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Scientists at Georgia State University suggest it is early experience with language - not special innate cognitive ability - that allows humans to process and perceive speech, while their closest evolutionary relatives, chimpanzees, do not.
Traditionally, the human brain has been thought to be uniquely adapted to perceive and process speech patterns, a trait widely regarded as an evolutionary phenomenon separating humans from other primates. However, a 25-year-old, language-trained chimp named Panzee recently demonstrated the ability to interpret highly distorted speech sounds in a similar manner to humans. These data provide evidence that the capacity for speech perception may have existed in a common ancestor.
According to this theory, it is not that chimpanzees in the wild lack the perceptual apparatus to understand human language. Rather, they don't develop speech because they lack experience using their capacity for it. They need practice.
"Our results reinforce the fact that experience matters," said Lisa Heimbauer, a researcher at the Language Research Center at Georgia State. "Humans may not perceive speech because they are human but because of the tremendous amount of experience they have with it from birth."
The research team presented its findings at the American Acoustical Society meeting held recently in San Diego.
Heimbauer and colleagues Michael Owren, PhD, and Michael Beran, PhD, had an ideal subject in Panzee. Raised by humans since she was 8 days old, the chimp was exposed to the English language early, as if she were a human baby. She is proficient in understanding more than 130 words and recognizes words in sine-wave form, a type of synthetic speech that reduces language to three whistle-like tones.
Understanding language, even when highly distorted, is a specialized capacity once attributed only to humans. However, Panzee processes words even when they are lacking many of the traditional cues present in normal speech. Now, similar to the 13 humans in the study, Panzee has shown better recognition of synthesized words when the first two sine-wave tones are present than when one of them is absent. This suggests she relies on the same information in sine-wave speech used by humans for language recognition of words in this form.
This research may help scientists better understand the mechanisms used by young children to process and produce speech.
"If non-humans given a richer, early speech experience can show human-like speech perception, it suggests that speech impairments may not be related to problems in specialized circuits," Heimbauer said. This insight ultimately could lead to a better understanding of why some children have hearing and language deficits.
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