morphosyntactic

Third, the results of a number of studies of morphosyntactic processing showed increased activation of the language areas of the left hemisphere, notably Wernicke’s
area (for grammatical processing) and Broca’s area (for formulating grammatically ordered speech output), as well as the parietal lobes. Likewise, the results of studies involving attempts to isolate semantic processing from syntactic processing showed distinct neuroanatomical correlates for processing complex syntax; these correlates correspond to the inferior left frontal lobe in Broca’s area (Bookheimer, 2002). This region appears to be specialized for not only processing the morphosyntactic elements of language, but also selectively attending to syntax, such as examining whether a sentence uses a “legal” syntactic structure even when the sentence is
devoid of meaning (e.g., “Twas brillig, and the slithy toves . . .”; Friederici, Opitz, &
von Cramon, 2000).

Nonetheless, researchers heartily disagree as to whether morphosyntactic processing should be represented as the function of a single domain-specific module, or by connectionist models that emphasize interactivity of various regions of the brain. Grammatical production and comprehension requires a person to combine
fixed semantic representations into novel and complex representations of sentences; it also involves nonlinguistic symbolic and conceptual thought, as well as planning and reasoning (Glezerman & Balkoski, 1999). When individuals engage in complex
linguistic tasks, left-hemisphere frontal, temporal, and parietal regions are activated, which shows an interaction of executive, semantic, and morphosyntactic processing (Bookheimer, 2002). In light of such evidence, morphosyntactic processing might best be conceived as a complex cognitive ability served by a variety of separate and
specialized cortical areas transcending the right and left hemispheres. In evolutionary terms, a primitive human grammar may have once resided in a distinct language
area of the brain (e.g., Broca’s area). However, the higher-level complex grammar in modern-day language requires integration of the traditional language areas of the brain with other cognitive systems via complex interconnections of the parietal,
temporal, and frontal lobes (Brennan & Pylkkänen, 2012).

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Phonology

Phonology
Processing speech sounds is qualitatively and quantitatively different from processing nonspeech sounds because speech comprises a series of overlapping, rapidly changing, and rapidly produced phonetic segments (Golumbic, Poeppel, & Schroeder,
2012). Whereas, the capacity of the human brain to process sequences of nonspeech sounds is fairly limited (about 7–9 units/second), speech processing occurs
at much higher rates (50–60 units/second; Lieberman, 1991)

phonetic module

Some experts contend that the human brain has evolved a specialized processor, sometimes called
the phonetic module, designed specifically for processing the phonetic segments
of speech (Liberman & Mattingly, 2014). Experts view this specialized processor as a “biologically coherent system, specialized from top to bottom” to process the phonetic segments of speech (Liberman, 2000, p. 115).

Experts view this specialized processor
as a “biologically coherent system, specialized from top to bottom” to process the phonetic segments of speech (Liberman, 2000, p. 115). The phonetic segments of spoken language are channeled through the human ears along the auditory pathway that culminates in the primary and secondary auditory cortices of the temporal
lobe. Rapid analysis of the temporal characteristics of the speech sounds occurs in the auditory centers of the left temporal lobe, whereas the spectral characteristics of speech sounds are processed in the right temporal lobe. Therefore, both hemispheres seem to be involved in speech–sound processing, although the auditory re-
gions of the left temporal lobe appear to be critical locations for phonetic analyses of speech sounds (Frackowiak et al., 2004).

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Once speech sounds are phonetically analyzed, they must be processed as
linguistic units, or phonemes

phonological processing

This level of processing, which occurs in Broca’s
area, is termed phonological processing; it involves analyzing phonological seg-
ments and working memory

neuroanatomical model

Neuroimaging data confirm historical neuroanatomical
models in which phonological processing and speech production are located at the site of Broca’s area in the motor cortex of the left hemisphere. Nevertheless, Broca’s area does not work alone to process and produce speech.

Heschl’s gyrus, Wernicke’s area, and Broca’s area of the left hemisphere are connected by a series
of anatomical pathways, even though they are anatomically remote, as shown in Figure 3.8 (Frackowiak et al., 2004). These interconnections support the interactions of processing mechanisms involved with auditory processing (Heschl’s gyrus),
language comprehension (Wernicke’s area), and phonological processing (Broca’s
area).

