John Siderov¹ (), Frantisek Pluhacek², Andrea Sláviková², Sarah Waugh¹; ¹University of Huddersfield, ²Palacky University Olomouc

The impact of horizontal/vertical asymmetry in crowding on visual acuity is not clear as reports differ. To assess crowding asymmetry on measurements of visual acuity, we measured visual acuity using custom letter charts arranged in vertical and horizontal alignment with varying inter-letter and inter-row/inter-column spacings. Specially constructed logMAR letter charts based on the formats of the ETDRS (Ferris et al, 1982) and Waterloo (Strong and Woo, 1985) charts were used. Charts comprised rows or columns of dark Sloan letters, ranging from 1.0 to -0.3 logMAR at the 6 m test distance, printed onto white cardboard backgrounds. Edge-to-edge spacings between adjacent letters of 100%, 75%, or 50% of the optotype size were used for each alignment condition (vertical and horizontal). Row and column spacing was fixed at the inter-optotype spacing of the row below or column to the right, in each chart, respectively. An additional set of charts was created to assess test-retest repeatability. Monocular best-corrected acuity was measured in 30 adult observers. Charts were presented in random order (i.e. vertical or horizontal alignment and spacing) and observers asked to read the letters from largest to smallest. Measurements continued until 3 or more errors were made on a single row/column. Final visual acuity was determined using letter-by-letter scoring. Measurements were repeated on a separate day. LogMAR acuity was significantly better for the horizontally aligned formats (F1,29 p<0.001), and worsened with decreasing letter spacing (F2,58 p<0.001), indicating more crowding for vertically aligned charts and closer inter-optotype separations, respectively. However, differences were small, about 0.04 logMAR (i.e. 2 letter difference). Test-retest results resulted in a significant interaction between letter spacing and alignment (F2,58 p<0.05), indicating a learning effect for the vertical aligned charts at the largest inter-optotype separation, suggesting that practice may overcome some of the differences due to crowding.

Acknowledgements: Supported, in part, by Grants URN020-01 from the University of Huddersfield and IGA_PrF_2017_003 and IGA_PrF_2018_007 from the Faculty of Science, Palacky University Olomouc, Olomouc, Czech Republic.