Recent work in blind human subjects has confirmed the presence of a non-visual ocular photoreceptive mechanism similar to that described in blind mice. This system appears to subserve circadian photic entrainment, the pupillary light response, and a number of other aspects of neurophysiology and behavior.
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"For non-visual luminance monitoring, there is thus no need for cilia or microvilli. Indeed, non-visual photoreceptors often lack these conspicuous structures altogether (Gooley et al. 2003; Gotow & Nishi 2007, 2008; Van Gelder 2008). Modelling also demonstrates that phototaxis, which requires directionality with wide angular sensitivities and intermediate integration times, can function without membrane stacking, but only during the day, and in rather shallow water (figure 4; see table S1, electronic supplementary material). "
[Show abstract][Hide abstract]ABSTRACT: The morphology and molecular mechanisms of animal photoreceptor cells and eyes reveal a complex pattern of duplications and co-option of genetic modules, leading to a number of different light-sensitive systems that share many components, in which clear-cut homologies are rare. On the basis of molecular and morphological findings, I discuss the functional requirements for vision and how these have constrained the evolution of eyes. The fact that natural selection on eyes acts through the consequences of visually guided behaviour leads to a concept of task-punctuated evolution, where sensory systems evolve by a sequential acquisition of sensory tasks. I identify four key innovations that, one after the other, paved the way for the evolution of efficient eyes. These innovations are (i) efficient photopigments, (ii) directionality through screening pigment, (iii) photoreceptor membrane folding, and (iv) focusing optics. A corresponding evolutionary sequence is suggested, starting at non-directional monitoring of ambient luminance and leading to comparisons of luminances within a scene, first by a scanning mode and later by parallel spatial channels in imaging eyes.
Full-text · Article · Nov 2009 · Philosophical Transactions of The Royal Society B Biological Sciences
"Though they are located within the retina, these cells do not function in image formation; instead, they appear to serve a circadian function. It is for this reason that some blind people lacking rods and cones can nonetheless respond to day–night cycles (Van Gelder 2007; Zaidi et al. 2007). The cases of Platynereis, Tripedalia, and humans suggest that most animals will turn out to exhibit both types of photoreceptor cells, or at least that they had both at some stage in their ancestry (Plachetzki et al. 2005). "
[Show abstract][Hide abstract]ABSTRACT: The origin of complex biological structures has long been a subject of interest and debate. Two centuries ago, natural explanations
for their occurrence were considered inconceivable. However, 150years of scientific investigation have yielded a conceptual
framework, abundant data, and a range of analytical tools capable of addressing this question. This article reviews the various
direct and indirect evolutionary processes that contribute to the origins of complex organs. The evolution of eyes is used
as a case study to illustrate these concepts, and several of the most common misconceptions about complex organ evolution
Preview · Article · Oct 2008 · Evolution Education and Outreach
[Show abstract][Hide abstract]ABSTRACT: Older, and even some modern, intraocular lenses (IOLs) transmit potentially hazardous ultraviolet radiation (UVR) to the retina. In addition, IOLs transmit more blue and green light to the retina for scotopic vision than the crystalline lenses they replace, light that is also potentially hazardous. The severity of UVR-blue type phototoxicity increases with decreasing wavelength, unlike the action spectrum of blue-green type retinal phototoxicity and the luminous efficiency of scotopic vision which both peak in the blue-green part of the optical spectrum around 500 nm. Theoretically, UVR+blue absorbing IOLs provide better retinal protection but worse scotopic sensitivity than UVR-only absorbing IOLs, but further study is needed to test this analysis. UVR is potentially hazardous and not useful for vision, so it is prudent to protect the retina from it with chromophores in IOLs. Determining authoritatively how much blue light an optimal IOL should block requires definitive studies to determine (1) the action spectrum of the retinal phototoxicity potentially involved in human retinal ageing, and (2) the amount of shorter wavelength blue light required for older adults to perform essential activities in dimly lit environments.
Full-text · Article · Jan 2004 · British Journal of Ophthalmology