Development: Linguistic ability and early language exposure

For more than 100 years, the scientific and educational communities have thought that age is critical to the outcome of language learning, but whether the onset and type of language experienced during early life affects the ability to learn language is unknown. Here we show that deaf and hearing individuals exposed to language in infancy perform comparably well in learning a new language later in life, whereas deaf individuals with little language experience in early life perform poorly, regardless of whether the early language was signed or spoken and whether the later language was spoken or signed. These findings show that language-learning ability is determined by the onset of language experience during early brain development, independent of the specific form of the experience. The ability to learn language, whether spoken or signed, declines with age. How the onset and type of the initial language experience contributes to this critical-period phenomenon is unclear. This question cannot be investigated by studying hearing individuals only, because the factors of age and experience are inseparable in these individuals--all hearing babies experience language from birth. But the question can be investigated by studying individuals who were born deaf, because they often do not experience any language until they are enrolled in special programmes. We therefore compared the language-learning capacities of deaf and hearing individuals as a function of early language experience.