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Binocular eye movement recordings and laser-target retinal cinematography were employed to identify a fixation bias in a case of hereditary congenital nystagmus. The oscillations of each eye varied with gaze angle and were, in general, unequal over a ± 30° range of gaze. Although the nystagmus frequency was also gaze-angle dependent, it was equal in both eyes. In the null region both nystagmus amplitude and frequency were minimal and equal for the two eyes.The lateral fixation bias was found to occasionally shift in direction simultaneously in both eyes; this preserved retinal correspondency and, in conjunction with the nulling effect of convergence and proper gaze angle, contributed to increased visual acuity. These findings support and further clarify the mechanisms by which the previously described prism lenses dramatically increased visual acuity.The attempt to fixate was identified as the adequate stimulus for the nystagmus.