Ocular artifacts in recording EEGs and event related potentials.

Abstract

The ocular artifacts derive from the potential difference between the cornea and the fundus of the eye. This can be represented by an equivalent dipole with its positive pole directed toward the cornea. The DC potential between the cornea and the forehead measures approximately +13 mV. The scalp-distribution of the ocular artifacts can be described in terms of propagation factors--the percentage of the EOG present at the EEG electrodes. These factors are significantly different for blinks and upward eye-movements. The source dipoles for blinks and saccades are different--blink dipoles point radially whereas saccade dipoles point tangentially, in the direction of the eye movement. Blink and eye movement potentials are generated by different mechanisms--blink potentials are generated by the eyelid sliding over the cornea, eye movement potentials by the rotation of the ocular dipole. A very small downward rotation of the eyes may occur during a normal blink. The "rider artifact" at the onset of upward saccade is caused by the eyelid as it lags behind the eyes at the beginning of the movement. Smaller rider artifacts, caused by the horizontal asymmetry of the eyelid, can be noted during horizontal but not downward saccades. Techniques that use scaled EOG to remove ocular artifacts from EEG recordings may remove some of the frontal EEG together with the ocular artifacts. Dipole source techniques allow the ocular generators to be distinguished from the nearby brain generators. A problem with dipole source techniques is that the head model used in the calculation is not accurate at the eyes. A new technique uses principal component analysis to estimate the ocular artifact at each electrode without using a head model. This technique is the most effective way to remove ocular artifacts from EEG recordings. (Abstract shortened by UMI.)