Even black-and-white bananas look yellow

Experiment reveals how expectation interferes with perception.

What colour are your bananas? Most people see a tinge of yellow even when the picture is grey. When we look at a banana, does our brain tell us it looks yellow, even if it isn’t? A recent study shows that it does.

Psychologists at the University of Giessen, Germany, report in Nature Neuroscience that our perception of an object’s colour depends on our memory of its typical colour.

Karl Gegenfurtner and his co-workers showed their subjects digitized images of fruit, presented in random colours against a grey background. They then asked observers to adjust the colour of the fruit on the computer screen until it too was grey.

Bananas are yellow
But the volunteers had a hard time doing this. With a picture of a banana, for example, they would adjust the colour to be slightly too blue when trying to achieve grey, as if compensating for a perception of yellow that wasn’t really there (blue is opposite yellow on the colour wheel). At the point at which the banana was truly achromatic, volunteers thought it still looked a bit yellow.

It made no difference what colour the picture of the banana started as. Volunteers might be struck or amused by the image of a red banana, says Gegenfurtner, but they still kept a yellow banana in mind.

When volunteers were shown a neutral shape — a uniformly coloured disk, for example — this problem disappeared: observers could accurately make it grey.

Grass is greener
Previous studies have also shown that our minds can play tricks on us when it comes to colour. We may remember colours as being more intense than they really were, for example. Volunteers asked to pick out the colour of grass from a variety of green cards often choose one that is ‘greener’ than real grass, says Gegenfurtner.

One study has even shown that we discriminate colour differently when it is seen to our left versus to the right (see ‘Language colours vision’).

Gegenfurtner says the new study highlights how memory of a colour can also have an impact on its perception.

Full text at newst @ nature.com