Some people think that numbers always have a certain color. For example, the number 5 appears red. A new study reveals that their eyes respond as if they were seeing those colors in the real world.
The study found that people with synesthesia, a neurological condition in which sensations mix, don’t just experience these colors in their heads. Instead, you’ll see real, measurable differences in your pupils, just as if you were seeing real colors. The findings, published March 6 in the journal eLife, suggest that the brains of people with synesthesia process internally generated colors and actual visual input in a similar way, the study authors said.
“This eLife study is based on [earlier] “Our findings show that even though the actual stimulus is always the same (gray), the pupil responds similarly to the perception, constricting for lighter synaesthetic colors and dilating for darker synaesthetic colors,” Rebecca Keogh, a researcher at Macquarie University in Australia and an expert on pupillary responses who was not involved in the study, told Live Science via email.
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Measuring internal experience
Synesthesia comes in many forms. For example, people with grapheme-color synesthesia perceive letters and numbers as specific colors, while others perceive sounds as colors and words as tastes. Scientists estimate that at least 4% of people around the world have some form of synaesthesia.
Because synaesthetic experiences are unique to each individual, researchers have long struggled to objectively measure them. In a new study, scientists tested whether pupil size can provide a physical indication of the color perceived internally.
Based on these findings, it follows that if synaesthetic colors are perceptual in nature, they should also generate pupillary photoresponses.
Rebecca Keogh, Research Fellow, Macquarie University
Dr. Krishnankutty “Krish” Sathyan, a cognitive neurologist and neuroscientist at Penn State College of Medicine who was not involved in the study, said the findings provide compelling evidence that the colors people with synesthesia see when they see gray letters and numbers are associated with actual physiological changes in their eyes.
“This study could lead to better and more objective ways to identify synaesthesia, using physiological measurements rather than relying solely on self-report,” Keogh said.
The pupil size changes automatically depending on the brightness. The pupil constricts in bright light to protect the retina from light damage and dilates in darkness to let more light into the retina. This reflex is especially noticeable in cats, whose pupils dilate and constrict dramatically. The same reaction occurs in humans, but it is more subtle, and the pupil usually only changes by a few millimeters. In rare cases, the size of the pupils can change spontaneously.
Previous research by Keogh et al. has shown that the same effect occurs when people imagine visual images. “Our pupils become smaller when we imagine bright images than when we imagine dark images,” she explained. “Based on these findings, it follows that if synesthetic colors are perceptual in nature, they should also generate pupillary light responses.”
To test this idea, researchers at Utrecht University and the University of Amsterdam recruited 16 people with grapheme-color synesthesia and had them look at gray numbers on a screen while measuring their pupil size with an eye tracker. Each participant reported the hue, saturation, and brightness of each color that came to mind. The researchers also recruited two control groups of 16 people without synaesthesia. One control group was asked to actively think about the color of each digit on the screen, and the other group passively observed the digits without making color associations.
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People with synaesthesia showed distinct patterns in their pupillary responses. Pupils constricted when looking at numbers associated with bright synaesthetic colors, and dilated when the associated color was dark. For example, most respondents considered 0 to be a bright color such as white or light gray, whereas 9 was more likely to be associated with a variety of colors such as navy blue, black, and brown. Neither control group developed significant color associations or experienced changes in pupil size like people with synaesthesia.
In the second half of the experiment, people with synaesthesia observed colored discs that matched the colors they had previously reported. Their pupil reactions were very similar to those seen when looking at gray numbers, suggesting that their brains were treating internally generated colors the same as real colors.
The researchers concluded that the findings show that people with synaesthesia use the same brain networks to perceive color in their heads as they do when they see color in real life.
Keogh agreed. The results “would not be expected if synaesthesia were purely associative,” Keogh said, adding that it “supports the idea that these experiences have a perceptual, image-like quality.”
It is not yet clear how generalizable these findings are to other forms of synaesthesia
Rebecca Keogh, Research Fellow, Macquarie University
The timing of student participant responses was also interesting. Students’ reactions to consciously imagined colors usually take longer than reactions to real colors. In this study, synaesthetic colors caused pupil adjustment to be approximately 0.5 seconds slower than the actual color, but when participants with synesthesia intentionally imagined the color, the delay was shorter than expected. This is evidence that synesthetic color perception is an involuntary rather than a conscious process, the study authors said.
In contrast, people without synesthesia showed greater pupil dilation than both those with synesthesia and those who were not asked to imagine the color of each digit. No change in pupil size was observed in the latter. Previous studies have shown that pupil dilation increases during demanding cognitive tasks and that it is a reliable measure of effort. This shows that when people without synesthesia are asked to associate numbers with colors, they are making color associations intentionally, and this requires mental work.
The differences in pupil size in each group suggest that synesthetes automatically perceive color, Sassian said.
But “one limitation is that this study focused on a specific type of synaesthesia: grapheme color,” Keogh said. “Thus, it is not yet clear how generalizable these findings are to other forms of synaesthesia.”
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