After studying this chapter, the student should be able to:
Describe the representation of concepts and identify the semantic hub(s).
Describe and diagram the speech perception and production systems.
Identify the white matter tracts most relevant to language processing.
Identify the neural system for reading and describe how it changes during reading development.
The classic model of the neural basis of language emerged from the work of 19th-century cognitive neurologists Paul Broca, Carl Wernicke, and Ludwig Lichtheim. Broca described patients with relatively preserved ability to understand speech but profound deficits in producing speech: slow, labored, telegraphic speech, often with speech errors or distortions of speech sounds and omission of function words (eg, articles and prepositions) and inflections (eg, –s for plurals, –ed for past tense). This disorder came to be known as Broca aphasia or expressive aphasia and to be associated with damage to the left inferior frontal gyrus (Broca area) and the underlying white matter. In contrast, Wernicke described patients with profound comprehension deficits but relatively fluent speech production. Although relatively rapid and well-articulated, the speech production in Wernicke aphasia (also called receptive aphasia) tends to have limited meaningful content due to use of a small set of high-frequency words and made-up words (also called “nonwords” or “abstruse neologisms”). Wernicke aphasia was thought to be associated with damage to Wernicke area, the posterior portion of the left superior temporal gyrus.
Lichtheim developed an integrated description of the language system and its instantiation in the cortical regions around the left hemisphere’s Sylvian fissure. This distinction between nonfluent expressive aphasia and fluent receptive aphasia and the role of peri-Sylvian regions in language remained largely unchanged into the late 20th century and remains deeply influential despite being challenged by 3 kinds of observations. First, many individuals with aphasia do not fit the classic aphasia subtypes; rather, they exhibit “mixed” or “unclassifiable” clusters of language deficits. Second, the symptoms are functional/behavioral and may be caused by a variety of very different underlying cognitive and neurologic deficits. For example, the telegraphic speech typical of Broca aphasia can be caused by a deficit in syntax (grammar) and sentence planning, a deficit in word retrieval and selection, or a deficit in articulatory motor planning and execution. Third, focal damage to the Broca area does not tend to produce Broca aphasia, and focal damage to the Wernicke area does not tend to produce Wernicke aphasia. Rather, those syndromes are caused by broader damage that includes the traditionally associated regions as well as other regions.
Partly motivated by these challenges, an alternative has emerged that focuses on the “primary systems”—the cognitive and neural systems that support language processing. These systems are as follows: (1) a distributed semantic system for representation of conceptual knowledge, with particularly important integrative hubs in the anterior temporal lobe and temporoparietal cortex (angular gyrus and posterior superior temporal gyrus) and ...