Word acuity threshold as a function of contrast and retinal eccentricity.

Abstract

We determined word acuity thresholds as a function of contrast and retinal eccentricity to determine the rate of threshold alteration in the normal retinal periphery. Subjects identified words presented foveally (0 degrees eccentricity) or above the point of fixation at retinal eccentricities of 0.5, 1, 2, 3, 6, and 8 degrees for three contrast levels of 10, 45, and 85%. A descending method of limits was used to determine thresholds for random four-letter words flashed for 90 ms at the different retinal eccentricities. For high-contrast letters, word acuity displayed threshold elevation in the periphery similar to previous reports and similar threshold elevation to those reported for vernier acuity. Lower contrast levels displayed different threshold change as a function of eccentricity, approaching levels reported for grating acuity. When comparing the relative elevation of word acuity thresholds for the different contrast levels (85 vs. 45% and 85 vs. 10%), both comparisons showed that the most rapid decline in word acuity threshold occurs within 2 degrees of the fovea. The peripheral retina displays a reduction in word acuity threshold that is dependent on letter contrast and shows a change similar to those reported for higher cortical functions such as vernier thresholds. The greatest word threshold elevation occurs within the central 2 degrees of the fovea.