in the infant in terms of hemispheric dominance, we can now consider more precise aspects of language knowledge and more precise aspects of its biological representation in development, e.g., its “localization.”
36Mills et al. 1993, Neville and Mills 1997; Mills, Coffey-Corina and Neville 1997. Behavioral research suggesting early differentiation of these categories (chapter 9) must be coordinated with these findings; also Shafer et al. 1998.
37Molfese et al. 2001; Segalowitz and Berge 1995 provide review.
38Dehaene-Lambertz and Dehaene 1994; Dehaene-Lambertz 2000.
39Different neuronal components were involved in responses to linguistic and non-linguistic stimuli within the left hemisphere.
Table 5.3 Examples of classical aphasia types: dissociations of components of language knowledge.
Clinician: What happened?
Broca’s Aphasia
Berko Gleason and Ratner 1993a, b
Patient: The brain is- see . . . headaches . . . first
Clinican: Your baby?
Patient: Yeah . . . [trIsm nts], uh one year ago . . . s- Chrismonts day . . . died
Wernicke’s Aphasia
Berko Gleason and Ratner 1993a, b
Patient: I got it in the- in the- in the brain and they him him in here and they hit him over here and this one here, I figure the next time they hit this will knock this off [unintelligible].
Apraxia
Dronkers 1996, 160
Patient(attempting to say “cushion”): “Oh, uh, uh, /chookun/uh, uh, uh, /dook/ I know what it’s called. It’s c-u, uh, no, it’s, it’s /chook/chookun/no”
Patients with apraxia of speech do not consistently articulate words correctly, and they struggle for correct pronunciation.
11. Localization
Determination of restricted cortical areas which subserve specific components of language knowledge.
As we have seen, language knowledge involves several components correspond- ing to several levels of representation, which must be integrated (figure 2.1).
Processing of a word or a sentence involves complex orchestration of all of this knowledge in very short time periods (Caplan 1992); this linguistic orchestration must be constantly related to general cognition. The organization of this complex computation in the brain is currently under investigation through several different areas (5.4.1–5.4.5).
5.4.1 Language pathologies
Various components of language knowledge are revealed when they are dissociated, i.e., when one component, (A), is lost, while another, (B), is retained. Even stronger evidence is revealed when there isdouble dissociation:
(B) may also be lost, while (A) is retained. Such dissociations reveal separable components of language knowledge; they provide clues as to how language is built and which components are independent.
Numerous studies of aphasia have revealed that language knowledge and gen- eral cognitive knowledge, as well as various components of language, may become dissociated when the brain is injured. Table 5.3 provides examples of several clas- sical aphasia types (Caplan 1992, 1995; Goodglass 1993). They have evidenced
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dissociationandlocalizationof various specific language functions within the adult left hemisphere.40
5.4.2 Classic aphasias
Broca’s Aphasia (agrammatism). In this aphasia type, patients with slow, labored speech and extreme difficulty in sentence construction often omit grammatical words and morphemes. The sentence, “no ifs ands or buts about it”
is most difficult. This type of aphasia reveals a loss of structure building while other forms of language knowledge, e.g., vocabulary, are intact; basic aspects of general cognition may be maintained. Language loss is not simply due to motoric deficits; patients can often sing words that they can’t speak or repeat.
This syndrome reflects an injury in left anterior areas, including the inferior frontal lobe, adjacent to the motor cortex (see figure 5.1).41
Wernicke’s aphasia(fluent aphasia). Patients talk fluently, even excessively, productively using sentence syntax. But their sentences make no sense. They pro- duce circumlocutions and paragrammatisms (choosing wrong words or wrong phonemes within words). They cannot fathom the language they hear, although their hearing is unimpaired. Wernicke’s aphasia is frequently observed in the elderly. In contrast to Broca’s aphasics, these patients appear to have retained structure-building abilities for sentence syntax, but to have lost much compre- hension and word level ability. This syndrome reflects injury in left posterior areas, in the rear temporal lobe, adjacent to the auditory cortex. (See figure 5.1.)
Both aphasias differ from apraxia (loss of motor ability).
