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A.R. Whatham, A. Déruaz, M. Goldschmidt, C. Mermoud, A.B. Safran; What Limits Word Acuity in Clinical Eccentric Fixation . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4584.
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Purpose: We investigated reading ability in normal vision, simulated visual loss and clinical eccentric fixation to investigate what factor limits word acuity, and what factor should be changed in order to improve reading, in patients with a central scotoma in whom only peripheral vision can be used. Methods: Twenty subjects with normal vision, eight subjects with normal vision and simulated vision loss with <0.1 diffusers (Ryser), and nine subjects with a central scotoma and eccentric fixation secondary to a macular lesion participated. Scotometry and reading of isolated words were assessed through a scanning laser ophthalmoscope (Rodenstock) in the patient group to establish involvement of the fovea in the absolute or dense scotoma and use of eccentric areas to read words. Acuities for uppercase SLOAN letters and lower case words of 4, 7 and 10 letters were measured using a computer–based technique. Results: Word acuities were equivalent across word lengths in normal vision (repeated measures ANOVA p = 0.14), simulated vision loss (repeated measures ANOVA p = 0.17) and clinical eccentric fixation (repeated measures ANOVA on normalized values; p = 0.35). Acuities for all word lengths were better than SLOAN letter acuities in normal vision (paired t–tests, p < 0.05), but considerably worse in eccentric fixation (paired t–tests, p < 0.001). Mean normal acuities (in logMAR ± SE) were – SLOAN letters: –0.08 ± 0.02 (4 letters): –0.15 ± 0.02; (7 letters): –0.16 ± 0.02 and –0.17 ± 0.02 (10 letters). Mean acuities (± SE) for the simulated loss group were and 1.30 ± 0.09 (SLOAN); 1.26 ± 0.09 (4 letters); 1.22 ± 0.09 (7 letters); 1.26 ± 0.09 (10 letters) for the patient group were 0.90 ± 0.04 logMAR (SLOAN letters), 1.20 ± 0.06 logMAR (4 letters), 1.17 ± 0.06 logMAR (7 letters) and 1.20 ± 0.06 logMAR (10 letters). Conclusions: Limitations on letter and word acuity change in moving from central to eccentric fixation. This is probably due to neural rather than optical factors limiting reading performance. As acuities for different word lengths are similar in eccentric fixation and in simulated visual loss with maintenance of central fixation, the neural factors that limit performance in clinical eccentric fixation might be either crowding or increased oculomotor instability but not a reduced visual span.
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