Chimpanzee brains may be hard-wired to evolve language even though they can't talk. That is the suggestion of a study which found chimps link sounds and levels of brightness, something akin to synaesthesia in people. Such an association could help explain how our early ancestors took the first vital step from ape-like grunts to a proper vocabulary.
Synaesthetes make unusual connections between different senses ? they might sense certain tastes when they hear music, or "see" numbers as colours. This is less unusual than you might think: "The synaesthetic experience is a continuum," explains Roi Cohen Kadosh of University College London. "Most people have it at an implicit level, and some people have a stronger connection."
Now, Vera Ludwig from the Charite University of Medicine in Berlin, Germany, and colleagues have shown for the first time that chimpanzees also make cross-sensory associations, suggesting they evolved early on.
The team repeatedly flashed either black or white squares for 200 milliseconds at a time on screens in front of six chimpanzees (Pan troglodytes) and 33 humans. The subjects had to indicate whether the square was black or white by touching a button of the right colour. A high or low-pitched sound was randomly played in the background during each test.
Chimps and humans were better at identifying white squares when they heard a high-pitched sound, and more likely to correctly identify dark squares when played a low-pitched sound. But performance was poor when the sounds were swapped: humans were slower to identify a white square paired with a low-pitched noise, or a black square with a high-pitched noise, and the chimps' responses became significantly less accurate.
Sensory crosstalk innate
The finding adds evidence to the idea that cross-sensory associations are innate, not learned. In 2001, V. S. Ramachandran and Edward Hubbard of the University of California at San Diego, asked adults to ascribe "kiki" and "bouba" to a spiky and a rounded shape. They almost all dubbed the spiky shape kiki and the rounded one bouba, regardless of culture or language. Children also find it easier to learn the names of rounded objects whose names use rounded vowels.
Ludwig reckons that an innate tendency to link sounds and sights could have been advantageous to our ancestors. "If two individuals both think a word that contains a high pitched vocal fits a lighter object, then it would be easier for them to develop a common vocabulary and to understand the words that others use," she says.
Ramachandran agrees that innate associations would have played a crucial role in language evolution. "These experiments suggest there were non-arbitrary similarities between sounds and sights which could have helped get it started," he says.
So why don't chimps talk? Crosstalk between the senses was likely just one part of the evolutionary toolkit, says Morten Christiansen who studies language evolution at Cornell University in Ithaca, New York. Other features humans do not share with fellow primates, including superior sequence-processing abilities, could have helped our ancestors to develop speech when chimps could not.
Ludwig believes that some of the neural processes that cause the type of connections in her experiments also underlie full synaesthesia in humans. But whether some chimps also find music by Bach tastes creamy remains to be seen.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1112605108
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