Three cone mechanisms in the primate electroretinogram: two with, one without off-center bipolar responses.

We have used ganzfeld stimulation, selective chromatic adaptation and computer averaging to identify three cone mechanisms in the corneal ERG of the macaque monkey. On a strong yellow background, two cone mechanisms can be distinguished. One has a peak spectral sensitivity near 460 nm with a relatively slow b-wave to light onset and a negative wave at offset. This response is transiently reduced after a yellow background is extinguished. The second has its peak near 560 nm with a quicker b-wave at onset and a positive wave (d-wave) at offset. On a strong blue background, the ERG appears univariant from 400 to 600 nm but not at longer wavelengths (640 nm). An even brighter blue background reduces the ERG to most of the spectrum but has almost no affect on the 640 nm response. Adding an additional red background greatly reduces the 640 nm response without significantly influencing the responses to the rest of the spectrum. By Granit's terminology, the cone ERG appears to be composed of an E-like ERG of short wave cones and an I-like ERG of at least two other cone mechanisms. The difference between E and I ERGs may be due to the absence or presence, respectively, of an off-center bipolar system. The intravitreal injection of 2-amino-4-phosphonobutyric acid (APB) supports this hypothesis. APB eliminates the b-wave of all three cone ERGs and the negative off-response of the S cone ERG but releases the positive off-response of the other cone ERGs.