Children versus adults in second language acquisition

It is an accepted fact that the younger the child, the easier it is for them to learn a second language. Children are able to understand words and hear small sound differences that adults often miss -- making understanding more difficult for adults. For example, Polish students of English have difficulty differentiating between vowels such as "pen" and "pan" while German students must learn to hear a difference between the v in "vest" and the w in "west".
Scientists used to believe that the adult brain could not be retrained later in life to distinguish between these sounds: in other words the brain's plasticity (or ability to change) was set.
Dr Iverson shows that adults can retune their brains to hear these differences again. Scientists now believe that the difficulties are caused by our experience which teaches us to ignore certain sounds so that we are able to give our full attention to the sounds that (in our native language) matter most to understanding a sentence.
Two studies jointly worked on by Dr Paul Iverson and Dr Valerie Hazan, UCL's Department of Phonetics and Linguistics, have examined whether it is possible to retune how the brain processes speech sounds, and hope that their findings will help make language learning easier for adults. In one study, Japanese subjects were retrained to hear the difference between r's and l's (something that Japanese students of English tend to find particularly difficult). The study tested 63 native Japanese subjects in Japan and London, and had them complete a 10-session training course. Before and after training, the subjects were given a number of perceptual tests to evaluate their perception of acoustic cues. Similar tests were carried out in London on Sinhalese (from Sri Lanka) and German speakers who had lived in the UK for more than 20 years.
In the Japanese training study, the subjects improved their recognition of l's and r's by an average of 18%. So, for example, if an individual could identify the difference between r and l 60% of the time, at the end of training they would be able to get this correct 78% of the time - supporting the view that the brain can be returned.
Talking at the UCL workshop, which brings together specialists in language, speech and speech perception, Dr Iverson said: "Adult learning does not appear to become difficult because of a change in neural plasticity. Rather, we now think that learning becomes hard because experience with our first language 'warps' perception. We see things through the lens of our native language and that 'warps' the way we see foreign languages.
"It is very difficult to undo this learning. That is, we change our perception during childhood so that it becomes specialized to hear the speech sounds in our first language. This specialization can conflict with our ability to learn to distinguish sounds in other languages. Through training, we can essentially change our 'perceptual warping' to make second-language learning easier. I hope that this research will lead to new ways of training adults to learn second languages."