At the age of 82 the impressionist painter Claude Monet complained to a friend that everything he saw was in fog and painting had become difficult for him. Monet suffered from cataracts, which increasingly blurred his vision and rendered colours muted and muddy. Thus, in 1923 he underwent standard cataract surgery of the time, which left him aphakic. The surgeon had removed the ocular lens of his right eye. But the surgery didn’t only relieve Monet from the fog, it also gave him a skill mostly just insects and birds possess: Monet may have started seeing UV light.
The ocular lens improves our acuity of vision. But it also protects the retina from the hazardous exposure to UV light. After the removal of the lens the UV filter is gone and the entire light spectrum can enter the eye. Although our retina only possesses three types of pigments that are not tuned to detect UV light, the blue cones can still pick up some, which becomes visible as a tint of blue or purple.
After his cataract surgery, Monet changed the coloration of his paintings, among them one of his most famous themes, water lilies in a pond. He used to paint water lilies in white, as they would appear to the healthy eye. As an aphakic, he added blue to the water lilies’ appearance.
Not only Monet but countless of other people who underwent traditional cataract surgery in the course of history may have experienced this change of vision. The technique of surgical removal of the lens dates back to ancient Greece and was frequently used in medieval times. In contrast to today’s almost pain-free procedure, traditional cataract surgery was supposedly excruciating, as analgesics were widely unavailable. In the 1940s, ophthalmologists started to replace the lens with an implant. It was only in the 1980s, that lens implants were introduced that filtered out UV light to protect the retina more efficiently.
As known for a long time, birds and insects use UV light to distinguish between male and female conspecifics to find a partner. For pollinating insects like bees, UV light provides guidance to the nectar of flowers. But what about mammals? Are aphakic humans the only mammals that can see UV light? This question remained unanswered until 2014. Ronald Douglas and Glen Jeffrey from the City University and the University College of London tested the lenses of a variety of mammal species for their UV permeability. The researchers discovered that in addition to insects, birds and fish, a large range of mammals like rodents, cats, dogs and bats also possess the ability for UV vison. Rodents for instance use this ability to follow otherwise invisible trails of urine to orient themselves. Researchers pondered why the human lens developed to filter out UV light, after all UV vision appears pretty advantageous. UV exposure certainly is hazardous to the eye, however, some long-lived animal species without a UV filter in their lens cope just fine. Eventually the same team of scientists discovered that having an in-built UV block in the lens comes with higher visual resolution. It turned out to be a simple trade-off. We might be blind to certain UV based signals and patterns in our natural environment, but in turn we gained one of the highest visual acuity among all mammal species.
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