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Recall from previous sections that Broca’s area is also the site of sensorimotor encoding of phonological (speech) output, with efferent pathways to organize the controlled, voluntary production of speech sounds. The shared neurophysiology for both phonological processing and phonological production suggests that
the motoric production of speech may play a role in phonological development
(Bookheimer, 2002).

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Pragmatics

Pragmatics
The pragmatics of language involves using language as a social tool. It concerns understanding the rules of communication, which include following conventions related to the quantity, quality, manner, and relevance of language during communication. Although the aspects of language discussed thus far involve a significant investment of the traditional language areas of the brain (e.g., Heschl’s gyrus,
Wernicke’s area, Broca’s area), pragmatic ability draws primarily on frontal lobebfunctions. In other words, an individual who sustains damage to the language areas of the brain that results in significant impairment of semantic, phonological, and morphosyntactic abilities may have fully intact pragmatic skills. Conversely, an
individual with frontal lobe damage may have intact semantic, phonological, and morphosyntactic abilities, yet use language in odd and idiosyncratic ways.

The frontal lobe

As discussed previously, one major function of the frontal lobe is to control human executive functions, including reasoning, problem solving, planning, hypothesizing, social awareness, and rationalizing. These functions involve the organized, goal-directed, and controlled execution of critical human behaviors. Pragmatic abilities involve the organized, goal-directed, and controlled use of language as a means
for communication with other people. Thus, when these more general executive functions are impaired, the social use of language is often undermined.

neurophysiological

The results of brain-imaging studies indicate that many human executive functions involve not only the frontal lobe, but also other neurophysiological functions
of the brain. For example, consider the case of willful attention

Willful attention

Willful attention is
what people use to maintain attention to a given task when competing stimuli are present (Frackowiak et al., 2004). Your attention to reading this chapter likely in- volves some degree of willful attention if competing thoughts (e.g., thinking about an upcoming exam) or events in the environment (e.g., friends talking, music play-
ing) exist

Both parietal and frontal lobe regions are involved in willful attention.
Together, they impose a hierarchy of control over the competing forces for attention in that the parietal lobe is involved with processing incoming stimuli, whereas then frontal lobe forces attention to the particular stimulus selected for attention (e.g.,
the words you are reading; Frackowiak et al., 2004).

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Consider an individual whose
frontal lobe functions are compromised, perhaps as a result of frontal lobe injury. During communication with another person (one competing force for attention), he or she may be distracted by other competing forces for attention (e.g., noises in the
environment), which thus degrades his or her ability to sustain the communication topic. Therefore, the pragmatic aspects of language are compromised

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What are Neurophysiological and
Neuroanatomical Sensitive Periods?

What are Neurophysiological and
Neuroanatomical Sensitive Periods?
Thus far in this chapter, we have presented the brain as if it were a static neuroanatomical structure. This representation is far from the truth. As a human develops prenatally and postnatally, the brain undergoes significant changes as a result of experience. In short, our experiences change our brain over time. In this section, we consider the brain as a dynamic organ that changes during growth,
dealing specifically with the concept of neurophysiological and neuroanatomical sensitive periods, with a particular focus on how these periods affect the capacity for language.

Sensitive Periods Defined

Sensitive Periods Defined
As applied to the development of the human brain, a sensitive period is a time frame of development during which a particular aspect of neuroanatomy or neurophysiology underlying a given sensory or motoric capacity undergoes growth or change. For instance, the results of a classic study showed that depriving kittens
of visual input during the first 6 weeks of life resulted in permanent blindness, indicating this developmental time frame is a critical window of opportunity for visual development in kittens (Hubel & Wiesel, 1970).

