Electrical responses and photopigments of twin cones in the retina of the walleye.

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Abstract
The properties of twin and single cones in the retina of the walleye (S. v. vitreum) were studied by intracellular recording, dye injection and microspectrophotometry. Twin cones generating hyperpolarizing responses to central illumination received depolarizing influences (feed-back) from the receptive field surrounding and showed no detectable dye coupling when injected with Procion yellow. In 17 of 18 dye-injected cones, fluorescence was intense in the inner segment and undetectable or weak in the cone pedicle. Both members of the twin cone contained the same photopigment in their outer segments. It absorbed maximally at about 605 nm. A 533 nm green-sensitive photopigment was found in single cones. No blue-sensitive cones were found. With the exception of a modest discrepancy in the violet, the absorptance spectrum of the 605 nm photopigment of twin cones agreed closely with the action spectrum measured by intracellular recording. The spectral properties established by the twin cone''s photopigment were not detectably altered by the hyperpolarizing influences arising from nearby cones or by the depolarizing influences arising from the receptive field surrounding. The twin cones of the walleye retina were identical twins photochemically and physiologically and seem designed to function as long-wave, spectrally univariant receptor units for color vision. Identical twin cones may differ functionally from double cones and non-identical twin cones. Although they outnumber single cones by about 3:1 in adults, identifiable twin cones were rarely observed in the cone population of retinas examined 3-5 days after birth. If walleye twin cones develop by fusion of single cones, this process apparently occurs only for cones containing the 605 nm photopigment.