04 Apr 2018
Speech production is a highly skilled behavior that requires rapid and coordinated movements of the orofacial articulators. Speech and language are often confused, but there is a distinction between the two. Speech is the verbal expression of language and includes articulation, which is the way sounds and words are formed. On the contrary, language is much broader and refers to the entire system of expressing and receiving information in a way that's meaningful. It is understanding and being understood through communication in the form of verbal, non-verbal, and written. As a matter of fact, speech and language acquisition are intimately connected and have traditionally worked in parallel.
In order to interpret the meaning of the spoken sounds, a coding mechanism is provided with set of rules for the listener and it is not straightforward. Speech production is a complicated process that undergoes several stages. The first stage is known as premotor process which means that the thought process is generally lumped together. The new ideas are converted into linguist structure that requires selections of suitable words and phrases. The brain functions by assembles the sounds that are needed to make each of the words. Next is the second stage which is called as the mechanical process. During this process, the brain sends signals to make those sounds.
The development of language and brain support some relation to each other as language develops with the increase of the brain cell connections. Although the specific language learned is completely determined by the environment, we cannot deny that the capacity to acquire language is biologically determined as well. Thus it is reasonable to assume there are some language function exists in the brain which enables language acquired is parallel with the development of human brain as a child grows. Thus, the critical period for language learning is regarded to be the biologically determined period in which the brain keeps its plasticity for acquisition of any language. Under normal situations, a child is introduced to language essentially at the moment of birth. The first 3 years of life is the most intensive period for acquiring speech and language skills when the brain is developing and maturing. Brain development during childhood allows the brain to be shaped and the child is given an ability to grasp complex patterns, including those underlie speech. These skills develop best in a world that is rich with sounds, sights, and consistent exposure to the speech and language of others. As a child grows, they start to classify the speech sounds that form the words of their language. Plasticity for speech perception changes during childhood and adolescence, resulting in critical period for second-language acquisition. Once the natural critical period has passed without exposure to language, it will be more difficult to learn.
The phenomenon of lateralization is extremely strong in humans. The brain of human beings consists of a left hemisphere and a right hemisphere, which different functions develop slowly in each parts of the brain as a child grows older. In the vast majority language areas are concentrated in the left hemisphere. The two major areas of human brain that are responsible for language are called Broca area and Wernicke area. Broca’s area is located in the left hemisphere and it is associated with speech production such as putting together sentences and using proper syntax. It is next to the area that controls facial muscle, jaw, tongue and throat function movement. If the area that controls movement is corrupted in any fashion, a person will experience difficulty in producing the actual sounds of speech. Our ability to articulate ideas, as well as use words accurately in spoken and written language, has been attributed to this crucial area. On the contrary, Wernicke's area is the critical language area in the posterior superior temporal lobe that connects to Broca’s area via a neural pathway. Wernicke’s area is thought to be partially responsible for language processing or comprehension such as untangling and analyzing sentences, whether it is written or spoken. Other areas in brain that involved in language are those that surround the Sylvian fissure, a cleavage line that separates the portions of brain which are exclusively human. Many in the scientific community have posited that both speech and language are lateralize, which means, we use only one side of our brains for speech, which involves listening and speaking; and language, which involves constructing and understanding sentences. However, the conclusions pertaining to speech generally stem from studies that rely on indirect measurements of brain activity, raising questions about characterizing speech as lateralized. According to reseachers at New York University, speech is in fact "bilateralized". In simple words, the brain areas in both hemispheres are harnessed in making words. The specific areas are the bilateral inferior frontal and the inferior parietal, superior temporal, pre-motor and somatosensory cortices.
Though both hemispheres of the brain are equal at birth, the function of language gradually settles in the dominant left hemisphere of the brain after biological maturation or the critical period which is proposed by Lenneberg (1967). Lenneberg concludes that the development of language is the result of brain maturation, which it holds that primary language acquisition must occur during critical period with the establishment of cerebral lateralization of function. A strong implication of this hypothesis is that the processes involved in any language acquisition which takes place after the age of puberty will be qualitatively different from those involved in first language acquisition. A commonly drawn, though not absolutely necessary, corollary of the CPH is that any language learning which occurs after the age of puberty will be slower and less successful than normal first language learning. There is a research obtained from people who suffered from brain damage also proved the evidence for the lateralization of brains. A brain is said to lose its plasticity after the lateralization. Some case studies are reported on impaired brains before and after the critical periods. Adults who suffered brain damage in the left hemisphere fail to recover their language if they do not recover within five months, whereas children show an ability to recover fully if they were very young at the period of damage. It is said that even total removal of the left hemisphere did not remove children’s ability to reacquire a language. Consequently Lenneberg’s hypothesis is based on this period of lateralization, which language acquisition relied on the plasticity of both hemispheres and that hemispheric specialization was complete at puberty. If the language acquisition had not occurred by the time a child reached puberty, full mastery would never be attained.
