Figure 113

Processing and repetition of spoken words. Adapted from Ellis and Young (1988).

If patients with pure word deafness have a severe deficit in phonemic processing, then their speech perception should improve when they have access to other kinds of information. This is the case. Okada et al. (1963) studied a patient with pure word deafness who could use contextual information. The patient found it much easier to understand spoken questions when they all referred to the same topic than when they did not. In another study, Auerbach et al. (1982) found that patients with pure word deafness had better speech perception when lip-reading was possible.

A crucial aspect of pure word deafness is that auditory perception problems are highly selective, and do not apply to non-speech sounds. Evidence that separate systems deal with speech and non-speech sounds was reported by Fujii et al. (1990). They studied a patient who had suffered brain damage within the right hemisphere. He found it very hard to name familiar environmental sounds, but his language abilities were only modestly affected.

Route 1

This route makes use of the auditory input lexicon, the semantic system, and the speech output lexicon. It represents the normal way in which familiar words are identified and comprehended by those with no brain damage. If a brain-damaged patient could use only this route (plus perhaps Route 2), then familiar words would be said correctly. However, there would be severe problems with saying unfamiliar words and non-

words, because they do not have entries in the auditory input lexicon, and therefore use of Route 3 would be required.

McCarthy and Warrington (1984) described a patient, ORF, who seems to fit the bill fairly well. ORF repeated words much more accurately than non-words (85% vs. 39%, respectively), indicating that Route 3 was severely impaired. However, the fact that he made a fair number of errors in repeating words suggests there was also some impairment to other parts of the system.

Route 2

If patients could use Route 2, but Routes 1 and 3 were severely impaired, they should be able to repeat familiar words but would often not understand their meaning. In addition, they should have problems with non-words, because non-words cannot be handled through Route 2. Finally, as such patients would make use of the input lexicon, they should be able to distinguish between words and non-words.

Patients suffering from a condition known as word meaning deafness fit this description. Unfortunately, very few patients with this condition have been studied, so "the existence of word meaning deafness is still a matter of controversy" (Franklin et al., 1996, p. 1140). However, one of the clearest cases of word meaning deafness, Dr O, was studied by Franklin et al. (1996) themselves, and we will focus on this case.

Dr O showed "no evidence of any impairment in written word comprehension, but auditory comprehension was impaired, particularly for abstract or low-imageability words" (Franklin et al., 1996, p. 1144). His ability to repeat words was dramatically better than his ability to repeat non-words, 80% vs. 7%, respectively. Finally, Dr O was very good at distinguishing between words and non-words: he was 94% correct on an auditory lexical decision task.

Dr O seems to have reasonable access to the input lexicon as shown by his greater ability to repeat words than non-words, and by his almost perfect ability to distinguish between words and non-words. He clearly has some problem relating to the semantic system. However, the semantic system itself does not seem to be damaged, as indicated by the additional finding that his ability to understand written words was intact. These various findings led Franklin et al. (1996, p. 1139) to conclude as follows: "Dr O has an impairment of the mappings between the lexical representations of spoken words and their corresponding semantic representations." Thus, there is damage to parts of Route 1. Tyler and Moss (1997) argued that Dr O may also have problems earlier in processing. They reported some evidence that he may have difficulties in extracting phonemic features from speech. For example, when he was asked to repeat spoken words as rapidly as possible, he made 25% errors.

Hall and Riddoch (1997) reported on KW, a man who had had a stroke, and who suffered from word meaning deafness. He showed impaired auditory comprehension of words even though his ability to understand written words was fairly intact. There was good evidence that KW made use of the input lexicon: (1) he spelled 60% of auditorily presented words correctly, compared to only 35% of non-words; (2) he was 89% accurate in distinguishing between auditorily presented words and non-words. Hall and Riddoch (1997, p. 1161) concluded as follows: "We have clearly demonstrated the use of a lexical nonsemantic spelling route."

Route 3

If a patient had damage to Route 3 only, he or she would show good ability to perceive and to understand spoken familiar words, but would be impaired at perceiving and repeating unfamiliar words and non-words. This is the case in patients with auditory phonological agnosia. Such a patient was studied by Beavois,

Derouesne, and Bastard (1980). Their patient, JL, had almost perfect repetition and writing to dictation of spoken familiar words, but his repetition and writing of non-words was very poor. However, he was very good at reading non-words. JL had an intact ability to distinguish between words and non-words, indicating that there were no problems with access to the input lexicon.

Deep dysphasia

Some brain-damaged patients have extensive problems with speech perception, suggesting that several parts of the speech perception system are damaged. For example, patients with deep dysphasia make semantic errors when asked to repeat spoken words (i.e., they say words related in meaning to those spoken). In addition, they find it harder to repeat abstract words than concrete ones, and they have very poor ability to repeat non-words. With reference to the model in Figure 11.3, it could be argued that none of the three routes between heard words and speech is intact. The presence of semantic errors can be explained by assuming there is some impairment in (or near) the semantic system.

Valdois et al. (1995) studied EA, a 72-year-old man who had suffered a stroke. He exhibited all the symptoms of deep dysphasia, including numerous semantic errors when trying to repeat spoken words having a synonym. In addition, EA had very poor short-term memory for auditory and visual verbal material. These latter findings led Valdois et al. (1995, p. 711) to the following theoretical interpretation: "The impairment responsible for both E.A.'s language performance and his short-term memory deficit is rooted in the inability to maintain a sufficiently activated phonological representation [in the response buffer]." They developed a connectionist model to explain the various symptoms of deep dysphasia. For example, the existence of semantic errors may occur because semantic information is often activated for longer than phonological information.

Which theoretical approach is preferable? It is possible that both approaches apply to some (but not all) deep dysphasics. Valdois et al. (1995) reviewed the literature, and discussed six deep dysphasics who had a very severe short-memory memory deficit (memory span of one or two items). These patients conform to the theoretical expectation of Valdois et al. (1995), in that there is evidence of damage to the response buffer. However, they also discussed three other patients who had only slightly impaired short-term memory. As Valdois et al. (1995, p. 719) concluded, "These overall data strongly suggest that different subtypes of repetition disorders of the deep dysphasia type do exist, which probably reflect different underlying deficits."

Section summary

There has been relatively little research on auditory word recognition and comprehension in brain-damaged patients. However, there are clearly different patterns of impairment in the ability to repeat and to understand spoken words. This encourages the belief that various processes are involved, and that there is more than one route between hearing a word and then saying it. Figure 11.3 represents a possible set of components and their interactions, but its validity will become clear only after much further research.

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