Scientists have found that brain in infants is hard-wired to respond to the calls from a non-human primate - an ability that quickly fades away in a few months.
Previous studies have shown that even in infants too young to speak, listening to human speech supports core cognitive processes, including the formation of object categories.
Alissa Ferry, lead author and currently a postdoctoral fellow in the Language, Cognition and Development Lab at the Scuola Internationale Superiore di Studi Avanzati in Trieste, Italy, together with Northwestern University colleagues, documented that this link is initially broad enough to include the vocalisations of non-human primates.
"We found that for 3- and 4-month-old infants, non-human primate vocalisations promoted object categorisation, mirroring exactly the effects of human speech, but that by six months, non-human primate vocalisations no longer had this effect - the link to cognition had been tuned specifically to human language," Ferry said.
In humans, language is the primary conduit for conveying our thoughts. The new findings document that for young infants, listening to the vocalisations of humans and non-human primates supports the fundamental cognitive process of categorisation.
From this broad beginning, the infant mind identifies which signals are part of their language and begins to systematically link these signals to meaning.
Furthermore, the researchers found that infants' response to non-human primate vocalisations at three and four months was not just due to the sounds' acoustic complexity, as infants who heard backward human speech segments failed to form object categories at any age.
"For me, the most stunning aspect of these findings is that an unfamiliar sound like a lemur call confers precisely the same effect as human language for 3- and 4-month-old infants," said Susan Hespos, co-author and associate professor of psychology at Northwestern.
"More broadly, this finding implies that the origins of the link between language and categorisation cannot be derived from learning alone," Hespos said.
"These results reveal that the link between language and object categories, evident as early as three months, derives from a broader template that initially encompasses vocalisations of human and non-human primates and is rapidly tuned specifically to human vocalisations," said Sandra Waxman, co-author and Louis W Menk Professor of Psychology at Northwestern.
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