Describe and discuss the neural basis of aphasia, as a syndrome associated with localized damage to the cerebral cortex.

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Describe and discuss the neural basis of aphasia, as a syndrome associated with localized damage to the cerebral cortex.

Language and the ability to speak are major tools of communication of human beings therefore disturbance or loss of speech and impairment in comprehension cause severe disadvantage in daily living. Researchers study aphasia for two reasons; firstly, to improve the success of treating aphasia, and secondly, to gain further understanding of the language-brain relationship. The main value of aphasia in neuroscientific studies is that it is a disorder that exclusively affects language ability without virtually any influence on other cognitive modules. Considering this point, aphasiology can help to explain the relationship between brain and language through those neurological mechanisms, which are implied in the language function.

 

Language is one of the most accessible parts of human cognition. The phenomenon that the brain injury or trauma to the left hemisphere leads to an impairment or a loss in the language ability whereas injury on the right hemisphere can lead to a loss of ability to express emotions, thereby  speech becomes flat, lifeless and mechanical, but leaves the main linguistic functions more or less intact, among other observations, led researchers to the conclusion that studying the effects of brain damage on adult language can help to clarify which neurological mechanisms are implied in the language function. One type of language impairments caused by damage to brain areas that are responsible for language  is Aphasia (MedicineNet.com, 2006) that manifests in an impairment or loss of the understanding or transmission of ideas by language in any of its forms; reading, writing, or speaking. Because language functions are lateral to the left hemisphere in 96-99% of right-handed people and 60% of left-handed people; and approximately one half of the remaining left-handed people has mixed dominance, and one half has right hemisphere dominance, left-handed individuals usually develop aphasia after a left hemispheric stroke, but the aphasia may be milder if their have mixed or right hemisphere dominance for language function (Jacobs, 2005). Aphasia may co-occur with other speech disorders such as disturbed language functions in writing or reading (agraphia or alexia) and also can co-occur with a disturbance of the articulation alone (dysarthria). Aphasia can arise suddenly, often as the result of a stroke or head injury, but it may develop slowly in the cases of a brain tumor or degenerative diseases.

Physicians' observations of aphasic symptoms extend back as early as 3500 B.C. (Smith Surgical Papyrus). Later in the Hippocratic Corpus (400 B. C.) numerous cases of loss of speech were described along with distinguish between types of speech loss, such as between aphasia and anarthria, and loss of vocal ability. In the last two centuries Gall (1825) was probably the first to suggest the existence of anatomo-functional relationships between specific cortical areas and different human capacities. Anatomo-clinical studies carried out in Western Europe in the nineteenth century have provided an empirical basis for contemporary idea about the cerebral representation for language (Basso, 2000). Based on clinical experiences well-known models of language processing and its neurological implementations were created by Broca (1861), Wernicke (1874), Lichtheim (1885), and Geschwind (1970). Among the many the Wernicke-Geschwind model of language (1965) has been very useful in order to make a classification of the five common aphasic syndromes, which are the Broca's aphasia, Wernicke's aphasia, Conduction aphasia, Global aphasia and Nominal aphasia.

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According to the Wernicke-Geschwind model of language (represented in Figure 1) the following seven areas of the left hemisphere mediate language-related activities (Pinel, 2003). The primary auditory cortex (1) mediates hearing the spoken word. The primary visual cortex (2) mediates seeing the written word. The mouth and throat area of the primary motor cortex (3) mediates the motor responses of speech. Wernicke’s area (4), an area in the left temporal lobe just posterior to primary auditory cortex, mediates comprehension of spoken language. The left angular gyrus (5), the parietal lobe gyrus located on its border with the temporal lobe, translates the ...

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