Effects of Blinks and Eyelid Closure on the EEG

Background: Identification and rejection of eye-related artefacts is important in the interpretation of clinical EEGs. Typical eye-blink artefacts with prominent downward deflection in the most anterior channel and a rapid amplitude fall-off posteriorly are traditionally explained by an upward rotation of the eyeball during lid closure. However, there are no reliable data about the contribution of lid and eyeball movements to the generation of these artefacts. The goal of the present study was to correlate eye and lid movements with EEG potentials using search coils in a magnetic field. Methods: Two healthy male subjects with no known history of neurological disease were studied. Horizontal and vertical eyeball movements and vertical eyelid movements were measured using the magnetic search coil technique. Eighteen scalp EEG electrodes were applied according to the international standard 10–20 system. Signals from the search coils were simultaneously recorded with the EEG machine, and were used to obtain a time correlation between the EEG and search coil signals. Results: A blink was characterized by stereotypical movements consisting of fast descent (8 to 20 degrees for 50 to 80 ms) and slower elevation of the eyelid. During the blinks, eyeballs were slightly rotated downward (1.5 to 3.5 degrees) and inward (0.9 to 3.1 degrees). No upward rotation was seen in either subject. The simultaneous EEG showed typical high-amplitude positive potential bifrontally maximum at electrodes Fp1 and Fp2. As revealed in the wide-band EEG, the time course of the EEG potential corresponded to the lid movement. Forced blinks were characterized by fast descent and slower elevation of the eyelid with a long plateau during the eye closure. The eyeball was rotated upward during the slower phase of the eyelids, although the onset of the eyeball movement was almost the same as that of the eyelids. This corresponds to Bells' phenomenon. Conclusion: EEG potentials during eye-and-lid movements are mainly determined by the eyelid and not by eye movements. During eye blinks, the eyeballs do not move upward but rather downward and nasalwards. During forced lid closure, the eyes deviate up (Bells' phenomenon). In both cases typical bifrontal positive EEG potentials occur. Movements of the eyes have only a minor influence on EEG signals.