Electrical responses of double cones in the turtle retina.

1. Responses to monochromatic flashes were recorded intracellularly from double cones in the retina of the turtle, Pseudemys scripta elegans. Double cones have been identified by intracellular marking with Procion Yellow dye.2. The direct light response of a double cone is a hyperpolarization graded with the intensity of a flash and similar to the response of single cones.3. When flashes were dim, responses were proportional to light intensity but varied in time course as a function of wave-length. They reached peak in about 120 msec for deep red stimuli and about 143 msec for green stimuli.4. When applied over red backgrounds, responses to red flashes became smaller, faster and frequently diphasic, but responses to green remained similar to those recorded from darkness. Green backgrounds made responses to all colours small and fast.5. Linear spectral sensitivity curves have peaks corresponding to the peak sensitivities of single red-sensitive and green-sensitive cones. Red and green backgrounds suppressed red and green sensitivity respectively.6. Large fields of illumination evoked composite responses which included the direct light response, its enhancement from illumination of nearby receptors and the depolarizing effect of luminosity horizontal cell impingement. In the green-sensitive element the depolarizing effect was larger for red than for green flashes, and stimulation with red annuli evoked net depolarizing responses.7. It is concluded that the responses of double cones may be explained by coupling of the responses from red-sensitive and green-sensitive elements each of which has properties otherwise similar to single red-sensitive and green-sensitive cones.