Abstract
Purpose: :
People with age-related macular degeneration (AMD) often report difficulty reading, which involves identifying letters in clutter. Recently, Pelli, Song and Levi (2011 VSS) proposed a simple analysis to tease apart the contribution of size and spacing on letter identification. In this study, we adopted their analysis to determine the primary limiting factor on letter identification in people with AMD.
Methods: :
Nine observers with AMD (age: 66-89, logMAR acuity: 0.26-1.10) were tested monocularly using their better-seeing eye. We measured the size threshold (acuity) for identifying a lowercase letter presented singly, or flanked by two other letters, at a center-to-center letter spacing (the spacing factor) that ranged from 1× to 3× the letter size (x-height). In each block of trials, we presented letters at different sizes, but at the same spacing factor such that the absolute spacing covaried with letter size. Acuity was defined as the letter size that yielded 52% identification accuracy. For comparison, we also measured the single and flanked acuities at 5 and 10° eccentricity in the nasal and lower visual fields in three observers with normal vision.
Results: :
In general, flanked acuity improved with spacing factor up to the critical spacing, beyond which acuity was unaffected by flankers and approached the unflanked acuity. While the critical spacing increased systematically with eccentricity in the normal periphery, it occurred at a range of spacing factors for AMD observers. Following Pelli et al., we evaluated the relationship between the threshold spacing (critical spacing factor × acuity) and acuity. In the normal periphery, threshold spacing increased faster than acuity (by ~1.75×) when eccentricity increased, indicating that spacing, not letter size, limited letter identification. For all AMD observers, the threshold spacing predicted based on the acuity was smaller than in the normal periphery, implying a weaker limitation of spacing. In fact, for six of the nine AMD observers, the change in threshold spacing with acuity was 1:1, implying that size, but not spacing, limited letter identification.
Conclusions: :
Unlike in the normal periphery, letter identification in people with AMD, who have to rely on their peripheral vision to identify letters, is limited primarily by letter size, and to a lesser extent, by spacing. This finding provides evidence that the preferred retinal locus (PRL) in people with AMD may become more "fovea-like" as a result of repeated usage of the PRL. Practically, our result suggests that people with AMD would benefit more from an increase in letter size (magnification) rather than larger letter spacing for letter identification.
Keywords: low vision • pattern vision • shape, form, contour, object perception