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Forbidden colours

Stories from Physics for 11-14 14-16 IOP RESOURCES

A model of colour perception, proposed in 1872 by Ewald Hering, argued colour vision involves two opponent pairs of colours: red-green and yellow-blue. It has long been believed that the opponency of colours is an inherent feature of vision and that it was impossible to see reddish-green or blueish-yellow. This effect arises because colour perception appears to use a red-green channel and a blue-yellow channel.

However, Californian visual scientists H D Crane and T P Piantanida showed that under special conditions people can experience these ‘forbidden’ colours. In 1983, they drew figures in which two touching vertical bands of colour run parallel to each other: a red and green line and, in a separate figure, a blue and yellow line. Using an eye tracker, the researchers moved the images in response to the participants’ eye movements in order to keep them in the same place in the viewers’ visual field. The participants reported that, as they stared at the bands of colour, the colours appeared to flood together allowing them to see colours which they had never seen before, including, for example, a colour that was both red and green at the same time.

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