The sCDD thresholds were measured at 4 eccentricities, with radius from the center of the ring to the fovea set at 4 degrees and 8 degrees and stimuli presented in both inferior and superior hemispheres (
Fig. 2). The choice for these stimulus locations was based on a number of considerations. First, rod-mediated function is altered along a gradient across the macula in patients with AMD,
13–15 with greater dysfunction in the superior retina, particularly in patients with RPD.
15 Second, sCDD thresholds from healthy volunteers are not dependent on the radius of the stimulus between 2 degrees and 8 degrees retinal eccentricity.
36 Therefore, we chose the four stimulus loci to evaluate whether sCDD thresholds varied with eccentricity and/or hemisphere in patients with iAMD and RPD. Note that all location descriptors in this paper refer to retinal space (i.e. not visual field space). The stimulus at 4 degrees was a 3/8 RF arc (3 visible from 8 radial cycles; see
Fig. 1). At 8 degrees, the stimulus was a 3/16 RF arc (3 visible from 16 radial cycles). These combinations of radii and radial frequencies ensure an equivalent amount of curvature information at each eccentricity as measured by circular contour frequency (CCF).
36,38 Circular contour frequency (units of cycles per contour length degree [cycles/cl-deg]) is a measure of the physical length on the retina of one cycle of modulation around the circumference of a radial frequency pattern. For both stimulus conditions used in the current study, circular contour frequency was 0.32 cycles/cl-deg (i.e. each cycle subtended 3.14 degrees on the retina and 9.4 degrees for the 3 visible cycles) which produces the best sCDD thresholds in healthy volunteers.
36 For all tests, the following stimulus parameters were fixed: contrast = 100%, spatial frequency = 2 cycles per degree, and maximum luminance = 0.002 cd/m
2. To achieve this luminance, the monitor was calibrated by setting the maximum luminance of the display to 1 cd/m
2 and placing 3 neutral density filters (0.9121, 0.9410, and 0.8452 log unit reductions) in front of the monitor.
36