In a human analog, studies of birth defects in children born to pregnant women exposed to radiation in Nagasaki
and Hiroshima during World War II showed brain damage (i.e., mental retardation, microcephaly) to be most serious when radiation exposure occurred between 56 and 105 days postovulation (Huttenlocher, 2002). This time frame corresponds to
a period of significant prenatal growth in neuron numbers in the forebrain (Schull,
1998). Thus, at least for in utero humans, the period between 56 and 105 days postovulation corresponds to a window of opportunity for supporting the child’s neural development prenatally; it is also a time of significant risk.

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As these examples show, sensitive periods have the following three features:

Sensitive periods correspond to a time of active neuroanatomical and neurophysiological change.

Sensitive periods correspond to a time of active neuroanatomical and neurophysiological change. Other terms used to describe this time include critical period, window of opportunity, critical moment, and sensitive phase (Bruer, 2001). Although the term critical period is prevalent in the literature, it carries the connotation that
changes occurring in a critical period are irreversible and permanent, which is often not the case. For instance, monkeys that experience visual deprivation during a visual critical period can regain nearly normal visual function with intense remediation
(e.g., suturing closed the normal eye; Bruer, 2001). Therefore, many scientists prefer the term sensitive period, which carries the “window of opportunity” connotation, but allows that change is possible beyond the sensitive period (Bruer, 2001).

Sensitive periods are a phase not only of opportunity but also of risk.

Sensitive periods are a phase not only of opportunity but also of risk. Some experts identify critical periods as a phase in which “normal development is most sensitive to abnormal environmental conditions” (Bruer, 2001, p. 9). Studies of sensitive periods are important not only for improving researchers’ fundamental understanding of human brain development, but also for identifying periods during which
the brain is most vulnerable to risks. This knowledge is useful for prevention—for instance, ensuring that women prior to and in the several months following conception ingest adequate levels of folic acid to support the embryo’s neural tube development. Sensitive periods therefore correspond to times in which an individual’s developmental trajectory can be changed for better or for worse.

Sensitive periods have a beginning and an end point, and the length of a period

Sensitive periods have a beginning and an end point, and the length of a period varies for different aspects of neuroanatomy and neurophysiology. In the previous example, the sensitive period for neural tube development in prenatal human embryos is about 32 days; thus, this period is one of significant risk to the developing embryo if neural tube development is compromised in some way, which
occurs with inadequate folic acid consumption by the mother (Huttenlocher, 2002). In contrast, the sensitive period for language acquisition is much longer, perhaps as long as 12 years for the development of grammar.

Neuroanatomical and Neurophysiological
Concepts Related to Sensitive Periods

Neuroanatomical and Neurophysiological
Concepts Related to Sensitive Periods

Synapses provide the means for neurons within the CNS to communicate, and the synaptic connections forged among neurons during development result in the complex neural circuitry that allows information processing in the human brain (Hut-
tenlocher, 2002).

synaptogenesis

Rather, synaptogenesis (the forma-
tion of synaptic connections) is driven by sensory and motoric experiences after birth and occurs most rapidly in the first year of life (Huttenlocher, 2002).

synaptic pruning

At about the end of the first year, the infant’s brain contains approximately twice as many synaptic connections as an adult’s; from this time to adolescence, excess synapses
are pruned, a process called synaptic pruning.

Neural plasticity

Neural plasticity is a term pertaining to the malleability of the CNS, and it relates primarily to the capacity of the sensory and motor systems to organize and reorganize themselves by generating new synaptic connections or by using existing synapses for alternative means.

Consider that infants with significant left-hemisphere
brain damage that destroys the language areas can achieve typical or near-typical
language abilities by recruiting other neural functions to serve the purposes of lan-
guage; neural plasticity accounts for this possibility (Huttenlocher, 2002).

Older children and adults who sustain a similar type of brain damage often cannot achieve normal language in their lifetime, which suggests that brain plasticity varies with
time. Hence, plasticity relates to sensitive periods because the plasticity of the brain for reorganizing itself and for resolving injury or damage to its neurophysiology and neuroanatomy varies during development.

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Plasticity is often categorized into two types: experience-expectant plasticity and experience-dependent plasticity. These two types of plasticity differentiate the effects of the environment on changes in the brain (Lent & Tovar-Moll, 2015).

