Identification of the R–G-cone difference signal in the corneal electroretinogram of the primate

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Abstract
The electroretinogram (ERG) can be used to evaluate retinal processes. Electrophysiologic studies of lower-order species indicate that important color-vision coding occurs in the outer plexiform layer. In the monkey, a color-opponent R-G-cone difference signal has been reported in the intraretinally recorded foveal local ERG. We have searched for a similar signal in the corneal ERG. Steady-state ERG waveforms were elicited with long-wavelength, sinusoidally flickering stimuli that subtended either 45 or 17°. Waveform analyses reveal two primary components that are differentially affected by changes in illuminance, temporal frequency, and chromatic adaptation. Similarities between the foveal local ERG and corneal ERG data with regard to amplitude and phase indicate that one of these corneal ERG components is the R-G-cone difference signal. Our findings raise the possibility that the integrity of the outer plexiform layer can be monitored by means of the corneally recorded ERG and that distal-retinal color-cod ng phenomena can be studied in man.