Effects of aging on shape discrimination

A large number of senior patients manifest eye diseases in addition to norm al aging changes in their visual system. Visual tests not affected by age w ould be very useful in early detection of age-related eye diseases. The pur pose of this study was to determine the effects of aging on the detection o f contour deformation. Seventy-six normal subjects were divided into three age groups: young adults 15 to 39 years old (N = 26); middle-aged adults 40 to 59 years old (N = 22); and senior adults 60 to 78 years old (N = 28). S timuli were circular or linear D4 (4th derivative of Gaussian) contours. A spatial two-alternative forced-choice staircase paradigm was used. In each trial, one deformed and one undeformed contour were presented on a screen. Subjects indicated which contour was deformed. Deformation detection thresh olds were estimated by a maximum likelihood fitting procedure. We found tha t at a low modulation frequency (four cycles), the performance of detecting deformation does not change with age, and it does not correlate with eithe r visual acuity or letter contrast sensitivity. At a higher modulation freq uency (eight cycles), a significant difference between mean thresholds was found for young and senior adults. For circular D4, the mean threshold was elevated by 18%, from 7.6 sec arc (young adults) to 9.0 sec are (senior adu lts). For linear D4, the mean threshold was elevated by 52%, from 5.8 sec a rc to 8.8 sec arc. As a comparison, minimum angle of resolution was increas ed by 33%, and letter contrast threshold was elevated by 81%. The performan ce for linear D4 at higher modulation frequency was found to be significant ly correlated with contrast threshold. These results provide further eviden ce to support the hypothesis that different mechanisms may be underlying th e detection of circular and linear D4 contour deformation. The very small e ffect of aging on hyperacuity for radial deformation suggests that a global shape discrimination task could be a sensitive test for quantifying the ea rly visual loss caused by eye diseases among seniors.