Arcuate Fasciculus

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Language is a means of transmitting and processing information, organizing sensory perceptions, and expressing thoughts, feelings, and intentions. The content of language encompasses the past, present, and future. The development of language does not necessarily require speech and audition: deaf-mutes learn to communicate with sign language. Language is most easily acquired in childhood. Linguistic messages are transmitted and received through speaking and hearing, writing and reading, or (in the case of sign language) the production and interpretation of gestures. The cerebral language areas are located in the left hemisphere in over 90% of right-handers and in 60% of left-handers; the remaining individuals have bihemispheric or (in 1-2%) exclusively right-hemispheric dominance for language. The left (dominant) hemisphere is responsible for the cognitive processing of language, while the right (nondominant) hemisphere produces and recognizes the emotional components of language (prosody = emphasis, rhythm, melody). Language is subserved by subcortical nuclei as well (left thalamus, left caudate nucleus, associated fiber pathways). Language function depends on the well-coordinated activity of an extensive neural network in the left hemisphere. It is simplistic to suppose that language is understood and produced by means of a unidirectional flow of information through a chain of independently operating brain areas linked together in series. Rather, it has been shown that any particular linguistic function (such as reading, hearing, or speaking) relies on the simultaneous activation of multiple, disparate cortical areas. Yet the simplified model of language outlined below (proposed by Wernicke and further elaborated by Geschwind) usually suffices for the purposes of clinical diagnosis.

Hearing and speaking. Acoustic signals are transduced in the inner ear into neural impulses in the cochlear nerve, which ascend through the auditory pathway and its relay stations to the primary and secondary auditory cortex (p. 100). From here, the information is sent to Wernicke's area (the "posterior language area"), consisting of Wernicke's area proper, in the superior temporal gyrus (Brodmann area 22), as well as the angular and supramarginal gyri (areas 39, 40). The angular gyrus processes auditory, visual, and tactile information, while Wernicke's area proper is the center for the understanding of language. It is from here that the arcuate fasciculus arises, the fiber tract that conveys linguistic information onward to Broca's area (areas 44 and 45; the "anterior language area"). Grammatical structures and articulation programs are represented in Broca's area, which sends its output to the motor cortex (speech, p. 130). Spoken language is regulated by an auditory feedback circuit in which the utterer hears his or her own words and the cortical language areas modulate the speech output accordingly.

Reading and writing. The visual pathway conveys visual information to the primary and secondary visual cortex (p. 80), which, in turn, project to the angular gyrus and Wernicke's area, in which visually acquired words are understood, perhaps after a prior "conversion" to phonetic form. Wernicke's area then projects via the arcuate fasciculus to Broca's area, as discussed above; Broca's area sends its output to the motor cortex (for speech or, perhaps, to the motor hand area for writing). This pathway enables the recognition and comprehension of written language, as well as reading out loud. Examination. The clinical examination of language includes spontaneous speech, naming of objects, speech comprehension, speech repetition, reading, and writing. The detailed assessment of aphasia requires the use of test instruments such as the Aachen aphasia test, perhaps in collaboration with neuropsychologists and speech therapists. Disturbances of speech may be classified as fluent or nonfluent. Examples of the former are paragrammatism (faulty sentence structure), meaningless phrases, circumlocution, semantic paraphasia (contextual substitution, e. g., "leg" for "arm"), phonemic para-phasia (substitution of one letter for another, e. g., "tan" for "can"), neologisms (nonexistent words), and fluent gibberish (jargon). Examples of the latter are agrammatism (word chains without grammatical structure), echolalia (repetition of heard words), and automatism (repeating the same word many times). Prosody and dysarthria (if present; p. 130) are evaluated during spontaneous speech. Anomia is the inability to name objects. Patients with aphemia can read, write, and understand spoken language but cannot speak.

Precentral gyrus

Arcuate fasciculus

Arcuate fasciculus

Broca's area

Arcuate Fasciculus

Wernicke's area

Auditory cortex (areas 41, 42)

Hearing spoken language

Angular gyrus

Wernicke's area

Auditory cortex (areas 41, 42)

Hearing spoken language

Auditory Cortex
Primary visual cortex

Secondary visual cortex

Reading written language

Broca's area cu m O cu

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