The Critical Period
Under normal circumstances, a child is introduced to language virtually at the
moment of birth. Adults talk to him and to each other in his presence. Children
do not require explicit language instruction, but they do need exposure to
language in order to develop normally. Children who do not receive linguistic
input during their formative years do not achieve nativelike grammatical competence.
Moreover, behavioral tests and brain imaging studies show that late exposure
to language alters the fundamental organization of the brain for language.
The critical-age hypothesis assumes that language is biologically based and
that the ability to learn a native language develops within a fixed period, from
birth to middle childhood. During this critical period, language acquisition
proceeds easily, swiftly, and without external intervention. After this period,
the acquisition of grammar is difficult and, for most individuals, never fully
achieved. Children deprived of language during this critical period show atypical
patterns of brain lateralization.
The notion of a critical period is true of many species and seems to pertain to
species-specific, biologically triggered behaviors. Ducklings, for example, during
the period from nine to twenty-one hours after hatching, will follow the first
moving object they see, whether or not it looks or waddles like a duck. Such
behavior is not the result of conscious decision, external teaching, or intensive
practice. It unfolds according to what appears to be a maturationally determined
schedule that is universal across the species. Similarly, as discussed in a later section,
certain species of birds develop their bird song during a biologically determined
window of time.
Instances of children reared in environments of extreme social isolation constitute
“experiments in nature” for testing the critical-age hypothesis. The most
Language and Brain Development 23
dramatic cases are those described as “wild” or “feral” children. A celebrated
case, documented in François Truffaut’s film The Wild Child, is that of Victor,
“the wild boy of Aveyron,” who was found in 1798. It was ascertained that he
had been left in the woods when very young and had somehow survived. In
1920 two children, Amala and Kamala, were found in India, supposedly having
been reared by wolves.
Other children have been isolated because of deliberate efforts to keep them
from normal social intercourse. In 1970, a child called Genie in the scientific
reports was discovered. She had been confined to a small room under conditions
of physical restraint and had received only minimal human contact from the age
of eighteen months until nearly fourteen years.
None of these children, regardless of the cause of isolation, was able to speak
or knew any language at the time they were reintroduced into society. This linguistic
inability could simply be caused by the fact that these children received
no linguistic input, showing that language acquisition, though an innate, neurologically
based ability, must be triggered by input from the environment. In the
documented cases of Victor and Genie, however, these children were unable to
acquire grammar even after years of exposure, and despite the ability to learn
many words.
Genie was able to learn a large vocabulary, including colors, shapes, objects,
natural categories, and abstract as well as concrete terms, but her grammatical
skills never fully developed. The UCLA linguist Susan Curtiss, who worked with
Genie for several years, reported that Genie’s utterances were, for the most part,
“the stringing together of content words, often with rich and clear meaning, but
with little grammatical structure.” Many utterances produced by Genie at the
age of fifteen and older, several years after her emergence from isolation, are
like those of two-year-old children, and not unlike utterances of Broca’s aphasia
patients and people with SLI, such as the following:
Man motorcycle have.
Genie full stomach.
Genie bad cold live father house.
Want Curtiss play piano.
Open door key.
Genie’s utterances lacked articles, auxiliary verbs like will or can, the thirdperson
singular agreement marker -s, the past-tense marker -ed, question words
like who, what, and where, and pronouns. She had no ability to form more complex
types of sentences such as questions (e.g., Are you feeling hungry?). Genie
started learning language after the critical period and was therefore never able
to fully acquire the grammatical rules of English.
Tests of lateralization (dichotic listening and ERP experiments) showed that
Genie’s language was lateralized to the right hemisphere. Her test performance
was similar to that found in split-brain and left hemispherectomy patients, yet
Genie was not brain damaged. Curtiss speculates that after the critical period,
the usual language areas functionally atrophy because of inadequate linguistic
stimulation. Genie’s case also demonstrates that language is not the same as communication,
because Genie was a powerful nonverbal communicator, despite her
limited ability to acquire language.
24 INTRODUCTION Brain and Language
Chelsea, another case of linguistic isolation, is a woman whose situation also
supports the critical-age hypothesis. She was born deaf but was wrongly diagnosed
as retarded. When she was thirty-one, her deafness was finally diagnosed,
and she was fitted with hearing aids. For years she has received extensive language
training and therapy and has acquired a large vocabulary. However, like
Genie, Chelsea has not been able to develop a grammar. ERP studies of the
localization of language in Chelsea’s brain have revealed an equal response to
language in both hemispheres. In other words, Chelsea also does not show the
normal asymmetric organization for language.
More than 90 percent of children who are born deaf or become deaf before
they have acquired language are born to hearing parents. These children have
also provided information about the critical age for language acquisition. Because
most of their parents do not know sign language at the time these children are
born, most receive delayed language exposure. Several studies have investigated
the acquisition of American Sign Language (ASL) among deaf signers exposed to
the language at different ages. Early learners who received ASL input from birth
and up to six years of age did much better in the production and comprehension
of complex signs and sign sentences than late learners who were not exposed to
ASL until after the age of twelve, even though all of the subjects in these studies
had used sign for more than twenty years. There was little difference, however,
in vocabulary or knowledge of word order.
Another study compared patterns of lateralization in the brains of adult
native speakers of English, adult native signers, and deaf adults who had not
been exposed to sign language. The nonsigning deaf adults did not show the
same cerebral asymmetries as either the hearing adults or the deaf signers. In
recent years there have been numerous studies of late learners of sign language,
all with similar results.
The cases of Genie and other isolated children, as well as deaf late learners of
ASL, show that children cannot fully acquire language unless they are exposed
to it within the critical period—a biologically determined window of opportunity
during which time the brain is prepared to develop language. Moreover, the
critical period is linked to brain lateralization. The human brain is primed to
develop language in specific areas of the left hemisphere, but the normal process
of brain specialization depends on early and systematic experience with language.
Language acquisition plays a critical role in, and may even be the trigger
for, the realization of normal cerebral lateralization for higher cognitive functions
in general, not just for language.
Beyond the critical period, the human brain seems unable to acquire the
grammatical aspects of language, even with substantial linguistic training or
many years of exposure. However, it is possible to acquire words and various
conversational skills after this point. This evidence suggests that the critical
period holds for the acquisition of grammatical abilities, but not necessarily for
all aspects of language.
The selective acquisition of certain components of language that occurs
beyond the critical period is reminiscent of the selective impairment that occurs
in various language disorders, where specific linguistic abilities are disrupted.
This selectivity in both acquisition and impairment points to a strongly modularized
language faculty. Language is separate from other cognitive systems and
Language and Brain Development