The mapping from sound structure to the lexicon in aphasia: evidence from rhyme and repetition priming

Two studies were conducted to explore the hypothesis that Broca's and Wernicke's aphasics have deficits arising from the processes involved in activating the lexicon from phonological form. The first study explored whether phonologically similar lexical entries differing only in their initial consonants show "rhyme priming." Results revealed that Broca's aphasics failed to show facilitation when the target was identical to the prime (i.e. identity priming) and they showed significant inhibition when targets were preceded by rhyming words. Wernicke's aphasics showed a pattern of results similar to that of normal subjects, i.e., identity priming and rhyme priming as well as significantly slower reaction-times in the rhyming condition compared to the identity condition. The second study investigated form-based repetition priming in aphasic patients at a number of intervals including when no other stimuli intervened between repeated stimuli (0 lag) or when 4, 8, or 12 stimuli intervened. Results showed that, unlike old normal subjects who showed repetition priming for both words and nonwords, both Broca's and Wernicke's aphasics showed repetition priming for word targets only. Moreover, in contrast to old normal subjects who showed a greater magnitude of priming at 0 lag for word targets, neither Broca's aphasics or Wernicke's aphasics showed priming at 0 lag. Implications of these findings are considered with respect to the hypotheses that Broca's and Wernicke's aphasics have deficits in the nature of the activation patterns within the lexicon itself and in auditory (working) memory.     

Mapping from Sound to Meaning: Reduced Lexical Activation in Broca''s Aphasics

Recent studies of lexical access in Broca's aphasics suggest that lexical activation levels are reduced in these patients. The present study compared the performance of Broca's aphasics with that of normal subjects in an auditory semantic priming paradigm. Lexical decision times were measured in response to word targets preceded by an intact semantically related prime word ("cat"-"dog"), by a related prime in which one segment was acoustically altered to produce a poorer phonetic exemplar ("c*at"-"dog"), and by a semantically unrelated prime ("ring"-"dog"). The effects of the locus of the acoustic distortion within the prime word (initial or final position) and the presence of potential lexical competitors ("cat" --> /gaet/versus "coat" --> "goat") were examined. In normal subjects, the acoustic manipulations produce a small, short-lived reduction in semantic facilitation irrespective of the position of the distortion in the prime word or the presence of a voiced lexical competitor. In contrast, Broca's aphasics showed a large and lasting reduction in priming in response to word-initial acoustic distortions, but only a weak effect of word-final distortions on priming. In both phonetic positions, the effect of distortion was greater for prime words with a lexical competitor. These findings are compatible with the claim that Broca's aphasics have reduced lexical activation levels, which may result in a disruption of the bottom-up access of words on the basis of acoustic input as well as increased vulnerability to competition between acoustically similar lexical items.     

Semantic processing in aphasia: Evidence from an auditory lexical decision task

W. Milberg and S. E. Blumstein (1981, Brain and Language 14, 371–385) demonstrated semantic facilitation effects in a visual lexical decision task administered to Wernicke and other aphasics with severe comprehension deficits. In an attempt to explore the generalizability of these findings in a task where the acoustic-phonetic system could not be bypassed to access meaning, Wernicke's, Global, Broca's, and Conduction aphasics were administered a lexical decision task in the auditory modality. The patients were also given a simple semantic judgment task using the word pairs from the lexical decision task. The aphasic patients showed evidence of semantic facilitation whether they were categorized by diagnostic group or comprehension level. Performance of the semantic judgment task correlated with the severity of auditory comprehension deficits, whereas the consistency of the semantic facilitation effect did not. Even patients with severe comprehension deficits showed semantic facilitation. These results decrease the likelihood that auditory comprehension deficits are due to semantic organization per se and increase the likelihood that the deficits lie in one of the many processes involved in access to that information.     

