Relative legibility and confusions of letter acuity targets in the peripheral and central retina.

Abstract

Patients with macular disease may image optotypes at a peripheral retinal locus during visual acuity testing. In this study, we asked if the relative legibility and confusions between letters are similar in the fovea and at 10 degrees in the peripheral retina. Twenty five upper-case alphabet letters (all except "I"), constructed to the same specifications as the Sloan letters, were presented one at a time on a computer monitor at the fovea and at 10 degrees in the temporal, superior and superior-temporal retina. The range of relative legibility at peripheral loci was generally larger than in the fovea, for all letters and for the subset of 10 Sloan letters. Although many confusion pairs were similar in the fovea and periphery, additional confusion pairs, preferentially involving curved letters, occurred uniquely in the periphery. The increased range of relative legibility for letter targets in the peripheral retina underscores the importance of using letter-by-letter scoring to obtain precise measures of visual acuity in patients without central vision.