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J. D. Pratt, H. E. Bedell, S. B. Stevenson; Reading Eye Movements With Simulated Central Scotomas Depend on Scotoma Type. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3070.
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The slow reading rates exhibited by patients with macular degeneration may be attributable in part to perceptual filling-in of their area of decreased vision (scotoma) and the consequent development of poor reading eye movement strategies. Previously we showed that reading speed was slower for a stabilized artificial central scotoma composed of random letters than for one composed of random dots. In this study we investigate whether the reading eye movements differ for the two scotoma types.
Four subjects with normal visual acuity read aloud quickly and accurately a series of MNRead style sentences with one of two simulated central scotomas. Sentences were presented to one eye as dark text on a light background on a CRT monitor at optical infinity. Each sentence was 52 characters, composed of four lines of fixed width, dark courier text with a lower-case x height of one deg. Horizontal and vertical eye movement signals from a dual Purkinje eye tracker were recorded and used to stabilize one of two 4-deg square central scotomas with a delay of one frame (8 ms). One scotoma was composed of random dots and was highly visible. The other, less visible scotoma was composed of random letters with the same spacing and background luminance as the text. The scotomas were presented in alternating blocks over a series of 40 - 60 sentences. Saccades with a velocity greater than 15 deg/s were grouped by direction as horizontal, vertical or oblique.
The proportion of non-horizontal saccades was approximately 3.5 times greater with the random-letter scotoma (mean: 0.45) than with the random-dot scotoma (mean: 0.13). The number of saccades per second was greater for the random-dot scotoma (mean: 3.29) than for the random-letter scotoma (mean: 2.48). For the random-letter scotoma, but not the random-dot scotoma, saccades per second correlated positively with reading duration.
Reading eye movements are different for the two scotoma types even when reading rates are similar, suggesting that different strategies are employed. The development of effective eye movement strategies may be facilitated in patients with macular degeneration if perceptual filling-in can be counteracted.
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