The ppERG signal was recorded while the subject viewed a reversing checkerboard presented on a hemispherical surface that filled a substantial portion of the peripheral the visual field (
Fig. 1A). Each check was illuminated by a “cool white” light-emitting diode (LED; BXRA-C0402, correlated color temperature of 5600 K, maximum output 450 lumens; Bridgelux, Fremont, CA) behind the translucent hemisphere (30-cm radius, 0.125-inch thick clear acrylic with both surfaces “frosted”); the borders of the checks were defined by thin metal fins perpendicular to the convex surface and were therefore fixed in size. A thin diffusing element was positioned between each LED and the hemisphere to improve uniformity of luminance within each check. The mean ON-luminance was varied from 90 to 1670 photopic cd m
−2; luminance of every check was measured periodically throughout the 2-year study, and LED driving currents adjusted as necessary to maintain calibration. For a given mean ON-luminance, the standard deviation (SD) of luminance across checks was approximately 16%; this larger than desired variation was due to wide manufacturing tolerances of the LEDs. Careful static luminance measurements using a photometer (IL 1700; International Light Technologies, Peabody, MA), and luminance versus time measurements using photodiodes, confirmed equal mean luminance for each phase of the pattern, and an absence of transient luminance artifacts (all-dark or all-bright periods accompanying each pattern reversal), respectively. The four rows and 30 columns formed a spherical section. Subjects were positioned at one of three viewing distances (distance from eye to dome apex, ±1 cm due to variations in face shape) via a moveable chin rest, and a target at the apex of the hemisphere was fixated during recording. The field subtended by the stimulus pattern is illustrated in
Figures 1B to
1E, where the limits of visual field for a right eye (red curve) is superimposed on a representation of the ppERG stimulus for each viewing distance used in this study. The physical check size was not adjustable (smallest checks 6.1 × 6.0 cm, largest checks 6.3 × 6.5 cm), and so apparent check size varied with viewing distance; checks were slightly trapezoidal in shape. Average apparent check size, luminance, and reversal rates investigated for the ppERG stimulus source are given in
Table 1. At the highest pattern luminance, contrast was measured as 97%. Unless otherwise noted, the conventional pERG system used in this study (Espion E
3 Electroretinography system with pattern stimulus generator; Diagnosys LLC, Lowell, MA) conformed to International Society for Clinical Electrophysiology of Vision (ISCEV) recommended settings,
23 also provided in
Table 1. Pattern luminance was not adjusted for pupil size.