Quantifying the Interplay of Semantics and Phonology During Failures of Word Retrieval by People With Aphasia Using a Multiplex Lexical Network

Investigating instances where lexical selection fails can lead to deeper insights into the cognitive machinery and architecture supporting successful word retrieval and speech production. In this paper, we used a multiplex lexical network approach that combines semantic and phonological similarities among words to model the structure of the mental lexicon. Network measures at different levels of analysis (degree, network distance, and closeness centrality) were used to investigate the influence of network structure on picture naming accuracy and errors by people with Anomic, Broca's, Conduction, and Wernicke's aphasia. Our results reveal that word retrieval is influenced by the multiplex lexical network structure in at least two ways—(a) the accuracy of production and error type on incorrect productions were influenced by the degree and closeness centrality of the target word, and (b) error type also varied in terms of network distance between the target word and produced error word. Taken together, the analyses demonstrate that network science techniques, particularly the use of the multiplex lexical network to simultaneously represent semantic and phonological relationships among words, reveal how the structure of the mental lexicon influences language processes beyond traditionally examined psycholinguistic variables. We propose a framework for how the multiplex lexical network approach allows for understanding the influence of mental lexicon structure on word retrieval processes, with an eye toward a better understanding of the nature of clinical impairments, like aphasia.     

The crucial role of tense for verb production

The case of an aphasic patient whose spontaneous speech contains very few lexical verbs is reported. Instead of sentences with lexical verbs, the patient produces many (grammatical) copular constructions. He also substitutes lexical verbs with the copula. Although this results in ungrammatical utterances, by doing so, a resemblance of sentence structure and a degree of grammaticality of his utterances are preserved. Although the patient is more impaired in naming action than object pictures, it is unlikely that lexical retrieval difficulties are solely responsible for the paucity of lexical verbs in his speech. A series of tests revealed a profound deficit in producing tense marking inflections and in understanding their significance. We argue that the unavailability of tense features is the primary reason for the lack of lexical verbs in his speech. An alternative possibility, that the tense deficit interacts with the verb retrieval deficit, is also discussed. The patient has a complex lesion and language profile, with features associated with both Broca's and Wernicke's aphasia. However, since the study focuses on his verb and tense deficits and the grammaticality of his utterances, issues that are often discussed in relation to agrammatic Broca's aphasia, the literature that is relevant to these topics and to Broca's aphasia is reviewed, despite the different diagnostic profile of the patient.     

The selective impairment of phonological processing in speech production

We report the naming performance of a patient (DM) with a fluent progressive aphasia who made phonological errors in all language production tasks. The pattern of errors in naming was strikingly clear: DM made very many phonological errors that resulted almost always in nonword responses. The complete absence of semantic errors and the very low ratio of formal errors relative to nonword errors (1.6:30.3) in DM's performance are discussed in the context of recent claims about the nature of naming deficits in fluent aphasics. We argue that DM's performance makes highly improbable the claim that fluent aphasia results from global lesions affecting all levels of the lexical access system equally.     

Do age-related word retrieval difficulties appear (or disappear) in connected speech?

We conducted a comprehensive literature review of studies of word retrieval in connected speech in healthy aging and reviewed relevant aphasia research that could shed light on the aging literature. Four main hypotheses guided the review: (1) Significant retrieval difficulties would lead to reduced output in connected speech. (2) Significant retrieval difficulties would lead to a more limited lexical variety in connected speech. (3) Significant retrieval difficulties would lead to an increase in word substitution errors and in pronoun use as well as to greater dysfluency and hesitation in connected speech. (4) Retrieval difficulties on tests of single-word production would be associated with measures of word retrieval in connected speech. Studies on aging did not confirm these four hypotheses, unlike studies on aphasia that generally did. The review suggests that future research should investigate how context facilitates word production in old age.     

Developmental dyslexia and word retrieval deficits

Developmental dyslexics, selected on the basis of very slow naming rates on the Rapid Automatic Naming Tasks (RAN), were compared to normal readers on oral language, picture categorization, and reading tasks. Findings indicated that the dyslexics' word retrieval deficits were one symptom of a more generalized, however subtle, oral language deficit which involved both receptive and expressive oral language functioning. The dyslexics' word retrieval problem also seemed chiefly related to language processing and not to deficits in semantic memory as there were no significant differences between dyslexics and controls on a nonverbal semantic memory task (picture categorization). In naming and identifying printed words, the dyslexics appeared to rely considerably upon the "indirect" or "assembly-of-phonology" route; they were slower in naming irregularly spelled words compared to regularly spelled words and on a lexical decision task, the dyslexics were slower in making negative decisions for "pseudohomophones" (e.g., "braik") than for other matched nonwords. Results are discussed in terms of the logogen model with some consideration of a developmental model as well.     

Are age-of-acquisition effects on object naming due simply to differences in object recognition? Comments on Levelt (2002)

Levelt (2002) argued that apparent effects of word frequency and age of acquisition (AoA) reported in recent picture naming studies might actually be confounded effects operating at the level of object recognition, rather than relevant to theories of lexical retrieval. In order to investigate this issue, AoA effects were examined in an object recognition memory task (Experiments 1 and 2) and a word-picture verification task (Experiment 3) and compared with those found in naming tasks using the same pictures. Contrary to Levelt's concerns, the results of the three experiments show that the AoA effect on picture naming has a lexical origin and does not simply result from a possible confound of object identification times.     