Lexical creativity during instances of word-finding difficulty: Broca's vs. Wernicke's aphasia

Lexical innovation--the creation of a word by combining existing morphemes in a novel way (e.g. "map ball" for "globe")--was evaluated as a method for circumventing word-finding difficulty in Broca's and Wernicke's aphasia. Aphasic groups were matched for naming performance and compared to a control group of normal adults matched for age and education. Lexical innovations were collected during the administration of a confrontation naming test, and were then analyzed in terms of the correctness of morpheme combination, semantic accuracy, novelty, and communicative effectiveness. An innovation was considered to be communicatively effective when its intended referent was understood by a naive judge. The lexical innovations of the two aphasic groups were diametrically opposed: as compared to both Broca's aphasics and normal adults, Wernicke's aphasics innovated significantly less often, and their innovations were significantly inferior in terms of: semantic precision, the proper construction of morpheme combination, and communicative effectiveness. This pattern suggests that lack of verbal fluency may be compatible with lexical creativity, while empty logorrheic speech may be an impediment to lexical creativity. Similarly, we conclude that the agrammatism of Broca's asphasia does not interfere with lexical innovation, while the paragrammatism of Wernicke's aphasia does interfere with lexical innovation, thus suggesting that paragrammatism affects morpheme combination at the word level as well as the sentence level.     

The noun-verb problem in Chinese aphasia.

Previous studies have shown that Broca's aphasics experience a selective difficulty with action naming inside or outside of a sentence context. Conversely, it has been suggested that Wernicke's aphasics are particularly impaired in object naming. A number of explanations have been offered to account for this double dissociation, including grammatical accounts according to which the main verb problem in agrammatic Broca's aphasics is viewed as a by-product of their syntactic and/or morphological impairment, due perhaps to the greater morphological load carried by verbs (compared with nouns). In the Chinese language, there are no verb conjugations and no declensions. Hence there is no reason to expect a relationship between morphological impairment and deficits in action naming. We examined comprehension and production of object and action names, outside of a sentence context, in a sample of Chinese-speaking Broca's and Wernicke's aphasics. There was an interaction between patient group and object/action naming, but no corresponding interaction on the comprehension task. We conclude that action-naming deficits in Broca's aphasia (and/or the corresponding sparing of action names in Wernicke's aphasia) cannot be attributed to morphological differences between nouns and verbs. We also found a sublexical variant of the noun/verb dissociation applied to the internal structure of compound words made up of a verbal and a nominal element: Broca's aphasics tended to lexicalize the verbal portion of these words more often than the nominal compound, while Wernicke's showed the opposite pattern. These sublexical effects are difficult to explain in syntactic terms nor do they fit the standard lexical view. A modified lexical account is proposed, emphasizing semantic/conceptual effects in a distributed lexicon.     

Deficits in thematic integration processes in Broca's and Wernicke's aphasia

This study investigated how normal subjects and Broca's and Wernicke's aphasics integrate thematic information incrementally using syntax, lexical-semantics, and pragmatics in a simple active declarative sentence. Three priming experiments were conducted using an auditory lexical decision task in which subjects made a lexical decision on a 'target' (the last word of each sentence) preceded by a 'prime' (a subject noun phrase and verb). The presence and magnitude of priming was compared to a baseline condition in which non-words systematically replaced real word primes. Normal subjects showed evidence for combinatorial thematics by exhibiting significant and larger amounts of thematic priming in the condition where two real words were present in the prime than in the conditions in which only one real word was present in the prime. Additionally, normal subjects showed sensitivity to both syntactic structures and pragmatics. In contrast, Broca's patients did not show significant priming for any condition nor did they show a difference in the magnitude of priming among the conditions. Nonetheless, they showed sensitivity to pragmatics. Wernicke's patients showed significant priming for all conditions, but did not show differences in the magnitude of priming among the conditions. However, they showed sensitivity to sentence grammaticality and pragmatics. The distinct patterns of performance of Broca's and Wernicke's aphasics are discussed in terms of the nature of their impairments in the processes of combinatorial thematics.     

Processing of lexical ambiguities: a comment on Milberg, Blumstein, and Dworetzky (1987)

In a study by Milberg, Blumstein, and Dworetzky (1987), normal control subjects and Wernicke's and Broca's aphasics performed a lexical decision task on the third element of auditorily presented triplets of words with either a word or a nonword as target. In three of the four types of word triplets, the first and the third words were related to one or both meanings of the second word, which was semantically ambiguous. The fourth type of word triplet consisted of three unrelated, unambiguous words, functioning as baseline. Milberg et al. (1987) claim that the results for their control subjects are similar to those reported by Schvaneveldt, Meyer, and Becker's original study (1976) with the same prime types, and so interpret these as evidence for a selective lexical access of the different meanings of ambiguous words. It is argued here that Milberg et al. only partially replicate the Schvaneveldt et al. results. Moreover, the results of Milberg et al. are not fully in line with the selective access hypothesis adopted. Replication of the Milberg et al. (1987) study with Dutch materials, using both a design without and a design with repetition of the same target words for the same subjects led to the original pattern as reported by Schvaneveldt et al. (1976). In the design with four separate presentations of the same target word, a strong repetition effect was found. It is therefore argued that the discrepancy between the Milberg et al. results on the one hand, and the Schvaneveldt et al. results on the other, might be due to the absence of a control for repetition effects in the within-subject design used by Milberg et al. It is concluded that this makes the results for both normal and aphasic subjects in the latter study difficult to interpret in terms of a selective access model for normal processing.     

