Kaplan + Sadock's Synopsis of Psychiatry, 11e

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1.2 Functional Neuroanatomy

frontal lobes. Phonological processing improves if lip reading is allowed, if speech is slowed, or if contextual clues are provided. Second, lexical processing matches the phonological input with recognized words or sounds in the individual’s memory. Lexical processing determines whether a sound is a word. Recent evi- dence has localized lexical processing to the left temporal lobe, where the representations of lexical data are organized accord- ing to semantic category. Third, semantic processing connects the words to their meaning. Persons with an isolated defect in semantic processing may retain the ability to repeat words in the absence of an ability to understand or spontaneously generate speech. Semantic processing activates the middle and superior gyri of the left temporal lobe, whereas the representation of the conceptual content of words is widely distributed in the cortex. Language production proceeds in the opposite direction, from the cortical semantic representations through the left temporal lexical nodes to either the oromotor phonological processing area (for speech) or the graphomotor system (for writing). Each of these areas can be independently or simultaneously damaged by stroke, trauma, infection, or tumor, resulting in a specific type of aphasia. The garbled word salad or illogical utterances of an aphasic patient leave little uncertainty about the diagnosis of left-sided cortical injury, but the right hemisphere contributes a somewhat more subtle, but equally important, affective quality to lan- guage. For example, the phrase “I feel good” may be spoken with an infinite variety of shadings, each of which is understood differently. The perception of prosody and the appreciation of the associated gestures, or “body language,” appear to require an intact right hemisphere. Behavioral neurologists have mapped an entire pathway for prosody association in the right hemi- sphere that mirrors the language pathway of the left hemisphere. Patients with right hemisphere lesions, who have impaired com- prehension or expression of prosody, may find it difficult to function in society despite their intact language skills. Developmental dyslexia is defined as an unexpected diffi- culty with learning in the context of adequate intelligence, moti- vation, and education. Whereas speech consists of the logical combination of 44 basic phonemes of sounds, reading requires a broader set of brain functions and, thus, is more susceptible to disruption. The awareness of specific phonemes develops at about the age of 4 to 6 years and appears to be prerequisite to acquisition of reading skills. Inability to recognize distinct pho- nemes is the best predictor of a reading disability. Functional neuroimaging studies have localized the identification of let- ters to the occipital lobe adjacent to the primary visual cortex. Phonological processing occurs in the inferior frontal lobe, and semantic processing requires the superior and middle gyri of the left temporal lobe. A recent finding of uncertain significance is that phonological processing in men activates only the left infe- rior frontal gyrus, whereas phonological processing in women activates the inferior frontal gyrus bilaterally. Careful analysis of an individual’s particular reading deficits can guide remedial tutoring efforts that can focus on weaknesses and thus attempt to bring the reading skills up to the general level of intelligence and verbal skills. In children, developmental nonverbal learning disorder is pos- tulated to result from right hemisphere dysfunction. Nonverbal learning disorder is characterized by poor fine-motor control in the left hand, deficits in visuoperceptual organization, problems with mathematics, and incomplete or disturbed socialization.

injury. These data indicate that certain regions of cortex may be nec- essary for a specific function, but they do not define the complete set of structures that suffices for a complex task. Anecdotal evidence from surface electrocorticography for the study of epilepsy, for example, sug- gests that a right parietal seizure impulse may shoot immediately to the left frontal lobe and then to the right temporal lobe before spreading locally to the remainder of the parietal lobe. This evidence illustrates the limitations of naively assigning a mental function to a single brain region. Functional neuroimaging studies frequently reveal simultaneous activation of disparate brain regions during the performance of even a simple cognitive task. Nevertheless, particularly in the processing of vision and language, fairly well-defined lobar syndromes have been confirmed. Language The clearest known example of hemispheric lateralization is the localization of language functions to the left hemisphere. Starting with the work of Pierre Broca and Karl Wernicke in the 19 th century, researchers have drawn a detailed map of language comprehension and expression. At least eight types of aphasias in which one or more com- ponents of the language pathway are inured have been defined. Prosody, the emotional and affective components of language, or “body language,” appears to be localized in a mirror set of brain units in the right hemisphere. Because of the major role of verbal and written language in human communication, the neuroanatomical basis of language is the most completely understood association function. Lan- guage disorders, also called aphasias, are readily diagnosed in routine conversation, whereas perceptual disorders may escape notice, except during detailed neuropsychological testing, although these disorders may be caused by injury of an equal volume of cortex. Among the earliest models of cortical local- ization of function were Broca’s 1865 description of a loss of fluent speech caused by a lesion in the left inferior frontal lobe and Wernicke’s 1874 localization of language comprehension to the left superior temporal lobe. Subsequent analyses of patients rendered aphasic by strokes, trauma, or tumors have led to the definition of the entire language association pathway from sen- sory input through the motor output. Language most clearly demonstrates hemispheric localiza- tion of function. In most persons, the hemisphere dominant for language also directs the dominant hand. Ninety percent of the population is right-handed, and 99 percent of right-handers have left hemispheric dominance for language. Of the 10 percent who are left-handers, 67 percent also have left hemispheric language dominance; the other 33 percent have either mixed or right hemi- spheric language dominance. This innate tendency to lateraliza- tion of language in the left hemisphere is highly associated with an asymmetry of the planum temporale, a triangular cortical patch on the superior surface of the temporal lobe that appears to harbor Wernicke’s area. Patients with mixed hemispheric dominance for language lack the expected asymmetry of the planum temporale. That asymmetry has been observed in prenatal brains suggests a genetic determinant. Indeed, the absence of asymmetry runs in families, although both genetic and intrauterine influences prob- ably contribute to the final pattern. Language comprehension is processed at three levels. First, in phonological processing, individual sounds, such as vow- els or consonants, are recognized in the inferior gyrus of the