Feeling grey? Now we know why

Feeling grey? Now we know why

Experts have known for years that many people perceive colour differently to others, with what we think of as "blue" often perceived as what we would class as "purple" by someone else, but researchers in the US have published a new study which examines the perception of colour and different shades in more detail than ever before.

According to experts at Rutgers–Camden, the state University of New Jersey, in collaboration with psychologists from the University of Pennsylvania, they have discovered how the brain is able to keep a stable perception of an object"s colour as lighting conditions change.

Sarah Allred, an assistant professor of psychology at Rutgers–Camden, along with Alan L Gilchrist, a professor of psychology at Rutgers–Newark, worked with professor David Brainard and post-doctoral fellow Ana Radonjic, both of the University of Pennsylvania, on the study, which will be published in the journal Current Biology.

"Although we recognise easily the colours of objects in many different environments, this is a difficult problem for the brain," Professor Allred explained.

"For example, consider just the greyscale that goes from black to white. A white piece of paper in bright sunlight reflects thousands of times more light to the eye than a white piece of paper indoors, but both pieces of paper look white. How does the brain do this?"

He noted that the process of seeing an object begins when light reflected off that object hits the light-sensitive structures in the eye, with the perception of an object"s lightness in terms of colour shade depending on the object"s reflectance.

According to the expert, objects that appear lighter reflect a larger percentage of light than those that appear darker, as the brain processes perceptual differences between black and white objects even when illumination of the object changes.

The expert explained that if the brain did not do this, it would fail to distinguish colour shade in different light and pose a major problem for people going about their daily lives.

Professor Allred said that, in general, white objects reflect around 90 per cent of the light that hits them and black objects reflect about three per cent, which is a ratio of 30 to one, though when they looked at the intensity of light that entered the eye from a typical scene, the ratio was over 10,000 to one.

The scientists explained that this happens because, in addition to having objects with different reflectance, real scenes also have different levels of illumination, with an example being the shadowed region underneath a tree.

According to the experts, they wanted to determine how the brain maps a large range of light intensity onto a much smaller reflectance range, as a long-term hypothesis is that the brain segments scenes into different regions of illumination and then uses ratio coding to decide what looks white.

To verify this, they conducted an experiment in which participants viewed images that had a very large range of light intensities and were asked to look at a 5x5 checkerboard composed of greyscale squares with random intensities spanning the 10,000 to one range.

They were then asked to report what shades of grey a target square looked like by selecting a match from a standardised grey scale.

This visual system relied only on ratios to determine surface lightness, then the ratio of checkerboard intensities the participants reported should have had the same ratio as that of the black and white samples on the reflectance scale, about 100-to-1, but instead the researchers found that this ratio could be as much as 50 times higher, more than 5,000-to-1.

Professor Allred explained: "This research is important because we have falsified the ratio hypothesis, which is currently the most widely invoked explanation of how we perceive lightness.

"We also were able to reject several similar models of lightness. We were able to do this because we measured lightness in such high-range and relatively complex images."

Additionally, the expert noted that despite using behavioural rather than physiological measures, the results provide insight into the neural mechanisms that underlie the behavioural results. 

by Martin Burns

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