We’ve all heard of (felt/ tasted/ saw/ smelled) it before. You know someone who listens to a song and thinks it tastes fruity, bitter, or salty. A friend of a friend associates each day of the week with a different personality – Tuesdays are rambunctious, Thursdays are bookish, and Saturdays are charismatic. Or perhaps you yourself associate colors with numbers and letters.
This is synesthesia, a neurologically based condition in which senses get crossed. For instance, stimulation of the visual pathway might automatically trigger an audio or tactical sensation.
Synesthesia is known to occur in humans. The statistics vary, partially because the experience is largely subjective, but some scientists estimate that one percent of people see numbers or letters in color. Some famous synesthetes include Vladimir Nabokov, Marilyn Monroe, and Stevie Wonder. Now, a new study published this week in Proceedings of the National Academy of Sciences (PNAS) suggests that chimpanzees may also experience synesthesia.
The study, led by Vera Ludwig, a cognitive neuroscientist at Charité Medical University (Berlin, Germany), found that chimps associate light shades with high-pitched sounds, and dark shades with low-pitched sounds.
To prove this, Ludwig and her colleagues trained chimpanzees to select black or white squares on a screen after being physically shown either a black or white box. While the chimps were making their selection on the screen, the scientists would play either a high or low tone.
The apes, aged 8-32, chose the correct square 93% of the time when the scientists showed the chimp a white box and then played a high tone, or a showed them a black box and played a bass pitch. In contrast, when the scientists reversed the colors and tones, the chimps were successful only 90% of the time.
Almost all humans perceive similar links between high pitches and light colors or low pitches and dark colors. For instance, people relate high-pitched vowels, as in the word “mill”, with light hues, and low-pitched vowels, as in “mole”, with dark hues. Ludwig believes that these associations demonstrate mild synesthesia, occurring as a result of crossed connections in the regions of the brain that process senses.
Ludwig and colleagues had humans perform the same exercise as the chimps. Although these 33 humans did not make enough mistakes to detect a significant difference in color perception, they did make decisions more rapidly when sounds and colors matched.
This study is significant because it implies that sound-color associations are not unique to humans, and therefore do not require language. Indeed, paired sound-color perceptions may not be a learned behavior, but rather an innate one that was present in the common ancestor of humans and chimpanzees six million years ago.
These very couplings between sounds and other senses may have in fact influenced the development of human language. If certain concepts trigger certain sounds, it makes sense for people to assign meanings to sounds. Put these sounds together, and you have words.
Lynne Nygaard, at Emory University, suspected just that. She thought that sounds on their own might give people an indication of what words mean, even if it’s in a language they don’t understand. She presented English speakers with pairs of antonyms in ten different languages, such as Albanian, Dutch, Gujarati, Mandarin, and Yoruba, and told them to match the pairs of foreign antonyms with corresponding pairs of English words. The subjects performed statistically better than they should have by chance, perhaps because they were able to make associations between words’ sounds and meanings.
For instance, words that denote slow movements tend to have sonorant sounds such as “l” or “w”, while words that represent rapid movements are characterized by sounds with more friction such as “ch” and “f”.
In any case, whether or not the sound-color associations that Ludwig and colleagues observed can be considered synesthesia is still up for debate. Danko Nikolic, a scientist at the Max Planck Institute for Brain Research (Frankfurt, Germany), argues that the linkages between sound and color may have to do with perceived associations between higher concepts rather than the neural cross-wiring that causes synesthesia.
Edward Hubbard, a neuroscientist at Vanderbilt University, explains that while non-human animals may experience intertwined visual and audio stimuli, they are not likely to encounter the large breadth of synesthetic experiences that humans do, because humans have many more cultural constructs to associate with their senses (such as words and numbers).
Perhaps it would be interesting to conduct a study with babies who have not yet developed an understanding of concepts or cultural constructs?
Ultimately, as scientific research is often apt to do, this study addresses one question, and raises several more. For instance, what is the actual mechanism underlying this pairing of senses? Do associations between visual stimuli and sound have to do with the development of spoken languages? Are sound-color associations innate or learned cultural constructs? Or perhaps a combination of both?
We await further insight. In the meantime, I’ll take my sounds with a generous side of color.