A 25-year-old chimpanzee named "Panzee" has just demonstrated that speech perception is not a uniquely human trait.
Well-educated Panzee understands more than 130 English language words and even recognizes words in sine-wave form, a type of synthetic speech that reduces language to three whistle-like tones. This shows that she isn't just responding to a particular person's voice or emotions, but instead she is processing and perceiving speech as humans do.
"The results suggest that the common ancestor of chimpanzees and humans may have had the capability to perceive speech-like sounds before the evolution of speech, and that early humans were taking advantage of this latent ability when speech did eventually emerge," said Lisa Heimbauer who presented a talk today on the chimp at the 162nd Meeting of the Acoustical Society of America in San Diego.
Heimbauer, a doctoral candidate and researcher at Georgia State University's Language Research Center, and colleagues Michael Owren and Michael Beran tested Panzee on her ability to understand words communicated via sine-wave speech, which replicates the estimated frequency and amplitude patterns of natural utterances. "Tickle," "M&M," "lemonade," and "sparkler" were just a few of the test words.
Even when the words were stripped of the acoustic constituents of natural speech, Panzee knew what they meant, correctly matching them to corresponding photos.
The findings refute what is known as the "Speech is Special" theory.
"This argument proposes that besides humans being the only species able to produce speech, due to their anatomy, they also have a specialized cognitive module to process speech," Heimbauer explained.
Supporters of the "Speech is Special" view have pointed to the fact that humans can understand speech, even when it is incomplete or highly distorted. The alternative view to the hypothesis, she said, is that auditory processing is fundamentally similar across most mammals, and that many animals have latent abilities for speech perception.
"What we are saying is that humans do not need unique cognitive abilities to process speech, and that instead, the general auditory processes that we share with apes, and probably a common ancestor, can be used to accomplish speech perception tasks," she said. "Panzee then is able to understand speech because of her early experience in a speech-rich environment, and because she was taught about the association between words and their meaning from a very early age."
"These are the same things that allow humans to learn how to understand speech," she said.
There is evidence that non-human primates use specific vocalizations in certain situations, but researchers hesitate in labeling these as being part of a "language." The communications are not believed to be very complex.
Nonetheless, the findings suggest that, with education and experience, chimpanzees can become proficient in aspects of human languages, allowing them to communicate certain feelings and desires with us.
Panzee "certainly does communicate about her wants and needs, sometimes putting two to three words together," Heimbauer said, adding that the female chimp does this using the visuo-graphic symbols that she learned when she was young.
Tecumseh Fitch, a professor in the Department of Cognitive Biology at the University of Vienna, said the research, "provides important evidence that human speech perception abilities are built upon a pre-existing auditory basis, shared with other animals," he said.
"Scientists have been too quick to assume that speech perception is 'special' without actually testing this assumption with animals," Fitch said.
Although Panzee's education puts her in a unique intellectual category, prior research on birds, chinchillas and monkeys suggests that these animals can also discriminate and categorize speech sounds.