The results suggest the need for more glaucoma screenings and proper patient education, particularly in Nigerian populations in which the consumption of caffeine, contained in kola nuts and in other raw and processed products, is endemic and long-term.
The finding of invariance of fixation disparity with line length for nonius targets < 1degrees extends those of earlier findings for larger targets. Because the precision of binocular nonius alignment is poorer for large nonius-line separations or with a peripheral fusion lock, fixation disparity should be most repeatable when measured with a central fusion lock and nonius lines separated by 20 min arc or less. However, a peripheral fusion lock and small nonius-line separation may provide the most effective diagnostic combination for fixation disparity.
Fixation disparity or vergence noise produce instantaneous vergence errors. These errors are analogous to the imposition of a pedestal disparity, which is known to elevate stereothresholds. In this study, stereothresholds were measured as a function of induced vergence errors in subjects with normal binocular vision. Stereo half-images were viewed in the dark through a custom mirror haploscope. Vergence constant error and vergence variability were induced by introducing horizontal disconjugate position offsets in a pair of moveable mirrors within the haploscope, resulting in forced vergence demands of 6(Delta) base-in to 12(Delta) base-out. In addition, vergence variability was simulated by producing oscillatory disconjugate retinal image motion via motion of the moveable mirrors. The motion of the mirrors was either sinusoidal (frequency=2-6 Hz) or random, with peak-to-peak amplitudes of 0 degrees -0.5 degrees per eye. Stereopsis worsened systematically with induced or simulated VV that exceeded approximately 1.5' disparity. The results were similar regardless of whether the vergence error was induced by forced vergence or was simulated by periodic or random disconjugate retinal image motion. Stereothresholds were invariant with the frequency of disconjugate oscillation, within the range of frequencies and amplitudes tested. Hence, the simulated vergence velocity is not the essential factor that limits stereopsis within Panum's fusional area. The results indicate that the stereothreshold is elevated if the vergence error exceeds a critical value.
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