The is something that has been theorised

The acquisition of language in
human beings is something that has been theorised by scholars for centuries,
from Hermogenes
who believed language was a product of convention to Cratylus ZS1 who
believed that language came in to being naturally, language acquisition is a
subject of much debate which still has no conclusive answer to date. Modern
researchers such as Noam Chomsky (1988) have built upon the theories of the
past and proposed a new theory commonly known as the nativist theory of
Language acquisition, which is the theory that human beings have an inbuilt
mechanism of language acquisition built in to their DNA. Chomsky’s nativist
theory is one of the most widely accepted theories regarding language
acquisition and many other researchers have accepted and attempted to
consolidate it. One such theory that has arisen from this research is that of the
‘Critical Period Hypothesis’ of language acquisitionZS2 .
 Pioneered by Penfield and Roberts
(1959), who’s main areas of study were neuroscience and language, they were the
first to introduce the idea of critical periods in language acquisition. A
critical period has been defined as a ‘window of time when an animal must
receive a certain kind of input in order to develop a skill or capacity; the
input will not have the same effect if presented outside that window of time’ (Byrd
& Mintz, 2010: 235). This idea of critical periods was studied extensively
in animals but was then later applied to human beings and in particular to the
acquisition of languages in human beings. The idea was that if during a certain period
of time a child is not exposed to language input they would not be able to acquire
language to normal levels at all in their lifetimeZS3 .
This is
most evident in feral and abused children who during their early lives were
exposed to little or no language input at all; this in most cases would result
in them never being able to adequately use language to ‘normal’ levels despite
efforts to try and teach themZS4 . Critical
Periods in Language acquisition are also supported by research done on brain
injuries in children and research regarding hemispherectomies, the removal ­of
either the left or right hemisphere of a person’s brain. This research was
based on the thought that if a child developed a brain injury within the
critical period for language acquisition or had a hemisphere removed within
this period and as a result lost the ability to use language, they would still
be able to reacquire ‘normal’ levels of language use with rehabilitation
however if either of the two happen after the critical period then regaining
the use of language would be a lot less likelyZS5 .
This evidence will be discussed in more detail later.

Before Critical Periods could be
applied to language, research was being done on critical periods in relation to
animal behaviours and traits which formed the foundation to studies in Language
acquisitionZS6 .
The main pioneers of research of critical periods in animals were Hubel and
Wiesel (1963) who experimented on the sight of kittens. Their experiments
involved keeping one eye of a nine month old kitten sutured shut for a period
of time (one month) and then after this period using a microelectrode they measured
the electrophysiological activity in each eye of the kitten. What they found
was abnormally low levels of neuron activity in the eye that had been sutured
shut and abnormally high levels of neuron activity in the eye that had been
left open. The kitten would permanently lose vision in the eye that was left
closed, in fact even keeping an eye closed for as little as three days in the
first 10-12 weeks after birth would cause the same drastic effects.  However they then repeated the same experiment
in adult cats, suturing one eye closed for a period of a few months, and found
that visual deprivation in adult cats produced no significant effect on their
vision and vision would return to normal levels after the eye was reopened
(Hubel and Wiesel 1964). What was concluded from this experiment is that the
same conditions can produce significantly different effects depending on when
it occurs in life. A kitten that is visually deprived in one eye in the first
10-12 weeks of life will permanently lose vision in that eye whereas an adult
cat even after prolonged periods of deprivation will still regain vision.

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The results of Hubel and Wiesel’s
experiment would strongly suggest that a critical period exists in animals; that there is
a specific time in an animal’sZS7 
life where a certain type of input is required for a particular trait or skill
to be acquired and if an animal does not receive the necessary type of input
within that time it will not have the same effect (Byrd and Mintz, 2010ZS8 ).
This definition is also supported by Saxton (2010) who similarly defined
critical periods as being ‘genetically determined periods of growth during
which the system is especially plastic, in the sense that it is especially
receptive to particular environmental inputs. Once development is complete,
there is no need for the system to be susceptible to key learning experiences
any longer. Hence, one encounters critical periods early in an organism’s life’
(Saxton, 2010: 56). Saxton’s definition like Byrd and Mintz’s speaks of needing
environmental inputs for a particular trait to develop however they differ in
that Saxton specifically mentions that critical periods are genetically
determined which is also mentioned by Lust (2006) who mentions that critical periods
are ‘assumed to be innately programmed’ (Lust, 2006: 93)ZS9 .
The scientific rigour of animal research provides a useful foundation which
should be considered before applying the idea of critical periods to language
acquisition because it shows how development can be seriously affected by the
absence of certain vital experiences during a critical period. The research on
animals therefore provides a direct parallel with critical periods in the
development of human beings and specifically in regards to the acquisition of the
unique human trait of language.