Aphasic naming in Spanish: predictors and errors

Sixteen Spanish aphasic patients named drawings of objects on three occasions. Multiple regression analyses were carried out on the naming accuracy scores. For the patient group as a whole, naming was affected by visual complexity, object familiarity, age of acquisition, and word frequency. The combination of variables predicted naming accuracy in 15 of the 16 individual patients. Age of acquisition, word frequency, and object familiarity predicted performance in the greatest number of patients, while visual complexity, imageability, animacy, and length all affected performance in at least two patients. High proportions of semantic and phonological errors to particular objects were associated with objects having early learned names while high proportions of no-response errors were associated with low familiarity and low visual complexity. It is suggested that visual complexity and object familiarity affect the ease of object recognition while word frequency affects name retrieval. Age of acquisition may affect both stages, accounting for its influence in patients with a range of different patterns of disorder.     

Lexical Anomia: Or the Case of the Missing Lexical Entries

This paper reports the case of an aphasic patient, EE, with a problem in word retrieval. He is consistently unable to produce specific lexical items, which tend to be items of low rated familiarity. His retrieval of these words is not aided by the provision of phonemic cues or extra time for word retrieval. His errors consist primarily of failures to respond, and the provision of semantic information without any attempt at the target. It is argued that this pattern of performance is consistent with the loss of specific lexical items from a phonological lexicon for speech production.

EE is shown to have no impairment in auditory recognition and comprehension of the lexical items that are unavailable for naming. This dissociation is problematic for theories that propose a single phonological lexicon for both word recognition and production, but is easily accounted for by separate input and output lexicons.     

Naming consistency in Alzheimer''s disease

Although lexical semantic deficits are postulated to play a prominent role in the anomia of Alzheimer's disease, it is unclear whether the primary disturbance is one of lexical access or one of lexical semantic loss. Response consistency on a naming task is one means of evaluating the underlying source of naming impairment. Access dysfunction usually implies variable word-finding difficulty, while a theory of lexical loss predicts that many word names would be consistently unavailable. Nineteen Alzheimer's disease patients were administered a visual confrontation naming task (the Boston Naming Test) on two occasions 6 months apart. Eighty percent of errors occurred consistently at both times; only 20% of errors occurred on only one occasion. Response consistency occurred significantly more often than expected under the assumption of no response consistency. Findings support the hypothesis that anomia in Alzheimer's disease is in part due to a loss of lexical semantic information.     

Hemispheric asymmetries in semantic processing: evidence from false memories for ambiguous words

We used fMRI to investigate competition during language production in two word production tasks: object naming and color naming of achromatic line drawings. Generally, fMRI activation was higher for color naming. The line drawings were followed by a word (the distractor word) that referred to either the object, a related object, or an unrelated object. The effect of the distractor word on the BOLD response was qualitatively different for the two tasks. The activation pattern suggests two different kinds of competition during lexical retrieval: (1) Task-relevant responses (e.g., red in color naming) compete with task-irrelevant responses (i.e., the object’s name). This competition effect was dominant in prefrontal cortex. (2) Multiple task-relevant responses (i.e., target word and distractor word) compete for selection. This competition effect was dominant in ventral temporal cortex. This study provides further evidence for the distinct roles of frontal and temporal cortex in language production, while highlighting the effects of competition, albeit from different sources, in both regions.     

Omissions and semantic errors in aphasic naming: is there a link?

Within a discrete two-stage model of lexicalization, semantic errors and errors of omission are assumed to be independent events. In contrast, cascading and interactive models allow for an influence of word form on lexical selection and thus for an inherent relationship in accounting for both error types. A group of 17 aphasic patients was assessed with a naming test controlling for semantic competition of the target items. Semantic errors were more frequent for targets with many competitors than for targets with few competitors while omissions were more frequent when few competitors were available. However, the overall sums of errors in both item groups were comparable. These results imply a common source of both error types and thus speak against a strictly serial model of naming.     

Optic aphasia, optic apraxia, and loss of dreaming

A 47-year-old man with a left temporo-occipital infarct in the area of the posterior cerebral artery is presented. The neuropsychological examination did not reveal aphasia or gross mental deficits. The patient presented with alexia without agraphia, color agnosia, but few visual perceptual deficits. The main impairment was in confrontation naming; he was incapable of naming objects and pictures, not from lack of recognition (excluding visual agnosia) but from lack of access to the appropriate word (optic aphasia). The patient also exhibited a deficit in the evocation of gesture from the visual presentation of an object (optic apraxia) and a difficulty in "conjuring up" visual images of objects (impaired visual imagery) and loss of dreams. The fundamental deficit of this patient is tentatively explained in terms of visuoverbal and visuogestural disconnection and a deficit of mental imagery.     

