Dealing with artifacts: The EOG contamination of the event-related brain potential

Abstract

Eye movements and blinks represent a major source of artifacts in the electroencephalogram (EEG) and event-related brain potentials (ERPs). The origin of this artifact is the large difference in potential that exists between the cornea and the retina. Eye movements and blinks produce shifts of the electric fields that propagate across the whole head and that can be several times larger than the activity generated by the brain, Ocular activity can be monitored by electrodes located near the eyes (electrooculogram, or EOG). The electric fields associated with eye movements and blinks are somewhat different. The simplest procedure for dealing with ocular artifacts is to eliminate trials on which EOG activity is detected (rejection). However, this technique may result in data loss and biased data samples, especially when one is comparing clinical populations or tasks involving large amounts of eye movements, Another approach involves estimation and correction of the ocular artifact on the EEG and ERP traces. Several techniques have been proposed. Some of them are reviewed in the present paper. Issues related to the accuracy of the various techniques, as well as other advantages and limitations, are also discussed. Finally, general guidelines for how to deal with ocular artifacts are proposed. 44The difference in electric potential between the front and the back ofthe eye generates a large dipole-like electric field (electro-oculogram, or EOG) that may interfere with the surface recording ofthe electrical activity of the brain (electroencephalogram, or EEG), If the EOG does not change over time, its effect on the EEG will be a constant. Since the EEG is measured as a change with respect to a baseline value, the effect of a constant EOG is null. Therefore, EOG contamination of the EEG trace occurs whenever the eyes move, or whenever the propagation ofthe EOG to the scalp electrodes varies over time. This type of contamination is labeled EOG artifact, ocular artifact, or eye-movement artifact.The EOG artifact is of great significance in the registration ofEEG and event-related brain potentials (ERPs). In fact, when measured at the scalp, especially at frontal locations close to the eyes, the EOG signal (several hundred microvolts in amplitude) can be several times larger than the brain-generated scalp potentials (typically, less than 50 JiV). Eye movements and blinks (both of which are associated with large EOG artifacts) are very frequent and may occur during the performance of various tasks or as the result of certain types ofstimulations. The frequency and size ofEOG artifacts may vary, depending I would like to thank Monica Fabiani and two anonymous reviewers for their comments on earlier versions of this manuscript. Send all correspondence to G. Gratton,

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