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Human children appear to begin communicative interactions at birth. However, they do not come equipped with all the physiological hardware necessary for development of communication. At birth, their nervous system is incomplete and immature. Throughout infancy, childhood, and into adolescence, neural fibers undergo a process of myelination that facilitates more efficient transmission of information. Meanwhile, neural structures grow in size, and synaptic networks between and among them are formed. Sensory receptors such as the ears and eyes become more finely tuned to relevant environmental stimuli. This development permits acquisition of the abilities of sensation, perception, and cognition, which serve as the foundation for language development. The mechanical areas of the human vocal tract such as the lungs, larynx, pharynx, oral cavity, and nasal cavities must also increase in size, and their associated muscles must develop strength, control, and coordination before the vocal tract can be employed for the verbal communication skills that distinguish human speech. As we learn more about communication among the species, we recognize that humans are not unique in the development and use of language communication. By virtue of our developing hardware, however, we do appear to possess a special potential for devising complex language systems and using verbal rules to express our communicative interactions. Even very young children display an ability to create new language structures to convey their needs, observations, and ideas. In tandem with physical development, a child's social experiences give rise to language learning. During the first year, children's experiences are largely self-centered. They are taking great leaps in motor development and becoming aware of their ability to produce movement and sound. They produce vocalizations and progress towards deliberate sound production through processes of babbling, reduplication, and imitation. Infants also begin to develop intentional communication during this period. They respond to a parent's voice, recognize familiar sounds and voices, and produce sounds in response to external stimulation. They use vocal behavior to play and to express pain and emotion. When these communication behaviors are reinforced through positive and consistent interactions with the environment, such as attention from caregivers or delivery of food, the behaviors increase in frequency and variety. During the first year, a developing child learns to use vocal and verbal communication to manipulate the world around him or her. Continuing throughout childhood, language structures, speech, and gestural inventories are acquired so that communication becomes more effective as a tool for life's interactions. The child expands the content of the language repertoire, perfects language skills in terms of an increasing number of forms, and applies these skills to a variety of uses. These abilities are developed in relation to the child's growing awareness of, and interaction with, the external world. In adolescence the adult model of language usage is attained; however, language skills continue to develop during an individual's lifespan. Adults can use language to create new languages, study the origins of language, and influence their children's development of language. In order to optimize language development, timing of physiological events and learning experiences is crucial. Research has revealed evidence of the existence of critical periods, especially in the first two years of life, during which a child's physical structures are optimized to take advantage of social environmental stimuli. If the structures are immature, or should the environmental stimulation be unavailable, language development may be delayed or even disrupted. Underlying developmental language disorders may be due to physical differences at the cellular level, structural anomalies in speech or hearing systems, synaptic inadequacy due to restricted environmental interactions, etc., that occur during these critical periods. Speech-Language Pathologists and Audiologists receive a strong foundation in human language development studies. They use this information to recognize the existence of language development and usage problems, to evaluate and diagnose the source of those problems, and to prescribe and deliver treatment services to ameliorate or compensate for any disorder. These professionals also act as a resource for parents and other professionals who seek an understanding of the process of child language acquisition and as researchers seeking to develop more accurate knowledge concerning the nature of this process.
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