ALL-SEEING EYES

Other animals perceive much that we can’t

Hypercolour Most of us don’t think of ourselves as colour-blind. But compared to the many animals blessed with superior colour vision, that’s exactly what we are. Strictly speaking, colours do not exist. Our brains generate the perception of colour by comparing the responses of colour-receptor cells in the eye tuned to different wavelengths. People with mutant receptors – which are surprisingly common – perceive light of a specific wavelength very differently. What’s more, the number of different colours we can distinguish depends on how many types of colour receptors we have. Early fish had four, which is why they and most of their descendants – including amphibians, reptiles and birds – have excellent colour vision. But during the many millions of years that early mammals spent hiding from the dinosaurs and coming out only at night, they lost two colour receptors. Most mammals therefore have poor colour vision. Our primate ancestors later re-evolved a third colour receptor,  so humans can see hundreds of thousands of hues that are invisible to dogs or horses. But with a fourth receptor we would be able to see millions more – as a few people can. As many as 1 in 10 women have mutated receptors for red light in their eyes, as well as the normal  one, effectively giving them four receptors in all. Neuroscientist Gabriele Jordan at Newcastle University in the UK has tested the perception of these women. Most have normal vision but one woman could see more colours than the rest of us. Jordan’s team has since identified others with this ability too. What it’s like for these people, of course, the rest of us can only imagine. Sight beyond sight Our colour vision stretches from the longer wavelengths we see as red to the shorter wavelengths we see as violet. But ultraviolet vision is fairly common among insects, fish, reptiles and birds, especially those with smaller eyes that filter out less UV light. Bees have excellent UV vision thanks to colour receptors optimised for detecting it, but at the cost of poorer vision at the red end  of the spectrum. UV vision is used  for different purposes by different species, from kestrels detecting the urine trails of prey to reindeer spotting polar bears. Reindeer were thought rare among mammals in having UV vision, but Ron Douglas of City University London reported this year that many mammals, including hedgehogs, cats, ferrets, seals, pigs and rabbits have lenses that let UV through. A few people can see into the near-UV spectrum. What do they have that the rest of us don’t? Well.

actually, it’s what they don’t have. The receptors in our retinas can detect UV but wavelengths shorter than 400 nanometres are normally filtered out by the eye’s lens. People who develop cataracts and have their lens replaced  with a UVtransparent one sometimes start seeing UV as a bluish or purplish glow. The painter Monet may have started seeing UV after a cataract operation at age 82, influencing his famous series of water lily pictures. The US defence department is working on several devices that people could wear to extend what they can see beyond the visible spectrum The colour of night Until recently, we thought nocturnal animals see at night the same way that we do – in shades of grey. But it turns out that several can see colours in the dark. Our colour vision depends on comparing the responses of three kinds of cone cells tuned to different wavelengths, but this works only when there is plenty of light. In dimmer conditions, we rely on rod cells that are far more sensitive to light but, because there is only one type of rod cell, they cannot help us to distinguish colour. Many nocturnal animals have the same two-part system as us, but max out on rods at the expense of cones, sacrificing colour perception in good light for better detail at night. Some animals do see colour in the dark, Almut Kelber of Lund University in Sweden discovered a few years ago. In the case of geckos, it happened by a kind of evolutionary accident. Their ancestors lost their rod cells altogether during millions of years as die-hard diurnal creatures, so maxing out on rods was not an option when they started to become nocturnal. Instead, their cone cells
grew bigger to work better in low light – becoming 350 times as sensitive as human ones. This ability to distinguish far subtler shades of colour gave them the ability to see colour in the dark. The superpower comes with a trade-off, though.  The bigger the cone cells, the less detail they can resolve, and so geckos have grainy vision in all light. Kelber and others have also  found colour night-vision in a handful of insects, including hawkmoths and nocturnal carpenter bees, which use it to find flowers in the dark. There may be many more species that see colour at night, says Kelber, who is now studying frogs and toads. These discoveries have inspired those trying to develop better cameras. A team at Toyota’s R&D centre in Belgium is working on night-vision systems that use ordinary camera sensors to produce a much better image in low light by mimicking the way night-flying dung beetles process signals. Toyota wants to provide drivers with a better view of the road ahead at night, but this kind of technology might one day make colour night-vision systems widely available Eyes in the back of our head Our forward-facing eyes move in unison, allowing us to judge distances accurately and see through solid objects. But with a little help from technology and our highly adaptable brains, we can swap these abilities for the 360-degree vision of a fly or the independently moving eyes of a chameleon. You can have a fly’s all-around vision using a head-mounted panoramic camera that beams a 360-degree view of the world onto a screen. According to its inventors at the ESIEA engineering school in Paris, France, “FlyVIZ” takes 15 minutes to get used to, doesn’t make you dizzy and enables wearers to dodge balls thrown from behind, grab objects they wouldn’t normally see and walk backwards while navigating doorways with ease. The device is an unwieldy prototype at present but potential beneficiaries could include police, security guards and, perhaps, even teachers. Another group, led by Fumio Mizuno at the Tohoku Institute of Technology in Sendai, Japan, is working on a different kind of 360-degree vision based on chameleon eyes. Two cameras move independently to provide totally non-overlapping views. In experiments, 11 out of 12 volunteers could make sense of this strange view of the world even though it is  so different from our own.  ■