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Experience-expectant plasticity

Experience-expectant plasticity refers to the ongoing sculpting of brain structures that occur as a result of normal experiences. As the infant develops, multitudes of synapses are present in the brain, expectantly waiting for certain normal experiences to occur for them to organize themselves into functioning circuits. This type of plasticity uses the basic hardware that is provided to sculpt the brain as
experiences amass. This type of plasticity develops “obligatory cortical functions” (Huttenlocher, 2002, p. 176) that organize basic sensorimotor neural systems, such as vision, hearing, and language. Most infants develop these experience-expectant functions because the basic stimuli needed to foster their development are pres-
ent in the typical environment. Once the sensitive period for a given experience expectant brain function has passed, though, environmental experiences no longer readily modify cortical circuits, possibly because few (if any) unspecified synapses remain. Acquisition of language grammar occurs as a function of experience-
expectant plasticity.

experience-expectant plasticity

In contrast with experience-expectant plasticity, experience-dependent plasticity is unique to a given individual; this type of functional brain modification requires highly specific types of experiences for change. This type of plasticity is
what permits humans to “learn from our personal experience, and store information derived from that experience to use in later problem solving” (Bruer & Greenough,
2001, p. 212)

dendritic sprouting

Learning new information (whether it is novel information or infor-
mation that must be relearned after brain injury) requires three mechanisms: the formation of new synaptic connections among neurons (dendritic sprouting), the generation of new neurons, and an increase in synaptic strength (Huttenlocher,
2002).

Unlike experience-expectant plasticity, experience-dependent plasticity is a brain capacity available independent of age because, through time, the human brain retains most of its capacity to learn through experience and to adapt to change

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Sensitive Periods and Language Acquisition

Sensitive Periods and Language Acquisition
You probably have some knowledge of how sensitive periods relate to language acquisition. For instance, if you attempted to learn a new language in high school and found it exceedingly difficult, you may have attributed the difficulty to your being past the “window of opportunity” for learning a new language. Moreover, you are
likely aware of (or have even attended) an immersion preschool program, in which children are exposed to two or more languages (e.g., English and Spanish) simultaneously in an effort to take advantage of sensitive periods for language acquisitio

In this section, we consider the evidence on whether sensitive periods for language acquisition are a scientific reality, that is, whether humans have a relatively brief window of time in which to acquire language, beyond which language cannot be learned. In some respects, identifying sensitive periods for language acquisition
is a scientific challenge because of the ethical impossibility of actively manipulating children’s language-learning environments to study the effects of language deprivation at different points to identify such periods. Nonetheless, some “natural” experiments have occurred that help scientists identify sensitive periods for language
acquisition by the brain

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Linguistic Isolation

Linguistic Isolation
Linguistic isolation occurs when a child develops with little or no exposure to a spoken or sign language. A few cases of “feral children” (children deprived of language exposure as a result of abuse and neglect) provide support for a sensitive
period for language acquisition. The most notable case is that of Genie, an adolescent in California who was discovered by social workers after having been locked in a bedroom for her entire life and presumably beaten for her attempts to vocalize or communicate. Despite substantial language therapy in subsequent years, Genie
never developed age-appropriate grammatical skills. However, the extent to which concomitant cognitive disabilities combined with years of neglect may have affected Genie’s capacity for language cannot be determined; thus, her case provides inconclusive support for sensitive periods for language.

Evidence on sensitive periods for language acquisition is more conclusive in studies of children who are deaf who are not exposed to a language, whether spoken or sign (e.g., American Sign Language [ASL]), until sometime beyond infancy and toddlerhood. Newport and her colleagues examined ASL fluency for three groups of individuals who were deaf: those who learned ASL from
birth, those who learned it between ages 4 and 6 years, and those who learned it after age 12 years. They found that age of ASL learning was associated with ASL fluency: Individuals who acquired ASL at birth exhibited nativelike language fluency, whereas those who acquired it later in life exhibited significant deficits in language ability, particularly in the area of grammar (see Newport,
Bavelier, & Neville, 2001). Such evidence points to the period of birth through early adolescence as a sensitive period for language acquisition. Although language skills can be acquired after this period, many individuals are unlikely to acquire
nativelike fluency.