The first researchers to
introduce the theory of critical periods to language were Penfield and Roberts
(1959) who specialised in the neuroscience of language. They theorised that up to the age of nine
children can learn multiple languages with ease compared to when they get older
and the concept that age is heavily linked to neural plasticity and that the
older one gets the more difficulty they have in learning language (Penfield and
Roberts, 1959). ZS10 This was then built upon by Eric Lenneberg
(1967) who compiled a large amount of evidence supporting the theory of
critical periods in language learning. His research involved drawing mainly on
evidence from aphasia caused by brain injuries in children but also on evidence
from feral children and those children who had been neglected and abused
growing up.

Lenneberg’s research revolved
mainly around the concept of lateralisation; the specialisation of each
hemisphere with its own specific tasks and processes, the left hemisphere being
for language (E Clark, 2003). One of the main sources of evidence for this was
research regarding brain injuries in children and adults of different ages and
hemispherectomies. It was found that damage to the Broca and Wernicke areas of
the brain, the two parts of the brain involved in speech production and
comprehension, would cause critical problems in an adultsZS11  ability to use language properly. However
it was found that children were a lot more resilient to this type of brain
damage and were able to regain normal (or near normal) levels of language
(Lenneberg, 1967). However, on rare occasions due to particular health issues
children or adults may need to have an entire hemisphere removed, a
hemispherectomy. Their language ability after this procedure depended largely
on which hemisphere was removed. It was found that the language of those
children who had their left hemispheres still intact were less affected than
those who had them removed (Dennis et al. in Clark, 2009: 364). This is also
reinforced by earlier research by Kohn and Dennis (1974) who tested a sample of
people with only one hemisphere removed ranging in age from fourteen to
twenty-eight on their visual and spatial abilities as well as their language
abilities. They found that those who had their right hemispheres removed
suffered in spatial abilities and analysis due to those traits only being controlled by the
right hemisphere. ZS12 In a following experiment of eight to
twenty eight year olds, four with right hemispheres removed and five with the
left, participants were asked to differentiate the meanings of statements which
varied in syntactic form. They found that some of the syntactic forms were
understood equally by all participants however others were more effectively
interpreted by those with an intact left hemisphere (Dennis and Kohn, 1975).
What was concluded from these tests is that the if a hemispherectomy occurred
at a younger age the more likely a child was to recover. This is due to the fact
that the brains of young children are not yet fully developed and therefore large
parts of the brain have not yet been assigned a task. This allows for those
areas of the brain which are not yet specialised for a specific task to take on
functions which may have otherwise been assigned to the hemisphere of the brain
which had been removed. This is much more likely to happen in younger children and the
older one gets the less likely it is to occur since older children/adult’s
brains have reduced neuroplasticity, however children are still a lot more
likely to recover from aphasia than adults (Clark, 2003)ZS13 . These experiments all suggest that
lateralisation is a phenomenon that occurs as a child grows older and both
sides of the brain have their own specialisations; the left side being the side
more responsible for language. They also suggest that as one gets older the
less likely it is that they will be able to reacquire language after a brain
injury or a hemispherectomy; Lenneberg (1967) even suggests that once a child
reaches puberty they will no longer be capable of adequately learning a first
languageZS14 . All of these evidences ZS15 strongly support the critical period
hypothesis because they all prove that neuroplasticity and therefore the
ability to learn a first language decrease significantly as a child gets older.ZS16  It is noticeable that none of the
researchers are able to give an accurate time frame of what the critical period
for language acquisition actually is in human beings and there is a slight
difference of opinion regarding this.