Knudsen (1999) distinguished a "sensitive" from a "critical" period, arguing that during a sensitive period, neuronal connections are particularly susceptible to environmental input, but later experience continues to influence neural development. In contrast, during a critical period, appropriate experience must occur to produce the neural connections necessary for normal function, and the resulting patterns are irreversible. During critical period, the neural system "awaits specific information...to continue to develop normally"(Knudsen, 1999). Thus, in both sensitive and critical periods, individuals are highly responsive to experience, but sensitive periods are ones in which later experience can also affect the individual, whereas during critical periods, experience is required for learning to occur and learning produces durable effects (Knudsen, 2004). Although phonetic learning can be affected by experience past childhood, phonetic learning exhibits the two principles cited by Knudsen for a critical period. According to Knudsen, a lack of exposure early in development to natural language, speech or sign results in the lack of normal language, and early experience with a particular language has indelible effects on speech perception. Phonetic perception might therefore be thought of as exhibiting a critical period in development.
There are three stages of speech development. The first stage is known as social speech or external speech. Luria (1992) points out that “In no way is this speech related to intellect or thinking.” In this stage, speech is used to express simple thoughts and emotions such as laughter, crying and shouting. A child tends to uses speech to control the behaviour of others. For example: I want ball. The second stage is egocentric speech which is typically the type of speech found in a child of age three to seven. According to Lefrancois (1994), he states that this stage ”serves as a bridge between the primitive and highly public social speech of the first stage and the more sophisticated and highly private inner speech of the third stage.” In this stage, the children often talk to themselves, regardless of someone listening to them. They are able to express and speak what they are doing as they do it, while voicing out loud in an attempt to guide their own behaviour. The final stage of speech development is known as inner speech, used by older children and adults. It is an inner, soundless speech that includes communication rehearsal, rehashing conversations and imagined interactions with media personalities. This stage of speech allows us to direct our thinking and behaviour.
Speech and language disorders
Speech production is a complex process, involving a networked system of brain areas that each contribute in unique ways. Difficulty with speech can be the result of problems with the brain or nerves that control the facial muscles, larynx, and vocal cords necessary for speech. Likewise, muscular diseases and conditions that affect the jaws, teeth, and mouth can impair speech. Some conditions that affect speech are present at birth, such as inborn muscular conditions and congenital anatomical abnormalities; while others are the result of metabolic diseases, infections, tumors, or injury. Abnormalities of the vocal cords such as inflammation, polyps, cysts, and tumors can affect the pitch and quality of the voice.
A number of different types of speech impediments can occur, and many terms have been used to qualify speech disorders. Aphasia is the loss of the ability to understand language, whether spoken or written, and occurs due to disturbances in the areas of the brain that are used in language processing. Benson (1967) has classified aphasia into two different types, which are Broca's aphasia and Wernicke's aphasia (Jeanne, 1998).
Broca's aphasia is also referred to as non-fluent or expressive aphasia. It is characterized by the loss of the ability to produce language. If someone has Broca's aphasia, they can still understand spoken language and read language, but they have a hard time speaking and writing due to movement problems. Typically they will talk using short phrases that make sense but they have extreme difficulty in moving their mouths and getting the sounds to come out. They also often omit words in order to shorten what they have to say. People who suffer from non-fluent aphasia understand the speech but because the can't get the words they want to say out they often get irritated and frustrated. People who suffer from Broca's aphasia also have right-sided paralysis or weakness that extends to their extremities.
In contrast, Wernicke's aphasia is characterized by superficially fluent, grammatical speech but an inability to use or understand more than the most basic nouns and verbs. Corruption of Wernicke's area causes an individual to make meaningless sounds. Basically they can make do speech sounds, but they do not make any sense. In other words, damage to the the side portion of the brain may cause fluent aphasia. In most cases this kind of damage occurs in the left side, although it the damage can be caused from the right side also. People that have this left side damage or temporal lobe damage often talk in long sentences that basically are meaningless, they often add unwarranted words, and can even make up words. Therefore it is often extremely difficult to understand what a person with this disturbance is trying to tell you. The people that are affected by this disorder are usually unaware that they have it and usually have no weakness in their movements because their problem is not near that area of the brain.
There are many other types of aphasia or speech and language disturbances and each form results from damage to different areas of the brain. Therefore it is hard for many neuroscientists to actually explain in words or pinpoint what area that truly conducts speech.
Jeanne, K. G. (1998). The fluency dimension in aphasia. Retrieved March 11, 2014, from http://aphasiology.pitt.edu/archive/00001131/01/27-13.pdf
Knudsen, E. I. (1999). Early experience and critical periods. In M. J. Zigmond (Ed.), Fundamental Neuroscience (pp. 637–654). San Diego, CA: Academic Press.
Knudsen, E. I. (2004). Sensitive periods in the development of the brain and behavior. Journal of Cognitive Neuroscience. Retrieved March 13, 2014, from http://www.mitpressjournals.org/doi/abs/10.1162/0898929042304796?journalCode=j ocn
Lenneberg. E.H. (1967). Biological foundations of language. Retrieved March 10, 2014, from http://www.ling.fju.edu.tw/biolinguistic/data/course/biological_foundations.htm
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