Study highlights important role of brain's color processing cells in color perception

When we view natural images the colors we perceive are due to color information at every local patch of an image, rather than how colors interact when they transition from one point to another, according to a new study from researchers at City, University of London.

The finding supports the role that color processing cells in the brain play when interpreting color, as previous it has been suggested that an effect called color 'filling-in' - in which the brain takes information at the edge between two colors and uses it to compute what the neighboring colors should be - influenced how we perceive colors in natural images.

Instead, the researchers found that this 'filling-in' effect only makes a small contribution to how colorful an images appears, as when natural images were restricted to such 'edge transitions' they did not appear to be very colorful.

To investigate the effect, the researchers carefully filtered natural images to remove the color differences except at the edges. When they carried out this process they found that the edge information was not sufficient to carry the color perception in the regions where the color had been removed.

As a result, the researchers conclude that while a weak 'filling-in' effect occurs, it only accounts for around 5%, and therefore cannot account for the rich colors we see in the natural world.

This finding is significant, as it provides evidence to support the vital role that color processing cells in the cortex play in color perception. Crucially, these cells are not sensitive to edges and only to the colors themselves present in such color fields. It was also seen that purely chromatic images with maximally graded ('edgeless') transitions look fully colorful.

This also has important practical implications as it shows that you cannot effectively compress image information to only the edge color information. The study is published in the journal i-Perception.

Professor Christopher Tyler, lead author of the study, said:

"While the mechanisms through which we perceive color when viewing natural images has been debated for a long time, our new study highlights the important role of color processing cells. As instead of the transitions between colors influencing the colors seen through 'filling-in', we instead found that the individual colors seen at each local point determine what we see."

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