Arguably the most important proof
of the critical period hypothesis is that of feral and abused children i.e.
those children who were raised with little to no language exposure. The most famous case of an abused child is that of ‘Genie’, a young American
girl who was raised by abusive parents and who received little to no auditory
language stimulation. She was raised in extreme confinement and was beaten when
she attempted to make any sounds. Her father was of the opinion that she was
severely mentally disabled and due to this from the age of twenty months she
was kept isolated from the rest of her family in a separate room and forcibly
bound to a toilet. This isolation and deprivation occurred until she was
discovered at the age of thirteen years and seven months (Curtiss 1977). It was
at this point that observations of genies linguistic capabilities began to be
recorded. The greatest proportion of data collected regarding Genie and her language
capabilities were collected by Susan Curtiss who had contact with Genie for up
to six years and six months (Jones, 1995). After being discovered it was found
that Genie already had understanding of a handful of words but not nearly
enough to form sentences or effectively communicate (Rymer, 1993). Rymer
suggests she was capable of producing two expressions at this stage, ‘stop it’
and ‘no more’. After being discovered Genie was removed from the care of her
parents and started to receive proper attention. The effects of this extra care
and nourishment were immediately noticeable both physically and mentally. The
onset of puberty occurred quickly after proper care and cognitive abilities developed
to the level of a six to eight year old a year after her release from
confinement. However her language use remained extremely deficient especially
in regards to syntax, although her vocabulary did develop fairly quickly and
she developed the ability to use single words in order to communicate. She then
progressed on to using two words to communicate e.g. want milk, then to three
to four words e.g. Genie loves Curtiss (Curtiss, 1977). ‘Her comprehension of
WH-questions, relative clauses, singular-plural distinctions, negatives etc.,
and her production of complex NP’s, sentence conjunctions etc. provide evidence
that there is a steady if modest progress in first-language acquisition’
(Curtiss, 1974: 544). This shows that Genies early language development was
fairly good and improvement occurred at a steady rate however nearer the end of
Curtiss’s research on Genie she found that her development slowed and by the
end of her research she concluded that Genies language use was ‘syntactically
primitive and underdeveloped’ (Curtiss, 1978: 29).

The case of Genie further supports the Critical Period Hypothesis because it
demonstrates how somebody deprived of language up until the age of thirteen,
which many would consider to be after the Critical Period for language
acquisition, is unable to acquire language to normal levels even with dedicated
teaching and proper care. Genie is a perfect example of this because she was
almost completely deprived of any linguistic input until the age of thirteen
and was only then exposed to language in a proper capacity. However even with
exposure to language she was unable to attain a proper grasp of its use and
understanding. It has been argued by some that the reason Genie and other
similar cases of abused children struggle to acquire normal levels of language
use is not because of them having passed the critical period but because of the
mental trauma caused as a result of their abuse (Pinker, 1994). The case of
Genie has also been criticised as not being an entirely reliable source of data
in proof of CPH, one of the main critics being Jones (1995). He argues that the
data was not presented in enough detail as Curtiss recorded most of her data in
standard American English without any information regarding ‘utterance
dynamics, hesitations, duration of utterances or syllable length etc.’ (Jones,
1995: 262). Jones also raised the issue of the small quantity of data collected
which is also an issue that could be argued against CPH as a whole. The lack of
empirical evidence in support of the theory has been cited as one of the main
weaknesses of CPH (Clark, 2003).

Although rare, there are some instances of fully grown adults acquiring a
first language. A young girl from the US known as “Chelsea” who was born deaf,
but wrongly diagnosed by doctors as being clinically retarded or emotionally
disturbed without noticing that she was deaf. Her family however never believed
her to be retarded and raised her as normally as they possibly could. Later in
her life at age thirty one she was referred to a neurologist who fitted her
with hearing aids which allowed her to hear to near-normal levels. After
intensive therapy ‘she scores at a ten-year-old level on intelligence tests,
knows two thousand words, holds a job in a veterinarians office, reads, writes,
communicates and has become social and independent (Pinker, 1994: 292). The
fact that Chelsea was able to acquire such levels of language at such a late
stage in her life would suggest that language can be learned after what most researchers
would consider to be the critical period. Although Chelsea has learned to
communicate and learn a significant amount of vocabulary her syntax is still very
poor (Pinker, 1994). This could suggest that critical periods do not simply
apply to language as a whole rather they apply to syntax only which is a view discussed
further by Clark (2003) who mentions that there may be different critical
periods for different aspects of language. The ‘Genie’ and ‘Chelsea’ case
studies also support this because both of them managed to acquire some level of
vocabulary and ability to communicate after what would generally be considered
the Critical Period for first language acquisition but yet neither of them
managed to acquire normal levels of syntax.   

The Critical Period Hypothesis is a theory of much debate amongst linguists
but given the significant evidence presented it would be hard to deny the
notion of a period of time existing in which if a child is not exposed to enough
language input and exposure they would struggle to later acquire language to
normal levels. The theories foundation is found in research relating to animals
however it is not at all farfetched to suggest that human beings also have
critical periods and specifically critical periods in relation to first language
acquisition. Lenneberg’s research gives supporting evidence through discussions
of lateralisation and hemisphere dominance, however it has been argued by some
such as Witelson & Pallie(1973)  that
lateralisation occurs at age five much earlier than Lenneberg’s proposal that
it occurs at puberty. Although it has been proven that children who were raised
away from the exposure of language e.g. Genie and Chelsea, can still learn how
to communicate and acquire vocabulary which may in some ways suggest the
opposite of CPH however it is noticeable that neither of them acquired syntax
to normal levels. This supports the theory that critical periods are specific
to the acquisition of syntax rather than language as a whole.   

            ZS17