Psychophysical and electrophysiological investigation of age-related variations in vernier acuity

Abstract

Hyperacuity tasks have proved to be useful in clinical situations. Selected stimulus configurations are extremely resistant to optical blurring, and this particular characteristic makes them valuable in the evaluation of the potential vision behind cloudy ocular media. Before a hyperacuity test can be used clinically, age norms must be established. A variety of visual functions decline with age. Previous studies, however, have suggested that vernier acuity may not vary significantly with age. The unit distance of displacement employed in these studies may not have been small enough to obtain accurate measures of vernier acuity. The present study set out to determine the effect of age on vernier acuity, using both psychophysical and electrophysiological approaches. The objectives of this study were: 1. to examine the variations in vernier acuity with age, 2. to establish normative data for the vernier VEP across the life span, and 3. to compare the intrinsic noise associated with vernier processing in younger and older subjects. No previous studies of the effects of age on vernier VEP or the intrinsic noise in vernier processing have been reported in the literature. Vernier acuity was examined in normal subjects ranging from 20 to 75 years old. Contrary to previous studies, significantly degraded performance in vernier alignment judgements was found in subjects aged above 60 years. The discrepancy can be attributed to the much smaller inter-pixel visual angle employed in this experiment. Electrophysiological studies showed significant differences in vernier VEP in older subjects compared to younger subjects, providing evidence of age-related neural changes. The electrophysiological estimates of vernier threshold were significantly elevated in subjects aged above 60 years. The theoretical intrinsic noise associated with vernier processing was examined in younger and older subjects, and it was concluded that the elevation of vernier threshold found in older subjects may be attributed to an increased level of intrinsic noise. The vernier task is remarkably resistant to optical degradation, and therefore the sensitivity loss is presumably due to neural loss between retina and cortex. Neuroanatomical and physiological evidence for neural changes in the aged visual system were discussed.