A comparison of three techniques to estimate the human dark-adapted cone electroretinogram.

The dark-adapted cone electroretinogram (ERG) is difficult to isolate because of unwanted rod intrusion. We compare dark-adapted cone estimates derived using three techniques. The first uses the cone response on a moderate rod saturating background to estimate the dark-adapted cone response. The second uses red and blue flashes to tease apart cone and rod responses (red-minus-blue technique, [Investigative Ophthalmology and Visual Science 31 (1990) 2283]). The third uses a bright flash to temporarily saturate rods, followed by a test flash that generates a putative cone-only response (2-flash technique [Investigative Ophthalmology and Visual Science 36 (1995) 1603]). By subtracting the cone estimates from 'mixed' ERG responses in the dark, rod isolated responses can be derived. The rod phototransduction parameters, derived using a computational model, are similar using the light-adapted and 2-flash cone estimates, but differ using the red-minus-blue estimates. The 2-flash cone estimate gives a cone waveform similar to the dark-adapted response of a patient with Oguchi stationary night blindness (a patient with no rod ERG responses and normal cone ERG responses). The growth of the cone response during light adaptation to steady backgrounds causes significant differences between the light-adapted and 2-flash cone waveforms at times beyond the first few milliseconds.