Neuropsychological Evidence on Lexical Processing

Evidence for independent modules in lexical processing can be obtained from individuals with brain damage due to a stroke or other injury who can competently produce or understand some types of linguistic information but not others. On the other hand, evidence that the same module is involved in performing two different tasks can be obtained by showing strong correlations between the factors affecting performance on each. In the domain of word recognition, double dissociations between written word and spoken word comprehension have been reported. That is, some patients who show a deficit in recognizing printed words can nonetheless recognize spoken words, whereas other patients show the reverse. For some of these patients, the deficit in spoken or written word perception cannot be attributed to difficulties with basic aspects of visual or auditory perception because the patient can recognize nonverbal materials in both modalities. Instead, the deficit is specifically in the phonological or orthographic processing systems. The existence of patients who can understand written words but who cannot understand spoken words because of disrupted phonological representations argues against the necessity of converting written words to phonological forms in order to access meaning. Evidence that the same lexical-semantic system is involved in comprehending spoken and written words comes from patients who show comprehension difficulties for certain words or certain semantic categories (such as animals or tools), and the same words are affected irrespective of whether the input is spoken or written. If there were separate semantic representations for spoken and written words, it would be highly unlikely that the same categories of words would be affected for both modalities.

Category-specific deficits have been noted for many patients and raise interesting questions concerning the nature of semantic representations in the brain. Although more specific deficits have been observed, these deficits tend to occur in the categories of animals, plants, and artifacts (i.e., man-made objects), with the most common deficit for animals or, more generally, for living things. One possible explanation of category-specific deficits is motivated by the fact that semantic properties of an object tend to be interrelated (e.g., having eyes usually occurs with having a nose) and that objects in the same superordinate category (e.g., tools or fish) share properties. If constellations of shared properties are organized together in the brain, then when damage occurs to a region in which semantic properties are stored, deficits that affect certain categories will result. The only difficulty with this account is that it does not explain why deficits tend to occur in three categories—that is, any constellation of shared properties should be subject to damage and we should observe patients with highly specific deficits such as a deficit for vehicles but not other artifacts.

Another possible explanation for category-specific deficits is that there are two separate semantic systems in the brain—one that represents sensory knowledge and another that represents functional knowledge (i.e., the functions that objects perform). Researchers have argued that knowledge of animals is mainly sensory, whereas knowledge of artifacts is mainly functional. Consequently, damage to the sensory knowledge system results in a deficit specific to animals, whereas damage to the functional system results in a deficit specific to artifacts. However, findings from some brain-damaged patients are problematic for this sensory/functional explanation. For example, some patients have been reported who have semantic deficits for only some subsets of living things (e.g., fruits and vegetables), and others have been reported who have a disruption of knowledge of both sensory and functional attributes of animals but a preservation of both sensory and functional knowledge for artifacts.

As discussed earlier, models of word production take either a discrete stage or interactive activation approach. Some data from speech errors in normal subjects (either spontaneous or experimentally elicited) support the interactive view. For example, sound exchange errors are more likely to occur if the exchange results in two words (saying "barn door'' for "darn bore'') than if the exchange results in nonsense words (saying "beal dack'' for "deal back''). This effect of lexical status of the resulting error can be attributed to feedback from the phoneme level to the lexical level. The word production errors of aphasic patients can also be better accounted for by an interactive approach. Such an approach provides a means of accounting for some patients' tendency to produce words phonologically related to a target word (so-called "formal errors,'' such as saying "mat'' for "cat") and for some patients' tendency to produce a large proportion of errors that are both semantically and phonologically related to a target (saying "rat" for "cat").

Double dissociations between deficits in written and spoken word production have been observed. Again, in many of these cases, basic deficits in the motor processes involved in speaking or writing can be ruled out, indicating that the modality-specific deficit is specific to phonological or orthographic output processing. Further evidence for the separation of phonology and orthography comes from patients who make semantic errors in only one output modali-ty—for example, producing "pillow'' as the name for a picture of a bed when speaking but producing "bed'' correctly in writing. Such a pattern would not be expected if it were necessary to use the phonological form to guide spelling.

Evidence from speech production deficits provides information about the representation of grammatical information. Deficits specific to certain grammatical categories have been reported because some patients have selective difficulties in the production of function words (i.e., words such as prepositions, pronouns, and auxiliary verbs that play primarily a grammatical role in a sentence). Such difficulties are remarkable given that these grammatical words are often quite short and easy to pronounce (e.g., "to" and "will'') and are the most frequently occurring words in the language. Some patients have demonstrated greater difficulty in producing nouns than verbs, and others have demonstrated the reverse. As with the semantic category deficits, there is no consensus on the explanation for these grammatical class deficits. In some cases, these apparent grammatical class effects have a semantic basis. For example, better production of nouns than verbs and better production of verbs than grammatical words may be observed because the patient is better able to produce more concrete words. However, for some patients, it appears that grammatical class effects cannot be reduced to a semantic basis; consequently, these deficits suggest that at some level in the production system words are distinguished neurally with regard to the grammatical role that they play in a sentence. The separability of grammatical information from other types of lexical information is supported by other findings showing that some patients with picture-naming deficits can provide grammatical information about a word, such as its gender (in a language such as Italian or French), even though they are unable to retrieve any of the phonemes in the word.

Neuropsychological research with brain-damaged patients, more so than research with normal subjects, has addressed the issue of the relation between the phonological processing systems involved in speech perception and production and the relation between the orthographic systems involved in reading and writing. Some patients show an excellent ability to recognize and remember input phonological forms (e.g., being able to decide whether a spoken probe word rhymes with any of the words in a preceding list but have great difficulty in producing output phonological forms (e.g., naming a picture). Other patients show the reverse pattern of great difficulty in holding onto input phonological forms (performing at chance on the rhyme probe task) but showing normal speech production. Similar double dissociations have been documented for orthographic processing. Thus, input and output forms in both speech and writing appear to be represented in different brain areas. However, although the input and output forms may be different, they are linked to each other. For example, individuals can repeat nonwords, converting an input to an output form. A close coupling between input and output forms appears to be involved in the development of speech production and in the maintenance of accurate speech production throughout adulthood.

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