Nasal consonant production in Broca's and Wernicke's aphasics: speech deficits and neuroanatomical correlates

The present study investigated the articulatory implementation deficits of Broca's and Wernicke's aphasics and their potential neuroanatomical correlates. Five Broca's aphasics, two Wernicke's aphasics, and four age-matched normal speakers produced consonant-vowel-(consonant) real word tokens consisting of [m, n] followed by [i, e, a, o, u]. Three acoustic measures were analyzed corresponding to different properties of articulatory implementation: murmur duration (a measure of timing), amplitude of the first harmonic at consonantal release (a measure of articulatory coordination), and murmur amplitude over time (a measure of laryngeal control). Results showed that Broca's aphasics displayed impairments in all of these parameters, whereas Wernicke's aphasics only exhibited greater variability in the production of two of the parameters. The lesion extent data showed that damage in either Broca's area or the insula cortex was not predictive of the severity of the speech output impairment. Instead, lesions in the upper and lower motor face areas and the supplementary motor area resulted in the most severe implementation impairments. For the Wernicke's aphasics, the posterior areas (superior marginal gyrus, parietal, and sensory) appear to be involved in the retrieval and encoding of lexical forms for speech production, resulting in increased variability in speech production.     

Syntactic priming effects in aphasia: an investigation of local syntactic dependencies

This study explored on-line processing of local syntactic dependencies in normal subjects and in Broca's and Wernicke's aphasics using a lexical decision paradigm. In addition, subjects performed a grammatically judgement task on the real word pairs used in the lexical decision tasks. Results of two experiments for normal subjects indicated different syntactic priming effects as a function of the type of local syntactic dependency. Word pairs that formed a single constituent phrase, i.e., a verb phrase, showed both facilitory and inhibitory effects, whereas word pairs that reflected local syntactic dependencies across a phrase boundary, i.e., pronoun verb, showed only inhibitory effects. Broca's aphasics failed to show facilitory effects when presented with word pairs forming a single constituent phrase but, similar to normals, did show inhibition when presented with word pairs that reflected local syntactic dependencies across a phrase boundary. In contrast, Wernicke's aphasics failed to show inhibitory effects in both experiments. The implications of these results for theories of language processing deficits in aphasia are considered.     

Speed of Lexical Activation in Nonfluent Broca''s Aphasia and Fluent Wernicke''s Aphasia

Rapid, automatic access to lexical/semantic knowledge is critical in supporting the tight temporal constraints of on-line sentence comprehension. Based on findings of “abnormal” lexical priming in nonfluent aphasics, the question of disrupted automatic lexical activation has been the focus of many recent efforts to understand their impaired sentence comprehension capabilities. The picture that emerges from this literature is, however, unclear. Nonfluent Broca's aphasic patients showinconsistent,notabsent,lexical priming, and there is little consensus about the conditions under which they do and do not prime. The most parsimonious explanation for the variable findings from priming studies to date is that the primary disturbance in Broca's lexical activation has something to do withspeedof activation. Broca's aphasic patients prime when sufficient time is allowed for activation to spread among associates. To examine this “slowed activation” hypothesis, the time course of lexical activation was examined using a list priming paradigm. Temporal delays between successive words ranged from 300 to 2100 msec. One nonfluent Broca's aphasic patient and one fluent Wernicke's patient were tested. Both patients displayed abnormal priming patterns, though of different sorts. In contrast to elderly subjects, who prime at relatively short interstimulus intervals (ISIs) beginning at 500 msec, the Broca's aphasic subject showed reliable automatic priming but only at a long ISI of 1500 msec. That is, this subject retained the ability to access lexical information automatically if allowed sufficient time to do so, a finding that may help explain disrupted comprehension of normally rapid conversational speech. The Wernicke's aphasic subject, in contrast, showed normally rapid initial activation but continued to show priming over an abnormally long range of delays, from 300 msec through 1100 msec. This protracted priming suggests failure to dampen activation and might explain the semantic confusion exhibited by fluent Wernicke's patients.     

