Perceptual Filling-in and Reading with Central Scotomas
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Purpose: Macular degeneration can be severely disabling as patients frequently develop central blind areas (scotomas) which impair their ability to read, drive, and recognize faces. To compensate for the loss of central vision, patients with absolute central scotomas must learn to use their peripheral retina to perform tasks normally performed with the fovea/parafovea. Because of perceptual filling-in, patients perceive characteristics of a scene, such as its color or texture, within the area corresponding to complete vision loss and generally are unaware of the scotoma border and location. I hypothesized that perceptual filling-in makes it more difficult for patients with central scotomas to read effectively using peripheral vision and that making the boundaries of the scotoma visibile would be beneficial as a training mechanism. The specific purpose of this dissertation was to investigate the effects of perceptual filling-in on fixation and reading speed in patients with bilateral central scotomas. Methods: In experiment 1, I investigated the retinal locus used and the stability of fixation in patients with bilateral central scotomas for six targets, three expected to fill-in and three with letters. In experiments 2 and 3, I examined whether the reading speed of normal subjects is affected by the visibility and information content of a simulated central scotoma. In experiment 4, I tested whether perceptually delineating the visual field location of the scotoma improves reading speed in patients with bilateral central scotomas. A gaze contingent display was used, first to map the scotoma, and then to display the scotoma location continuously as a high-contrast polygon while patients read computer-presented text. Results: Eleven of twelve subjects in experiment 1 used a retinal location closer to the vestigial fovea to fixate targets expected to fill-in, compared to letters. Target type produced no overall significant difference in fixation stability, which was measured as bivariate contour ellipse area (BCEA). However, for some individual subjects, fixation on letter targets tended to be more stable. In experiments 2 and 3, elapsed reading times were longer for simulated scotomas that were less visible and contained more linguistic information. Subjects adopted different eye-movement strategies for the more and less visible scotoma types, and the improvement in reading speed for the least visible scotoma type was associated with a decrease in saccadic amplitude and an upward shift of the mean fixation locus. Older subjects had more difficulty than younger subjects reading with a simulated central scotoma, especially the scotomas that were less visible and contained more linguistic information. In experiment 4, reading speed improved in all but one of the patients with central field loss after a brief period reading while viewing a polygon that marked the scotoma location. After experience with the overlaid polygon, five of seven patients shifted their fixation location to position the scotoma further from the center of the text, thereby, imaging more of the text on viable retina. Conclusions: The results of experiment 1 suggest that in patients with central field loss, letter targets generate more consistent fixation behavior than fill-in targets. The results also indicate that fixation using fill-in targets does not allow clinicians to estimate the visual field location of a central scotoma reliably. The data obtained from experiments 2 – 4 suggest that enhancing scotoma visibility has the potential to improve reading speed and to train effective eccentric viewing in patients with central scotomas. The vertical shift of the mean fixation locus demonstrated by normal subjects and by most patients with central field loss suggests that upward eccentric viewing is an adaptive oculomotor strategy in the presence of a central scotoma.