New York: Scientists have explained why people were divided over the true colours of "the dress" that went viral in 2015, attributing the difference in perception to our assumptions about how the dress was illuminated.


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Millions of people were divided whether the true colours of the dress, worn by the mother of a bride at a wedding in Scotland, were gold and white or black and blue.


Researchers at New York University (NYU) in the US found that those who thought that the dress was photographed in a shadow likely saw the garment as gold and white.


By contrast, those who thought it was illuminated by artificial light were more likely to see it as black and blue.


"The original image was overexposed, rendering the illumination source uncertain," said Pascal Wallisch from NYU.


"As a result, we make assumptions about how the dress was illuminated, which affects the colours we see," said Wallisch.


"Shadows are blue, so we mentally subtract the blue light in order to view the image, which then appears in bright colours - gold and white," he said.


"However, artificial light tends to be yellowish, so if we see it brightened in this fashion, we factor out this colour, leaving us with a dress that we see as black and blue.


"This is a basic cognitive function: to appreciate the colour on an object, the illumination source has to be taken into account, which the brain does continuously," Wallisch said.


The findings, based on an online study with more than 13,000 participants, who had previously seen the dress, and were asked whether or not they believed it was in a shadow.


Among those who saw it in a shadow, four out of five participants believed it to be white and gold; by contrast, only about half of participants who did not see it in a shadow saw the garment bearing these colours.


Wallisch hypothesised that differing perceptions could be linked to one's exposure to daylight - quite simply, people who rise and go to bed early, and spend many of their waking hours in sunlight (ie, under a blue sky), are more likely to see the dress as white and gold than are night owls, whose world is illuminated not by the sun, but, rather, by long-wavelength artificial light.


To test this, he asked participants if they go to bed early and feel best in the morning (ie, "larks") or if they like to sleep in and feel best at night ("owls"), then matched this self-identified circadian type with how they saw the dress.


Consistent with the hypothesis, larks were significantly more likely to see the dress as white and gold - relative to owls - underscoring the relative effects of exposure to daylight.


"This suggests that whatever kind of light one is typically exposed to influences how one perceives colour," Wallisch said.