Scotopic sensitivity during adulthood.

Older adults typically exhibit about a half log unit loss in scotopic sensitivity that cannot be attributed to optical factors and retinal disease suggesting a neural origin. Little is understood about the developmental course of this neural deficit as to whether it first appears in late life or gradually emerges during the course of adulthood. To address this developmental issue, scotopic sensitivity was measured in 94 adults ranging in age from the 20s to the 80s. Thresholds were measured at 27 test loci within a 18 degrees radius field. Analogous measurements were made for photopic sensitivity. Fundus photography and a grading scale were used to characterize macular health in subjects over age 49 in order to control for macular disease. Scotopic sensitivity decreased at a rate of 0.08 log units per decade; this decline was better fit by a single line model, not a bilinear model, implying that the impairment does not suddenly emerge in late life but gradually appears over the course of adulthood. Photopic sensitivity also decreased in a linear fashion at a rate of 0.04 log units per decade. Under these test conditions, the rate of scotopic sensitivity decline during adulthood was about double the rate of photopic sensitivity decline.