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There are, regrettably, also situations today in which children are reared in institutionalized environments in which they receive very little linguistic stimulation (van Ijzendoorn, Luijk, & Juffer, 2008). Children most typically left to institutional- ized care are those who are born with a developmental disability that makes care
for them difficult (e.g., blindness, cerebral palsy, Down syndrome), or creates a social stigma, when the parents do not have the financial or emotional resources to care for the child, or when the parents are deceased (as in cases of orphans with AIDS/HIV). Countries with high rates of poverty, and that experience traumatic events are particularly affected. In Haiti, for instance, it is estimated that there
are more than 750,000 orphans since the major earthquake of 2010. Although the provision of institutionalized care for young children, whether in orphanages or foster homes, has long been a reality, the linguistic environments of some of these children may well constitute a type of linguistic isolation. This stems from cultural
patterns of care within institutionalized settings (in which adult–child conversation seldom occurs), but is also due to discontinuity in care, in which a child may receive care from up to 50 different caregivers in a 24-hour period and have little opportunity to form stable attachments (Vorria et al., 2003).

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Second Language Learners

Second Language Learners
One interesting approach to estimating sensitive periods for language acquisition is to compare the language abilities of groups of individuals who learned a second language at different times of life. Flege and colleagues conducted such an investigation (Flege, Yeni-Komshian, & Liu, 1999) by examining two aspects of language
skill among Koreans who varied in their age of arrival (AOA) to the United States: phonology (specifically, the extent to which they exhibited a foreign accent) and grammar (that is, skills in applying syntactic rules). Interestingly, accents seemed to
be governed by a sensitive period, in that later AOAs were associated with stronger foreign accents. On the other hand, this was not the case for syntax. AOA was less strongly associated with English syntactic skill than other variables, such as one’s
use of English and one’s amount of education in the United States. The authors suggest that experience using one’s second language, as well as educational experience are more important to second language acquisition than constraints imposed
by a sensitive period.

Sensitive Periods and Early Intervention

Sensitive Periods and Early Intervention
Children exhibit tremendous growth in their language
abilities in the first several years of development. This
period largely coincides with the explosion of synap-
togenesis within the cerebral cortex, which begins in
the weeks just prior to birth (during the third prenatal
trimester) and then declines around the third birthday.
In this so-called sensitive period of development, chil-
dren exhibit the greatest ease in acquiring language.
As discussed in Chapter 2, children examine the
child-directed speech (CDS) that occurs around them
to develop their lexicon, grammar, and phonology and
to learn how language is used pragmatically as a so-
cial tool within their cultural community. However, after
the third birthday, synapses that were not formed be-
gin to be eliminated through the process of synaptic
pruning. Consequently, children’s ability to acquire
language also declines as their brains become less
plastic.

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What does the notion of sensitive period mean to
the practice of early intervention? Early intervention
is the implementation of practices and programs to
enhance the early development of children experi-
encing risk. This risk may be environmental, such as
being reared in extreme poverty, or it may be devel-
opmental, such as having a profound hearing loss.
Regardless, the sensitive period implies that interven-
tion should be implemented as early as possible so it
coincides with the explosion of synaptogenesis within
the developing brain. Theoretically, early intervention
implemented intensively in the first few years of life will capitalize upon the natural advantages that synapto-
genesis affords and therefore will be more effective

Scientists have tested whether this is indeed the
case when applied to early intervention practices. In
the Bucharest Early Intervention Project (Nelson et al.,
2007), which we discussed in Chapter 1, scientists
tested the hypothesis that early intervention imple-
mented earlier in children’s lives has greater impacts
on children’s language and cognitive abilities as com-
pared to intervention offered later. In this study, 136
abandoned children residing in Romanian orphan-
ages were randomly assigned to either stay in insti-
tutionalized care (68 children) or be moved into foster
homes. For the latter group, the age of placement oc-
curred at different times: from birth to 18 months (14
children), between 18 and 24 months (14 children),
between 24 and 30 months (22 children), or after 30
months (9 children). When the children were 42 and
54 months of age, the scientists assessed their lan-
guage and cognitive abilities and found that children
placed in foster homes earliest received significantly
higher scores than those placed in foster homes later.
In general, placement prior to 2 years of age seemed
to provide the greatest developmental advantage (a
difference of about 10 IQ points), a finding consistent
with what we might expect based on development of
the cerebral cortex. This study provides exceedingly
strong evidence of the importance of early interven-
tion as a means for mitigating early risks to develop-
ment of language and cognition.

