Cone difference signal in foveal local electroretinogram of primate.

Psychophysical and electrophysiological studies have shown that the perception of color is in part dependent upon an opponent signal between the long (R) and middle (G) wavelength--sensitive cone systems. Models of human color vision hypothesize that this signal is derived from the difference between sensitivities of the R and G classes of cones. We report here a slow potential in the foveal local electroretinogram (LERG) of primate that correlates well with the absolute logarithmic difference between psychophysically deduced R and G primaries. The foveal LERG is recorded from cynomolgus macaque monkeys with the use of low-frequency sinusoidally flickering stimuli. Responses obtained at the neural wavelength, typically in the region between 540 and 570 nm, or less like log-saturated sinusoids, whereas responses obtained to other wavelength stimuli have a negative component. The amplitude of the negative-going component is deduced by fitting waveforms obtained at the neutral wavelength to responses obtained to the other wavelengths. The validity of this nonlinear analysis is supported by fitting the deduced hyperpolarizing response vs. intensity (RvI) functions with the relationship, V/Vmax = I/(I r sigma), as previously found for single retinal units. The negative component RvI function does not follow this relationship--as expected for an R-G difference signal; a decrease in amplitude at high illuminances could account for perceptual luminance dependent hue shifts.