The contrast characteristic of the pattern electroretinogram depends on temporal frequency.

BACKGROUND: The pattern electroretinogram (PERG) amplitude is believed to be linearly related to contrast. In the context of analyzing the effects of media opacities on the PERG, we measured its contrast-amplitude function at various temporal frequencies.
METHODS: PERGs were recorded in nine subjects with a checksize of 0.8 degree and a mean luminance of 45 cd/m2. Experiment 1 covered six temporal frequencies [checkerboard pattern onset/offset in a transient (4 Hz) condition and checkerboard pattern reversal at 7, 10, 13, 16, 21 rev/s] at three contrast levels (25%, 50% and 100%). A second experiment covered two frequencies (7 and 21 rev/s) at five contrast levels from 25 to 100%, in a large field (27 degrees x 32 degrees) and a perifoveal condition (central mask of 7.5 degrees radius).
RESULTS: Experiment 1: At all temporal frequencies the PERG amplitude increases with contrast, but the shape of the contrast-amplitude function varies markedly: Under transient conditions and at 7 rev/s, the PERG increases linearly with contrast, but this function displays a progressively positive curvature at higher frequencies (P < 0.001). At 21 rev/s a reduction of the contrast from 100% to 50% reduces the amplitude to 1/5. Experiment 2: Experiment 1 was replicated. The amplitude-contrast characteristic was found to be linear at 7 rev/s and smoothly accelerating at 21 rev/s; the same characteristics were found when stimulating the perifoveal area alone.
CONCLUSION: This dependency of the contrast characteristic on temporal frequency is contrary to what would be expected from a magno/parvo model. Further, this contrast dependency needs to be taken into account when designing stimuli for use in patients that may have media opacities.