Mayberry, director of McGill's School of Communication Sciences and Disorders, along with Elizabeth Lock of the University of Ottawa and Hena Kazmi of the University of Western Ontario, studied groups of deaf and hearing adults to see how the onset and type of initial language experience affects the ability to learn a new language.
The results of the study, which will appear in the May 2 issue of the prestigious journal Nature, show that deaf and hearing adults who experience language in early life perform similarly well in learning a new language later in life - whereas deaf adults who had little language experience in early life showed low levels of performance in a later learned language. These findings are not affected by whether the early language or the later language was signed or spoken.
"The timing of our initial language experience during our development - whatever the form of those experiences - strongly influences our capacity to learn language throughout our lives," said Mayberry.
"People have always thought that the human capacity to learn language simply disappears as the brain ages," she said. "Our research shows that when the young brain learns language, it develops a lifelong capacity to learn language. When the young brain does not experience language, this language learning capacity does not fully develop."
The researchers could not work with hearing subjects only, because all hearing babies experience language from birth. It was necessary to also study individuals who were born deaf, because they often do not experience any language until they are enrolled in special programs.
The study was carried out in two parts. First, two groups of deaf adults, one of which was born hearing, were tested for their performance in American Sign Language (ASL), which all had learned at school between the ages of 9 and 15 and had used for more than 20 years. Adults who were born deaf and had little experience of language in early life showed low levels of ASL performance, whereas later deafened adults had high ASL results.
The second part of the study compared three groups of adults who had learned English in school between the ages of 4 and 13 and had used it for more than 12 years. Deaf and hearing adults who had experienced either a signed or a spoken language in early life showed similar high levels of performance in the later-learned language, English. However, deaf adults with little early language experience performed poorly in English.
A total of 58 adults participated in the study, which was carried out in four Canadian and two U.S. cities over the past four years. The researchers are now looking more specifically at how early language experience affects brain development and how it impacts later reading development.