[Aging of the blue mechanism studied by measuring the increment threshold on flashed background]

We studied aging of the blue cone mechanism by measuring increment thresholds for detection of a small blue flash of light (439 nm, 1 degree, 50 ms duration) on a second flashed light of identical color, (139 nm, 2 degrees, 500 ms duration) in 30 subjects. Simultaneously, a large yellow adapting field was presented. Subjects, consisting of 17 normal cases, 13 cases with refraction errors under 3 diopters, 11 myopes, and 2 hyperopes, with 1.0 or better corrected visual acuity, were divided into 4 groups by age. The control group consisted of 11 cases under 30 years, group I, 7 cases in their forties, group II, 6 cases in their fifties, and group III, 6 cases over 60 years. Best fitting exponential curves were calculated for this phase of the blue mechanism and averaged threshold curves for each group were evaluated. The averaged threshold curve in group I showed an upper shift, that is, an increase in the threshold alone. Curves in groups II and III demonstrated an upper right shift, with increased thresholds in both the blue flash and the yellow background. Together with decreased function of the blue cone mechanism, the shift of curves in the elderly groups is probably caused largely by yellowing of the crystalline lens.