Production deficits in aphasia: A voice-onset time analysis

An experimental study was conducted to examine phonetic and phonemic deficits in the speech production of aphasics. Subjects included four Broca's aphasics, four Conduction aphasics, five Wernicke's aphasics, one nonaphasic dysarthric patient, and four normal controls. The subjects read a list of words containing word-initial stop consonants which were subsequently measured acoustically for voice-onset time. The results showed that Broca's aphasics exhibit a more severe production disorder than Conduction aphasics who in turn exhibit a more severe disorder than Wernicke's aphasics, in accord with clinical observations. In addition, although Broca's aphasics produced both phonetic and phonemic errors, the results showed that they have a pervasive phonetic disorder which affects their correct target productions as well as the total number of phonetic errors produced. This deficit however seems to be a speech deficit rather than a low-level motor control problem. In contrast, the Wernicke's aphasics show a deficit characterized by isolated phonemic mistargeting errors. Finally, the pattern of productions for the Conduction aphasics indicates that some patients show a predominantly phonetic disorder similar to the Broca's aphasics and others show predominantly a phonemic disorder similar to the Wernicke's aphasics.     

Comprehension in aphasia: a cross-linguistic study

Broca's aphasics and normal controls were tested to determine relative sparing and impairment of word order, grammatical morphology, and semantic information in a sentence interpretation task. Patients were native speakers of English, German, or Italian, languages that vary drastically in the "cue validity" or information value of these three sources of information. Word order was selectively spared while grammatical morphology was selectively impaired in all three languages. Nevertheless, language-specific patterns of sentence interpretation remained in Broca's aphasics, even within the impaired morphological component, supporting an interpretation in terms of "accessing" rather than a "loss." Testing with Wernicke's aphasics, anomics, and some additional age-matched controls suggested that the selective vulnerability of morphology is not specific to agrammatic patients, at least in this paradigm.     

The role of lexical competition and acoustic-phonetic structure in lexical processing: evidence from normal subjects and aphasic patients

This study examined the effects that the acoustic-phonetic structure of a stimulus exerts on the processes by which lexical candidates compete for activation. An auditory lexical decision paradigm was used to investigate whether shortening the VOT of an initial voiceless stop consonant in a real word results in the activation of the lexical-semantic network of its voiced competitor, i.e., does acoustically modified time prime penny via dime. Results for normal subjects showed semantic priming for related pairs and mediated priming for voiced competitors, consistent with cascade models of language processing allowing for interaction between phonological and semantic levels of processing. Although Broca's aphasics showed semantic priming (dime primed penny), they failed to show priming in the context of a lexical competitor. These results are consistent with the hypothesis that these patients have a lexical processing deficit characterized by an overall reduction in lexical activation.     

Selective word-learning deficits in aphasia

The purpose of this study was to determine whether patients with focal brain damage can learn anything about a new word, and if so, whether selected aspects of the new word are acquired depending on the nature of the patient's language processing deficit. In the context of drawing pictures with a number of felt pens identical in all respects except color, agrammatic Broca's aphasics and fluent aphasics were exposed to the new word "bice," an adjective referring to the dark green portion of the color spectrum. Our findings revealed that brain-damaged patients can engage in lexical acquisition, but Broca's aphasics and fluent aphasics apparently learn about different aspects of the new word. Specifically, on successive exposures to "bice," both Broca's aphasics and fluent aphasics exhibit progressively more accurate hypotheses for identifying a bice-colored object. During subsequent assessments, agrammatic aphasics reveal on a metalinguistic judgment task their significant difficulty appreciating the grammatical form class of "bice"; on an object classification task, fluent aphasics are significantly impaired in their classification of bice-colored objects as "bice." Taken together, these unique word-learning profiles reinforce earlier observation on the selective nature of language processing deficits after focal left-hemisphere insult, and support the claim that separate language processing devices may be selectively compromised in different groups of aphasics. The relationship between syntactic and lexical semantic processing is discussed.     

