The distinct absorbance spectra of the cone photopigments form the basis of color vision, but ultrastructural and physiological evidence shows that mammalian cones are electrically coupled. Coupling between cones of the same spectral type should average voltage noise in adjacent photoreceptors and improve the ability to resolve low-contrast spatial patterns. However, indiscriminate coupling between spectral types could compromise color vision by smearing chromatic information across channels. Here we show, by measuring the junctional conductance between green-green and blue-green cone pairs in slices from the dichromatic ground-squirrel retina, that green-green cone pairs are routinely coupled with an average conductance of 220 pS, whereas coupling is undetectable in blue-green cone pairs. Together with a lack of tracer coupling and the selective localization of connexin proteins, our results show that signals in blue and green cones are processed separately in the photoreceptor layer.
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