title: Why do green rods of frog and toad retinas look green?
abstract: Amphibian “green” rods express a blue-sensitive cone visual pigment, and should look yellow. However,when observing them axially under microscope one sees them as green. We used single-cell microspectrophotometry (MSP) to reveal the basis of the perceived color of these photoreceptors. Conventional side-on MSP recording of the proximal cell segments reveals no selective longwave absorbing pigment explaining the green color. End-on MSP recording shows, in addition to the green rod visual pigment, an extra 2- to 4-fold attenuation being almost flat throughout the visible spectrum. This attenuation is absent in red (rhodopsin) rods, and vanishes in green rods when the retina is bathed in high-refractive media, and at wide illumination aperture. The same treatments change the color from green to yellow. It seems that the non-visual pigment attenuation is a result of slender green rod myoids operating as non-selective light guides. We hypothesize that narrow myoids, combined with photomechanical movements of melanin granules, allow a wide range of sensitivity regulation supporting the operation of green rods as blue receptors at mesopic-to low-photopic illumination levels.End-on transmittance spectrum of green rods looks similar to the reflectance spectrum of khaki military uniforms. So their greenness is the combined result of optics and human color vision.
authors.forename: Victor I
authors.initials: VI
authors.lastname: Govardovskii
authors.forename: Tom
authors.initials: T
authors.lastname: Reuter
mesh_terms: D000818:Animals; D002023:Bufo bufo / Q000033:anatomy & histology / Q000502:physiology*; D015591:Microspectrophotometry; D008959:Models, Neurological; D010775:Photic Stimulation; D011895:Rana ridibunda / Q000033:anatomy & histology / Q000502:physiology*; D011896:Rana temporaria / Q000033:anatomy & histology / Q000502:physiology*; D012168:Retinal Pigments / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000166:cytology / Q000502:physiology*
chemical_list: D012168:Retinal Pigments