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

[Scott Makeig]

Eugen,

Yes, thanks for correcting me - I was indeed reading from the tutorial
example(@ 25 Hz)  rather than the actual data (@ 20 Hz) ... I believe you
are correct that some nearby electronic source must have been the cause. I
believe it would be worthwhile to apply an initial ICA to see whether its
scalp projection (scalp map) is stable enough to be largely (or totally)
accounted by a single independent component, which then could be removed
neatly from the data -- or ignored in further analysis of the (brain)
independent source distribution.

Scott

On Mon, May 11, 2026 at 3:19 PM Евгений Машеров <emasherov at yandex.ru> wrote:

> If you're referring to this spectrum,
> https://urldefense.com/v3/__https://ibb.co/5WbNqYCW__;!!Mih3wA!CULZcfFkieH_bUE1w0wwnlMTADY94DKwrrOAJL4cnmw9hjrWFGypYhZLJUl1ZVoht5v4sXALi-8AvrBSXjma$ 
> I see a peak at 20 Hz (around 30 dB), 40 Hz (around 40 dB), and 60 Hz
> (around 30 dB). Possible sources include MRI (pulse frequency) or
> physiotherapy equipment. It could also be some industrial equipment. In one
> case, the interference was from a nearby airport's radar.
>
> > 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 
>


-- 
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