Need to check this out later...
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"Little more than a decade ago discussion that the eye of the vertebrates might contain a non-rod, non-cone photoreceptor generated either polite disinterest or hostile rebuttal by the majority of referees and funding bodies alike. It seemed inconceivable that something as important as an unrecognised ocular photoreceptor could have been missed. The rationale was that the eye has been the subject of serious study from some 200 years (Schultze, 1866; Young, 1802), and in broad terms we understand how the eye functions. Photosensory rods and cones of the outer retina transduce light, and the cells of the inner retina provide the initial stages of signal processing before topographically mapped signals travel down the optic nerve to specific sites in the brain for advanced visual processing. However, a small group of us who were interested in how circadian rhythms are adjusted to light began to question the assumption that the rods and cones are the only photosensory cells of the retina. Part of the reason for this more broad-minded approach to the eye can be attributed to an ignorance of the visual system! Our backgrounds and training were not rooted in vision research, but in fields such as circadian and reproductive physiology, and animal behaviour. Combined with this naiveté, however, there was an acute awareness that the vertebrate central nervous system is packed with "enigmatic" photoreceptors that help adjust circadian rhythms to the local light environment. Knowing that birds, reptiles, amphibians and fish utilise specialised photosensory cells in the basal brain and pineal to regulate their circadian rhythms, made it much easier to ask whether there might be dedicated photoreceptors in the retina for the same task. It seemed perfectly reasonable to ask whether, rods, cones, uncharacterised retinal photoreceptors, or a combination thereof might regulate the circadian rhythms of mammals."