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    The shape of colour

    The age-old question of whether we all see colour the same may now have an answer.

    Researchers at the National Eye Institute in the US have found that human brains process specific colours in a similar way, and the “shape” of this brain map can predict what colour people are seeing.

    These shapes also were influenced by looking at different brightness and hues, they report in a paper in the journal Current Biology.

    “This is one of the first studies to determine what colour a person is seeing based on direct measurements of brain activity,” says lead researcher Bevil Conway. “The approach lets us get at fundamental questions of how we perceive, categorise and understand colour.”

    To do this they used magnetoencephalography (MEG), which maps the geometry of brain action using magnetic fields. These fields are generated from the electrical signals that happen when the brain thinks.

    This essentially helped them “read the mind” of each person as they looked at colours, to map the shape of neural pathways.

    Each volunteer looked at pink, green, blue and orange at different brightness and reported what hue they saw. The team measured their brain activity with MEG. The colour processing happened in less than 400 milliseconds when looking at colours.

    Interestingly, they found each person’s processing was similar enough to be able to predict what colour volunteers were seeing based only on the MEG data.

    “People have been wondering about the organisation of colours for thousands of years,” says Conway. “The physical basis for colour – the rainbow – is a continuous gradient of hues, but people don’t see it that way. They carve the rainbow into categories and arrange the colours as a wheel.

    Coloured stimuli in yellow and blue. Light luminance level versions at left; dark versions, right. Volunteers used a variety of names for the upper stimuli, such as “yellow” (left) and “brown”, but consistently used “blue” for both lower stimuli. Credit: Bevil Conway, National Eye Institute

    “We were interested in understanding how the brain makes this happen; how hue interacts with brightness, such as to turn yellow into brown.” 

    They also witnessed many brain activity shapes when recognising the differences in warm colours at varying brightness. However, there were fewer brain activity shapes when looking at cold colours, regardless of cone activity in the eye.

    “First, there is a greater precision in naming warm colours compared to cool colours,” the researchers write. “This colour-naming efficiency reflects an interaction of hue and luminance: among the basic colour categories (red, pink, orange, yellow, brown, green, blue, and purple), the cool colours (green and blue) are not distinguished from each other by differences in lightness, whereas the warm colours are.”

    It is easy to think that language influences the colours we report: we might say a light and dark shade are both blue, but wouldn’t call pink light red. However, this research suggests there may be a neurological reason for this, particularly when it comes to red.

    “[T]he hue associated with red is the most salient chromatic colour, reflected in the preeminence of red among basic colour terms across languages.”

    This means that our colour perception transcends language and culture and is instead most likely based on the shape of our brain activity. It is therefore likely we are probably seeing colours the same.

    Why might brains process warm and cold colours differently?

    “We wonder whether the result reflects the adaptation of visual processing to the colour statistics of objects: warm colours, regardless of the specific hue, are associated with parts of scenes we label as objects,” they hypothesise in their paper.

    This could be important to evolution as objects that stand out from the environment are more likely to hold a threat, so a quick, refined assessment of these object colours could help the brain decide what to do.

    Remarkably, the researchers also found a difference in how people processed colours depending on whether they looked at a coloured shape or the name of the colour.

    They processed colour spirals more quickly, suggesting that that brain needs to process colours and words separately, so it takes longer. This points to the highly complex nature of colour processing in the brain, and so is just the beginning of learning how our eyes and minds perceive colour.

    “For us, colour is a powerful model system that reveals clues to how the mind and brain work. How does the brain organise and categorise colour? What makes us think one colour is more similar to another?” says Conway. “Using this new approach, we can use the brain to decode how colour perception works – and in the process, hopefully uncover how the brain turns sense data into perceptions, thoughts, and ultimately actions.”

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