The contrast characteristic of the pattern electroretinogram depends on temporal frequency

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.

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).

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.

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.