Monthly Archives: February 2010

chicken colour vision

Human colour vision under normal lighting levels is mediated by three cones (light-sensitive cells) in the retina. Each class of cone has peak sensitivity at a different wavelength and thus the cones are known as L (long-wavelength sensitive), M (medium-wavelength sensitive) and S (short-wavelength sensitive) cones or (sometimes) as red, green and blue cones. Both colour and luminance are captured by the same cone mechanism. The L and M cone responses are combined to give luminance and various cone responses are compared to give rise to hue and chroma. Interestingly, the distribution of L, M and S cones in the retina is not uniform but is random.

A recent paper – – by scientists at Washington University School of Medicine (St Louis, USA) reveals that chickens have five types of cone. Interestingly, one of these types of cones (so-called double cones) seems to encode luminance, whereas the other four cones (red, green, blue and violet) give rise to tetrachromatic vision. The cones are very regulary spaced in the retina.

The spacing of cones in the human retina may result from a compromise – the same cones need to encode colour and luminance. The avian colour vision system seems to be more sophisticated. One can only wonder at what benefit was bestowed in avians by separating the processing of colour and luminance information.

yellow line is offensive

Atlanta’s transit system will rename a train route into the heart of the city’s Asian community in response to complaints that calling it the “yellow line” showed a lack of racial sensitivity. The Metropolitan Atlanta Rapid Transit Authority issued a statement Thursday afternoon announcing it would change the name of the line to the “gold line.”

Iran green

The colour green has become synonymous with protests in Iran. Apparently, the Iranian government has been surprised by the number of protesters (greens) waving their flags in public places making it hard to show pro-government supporters because of all the green. Ihave read reports that they have developed a colour filter that would reduce the amount of green on their TV broadcasts. Does anyone know any more about this?

depressed people feel more gray than blue

People with anxiety and depression are most likely to use a shade of gray to represent their mental state. 

Peter Whorwell, Professor of Medicine and Gastroenterology at University Hospital South Manchester, worked with a team of researchers from the University of Manchester, UK, to create an instrument that would allow people a choice of colours in response to questions. He said, “Colours are frequently used to describe emotions, such as being ‘green with envy’ or ‘in the blues’. Although there is a large, often anecdotal, literature on color preferences and the relationship of color to mood and emotion, there has been relatively little serious research on the subject”.

The researchers have developed a colour chart, The Manchester Color Wheel, which can be used to study people’s preferred colour in relation to their state of mind.

For more information see

why is hue circular?

Everyone is familiar with the colour spectrum. If you pass white sunlight through a prism then it splits into the component wavelengths. The shorter wavelengths appear blue, the longer wavelengths appear red, and in between we have the familiar colours that I learned as school as Richard Of York Gave Battle In Vain, for the sequence red, orange, yellow, green, blue, indigo and violet, and that I have since understood is taught in the US as a person: Roy G Biv. I wonder if there are any other mnemonics that people know of? Of course, many people believe that Newton was in error when he identified 7 colours in the spectrum – he was probably influenced by Aristotle who wrote about there being 7 fundamental colours as there are 7 tones in the musical octave. I’ve posted about the indigo issue before – – so won’t repeat that here.

Newton was probably the first person to create a hue circle (others, such as Forsius, created colour cicles but often included white and black in the circles). Newton created a true hue cirlce where he took the colour spectrum and wrapped it around, noticing that the two ends of the spectrum (where the reds become bluish and the blues become reddish) look rather similar.

Of course, there was a gap because the two ends of the spectrum did not quite match and thus Newton had to add in some purplish colours – these are hues that are never seen in the spectrum (and are sometimes called extra-spectral hues or non-spectral hues). The hues in the spectrum can be created by a single wavelength; however, the extra-spectral hues only occur when we see several wavelengths at the same time. For example, when we see short and long wavelengths together we can see purple.

In my lecture at the University of Leeds ( this week someone asked “Why do the two ends of the spectrum look similar at all when the light is so different physically (at one end the waves are short and high energy and at the other they are long and low energy)?” Very very good question – if changes in wavelengths change the hue why should wavelengths that are so different look so similar?

So, why is hue circular? The answer is that it has very little to do with wavelengths and physics and more to do with human physiology. The human visual system captures light with three classes of cell (called cones) in the retinae of the eye. The signals from these cones are processed by the human visual system to create opponent signals (red-green and yellow-blue). This puts red and green opposite each other and yellow blue opposite each other and results in the perception of hue being circular. It also explains why some hues particularly contrast – sometimes called complementary colour harmony.