Effect of monocular visual loss upon stability of gaze.

Using the eye-coil/magnetic field method, we measured horizontal and vertical movements of both eyes in four patients with monocular loss of vision while they attempted steady, binocular fixation of a visual target. We also measured gaze stability in two normal subjects while they fixed upon a target monocularly, and in one patient with congenital, bilateral blindness. In the patients with monocular visual loss, gaze instability was greater in the blind eye, both vertically and horizontally, compared either with their seeing eye or with nonviewing eyes of control subjects. Gaze instability due to monocular blindness resulted from: (1) low-frequency, low-amplitude, bidirectional drifts that were more prominent vertically; and (2) unidirectional drifts, with nystagmus, that were more prominent in the horizontal plane. Gaze-evoked nystagmus, however, was not a feature of monocular blindness. Thus, the gaze instability of monocular blindness may reflect disruption of: (1) a monocular visual stabilization system; (2) fusional vergence mechanisms; or (3) both. In contrast, bilateral congenital blindness led to nystagmus with horizontal and vertical components and a wandering null point, indicative of an abnormal neural integrator.