[Eeglablist] Problem understanding my EEG recorder filters

[Enrico Fratto]

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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