Dark adaptation responses were obtained in all subjects using the AdaptDx dark adaptometer (MacuLogix, Inc., Hummelstown, PA, USA) using the protocol described by Jackson et al.
4 Subjects' eyes were dilated with 1% tropicamide and 2.5% phenylephrine hydrochloride. Briefly, at the beginning of the test, subjects were bleached with a 2-ms 5.8 × 10^4 cd m
−2 s
−1 flash, equivalent to 82% bleaching level for rods, after which sensitivity recovery was measured. The stimulus light was a 505-nm, 2° diameter circular test spot located 5° superior to the fovea, a point that is not ablated by PRP. The zero log unit stimulus intensity was 5 scotopic cd/m
2. Thresholds were measured approximately every 30 seconds. Thresholds were repeatedly measured until the subject's sensitivity consistently exceeded below 5 × 10
−3 cd/m
2. These settings were chosen to test macular outer retinal function, including cone, rod, and RPE cells. The resulting DA curve was characterized by three parameters: cone sensitivity level, rod recovery rate, and rod intercept time, which were our principal dependent variables (
Fig. 2). The cone sensitivity level is the threshold of the cone photoreceptors derived from the cone plateau of the DA curve. The rod recovery rate is the slope of the second component of rod-mediated DA.
20 The rod intercept is the time required for sensitivity recovery to reach a criterion sensitivity level of 5 × 10
−3 cd/m
2, a level within the second half of the second component of rod-mediated DA, and is derived by regression analysis of the second half of the second slope of rod-mediated DA. Because the rod intercept is expressed in minutes, it is more intuitive to understand than a slope and has been found useful in 12 peer-reviewed published experimental studies and proof-of-concept clinical trials in patients with DR and AMD.
4,12,14,21–29