Processing passive sentences in aphasia: deficits and strategies

Broca's and Wernicke's aphasics' ability to process passive sentences in the absence of semantic cues was investigated in an experiment which varies syntactic complexity and word order. The results indicate that Broca and Wernicke patients use different strategies for sentence comprehension. Wernicke patients use rather general strategies for interpretation, which assign syntactic function according to sequential arrangements of words. Broca's aphasics, by contrast, base their interpretation on specific structural elements of the sentence's surface form, without, however, being able to exploit the full syntactic information of these elements. The different strategies are interpreted to reflect differential underlying deficits.     

Altered patterns of word associations in dementia and aphasia

The word associations of 38 demented, 17 aphasic, and 22 normal subjects were studied. Both normal and brain-injured subjects appear to make judgments about the grammatical class of the word stimulus. Certain stimulus words (especially nouns and adjectives) elicit paradigmatic responses whereas other words (especially verbs and adverbs) elicit syntagmatic responses. The mechanism producing syntagmatic responses seems relatively resistant to deterioration in dementia or aphasia. However, in dementia the mechanism that generates paradigmatic responses becomes progressively less efficient (possibly due to a loss of semantic markers) and consequently more random (idiosyncratic) responses emerge. Perseverative responses, inhibited in normal subjects, are more prevalent in dementia. Anomic aphasics show a pattern of word associations similar to that of subjects with mild dementia. Broca's aphasics, while making fewer paradigmatic associations than normals, retain enough self-monitoring mechanisms so that few idiosyncratic and perseverative responses are made while more null responses occur. Wernicke's aphasics show a marked shift away from a paradigmatic word association strategy, possibly due to an inability to access semantic markers or a true loss of these markers. Metalinguistic deficits (i.e., a failure to adopt an appropriate strategy) may also contribute to this shift away from paradigmatic associations. Furthermore, a disruption of self-monitoring mechanisms in Wernicke's aphasia leads to an increase in perseverative and idiosyncratic responses.     

Syntactic and semantic processes in aphasic deficits: The availability of prepositions

The availability of a specific category of closed class items (prepositions) was examined in Broca's and Wernicke's aphasia. The role of prepositions was varied (syntactic/semantic) in two tasks: one requiring production of prepositions and the other acceptability judgments. Overall, it was easier to perform the acceptability task than to produce the target item. In the judgment task, Wernicke's aphasics more accurately determined the acceptability of prepositions in their syntactic role than in their semantic role, but Broca's aphasics showed no such difference. In the production task, Wernicke's aphasics again were more likely to produce prepositions which were in a primarily syntactic role. Broca's aphasics, however, failed to produce such syntactically based prepositions. The results suggest that the availability of a given closed class vocabulary form is not merely a function of its class membership but also of its functional role, and this fact serves to distinguish aspects of Broca's and Wernicke's aphasics' deficit.     

Lexical decision and aphasia: Evidence for semantic processing

Wernicke's and Broca's-Conduction aphasics and a Global aphasic were presented with a lexical-decision task in which English words and pronounceable nonwords were preceded by semantically related, unrelated, or nonword primes. The patients were also given a simple semantic-judgment task using the word pairs from the lexical-decision task. Wernicke's aphasics performed similar to normals and Broca's-Conduction aphasics showing significantly shorter latencies in making real-word identifications when preceded by a semantically related word. In addition, both superordinate and coordinate associates showed semantic-priming effects. Performance on the semantic-judgment task showed significantly more impairment in the aphasic group than in the normal controls. These results suggest that aphasics with even severe language impairments retain stored semantic information that may be automatically activated, yet is inaccessible to conscious semantic decision during metalinguistic tasks.     

Dissociation of algorithmic and heuristic processes in language comprehension: Evidence from aphasia

Three groups of aphasic patients, Broca's, Conduction, and Wernicke's, and a nonaphasic patients control group were tested for comprehension of object-relative center-embedded sentences. The sentences were of three types: sentences in which semantic constraints between words allowed the subjects to assign a correct semantic reading of the sentence without decoding the syntax, sentences in which semantic constraints were relaxed and for which a correct reading was only possible with knowledge of syntactic relationships among words, and sentences which described highly improbable events. The subjects' task was to choose which of two pictures captured the meaning expressed in the sentence. Broca's and Conduction aphasics performed near perfectly on sentences where they could use semantic information. Their performance dropped to chance when they had to use syntactic information. These results support a neuropsychological dissociation of heuristic and algorithmic processes based primarily, though not exclusively, on semantic and syntactic information, respectively.