What evidence is there that the ability to learn a language natively may decline after the first few years of life?

Mehler (1998) and Christophe and Morton (1998) also investigated infant's later loss of ability to discriminate between other languages. The infants were two months old with ambient languages of American English or British English. The infants heard French then Russian or French then Japanese. The results showed that the infants could not distinguish between the languages. This indicates that language properties that are not useful for the ambient language are discarded and infants lose prosodic properties and begin listening to segmental information. However Guasti observes that there is no experimental evidence from the same infants at four days and two months to confirm this.

When infants reach about one month old they begin to discriminate between consonant contrasts. Werker and Tees (1984) used a 'head turn procedure' to look at the discrimination of retroflex and pharyngeal found in Hindi and Salish but not English. For example, when a new stimulus is heard over another stimulus, the infants will turn their heads in the direction of the new stimulus. English and Hindi children were able to differentiate between the two sounds when they were six months old but the English children had lost the ability to discriminate between the sounds by the time they reached twelve months of age. The Hindi children however could still recognise the difference between the two sounds. The English children lost the ability because it was not important for the ambient language.

It has been suggested by Eric Lenneberg (1967) that the acquisition of a language is an innate process determined by biological factors, which is subject to a critical period. The Critical Period Hypothesis claims that if no language is learned from two years of age and before puberty, then it will never be learned in a normal and fully functional way. Lenneberg believed this to be as a result of lateralisation (a process by which two sides of the brain develop specialised functions) after which the brain loses plasticity. Lenneberg claimed that lateralisation of the language faculty is normally completed by puberty thus making post-adolescent language acquisition difficult. There are some cases which help to support this theory.

Genie was twenty months old and about to begin discovering language when doctors hinted that she was slightly slow and may be partially retarded. Her father took this to the extreme and viewed her as fully retarded so he decided to protect her by isolating and ill-treating her. Genie was discovered when she was thirteen years and seven months old. She had been confined in a small bedroom for over a decade without any human interaction or visual, tactile or auditory stimulation. She was found tied to a potty wearing a nappy and she had only ever been fed infant food. She was made to sleep in a sleeping bag, which restrained her arms and then placed in an over-sized crib with a cover made of metal screening. Genie was therefore unable to walk, chew, bite or swallow. When Genie did start to walk, she had a strange bunny-like style: she held her hands up in front of her like paws and moved in a halting way. She spat and sniffed constantly, was not toilet-trained and could not focus her eyes beyond 12 feet. Genie weighed just 59 pounds and was only 54 inches tall. She possessed very little language skills although she could babble like a baby and make high-pitched squeaks, and she understood a few words. The only words she could pronounce were 'no more' and 'stop it'. It was discovered her father had beat her every time she tried to vocalise, even barking and growling at her to keep her quiet. It was hard to know if her inability to talk was a result of living so long without interacting with other humans, being in an impoverished environment with little sensory stimulation, or because she had been abused.

Genie was rescued and put in a Children's Hospital. Her mental and physical development began almost immediately. By the third day in the hospital, Genie began helping dress herself, using the toilet voluntarily and moving more smoothly. Her mental growth increased by a year for each year after discovery, and four years later Genie had a six to seven year old ability in drawing, a logical sequencing of an eight or nine year old level, a spatial awareness of a twelve year old and a figure ground test ability of a ten to eleven year old. However her auditory short-term memory was that of a three year old. Genie was hungry to learn words, pointing at things until people would give her a word for them. Within several months she had a vocabulary of over one hundred words that she understood, though she was still very silent. These words included colour, number, emotional state, superordinates, hyponyms and distinctions within categories. However as her sentences increased in length they still lacked functional grammatical markings. Even after eight years Genie still failed to use much grammatical morphology and could not grasp word order, although she did acquire category information, subcategorisation and different types of wh-question.

During tests to measure what parts of Genie's brain were active whilst completing different tasks, scientists discovered the left side of Genie's brain was almost inactive. Some scientists thought this explained her inability to learn language. Whether this was correct or not, it raised the question: Why was her brain activity so lopsided? Does the left brain develop in those critical early years of life when Genie was so isolated? Does the left brain need to receive stimulation and hear language to develop? Whether Genie's lack of language ability is actually due to being underexposed to language or due to her awful upbringing is unclear.

Kaspar Hauser gives more support for the critical learning period. He was isolated from the age of three until he was sixteen in a small cell in which bread and insufficient water was left for him whilst he slept on a bed of straw. When he was discovered he behaved like a small child and could only say one phrase 'I want to be a rider like my father'. When Kaspar was released from captivity, he barely had the use of his fingers and walked like a toddler. However, Kaspar did learn to talk, read and write. Within months of discovery he could draw exceptionally well, displayed remarkable ability in memory, reasoning capacity, horsemanship and mathematics. Nevertheless, the development of his brain was profoundly affected. Kaspar's linguistic development failed to incorporate grammatical morphology such as participles, conjunctions, adverbs, pronouns and verb infinitives and he spoke of himself in third person. However, Kaspar did learn words easily and was able to participte in 'philosophical and intellectual discussions'.

The case of Chelsea is another example which helps provide evidence for the Critical Period Hypothesis. Chelsea had a hearing impairment but it was not diagnosed until she was in her thirties. After hearing aids had been fitted, she began to gain lexical knowledge and she scored above the twelfth grade level on a Word Association Test, but her sentences lacked grammatical properties. I.e. 'Orange bill car in'. This inability to master grammatical morphology, which is due to the long delay before hearing language, supports the critical learning period theory.

Isabelle is another example which helps support the Critical Learning Hypothesis. Isabelle was six and a half when she was discovered. She had been in contact with her mother - a deaf mute, but her grandfather had isolated them, although she had not been mistreated. Isabelle had learnt to communicate with her mother through a series of gestures but she was still mentally and physically retarded. Doctors and psychologists used a systematic and skilful training programme to help Isabelle acquire normal language and intelligence abilities. She undertook the usual stages of learning characteristics of the years one to six within two years. In a little over two months after her first vocalisation she was putting sentences together. Nine months later she could identify words and sentences on the printed page, could write well, could add to ten and could retell a story after hearing it. Seven months beyond this point she had a vocabulary of 1500-2000 words and was asking complicated questions. Isabelle's case seems to challenge the idea that the first critical period ends at two years of age, unless due to her exposure to sign language she underwent the kind of brain development that is hypothesised to occur during that stage.

Gemma Wilkinson