New equally readable charts based on anisotropy of peripheral visual acuity

Anstis' equally readable chart for visual acuity has been widely quoted in textbooks on visual perception. However, this chart does not reflect the anisotropy of peripheral visual acuity that has been reported by previous studies. Here, the authors reexamined peripheral visual acuity by measuring resolution thresholds for Landolt rings and recognition thresholds for hiragana letters as a function of retinal eccentricity across two principal retinal meridians: horizontal and vertical. Observers were required to identify the orientation of a gap in the Landolt ring or recognize a hiragana letter while the stimulus was moved slowly from the periphery toward the fovea across each of four meridians: nasal, temporal, superior, and inferior. The results revealed the horizontal-vertical anisotropy of visual acuity in accordance with the results of previous studies. The mean thresholds obtained in the vertical meridian were approximately 1.51-fold (for Landolt rings) and 1.42-fold (for hiragana letters) higher than those obtained in the horizontal meridian at the same retinal eccentricity. The authors propose new equally readable charts for Landolt rings and hiragana letters.