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The external horizontal cells (EHCs) in the goldfish retina are known to respond with hyperpolarization to monochromatic flashes of all wavelengths. From their spectral responses they are referred to as "monophasic" horizontal cells. However, when the retina is exposed to an intense red background, EHCs showed a depolarizing response to flashes of intermediate and short wavelength of moderate intensity. The depolarizing component is presumably driven by the input from green-sensitive cones.

The types of photoreceptors converging onto L-type external horizontal cells (LEHCs) were studied intracellularly in live, immobilized goldfish under dark-adapted conditions, and interactions between inputs from these photoreceptors were demonstrated. Our analysis of the color-dependence of response waveform, and the spectral sensitivity of LEHCs during various phases of their responses, suggests that LEHCs receive inputs not only from red-sensitive cones, but also from green-sensitive cones. Furthermore, inputs from red- and green-sensitive cones were found to interact so as to enhance LEHC responses. By presenting two successive flashes of different colors, the response enhancement was demonstrated only when green flashes preceded red flashes, indicating that responses to the input from green-sensitive cones affected the input from red-sensitive cones. A hypothetical model which incorporates reciprocal connections between green-sensitive cones and LEHCs is proposed, and shown to be consistent with most of the phenomena described in this paper, including the response enhancement.