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Other studies, including those on the language development of children who must acquire a new language (and lose their first) following a foreign-birth adoption, have also failed to identify a sensitive period for language acquisition, relative
to learning a second language. In fact, these studies have shown that “even by 7 or 8 years of age, plasticity in language areas is still sufficiently high to promote an essentially complete recovery of normal language” (Pallier et al., 2003,
p. 159). Thus, although young children unequivocally exhibit a unique propensity for learning language, and although the capacity of the brain for rapid language acquisition slows with time, a growing number of scientists argue that “the view of a biologically constrained and specialized language acquisition device that is turned
off at puberty is not correct” (Hakuta, 2001, p. 204). See Language Diversity and Differences: International Adoption and Language Acquisition for a discussion of
foreign-birth adoption and sensitive periods.

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Plasticity and Language

Plasticity and Language
Evidence on sensitive periods for language acquisition suggests that researchers must consider both experience-expectant and experience-dependent plasticity to understand the capabilities of the brain for language during the life span.
Whereas experience-expectant plasticity provides the immature brain with capacities well beyond those evident as people age, experience-dependent plasticity provides the human brain—even at advanced ages—with the capacity to grow and adapt not only to new experiences, but also to illness, disease, and injury to the brain. Although some development periods correspond to time
frames in which language learning is easiest (particularly infancy through early adolescence), researchers’ inability to identify a putative end point to the sensitive period for language acquisition likely reflects the experience-dependent abilities of the human brain to adapt and modify itself in response to the environment.

Foreign-birth adoptions in the United States—an es-
timated 7,000 such cases occurred in 2013 (U.S.
Department of State, 2014)—provides an important
avenue for scientists to explore the possibility of iden-
tifying sensitive periods for language acquisition. In a
foreign-birth adoption, a child is adopted from over-
seas, often from an institution. In the United States,
most foreign-birth adoptions are from China, Ethiopia,
Russia, South Korea, and Ukraine (U.S. Department
of State, 2014). In addition to the developmental chal-
lenges children experience during institutionalized
care, in which they may have relatively little contact
with adults and thus few experiences with healthy at-
tachment and language–cognitive stimulation, these
children often come from countries plagued by limited
prenatal care and maternal exposure to infectious dis-
eases (Glennen, 2015). Although the risks these chil-
dren encounter early in life are substantial, the results of
studies of outcomes for foreign adoptees suggest that
many will achieve healthy developmental outcomes in
cognitive and physical achievements (Glennen, 2015)

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In this chapter, we discuss the concept of expe-
rience-expectant brain plasticity. In contrast with ex-
perience-dependent plasticity, experience-expectant
plasticity is the developmental mechanism of the brain
for achieving basic processes, including language,
in relatively short time periods. Experts have failed to
identify a specific end point for the sensitive period for
language acquisition, which would presumably cor-
respond to a loss of experience-expectant plasticity.
Nevertheless, this sensitive period extends from at
least birth to age 5 years, if not beyond, and during
this period the brain exhibits an amazing capacity to
make amends for early delays in language, as shown
by studies of foreign-birth adoptees

