The Effects of Letter Design Features and Aging on Legibility

We examined how letter legibility is affected by letter design features (size, contrast, font-type, and complexity), and how this changes with age (by comparing younger [ N = 54] and older participants [ N = 54]). Types of letters in the study were Japanese characters, Latin alphabets, and digits. Participants verbally reported the letter presented on a Cathode Ray Tube (CRT) display in natural viewing conditions. The legible-contrast threshold was calculated as the contrast boundary of 50% correct answer. We found that letter complexity (i.e., number of strokes) did not affect legible contrast threshold, except for small (<0.35°) letters. Moreover, letter size even affected the threshold with a function similar but not identical to contrast sensitivity function. This supports the previous hypothesis proposed by Majaj et al. that optimal spatial frequency band exists for letter identification and shifts with its size. Another novel finding was the effect of aging on the legible-contrast threshold and how it related to other letter features. Constant aging effect on legible-contrast was found across letter sizes. Among older adults, the legible-contrast thresholds were 1.2 to 2 times those of younger adults. Moreover, the aging effects were greater for positive-contrast than for negative.