5.4.3 Refining dissociations
Aphasic patients may show impaired processing of verb forms of words like “watch,” “crack,” or “dress,” but not of their noun forms; other patients the reverse (Damasio and Tranel 1993). Patients with frontal lobe lesions may have more difficulty producing verbs, while those with temporal lobe lesions more difficulty with nouns. In one patient, ability to define “animals” was spared, while other categories were deficited; in another, the category of “animals” was disproportionately impaired (Hillis and Caramazza 1991).
Linguistic analyses of the speech of an Italian aphasic showed negligible derivationalerrors, but productiveinflectionalerrors in morphology (Badecker and Caramazza 1989). Different neurological systems have been implicated for processing English regular and irregular verbs (Pinker 1991).
These “double dissociations” provide evidence for a modular and categorical representation of language knowledge. How does the brain organize this?
40Original studies in this area were conducted by observation and description of specific syndromes of language behavior, followed by post-mortem examination of brain damage.
41Broca’s original patient suffered damage to a wide area including not only the lower rear portion of the frontal lobe, but portions of the parietal and temporal lobe (Calvin and Ojemann 1994, 43).
Fig. 5.3 Mapping the left hemisphere
Source: Wilder Penfield and Lamar Roberts 1959. Speech and Brain Mechanisms(Princeton, NJ: Princeton University Press), fig. vii-3, p. 122.
Reprinted by permission of Princeton University Press.
5.4.4 Modeling the brain’s organization
Aphasia studies led to a classic neurological model (Geschwind 1972, 5). Although this model has been extended and revised over time, it provides the foundation for more current modeling of brain organization for language.
5.4.4.1 The Wernicke Geschwind Model
According to the Wernicke Geschwind model (figure 5.1B), a word heard is processed in the Primary Auditory Area and passed to Wernicke’s area, where it is understood. If a word is to be spoken, this is transmitted (by the con- necting band of nerve fibers, the arcuate fasciculus) from Wernicke’s to Broca’s area, where its articulation is organized and passed to the motor area controlling muscles related to speech. When a word is read, the primary visual areas first pass it through the angular gyrus to Wernicke’s area, where its auditory form and comprehension is aroused.
This model, based on focal localization of brain injury in each of several classic types of aphasia, proposed that a sensory–motor distinction and a comprehension–
production distinction were fundamental to the brain’s organization of language.
Broca’s aphasia was described as a production deficit; Wernicke’s as a compre- hension deficit. The model proposed a serial posterior–anterior process. It made a number of successful predictions regarding aphasia types (Geschwind 1972).
In general, the classic Wernicke Geschwind model centralizes the perisylvian area, known through direct electrical stimulation to provoke interference with speech (arrest, slurring, repetition, anomia; figure 5.3), and generally coheres
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with results of direct cortical stimulation, concluding that “[L]anguage functions are discretely and differentially localized” in cortex (Ojemann 1983, 189).
5.4.4.2 Current modeling
Based on new brain imaging with normal subjects, new models of the brain’s organization for language cohere with much of the Wernicke Geschwind model, but revise and extend it. The perisylvian area of the dominant left hemi- sphere remains central, and it continues to be recognized that “the cortical area dedicated to language is not unitary” (Ojemann 1991, 2281).
However, in contrast to the classic Wernicke Geschwind model, more recent research has revealed the brain’s organization does not simply involve a production–comprehension or sensory–motor division. Broca’s aphasia does not simply involve a deficit in language production, but comprehension is also deficited. When complex syntax was tested, a patient diagnosed with Broca’s aphasia post-stroke, who demonstrated good vocabulary and good basic syntax in simple sentences, was unable to answer questions like “Who killed the leop- ard?” when given a sentence like (12) by Geschwind:
12. The leopard was killed by the lion
The same patient had difficulty with interpreting the meaning of a sentence which had embedding, like (13). Broca’s aphasics are deficited in certain grammatical knowledge necessary for language comprehension.
13. That’s [my [brother’s [sister]]]
More sophisticated linguistic and psycholinguistic testing was necessary to dis- cover these language deficits connected with brain injury (Zurif 1980 and 1983).
