Ocular artifact

Electrical contamination of EEG and MEG produced by the eyes. The eyeball is a standing electrical dipole (positive cornea, negative retina), so any rotation — a blink, a saccade, or slow drift — sweeps this dipole and projects large, low-frequency deflections onto frontal channels that fall off toward posterior sites. Blinks add a stereotyped deflection of roughly 100 to 400 ms, largest at Fp1 and Fp2; saccades add step-like shifts plus a brief spike potential from the extraocular muscles at movement onset.

Amplitudes routinely reach tens to hundreds of microvolts — an order of magnitude above cortical signals — so ocular artifact is usually the single largest non-brain source in scalp recordings and the first target of any cleaning pipeline. Because eye and brain activity overlap in time and in the low-frequency band, simple band rejection cannot separate them; correction instead exploits the fixed spatial topography (regression, ICA, SSP) or, in ERP work, rejects contaminated epochs. Co-registered eye tracking or dedicated EOG channels supply the reference signal that most correction methods need.