Line-noise removal and notch filtering

Power mains couple into recordings capacitively and inductively, producing a strong narrowband component at the line frequency (50 Hz in most of the world, 60 Hz in North America and parts of Asia) plus harmonics. The textbook fix is a notch (band-stop) filter at the line frequency; IIR notches are cheap but introduce phase distortion and ringing that can smear transient ERPs, and a wide notch removes genuine neural activity at that frequency.

Better practice removes only the deterministic sinusoid: spectral interpolation, or regression of a fitted sine and cosine at the exact line frequency and its harmonics (as in the CleanLine approach), leaves the surrounding spectrum intact. None of this substitutes for prevention — proper grounding, shielding, balanced high common-mode-rejection amplifiers and low electrode impedance attack the coupling at its source, and line noise that survives good hardware usually signals an impedance or grounding fault.