Summary: Researchers were able to ascertain the colors people were seeing by looking at their brain activity. The study reveals we have unique brain activity associated with specific colors.
Researchers at the National Eye Institute (NEI) have decoded brain maps of human color perception. The findings, published today in Current Biology, open a window into how color processing is organized in the brain, and how the brain recognizes and groups colors in the environment. The study may have implications for the development of machine-brain interfaces for visual prosthetics. NEI is part of the National Institutes of Health.
“This is one of the first studies to determine what color a person is seeing based on direct measurements of brain activity,” said Bevil Conway, Ph.D., chief of NEI’s Unit on Sensation, Cognition and Action, who led the study. “The approach lets us get at fundamental questions of how we perceive, categorize, and understand color.”
The brain uses light signals detected by the retina’s cone photoreceptors as the building blocks for color perception. Three types of cone photoreceptors detect light over a range of wavelengths. The brain mixes and categorizes these signals to perceive color in a process that is not well understood.
To examine this process, Isabelle Rosenthal, Katherine Hermann, and Shridhar Singh, post-baccalaureate fellows in Conway’s lab and co-first authors on the study, used magnetoencephalography or “MEG,” a 50-year-old technology that noninvasively records the tiny magnetic fields that accompany brain activity. The technique provides a direct measurement of brain cell activity using an array of sensors around the head. It reveals the millisecond-by-millisecond changes that happen in the brain to enable vision. The researchers recorded patterns of activity as volunteers viewed specially designed color images and reported the colors they saw.
The researchers worked with pink, blue, green, and orange hues so that they could activate the different classes of photoreceptors in similar ways. These colors were presented at two luminance levels – light and dark. The researchers used a spiral stimulus shape, which produces a strong brain response.
The researchers found that study participants had unique patterns of brain activity for each color. With enough data, the researchers could predict from MEG recordings what color a volunteer was looking at – essentially decoding the brain map of color processing, or “mind-reading.”
“The point of the exercise wasn’t merely to read the minds of volunteers,” Conway said. “People have been wondering about the organization of colors for thousands of years. The physical basis for color–the rainbow–is a continuous gradient of hues. But people don’t see it that way. They carve the rainbow into categories and arrange the colors as a wheel. We were interested in understanding how the brain makes this happen, how hue interacts with brightness, such as to turn yellow into brown.”
As an example, in a variety of languages and cultures, humans have more distinct names for warm colors (yellows, reds, oranges, browns) than for cool colors (blues, greens). It’s long been known that people consistently use a wider variety of names for the warm hues at different luminance levels (e.g. “yellow” versus “brown”) than for cool hues (e.g. “blue” is used for both light and dark).
The new discovery shows that brain activity patterns vary more between light and dark warm hues than for light and dark cool hues. The findings suggest that our universal propensity to have more names for warm hues may actually be rooted in how the human brain processes color, not in language or culture.
“For us, color is a powerful model system that reveals clues to how the mind and brain work. How does the brain organize and categorize color? What makes us think one color is more similar to another?” said Conway. “Using this new approach, we can use the brain to decode how color perception works – and in the process, hopefully uncover how the brain turns sense data into perceptions, thoughts, and ultimately actions.”
Funding: The study was funded by the NEI Intramural Program.
Color Space Geometry Uncovered with Magnetoencephalography
•Stimulus color can be decoded from surface magnetoencephalography (MEG) recordings
•Perceptual representations give rise to semantic representations, but not the reverse
•The results reveal a neural geometry of color space that is dynamic
•The geometry explains universal color-naming patterns and generates new hypotheses
The geometry that describes the relationship among colors, and the neural mechanisms that support color vision, are unsettled. Here, we use multivariate analyses of measurements of brain activity obtained with magnetoencephalography to reverse-engineer a geometry of the neural representation of color space. The analyses depend upon determining similarity relationships among the spatial patterns of neural responses to different colors and assessing how these relationships change in time. We evaluate the approach by relating the results to universal patterns in color naming. Two prominent patterns of color naming could be accounted for by the decoding results: the greater precision in naming warm colors compared to cool colors evident by an interaction of hue and lightness, and the preeminence among colors of reddish hues. Additional experiments showed that classifiers trained on responses to color words could decode color from data obtained using colored stimuli, but only at relatively long delays after stimulus onset. These results provide evidence that perceptual representations can give rise to semantic representations, but not the reverse. Taken together, the results uncover a dynamic geometry that provides neural correlates for color appearance and generates new hypotheses about the structure of color space.