[Eeglablist] Problem understanding my EEG recorder filters

[Enrico Fratto]

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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