Chimp recognises synthetic speech
A talented chimpanzee called Panzee can recognise distorted and incomplete words spoken by a computer, scientists have discovered.
That suggests that apes may be more capable of perceiving spoken sounds than previously thought, and that the common ancestor of humans and chimps may also have had this ability.
It also refutes the idea that humans have brains uniquely adapted to process speech, say the scientists who have published their findings in the journal Current Biology.
Panzee was raised from 8 days old, by humans, and was spoken to and treated as if she were human. At the same time, she was taught to use symbols called lexigrams to communicate.
"This has resulted in Panzee showing proficiency in understanding approximately 130 English words," researcher Lisa Heimbauer told BBC Nature.
That made her an ideal subject to test hypotheses about how well other species, rather than humans, might be able to understand speech.
"There is a view about the human ability to produce and perceive speech that is called 'Speech is Special'," said Ms Heimbauer, who is studying for her PhD.
"This argument proposes that, besides humans being the only species able to produce speech, due to their anatomy, they also have a specialised, cognitive module to process speech."
Evidence for that comes from studies showing that humans can understand speech even when it is incomplete or highly distorted.
"However, an alternative view is that auditory processing is fundamentally similar across most mammals, and that animals therefore have latent abilities for speech perception," said Ms Heimbauer.
So she and her colleagues Michael Beran and Michael Owren, all from Georgia State University in Atlanta, US, tested Panzee to find out if she too could recognise incomplete or distorted spoken words.
They played Panzee noise-vocoded speech, which alters the frequencies of the spoken words. This produces a sound similar to what people with cochlear implants hear.
They also played Panzee so-called sine-wave speech, which is synthesised from just three pure tones.
Both types of degraded speech have been shown to be understandable by people.
"She is only one of a few animals who could be tested in this way, to reveal what the speech perception abilities of a common chimp/human ancestor may have been," said Ms Heimbauer.
The researchers discovered that Panzee recognised these degraded spoken words far more often than she should by chance.
Her upbringing, say the scientists, appears to have given her enough experience of hearing and understanding spoken words to allow her to recognise them when they are distorted.
That "highlights the importance of early experience in shaping speech perception," said Ms Heimbauer.
It also provides evidence that a common ancestor of humans and chimpanzees would have had the ability to perceive speech, she says.
"If humans do possess a specialised, cognitive, speech-processing brain module, it would be something that evolved later in humans, making us more efficient at what we do."