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

[Евгений Машеров]

Can two aspects be clarified?
1. What is the quantization frequency?
2. Is the power frequency 50 Hz or 60 Hz?
A hypothesis: combination frequencies. With a sampling frequency of 200 Hz and a power frequency of 60 Hz, the main noise is eliminated because it lies below the Nyquist-Kotelnikov-Shannon frequency (100 Hz). However, nonlinear transformations can generate harmonics with a higher frequency, which will be recorded as having a lower frequency. Rectification of alternating current by a diode bridge converts alternating 60 Hz into a pulsating 120 Hz and, despite the filters, the 120 Hz harmonic and other even harmonics (240 Hz, 360 Hz, etc.) break through and create interference. Saturation of the transformer core can produce odd harmonics (180 Hz, etc.). A similar effect was observed by two talented and competent researchers, who found that all patients without exception exhibited peaks at frequencies of 22 and 28 Hz, absent in healthy subjects. However, the fact was that their sampling frequency was 128 Hz, and the fundamental harmonic of 50 Hz was successfully removed, unlike the harmonics of 100 and 150 Hz, due to the superposition of frequencies that appeared as 22 and 28 Hz (they wrote down patients in a clinic, among numerous equipment, and healthy subjects in a shielded chamber in (University laboratory).
Perhaps your 20 and 40 Hz are actually interference from 120 and 240 Hz. But if the sampling rate is not 200 Hz, but significantly higher, this hypothesis should be discarded.

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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