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J.-B. Bernard, B. Yik, J. Kao, S. T. L. Chung; Acuity and Fixation Stability Around a Simulated Scotoma in Normal Observers Does Not Predict the PRL Location for Patients With Central Vision Loss. Invest. Ophthalmol. Vis. Sci. 2010;51(13):978.
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Patients with long-standing central vision loss often adopt a peripheral retinal location just outside their central scotoma as a new reference locus for oculomotor and other visual tasks - the preferred retinal locus (PRL). The location of the PRL with respect to the central scotoma varies greatly among patients, prompting the question of how the location of a PRL is selected. We hypothesized that the PRL location corresponds to the retinal location outside a scotoma that offers the best visual acuity and/or fixation stability. In this study, we tested this hypothesis using simulated scotomas in observers with normal vision.
Using a scanning laser ophthalmoscope, visual acuity and fixation stability were assessed around the circumference of three simulated scotomas stabilized on the retina of three young adult observers with normal vision. Visual acuity was measured using briefly presented (200 ms) Tumbling-E targets. Stimuli for evaluating fixation stability were sequences of five uppercase letters (0.8° letter-height) each presented for 4s. Each simulated scotoma was adopted from the actual scotoma map of a patient with long-standing central vision loss and a well-defined PRL. We tested 36 locations around each simulated scotoma - 3 distances from the edge of the scotoma (0.5°, 1° and 2°) along each of 12 meridians (30° apart) centered on the anatomical fovea. Each measurement was repeated three times.
Visual acuity around each simulated scotoma was best when the testing location lied along the meridian with the shortest distance between the edge of the scotoma and the anatomical fovea. Along any given meridian, acuity worsened with increased distance from the edge of the scotoma. Fixation stability, quantified by the bivariate contour ellipse area, did not show systematic differences across the 36 testing locations around each simulated scotoma. For a given simulated scotoma, the location associated with the best acuity or the most stable fixation did not match the location of the PRL of the patient from whom the scotoma map was obtained.
Our results imply that the variations of acuity and fixation stability across the normal healthy retina, coupled with merely the shape and size of a scotoma, are not sufficient to predict the location of the PRL that a patient with the scotoma ultimately adopts.
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