Has there been any work done on why the long, medium and short wavelength cone photo-receptors of the human eye contribute so differently to the perception of lightness? I was wondering if it would fit with attempting a consistent perception of lightness across the varying illumination levels for scotopic, mesopic and photopic vision. As the light level drops and the cone photo-receptors cease responding, the perception of lightness would be determined solely by the wavelength sensitivity of the rod photo-receptor response. The rod photo-receptor responds more strongly to light in the medium wavelength region and less strongly to light in the long and short regions, so this would broadly match the contribution of those wavelengths to the photopic perception of lightness. The only change to lightness that has been noted between photopic and scotopic vision is reds darkening to black due to rod receptors not responding to the far red wavelengths (Purkinje effect). This suggests that otherwise colours generally retain the same perceived lightness at varying illumination levels, but I have not found any work or discussion on this subject.
The difference in the sensitivities of rods and cones are due to the difference in their photon detection characteristics (rods can detect as low as a single photon) and the neural convergence. While around 120 rod cells send signals to one ganglion cell, only about 6 cones send signal to a single ganglion cell (Goldstein, 2010). Therefore, less light (photon) is needed to initiate vision with rods. However, cone cells provide higher visual acuity, especially in the fovea, where a cone can send signals to an individual ganglion cell.
Hi Alp, thanks for your response, but I was interested in why the different types of cone cells produce different perceptions of lightness. From all I have read the different types of cone cells differ only in the wavelengths of light which initiate the response, but stimulation of the short wavelength cones produces a blue colour sensation which has a perceived lightness many times lower than e.g. the yellow colour sensation from stimulation of the medium and long wavelength cones. The various perceptual colour models such as CIELuv, CIElab and the Munsell colour system all reflect this, but I can't find any theories about why the perception of lightness is not equally dependent on the different cone types.
A high-level response to your question is the impact of environmental factors and the advantage of detecting certain wavelengths of energy.
Majority of the mammals have dichromatic vision, which suggests that red and green pigment genes evolved relatively recently. A theory explains the separation of green-red sensitivity with perceived advantages, such as food detection - identifying ripening fruits (which are mostly red) against a green background (foliage). Another theory suggest that social signalling (subtle colour change in skin, e.g. red when angry, green when sick) might have played a role in the evolution of human colour vision.
It is suggested that S-cone wavelength sensitivity shifted to accomodate human ancestors' switch from a nocturnal lifestyle to a diurnal lifestyle. S-cone sensitivity is similar to M and L cone sensitivity. However, there are far fewer S cones, and some of the energy in short wavelengths (including UV) is filtered by the lens. Evolutionarily this makes sense since shorter wavelengths have more energy per photon, therefore they have higher damage potential. The signals transferred to visual cortex from S-cones are not at the same level compare to the other cone cell types. In short, the difference is not due to the wavelength sensitivity of the cone cells themselves, but due to the evolution of the human visual system.
For more info, you can refer to
Bowmaker, J. K. (1998). Evolution of colour vision in vertebrates. Eye, 12(3b), 541.
Werner, J. S., Bieber, M. L., & Schefrin, B. E. (2000). Senescence of foveal and parafoveal cone sensitivities and their relations to macular pigment density. JOSA A, 17(11), 1918-1932.
Vorobyev, M. (2004). Ecology and evolution of primate colour vision. Clinical and Experimental Optometry, 87(4‐5), 230-238.
Yokoyama, S., Xing, J., Liu, Y., Faggionato, D., Altun, A., & Starmer, W. T. (2014). Epistatic adaptive evolution of human color vision. PLoS genetics, 10(12), e1004884.
Dear Timothy James Maguire , this may not be the proper way to answer, but the following link might be helpful as this research is working on something like you have asked.
https://m.phys.org/news/2018-10-human-retinas-grown-dish-vision.html?fbclid=IwAR21YoWBZ0HZMzs5g9mcB2GwBodVCMQLXlyxb4Swc_lTdNk25SXN-tEgRGw
I hope this will help you.
Not sure if you're still looking for this, but I think I can help you here.
The three types of cones have different sensitivities because they differ their position on the retina as well as their relative numbers. The fovea, the spot on the retina with maximum acuity (and responsible for color vision) is entirely packed with green and red cones whereas the blue cones are located around the periphery of the fovea. (http://michaeldmann.net/mann7.html) In addition, while about 64% are red cones and 32% are green cones, only about 2% are blue cones.
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