Different effect of dioptric defocus vs. light scatter on the Pattern Electroretinogram (PERG)

To assess the quantitative effect of optical deficiencies on the Pattern Electroretinogram (PERG), we degraded the optical imaging onto the retina by dioptric defocus (+ 1.75-5 D), and by scatter transparencies. With optimal and degraded optics we measured the visual acuity and the steady-state PERG, evoked by checkerboards (check size 0.2-16degrees), and flash stimuli. When the two optical degradation methods were equated for acuity, scatter transparencies reduced the PERG amplitude more than defocus. For scatter transparencies, there was a significant (p < 0.001) effect at 16 degrees, escalating for smaller checks. For defocus, significant effects were only observed for check sizes of 1.6 degrees and smaller. We explain the differences between the two blur techniques by their differing angular scattering functions. Dioptrical defocus leads to an underestimation of the effect of media opacities on the PERG. For the clinically relevant check size of 0.8 degrees, halving acuity reduces the amplitude by 15% with defocus and by 30% with scatter. With an acuity of 0.8 or higher, no clinically relevant effect of suboptimal refraction is seen.