Focal long duration ERGs of increasing stimulus sizes were recorded using a red circular stimulus (peak wavelength 660 nm, 250 ms, 55 cd/m
2, 2 Hz) on a rod saturating blue background (peak wavelength 469 nm, 100 scot cd/m
2). The stimulus and background luminance were chosen to minimize stray light effects in focal ERG recordings as proposed in an earlier study by Kondo et al.
14 Focal stimulation was produced by mounting the miniature Ganzfeld LED tube in the middle of a light box (44 × 44 × 10 cm) to produce a circular stimulus (
Fig. 1A). The light box contained a strip of white LEDs (color temperature >7000 K) passed through a blue filter (Lee Filter 068 Sky Blue, Lee Filters, Hampshire, UK), which produced a rod desensitizing blue surround (peak wavelength 454 nm, 140 scot cd/m
2). The angular diameters of the focal stimuli were 5.5°, 10°, 15°, 20°, 30°, 45°, and 60°; they were obtained by varying the viewing distance (VD;
Fig. 1A,
Table 1). The focal stimuli were centered on the fovea by asking the participant to fixate the intersection of a pair of cross-hairs centered in the middle of the stimulus. Head movements were restrained with a headrest. A full-field (110°) ERG was recorded by holding the miniature Ganzfeld directly to the eye (
Fig. 1B). The dimensions of the circular stimuli are shown in
Table 1.