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- Title
Why do green rods of frog and toad retinas look green?
- Authors
Govardovskii, Victor; Reuter, Tom
- 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 long-wave 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.
- Subjects
RETINAL rod photoreceptor cells; MICROSPECTROPHOTOMETRY; RETINAL cone photoreceptor cells; PHOTORECEPTORS; VISUAL pigments; COLOR vision
- Publication
Journal of Comparative Physiology A: Neuroethology, Sensory, Neural & Behavioral Physiology, 2014, Vol 200, Issue 9, p823
- ISSN
0340-7594
- Publication type
Article
- DOI
10.1007/s00359-014-0925-z