Current research pursues an exact characterization of linguistic and processing deficits involved in different forms of aphasia.42
Current models of the brain’s organization for language integrate cognitive psychology with neuroanatomy and physiology.43Using imaging methods, they provide more precise evidence on both localization and timing (within millisec- onds) of various aspects of language processing during the complex orchestration of language use in real time (Posner 1997 and 1995). They find that incoming sen- sory stimuli leading to language production and comprehension are processed in more than one neural pathway (Posner et al. 1988). Different areas of left hemi- sphere cortex are revealed whenviewingwords than whenlistening to words,
42Broca’s aphasia (Linebarger 1995; Mauner 1995; Grodzinsky 1990; Caplan 1992, 296f.); fluent aphasia (Caramazza, Papagno and Ruml 2000).
43Imaging studies allow researchers to begin to resolve indeterminacy surrounding interpretation of brain injury. For example, absence of a function which may correlate with a particular lesion site does not necessarily identify the location of the program underlying this function, any more than knocking out a light bulb on a car identifies the electrical system or program underlying the car’s lighting (Ojemann 1991; Posner and Raichle 1994/1997; Caramazza 1997a; Caplan 1992).
although both visual and auditory pathways converge on Broca’s area (Kandel et al. 2000, 14). Both parallel processing and serial processing are involved:
accessing meaning activates an area of left frontal cortex as well as Wernicke’s area (Posner and Raichle 1994, 115; Ojemann 1991, 2282). Current models con- sider parallel processes in addition to the serial processing involved in the Wer- nicke Geschwind model, and top–down as well as bottom–up processes (Posner and Raichle 1997, 111). Compartmentalization of perception and production is not the basis for brain organization of language. Cortical organization for lan- guage knowledge does not only involve sensory–motor components of language processing, but different knowledge components. The “functional role of the language-related areas is more accurately characterized in terms of linguisti- cally relevant systems including phonology, syntax and semantics than in terms of activities such as speaking, repeating, reading or listening” (Posner et al.
2001, 297).44
Mapping language knowledge to its biological foundations is complicated by many factors.
5.4.5 Individual variance
No two brains and no two brain injuries are identical. Substantial indi- vidual differences exist in the exact localization of language functions within the dominant left hemisphere, within and around the perisylvian area, e.g., differ- ences in localization of naming (Ojemann 1991). Factors such as attention and practice can affect patterns of cortical activity (Posner 1995).
5.4.5.1 Gender differences
Basic cortical organization for language does not appear to differ fundamentally across male and female brains; although some sex differences have been noted in electrophysiological tests of language processing.45Macaulay 1977 challenged the “myth of female superiority in language” on the basis of a review of the literature, concluding that “the evidence of consistent sex differences in language development is too tenuous and self-contradictory to justify any claims that one sex is superior to the other” (361). When infants between eight and twenty months were tested for their comprehension of words in a visual preference task, there was “no total difference in comprehension for boys and girls” (Goldfield and Reznick 1990, 163).46
44One example of how combining new imaging methods with sophisticated linguistic design of stimulus sentences and precise experimental methods has led to new precision in our understanding of the neural basis of sentence processing is Friederici 2002.
45Kimura 1992; Shaywitz et al. 1995; Segalowitz and Berge 1995.
46Tomlin 1999 reports that boys may be more likely to show behavioral problems connected with language disorders, and thus be identified more readily.
90 c h i l d l a n g ua g e 5.4.5.2 Multilingualism
Bilinguals, like monolinguals, demonstrate left hemisphere domi- nance for language knowledge.47 But intraoperative direct cortical stimulation of bilinguals has suggested some dissociated sites across multiple languages, i.e., cortical stimulation at a particular point may interfere with naming in one lan- guage, but not the other (Ojemann 1983, 189), and aphasia types in multilinguals may dissociate languages (Paradis 1990). Study of the brain’s representation of multiple languages and their acquisition remains a central area of research inquiry today (Kim et al. 1997).48
In summary, new brain imaging methods are empowering new more precise models for the brain’s processing of language.