Human eye movements associated with blinks and prolonged eyelid closure

Abstract

Eye movements associated with eyelid closure were recorded in human subjects with search coils, embedded in self-adhering scleral annuli, in a magnetic field. In contrast to classical notions, voluntary as well as reflex blinks were consistently accompanied by transient downward and nasalward movements of both eyes with amplitudes 1-5 degrees. These eye movements had a shorter duration than the upper lid movements, and the shapes of the spatial trajectories of eye and lid movements were not similar. The trajectory of the eye movements was only modestly affected by gaze eccentricities up to 15 degrees; there was a tendency for the downward component to be enhanced by looking upward, and vice versa. Restraining of the lids of one eye in the open or closed position did not significantly alter the eye movements during (attempted) blinks. Velocity-amplitude-duration relations of the down- and upward components were similar for the same eye before and after closure and for the closed eye and the contralateral unrestrained eye. The velocity-amplitude-duration characteristics of saccades were also unaffected by prolonged closure of the lids of one eye. Prolonged, voluntary closure of the lids was followed by a slow, tonic ocular deviation, which was consistently upward in half of the subjects and consistently downward in the other half. Additional horizontal components were highly variable even within subjects. In one subject the downward deviation was converted into upward deviation when lid closure was mechanically impeded. We conclude that elevation of the eye ball (Bell's phenomenon) does not occur during short blinks and only in about half of the subjects during voluntary unrestrained prolonged lid closure. Our evidence does not support the possibility that the transient eye movements during blinks are caused primarily by a mechanical interaction between the lids and the eye (or the scleral annulus). More likely, they are a secondary effect of an active cocontraction of extraocular muscles that primarily results in retraction of the eye.