Anatomy of Language

OVERVIEW

Language processes are lateralized to the left hemisphere. This has been determined through anatomoclinical correlation in patients with language disturbances and associated brain pathology. It has also been demonstrated by electrical stimulation and seizure activity in the left hemisphere, as well as injection of sodium amytal in the left internal carotid, all causing language disturbances and speech arrest. Increased cerebral blood flow to the left hemisphere during language processing also confirms this lateralized brain function. In addition, there are anatomical differences between the left and right hemispheres. The planum temporale, the region on the superior surface of the temporal lobe posterior to Heschl's gyrus and extending to the posterior end of the sylvian fissure, is slightly larger on the left which is consistent with Wernicke's area. y Computerized

tomography (CT) has shown the left occipital lobe to be longer than the right.

Approximately 90 percent of the general population are right-handed. y They prefer using their right hand for most motor activities, especially skillful tasks. Approximately 99 percent of right-handed individuals have language functions in the left hemisphere. There is a tendency for left-handed and ambidextrous individuals to have varying degrees of bilateral representation of language and visuospatial functions. y Left-handers are more likely to become aphasic because of the bilateral representation, but their aphasia tends to be milder and more brief than that of right-handers. [4 Specific cortical areas in the left hemisphere subserve distinct language functions. For example, the production or comprehension of a word requires serial and parallel processing of information through cortical and possibly subcortical circuits (Fig. 6-1 (Figure Not Available) ).

SPEECH COMPREHENSION AND PRODUCTION

Speech comprehension and production are complex processes. Words first reach the peripheral auditory apparatus, which transforms the mechanical input from the tympanic membrane into electrical impulses that travel to the brain stem cochlear nucleus, then to the medial geniculate nucleus of the thalamus via the trapezoid body and lateral lemniscus, finally reaching the primary auditory cortex (Heschl's gyrus) in the superior temporal gyrus. The auditory language content undergoes preliminary decoding in the auditory association cortex, Wernicke's area (Brodmann's area 22) located in the posterior third of the left superior temporal gyrus (see Fig. 6-1 (Figure Not Available) ). The sounds undergo further processing in the heteromodal association cortex, the angular gyrus, to provide the semantics (the meaning) and relate it to other incoming words, other sensory modalities, and past experiences. If the words are to be repeated, the auditory information is transmitted forward to Broca's area without necessarily passing through the angular gyrus. This information travels from Wernicke's area to the motor association cortex, Broca's area (Brodmann's area 44, 45 located in the posterior part of the inferior left frontal covolution), through the arcuate fasciculus, a band of white matter deep to the supramarginal gyrus connecting both language areas. Broca's area initiates a motor plan that is transmitted to the primary

Figure 6-1 (Figure Not Available) Primary language areas of the braifFrom Mayeux R: Disorders of language: The aphasias. In Kandel ER, Schwartz JH (eds): Principles of Neural Science, 3rd ed. New York, Elsevier, 1991.)

motor cortex (Brodmann's area 4) to pronounce the words. The motor cortex, in coordination with the supplementary motor area, basal ganglia, and cerebellum, sends corticobulbar fibers to implement speech sounds. (...Fi.gure..6:2 illustrates the four principal areas involved in comprehension of spoken language.)

READING AND WRITING

Reading depends on visual stimuli (written words) reaching the primary visual cortex (Brodmann's area 17). The visual stimuli are then transmitted from the primary visual cortex, where they are decoded, to the visual association cortex, or unimodal association cortex, to be processed into words. The words are further processed in the heteromodal association cortex, the angular gyrus, for semantic meaning and integration with other sensory modalities and passed experiences. If the words are to be read aloud, then the information is transmitted forward from the visual association cortex to Broca's area through the arcuate fasciculus, without necessarily passing through the angular gyrus. From Broca's area they are spoken using the same circuits as described earlier. A clinically pertinent fact is that visual information from the right visual cortex must travel through the posterior part of the corpus callosum to be further processed and read in the language regions of the left hemisphere.

Writing requires the transfer of language information possibly to the motor association cortex superior to Broca's area and then to motor neurons in the primary motor cortex (Brodmann's area 4), projecting to the arm and hand. Writing to dictation involves the transfer of auditory information from Wernicke's area to the anterior motor areas; copying written material involves transfer of information from visual association cortex to the anterior motor areas for execution. Visuospatial input is also important in the orientation of written language.

Figure 6-2 Artist's rendition of the four areas involved in comprehension of spoken language. R = receptive area (Heschl's gyrus); P area (Broca's area).(From Benson DF, Ardila A: Aphasia: A Clinical Perspective. New York, Oxford University Press, 1996.)

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