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      is faulty. With reduced sensitivity at certain wavelengths, they struggle   
      to tell the difference between reds and greens, for instance.    
      
      In theory, though, it could go the other way: according to some estimates,   
      an extra cone would offer a hundred different variants to each colour that   
      humans normally see. We know that this happens in nature: zebrafinches and   
      goldfish both have a fourth cone that seems to help them differentiate   
      apparently identical colours. About 20 years ago Gabriele Jordan at the   
      University of Newcastle and John Mollon at the University of Cambridge   
      proposed a way that it might be possible in humans too.   
      
      The crux of Jordan’s argument lay in the fact that the gene for our red   
      and green cone types lies on the X chromosome. Since women have two X   
      chromosomes, they could potentially carry two different versions of the   
      gene, each encoding for a cone that is sensitive to slightly different   
      parts of the spectrum. In addition to the other two, unaffected cones,   
      they would therefore have four in total – making them a “   
   etrachromat”.   
      For these reasons, it’s thought to be a condition exclusive to women,   
      though researchers can’t totally rule out the possibility that men may   
      somehow inherit it too.   
      
      (nnice/Flickr/CC BY 2.0)   
      (nnice/Flickr/CC BY 2.0)   
      
      Proving that these people actually see the world differently has involved   
      a two-decade journey, however. Although the relevant combination of genes   
      does not seem to be especially rare – perhaps 12% of women might have four   
      distinct cones – many of the people that Jordan tested just didn’t seem   
   to   
      show any differences in their perception. But by 2010, she had found a   
      subject who perfectly acted the part of a tetrachromat. Jordan’s “acid   
      test” involved coloured discs showing different mixtures of pigment, such   
      as a green made of yellow and blue. The mixtures were too subtle for most   
      people to notice: almost all people would see the same shade of olive   
      green, but each combination should give out a subtly different spectrum of   
      light that would be perceptible to someone with a fourth cone. Sure   
      enough, Jordan’s subject was able to differentiate between the different   
      mixtures each time. “When you ask them to discriminate between the two   
      mixtures, a tetrachromat can do it very quickly. They don’t hesitate,”   
      says Jordan.   
      
      Men are thought to be incapable of seeing this hidden world of colour   
      (Magdalena Roeselerby/Flickr/CC BY 2.0)   
      Men are thought to be incapable of seeing this hidden world of colour   
      (Magdalena Roeselerby/Flickr/CC BY 2.0)   
      
      But what do those colours actually look like? Unfortunately, Jordan’s   
      much-prized subject has not been available for media interviews. But once   
      the abilities of the woman “with super-human vision” became known, many   
      more potential tetrachromats have come forward who might be able to give   
      us an insight.   
      
      One of those was Maureen Seaberg, a journalist and author in New York who   
      took the genetics test after hearing Radiolab’s podcast on the subject.   
   “I   
      have always had polite disagreements with people about shades of colours,”   
      she says. When clothes shopping for instance, she often finds that   
      apparently matching tops and skirts seem to be a different shade to her,   
      clashing horribly – even though no one else seems to notice it. Her   
      sensitivity can sometimes be baffling to those around: when helping to   
      restore a house, she once rejected 32 paint samples before settling on the   
      right shade. “The beiges were too yellow and not blue enough, not cool   
      enough; some of the almonds were too orangey,” she says – distinctions   
      that were much to the confusion of her building contractor. Of course,   
      that is only anecdotal evidence, but it perhaps tells us a little about   
      the way that apparently identical shades could appear strikingly different   
      to a tetrachromat.   
      
      Antico has a similar story. She says she has always known that her eyes   
      saw the world differently to other people – a talent that was soon noticed   
      by her family. “When I was a very little girl, my mother looked at me and   
      said ‘You are going to be an artist and art instructor’.” Today,   
   Antico   
      has fulfilled that prediction, with her own gallery in San Diego,   
      California, where she uses her enhanced vision to create vibrant paintings   
      bursting with colour. Consider the following painting of a rainbow   
      eucalyptus. “The tubes of paint were flying. The yellows, the violets, the   
      lime greens – I was ferociously mixing on the palette trying to produce   
      all the streams of colour in the bark,” says Antico. When compared to the   
      real thing, the resulting canvas suggests that Antico is seeing more than   
      the average eye.   
      
      Comparing Antico’s painting with the original scene could give a hint of   
      the extra shades she is seeing (Concetta Antico)   
      Comparing Antico’s painting with the original scene could give a hint of   
      the extra shades she is seeing (Concetta Antico)   
      
      It was when looking at one of those paintings one day that a customer   
      suggested that Antico contact some researchers who work on tetrachromacy.   
      The genetics test came back positive, and she began collaborating on a   
      series of experiments with Jameson and her colleagues, including Alissa   
      Winkler at the University of Nevada, Reno.   
      
      Jameson immediately suspected the Antico’s genes might also give her   
      enhanced vision in dim lighting. “If you look at her pictures of dawn, she   
      paints a lot of colours and renders them in very low lighting,” she says   
   –   
      conditions that normally reduce our vision to greyscale. That could just   
      be creative licence, but Antico claims she is actually seeing the colours   
      there in front of her. Sure enough, Jameson’s experiments have shown that   
      the luminance of different colours is indeed altered for Antico, making   
      them pop out in the half light.   
      
      Antico’s vision is particularly enhanced at night, as seen in “Shiny   
   moon   
      in La Jolla” (Concetta Antico)   
      Antico’s vision is particularly enhanced at night, as seen in “Shiny   
   moon   
      in La Jolla” (Concetta Antico)   
      
      Enhanced sensitivity is not always a blessing, though. “The grocery store   
      is a nightmare,” she says. “It’s like a trash pile of colour coming   
   in at   
      every angle.” That enhanced sensitivity might explain why she finds solace   
      in plain white surfaces. “People find that extraordinary that white is my   
      favourite colour, but it makes sense because it is so peaceful and restful   
      for my eyes. There is still a lot of colour in it, but it’s not hurting   
      me.”   
      
      Not all tetrachromats have striking abilities. Jameson has found that   
      Antico’s perception outstrips other tetrochromats who have not trained   
      artistically. “Concetta is the perfect storm for tetrachromacy because she   
      has a huge amount of perceptual learning experience by working with colour   
      on a daily basis.” If that is confirmed with further research, Antico   
      hopes that she might be able to develop a training system to help all   
      tetrachromatic children realise their potential.   
      
      Antico’s ultimate ambitions are even greater: to help us all see the world   
      
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