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Studies of children adopted from Eastern Eu-
ropean orphanages into homes within the United
States reveal that most of these children exhibit early
and significant lags in language development, cor-
responding to their apparently limited exposure to
language stimulation during their period of institu-
tionalized care (Glennen & Masters, 2002). However,
when these children are followed over time, studies
tend to show that their skills in their second language
eventually become in line with typical non-adopted
children. Glennen (2015) has followed 44 children
who were adopted from Russia, Kazakhstan, Hun-
gary, and Romania into American homes. All had
lived in institutionalized care for at least one year
prior to being adopted. At ages 5 to 7 years, the chil-
dren were given a battery of language assessments;
as a group, the children had language skills in the
average range across multiple measures of vocab-
ulary, syntax, and morphology. What’s particularly
remarkable about these children’s performance on
language assessments in the early primary grades
is that a larger percentage of children than would
be expected scored in the above average range on
these measures. For instance, on a grammar test,
nearly 25% of children scored in the above average
range; normative references would suggest that 16%
of children would score in this range. Among this
group of international adoptees, a large proportion
of children had superior language skills than would
be expected! To understand this phenomenon, it
is important to point out that children who are ad-
opted internationally tend to be adopted into homes
that are quite advantaged. Parents who adopt chil-
dren internationally tend to be financially well off and
highly motivated towards parenting. Consequently,
the language advantages for international adoptees
may, in part, reflect their arrival to highly stimulating
language-learning environments. Nonetheless, data
such as these reveal the experience-expectant plas-
ticity of the brain for acquiring language during

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Language, a complex and distinctly human behavior,
resides in the neuroanatomical and neurophysiologi-
cal architecture of the human brain. Neuroscience is
a branch of science that focuses on the anatomy and
physiology of the nervous system, described respec-
tively as neuroanatomy and neurophysiology. The
human nervous system includes the central nervous
system (comprising the brain and the spinal cord) and
the peripheral nervous system (comprising the cranial
and spinal nerves, which carry information inward to
and outward from the brain and the spinal cord). The
billions of highly specialized cells that compose the
nervous system are neurons. A neuron is functionally
divided into four components: cell body, axon, presyn-
aptic terminal, and dendrites. The cell body is the cen-
ter of the neuron, containing its nucleus. The axon and
the dendrites are extensions from the cell body. The
axon transmits information away from the cell body;
the presynaptic terminals of the axon are the sites at
which the axonal connection of one neuron corre-
sponds with another neuron. Dendrites are the afferent
extensions of a neuron, bringing nerve impulses into
the cell body from the axonal projections of other neu-
rons. The synapse is the site where two neurons meet.
For two neurons to communicate, the nerve impulse
must cross the synapse.

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The brain, which contains more neurons than any
other organ in the human body, consists of two mir-
ror-image hemispheres. Aptly named, the right hemi-
sphere and the left hemisphere are separated by a long
cerebral crevice (or fissure) called the longitudinal
fissure. The corpus callosum is a band of fibers that
connects the two hemispheres, serving as a conduit for
communication between the hemispheres. The brain
is further divided into six lobes: one frontal lobe, one
occipital lobe, two temporal lobes, and two parietal
lobes. Each lobe has functional specializations. The
frontal lobe is the site of complex executive behaviors
(e.g., reasoning, planning, problem solving), and con-
tains in its left hemisphere an important site for speech
production and phonological processing: Broca’s area.
The occipital lobe is the site of visual perception and
processing. The two parietal lobes are the site for not
only perceiving and integrating sensory and percep-
tual information, but also comprehending oral and
written language and performing mathematical calcu-
lations. The two temporal lobes contain sites critical to
auditory processing, as well as language comprehen-
sion; language is lateralized to the left hemisphere in
Wernicke’s area.

.

Many theorists have argued that the brain exhibits
a sensitive period for language acquisition because the
experience-expectant brain plasticity used in language
development is available for a relatively short duration.
In contrast, experience-dependent plasticity is the ability
of the brain to adapt itself to new information with time.
Some evidence—including that attained from studies of
feral children, children who are deaf, and second lan-
guage learners—suggests that birth to early adolescence
is a sensitive period for language acquisition. Neverthe-
less, researchers have not yet been able to identify a
putative end point for this sensitive period, probably be-
cause the experience-dependent plasticity of the brain
endures (more or less) throughout life. Thus, although
infants, toddlers, and young children acquire language
remarkably easily, the capacity to learn language (or re-
learn language following brain damage) is present for
the entire human life span.

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