[Eeglablist] Persistent 20 Hz Harmonic Line Noise in EEG Data

[Scott Makeig]

The plots here clearly show the noise is at 25 Hz and its harmonics ...

On Sat, May 9, 2026 at 11:08 AM Евгений Машеров via eeglablist <
eeglablist at sccn.ucsd.edu> wrote:

> This isn't power line interference. It would be 50 or 60 Hz, and possibly
> harmonics at multiples of 100-150-200... or 120-180-240-... frequencies.
> Subharmonics wouldn't appear. It's undoubtedly a technical artifact,
> although I'm unsure of the source. Railways sometimes use other frequencies
> (16 2/3 Hz, others—20, 25, 45, and others no longer in use)—but not 20 Hz.
> A television screen will show 50 (PAL/SECAM) or 60 (NTSC) Hz, computer
> monitors even higher, and it could be some obsolete or specialized video
> equipment (with a lower frame rate). I'd assume the source in your case is
> medical equipment (although I can't rule out, for example, a security
> alarm).
> The best course of action is to find the source of the interference and
> turn it off. However, this isn't always possible, especially with medical
> equipment. A similar situation occurred with an infusion pump in the
> intensive care unit: the intermittently activated drive solenoid created
> regular pulsed interference. A possible solution, also used to combat a
> physiological artifact—an electrocardiographic signal interfering with the
> EEG—is to evaluate the shape of the interference (the presence of harmonics
> indicates that the shape is clearly not sinusoidal) and subtract it from
> the signal. To do this, narrowband filtering isolates the fundamental 20 Hz
> harmonic, its peaks (or zero crossings) are used as a trigger for
> averaging, the averaged response is calculated for each channel, and then
> subtracted from the signals for the individual channels. Naturally, this is
> a post-processing method and is not suitable for online processing.
>
> Your truly
>
> Eugen Masherov
>
> > Dear EEGLAB List,
> >
> > I am currently dealing with persistent line-noise contamination in my EEG
> > recordings, specifically strong peaks around 20 Hz and its harmonics (40
> > Hz, 60 Hz, etc.). The contamination remains substantial even after
> multiple
> > preprocessing attempts.
> >
> > Raw Data PSD for reference:
> https://urldefense.com/v3/__https://ibb.co/5WbNqYCW__;!!Mih3wA!EfdhaFV_tpprT4Wc8_HcfK7KDVTXA0O0NPLW3Ws732nxP-TmPxHUXI_qXevcPIO8hfndJk8euI6BWbB1_dCuBxHG_A$
> >
> > I have already tried:
> >
> > -
> >
> > Average re-referencing before notch filtering
> > -
> >
> > Average re-referencing after notch filtering
> > -
> >
> > Multiple full-rank average reference implementations, including:
> > -
> >
> > Makoto’s fullRankAveRef()
> > -
> >
> > Manual full-rank average reference using (nchan + 1) denominator
> >
> > Despite this, a large amount of the harmonic noise is still present in
> the
> > spectra.
> >
> > For reference, the issue looks somewhat similar to this discussion:
> >
> https://urldefense.com/v3/__https://mne.discourse.group/t/line-noise-eeg/11817__;!!Mih3wA!EfdhaFV_tpprT4Wc8_HcfK7KDVTXA0O0NPLW3Ws732nxP-TmPxHUXI_qXevcPIO8hfndJk8euI6BWbB1_dB2ZPVDPg$
> >
> > I would greatly appreciate advice on how people in the EEGLAB community
> > typically handle this kind of persistent harmonic contamination.
> >
> > Would you recommend CleanLine, Zapline-plus, spectrum interpolation, ASR,
> > ICA-based approaches, or something else?
> >
> > Thank you very much for your help.
> >
> > Best regards,
> > Maeghal Jain
> > BISE Lab
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-- 
Scott Makeig, Research Scientist and Director, Swartz Center for
Computational Neuroscience, Institute for Neural Computation, University of
California San Diego, La Jolla CA 92093-0559, http://sccn.ucsd.edu/~scott