Washington, Oct 21 : Five-month-old infants can tell human voice from monkey sounds, say psychology researchers.

Their finding provides the first evidence that human infants are able to correctly match different kinds of vocalisations to different species.

While young children know that humans speak, monkeys grunt, and ducks quack, it's not clear when we come to know which vocalisations each of these animals produce.

The team of psychologists explored this question by asking whether young infants expect humans, but not other animals, to produce speech, and also, whether infants can identify the sources of vocalisations produced by other species.

To do so, the researchers showed five-month-old infants from English- and French-speaking homes a sequence of individually presented pictures of human faces and rhesus monkey faces paired either with human speech or with rhesus vocalisations.

They then examined whether infants preferentially attended to the human faces when human vocalisations were presented (two Japanese single words "nasu" and "haiiro"), and whether infants preferentially attended to the rhesus faces when rhesus vocalisations (a coo and a gekker call) were presented.

As the researchers had predicted, the results showed that the infants looked longer at the pictures of human faces when human speech was presented and looked longer at pictures of rhesus monkey faces when rhesus vocalisations were presented.

Surprisingly, however, infants weren't able to match human-produced non-speech vocalisations, like laughter, to humans, suggesting that infants are especially tuned at an early age to some of the functional properties of speech.

The fact infants were able to correctly attribute even unfamiliar Japanese speech to humans bolstered the significance of the results, says a New York University release.

The study was led by Athena Vouloumanos, assistant psychology professor in New York University, with Madelynn Druhen, doctoral candidate, psychology, at the University of North Carolina; Marc Hauser, psychology professor at Harvard and Anouk Huizink, psychology researcher at McGill University.

These findings appeared in the latest issue of the Proceedings of the National Academy of Sciences (PNAS). (IANS)