appropriate) to the scalp average and use the CleanLine() plugin to
> > attenuate the line noise even further.
> > 
> > ICA will provide even further signal conditioning by allowing you to
> > remove strong, non-brain artifacts such as EMG, EYE, ECG etc
> > 
> > I personally always run a high pass filter at 0.5 Hz and only lowpass
> > filter if I want to stay under line noise or resample the data to a lower
> > rate (cutoff set below the nyqist of 0.5*resample rate)
> > 
> > Hope this helps,
> > 
> > Brian
> > 
> > ------------------------------
> > From: eeglablist eeglablist-bounces at sccn.ucsd.edu on behalf of Enrico
> > Fratto via eeglablist eeglablist at sccn.ucsd.edu
> > Sent: Monday, December 2, 2024 1:47 PM
> > To: eeglablist at sccn.ucsd.edu eeglablist at sccn.ucsd.edu
> > Subject: [Eeglablist] Problem understanding my EEG recorder filters
> > 
> > EXTERNAL SENDER
> > 
> > Good afternoon,
> > First, thank you the EEGLAB developers and the community all for the
> > precious opportunity of gathering expert advice.
> > 
> > I am a clinical clinical neurologist, and in the last few weeks I am
> > beginning to study EEG spectral analysis. My unit uses CADWEL ARC Apollo 32
> > channels amplifier for EEG recording. I know from the producer that the
> > amplifier bandpass can be set as wide as 0.16 to 100 Hz. Specifically, I
> > have been working with EEGs originally recorded with a 0.5-70 bandpass, 256
> > Hz sampling rate and notch filter ON. I have been told these recording
> > settings to correspond to the actual hardware filters affecting the
> > recorded EEG, so heavily cutting frequencies below 0.5 and above 70.
> > However, when I extract the full edf from the software and I analyse it
> > with EEGLAB or EDFBrowser, the spectrum seems to contain the whole array of
> > frequencies below 0.5 and above 70 without any true slope or filter effect.
> > For instance, 50 Hz peak and its 100 Hz harmonic are dominant in the
> > spectrum and the actual EEG is unreadable at its raw state.
> > 
> > For these reasons it seems to me that the filters that I set during the
> > recording are only active at the post-recording state (I.e. digital
> > filter).
> > I know that digital systems often use wide bandpass (i.e. 0.1 -100 Hz)
> > allowing more signal in prior to filter it digitally.
> > 
> > My concern is that I do not know what HF and LF physical filters my
> > amplifier uses at the acquisition stage and so what frequency array truly
> > is allowed, i.e. what is the passband prior to digital filtering.
> > 
> > Does Anyone have an idea on how I might proceed? Am I overlooking some
> > other issues?
> > 
> > I sincerely apologise if I have said anything wrong, or if I asked a stupid
> > question; I am new to signal analysis and possibly I need to build some
> > basic knowledge, but I have not been able to solve this doubt by myself. I
> > basically have no informatic/engineering skills.
> > 
> > I thank whoever will answer in advance.
> > Enrico Fratto, MD
> > Institute of Neurology,
> > Department of Medical and Surgical sciences
> > Università Magna Graecia, Catanzaro, Italy
> > _______________________________________________
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