Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 2)

Language allows the communication and elaboration of thoughts and experiences by linking them to arbitrary symbols known as words. The neural substrate of language is composed of a distributed network centered in the perisylvian region of the left hemisphere.The posterior pole of this network is located at the temporoparietal junction and includes a region known as Wernickes area. An essential function of Wernickes area is to transform sensory inputs into their lexical representations so that these can establish the distributed associations that give the word its meaning. ...
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Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 2) Chapter 027. Aphasia, Memory Loss, and Other Focal Cerebral Disorders (Part 2) THE LEFT PERISYLVIAN NETWORK FOR LANGUAGE:APHASIAS AND RELATED CONDITIONS Language allows the communication and elaboration of thoughts andexperiences by linking them to arbitrary symbols known as words. The neuralsubstrate of language is composed of a distributed network centered in theperisylvian region of the left hemisphere. The posterior pole of this network is located at the temporoparietaljunction and includes a region known as Wernickes area. An essential function ofWernickes area is to transform sensory inputs into their lexical representations sothat these can establish the distributed associations that give the word its meaning. The anterior pole of the language network is located in the inferior frontalgyrus and includes a region known as Brocas area. An essential function of thisarea is to transform lexical representations into their articulatory sequences so thatthe words can be uttered in the form of spoken language. The sequencing functionof Brocas area also appears to involve the ordering of words into sentences thatcontain a meaning-appropriate syntax (grammar). Wernickes and Brocas areas are interconnected with each other and withadditional perisylvian, temporal, prefrontal, and posterior parietal regions, makingup a neural network subserving the various aspects of language function. Damageto any one of these components or to their interconnections can give rise tolanguage disturbances (aphasia). Aphasia should be diagnosed only when thereare deficits in the formal aspects of language such as naming, word choice,comprehension, spelling, and syntax. Dysarthria and mutism do not, by themselves, lead to a diagnosis ofaphasia. The language network shows a left hemisphere dominance pattern in thevast majority of the population. In ~90% of right handers and 60% of left handers,aphasia occurs only after lesions of the left hemisphere. In some individuals nohemispheric dominance for language can be discerned, and in some others(including a small minority of right handers) there is a right hemispheredominance for language. A language disturbance occurring after a righthemisphere lesion in a right hander is called crossed aphasia. Clinical Examination The clinical examination of language should include the assessment ofnaming, spontaneous speech, comprehension, repetition, reading, and writing. Adeficit of naming (anomia) is the single most common finding in aphasic patients.When asked to name common objects (pencil or wristwatch), the patient may failto come up with the appropriate word, may provide a circumlocutious descriptionof the object (the thing for writing), or may come up with the wrong word(paraphasia). If the patient offers an incorrect but legitimate word (pen forpencil), the naming error is known as a semantic paraphasia; if the wordapproximates the correct answer but is phonetically inaccurate (plentil forpencil), it is known as a phonemic paraphasia. Asking the patient to name bodyparts, geometric shapes, and component parts of objects (lapel of coat, cap of pen)can elicit mild forms of anomia in patients who can otherwise name commonobjects. In most anomias, the patient cannot retrieve the appropriate name whenshown an object but can point to the appropriate object when the name is providedby the examiner. This is known as a one-way (or retrieval-based) naming deficit.A two-way naming deficit exists if the patient can neither provide nor recognizethe correct name, indicating the presence of a language comprehensionimpairment. Spontaneous speech is described as fluent if it maintainsappropriate output volume, phrase length, and melody or as nonfluent if it issparse, halting, and average utterance length is below four words. The examinershould also note if the speech is paraphasic or circumlocutious; if it shows arelative paucity of substantive nouns and action verbs versus function words(prepositions, conjunctions); and if word order, tenses, suffixes, prefixes, plurals,and possessives are appropriate. Comprehension can be tested by assessing thepatients ability to follow conversation, by asking yes-no questions (Can a dogfly?, Does it snow in summer?) or asking the patient to point to appropriateobjects (Where is the source of illumination in this room?). Statements withembedded clauses or passive voice construction (If a tiger is eaten by a lion,which animal stays alive?) help to assess the ability to comprehend complexsyntactic structure. Commands to close or open the eyes, stand up, sit down, or rollover should ...