cannicemuamua 2012-03-21

Our colour vision starts with the sensors in the back of the eye that turn light information into electrical signals in the brain – neuroscientists call them photoreceptors。 We have a number of different kinds of these， and most people have three different photoreceptors for coloured light。 These are sensitive to blues， greens and reds respectively， and the information is combined to allow us to perceive the full range of colours。 Most colour blind men have a weakness in the photoreceptors for green， so they lose a corresponding sensitivity to the shades of green that this variety helps to distinguish。

At the other end of the scale， some people have a particularly heightened sensitivity to colour。 Scientists call these people tetrachromats， meaning “four colours”， after the four – rather than three – colour photoreceptors they possess。 Birds and reptiles are tetrachromatic， and this is what allows them to see into the infrared and ultraviolet spectra。 Human tetrachromats cannot see beyond the normal visible light spectrum， but instead have an extra photoreceptor that is most sensitive to colour in the scale between red and green， making them more sensitive to all colours within the normal human range。 To these individuals， it is the rest of us who are colour blind， as while most of us would be unable to easily distinguish an exact shade of summer-grass-green from Spanish-lime-green， to a tetrachromat it would seem obvious。