An open-label trial of bromocriptine in nonfluent aphasia: a qualitative analysis of word storage and retrieval

Anomia is a commonly found in aphasia and has been attributed to a loss of representations (storage deficit) or to a loss of access to these representations (retrieval deficit). Bromocriptine, a dopamine agonist, was tested on four patients, two men and two women, with nonfluent aphasia. The patients were tested in an open-label ABBA design using a stochastic model that measured the degree of storage and retrieval deficits. All patients showed significant improvements in word retrieval. Bromocriptine may be a useful adjunct in the treatment of selected patients with a nonfluent aphasia in which retrieval deficits play a major role.     

Modeling lexical decision and word naming as a retrieval process.

We argue that rule-like phenomena in naming and lexical decision reflect the collapsing of information that occurs during retrieval from the lexicon, and that complex patterns in performance reflect the pattern of correlation that exists in the reader's lexicon rather than mapping rules wired into, or learned by, the processing apparatus. By using a lexicon built to scale, we show that simple retrieval operations applied to a large corpus of words correctly predict an interaction of word frequency by spelling-to-sound regularity in naming, a frequency main effect in lexical decision, sensitivity to orthographically defined syllable-like structures in lexical decision, and an interaction of number of syllables with word frequency in naming.     

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.     

Contrasting effects of phonological priming in aphasic word production

Two fluent aphasics, IG and GL, performed a phonological priming task in which they repeated an auditory prime then named a target picture. The two patients both had selective deficits in word production: they were at or near ceiling on lexical comprehension tasks, but were significantly impaired in picture naming. IG's naming errors included both semantic and phonemic paraphasias, as well as failures to respond, whereas GL's errors were mainly phonemic and formal paraphasias. The two patients responded very differently to phonological priming: IG's naming was facilitated (both accuracy and speed) only by begin-related primes (e.g. ferry-feather), whereas GL benefited significantly only from end-related primes (e.g. brother-feather), showing no more than a facilitatory trend with begin-related primes. We interpret these results within a two-stage model of word production, in which begin-related and end-related primes are said to operate at different stages. We then discuss implications for models of normal and aphasic word production in general and particularly with respect to sequential aspects of the phonological encoding process.     

Lexical Retrieval Deficit in Picture Naming: Implications for Word Production Models

In models of word retrieval, it is common to differentiate lexical-semantic (word meaning) and lexical-phonological (word form) levels. There has been considerable interest in the relationship between these two levels. The so-called discrete two-stage model claims that phonological activation follows selection at the lexical-semantic level and is limited only to the selected item. In contrast, nondiscrete (interactive and cascade) models assume that all activated lexical-semantic candidates are also phonologically activated to some extent. We addressed this issue by studying an anomic patient who suffered from a partial functional disconnection between lexical-semantic and lexical-phonological levels. A multitarget repeated naming task with phonological training was employed. Systematic manipulation of semantic and phonological relatedness between the to-be-named items indicated that our patient's word error patterns were sensitive to both types of lexical relatedness. A delayed repetition task employing the same items failed to show similar effects, suggesting that they were specific to naming. The discrete two-stage model is unable to explain the observed effects of semantic and phonological relatedness. However, they are consistent with assumptions of nondiscrete models of lexical retrieval. In addition to the theoretical implications of this study, the observed effects of lexical context on word retrieval have implications for treatment of anomia.     

Fluent versus nonfluent primary progressive aphasia: a comparison of clinical and functional neuroimaging features

To better characterize fluent and nonfluent variants of primary progressive aphasia (PPA). Although investigators have recognized both fluent and nonfluent patients with PPA, the clinical and neuroimaging features of these variants have not been fully defined. We present clinical and neuropsychological data on 47 PPA patients comparing the fluent (n=21) and nonfluent (n=26) subjects. We further compared language features with PET/SPECT data available on 39 of these patients. Compared to the nonfluent PPA patients, those with fluent PPA had greater impairment of confrontational naming and loss of single word comprehension. They also exhibited semantic paraphasic errors and loss of single word comprehension. Patients with nonfluent PPA were more likely to be female, were more often dysarthric, and exhibited phonological speech errors in the absence of semantic errors. No significant differences were seen with regard to left hemisphere abnormalities, suggesting that both variants result from mechanisms that overlap frontal, temporal, and parietal regions. Of the language measures, only semantic paraphasias were strongly localized, in this case to the left temporal lobe. Fluent and nonfluent forms of PPA are clinically distinguishable by letter fluency, single word comprehension, object naming, and types of paraphasic errors. Nevertheless, there is a large amount of overlap between dysfunctional anatomic regions associated with these syndromes.     

Origins of Paraphasias in Deep Dysphasia: Testing the Consequences of a Decay Impairment to an Interactive Spreading Activation Model of Lexical Retrieval

This study investigates an account of atypical error patterns within the framework of an interactive spreading activation model. Martin and Saffran (1992) described a patient, NC, whose error pattern was unusual for the occurrence of higher rates of form-related than meaning-related word substitutions in naming and the production of semantic errors in repetition. They proposed that NC′s error pattern could be accounted for by a pathologically rapid decay of primed nodes in the semantic-lexical-phonological network that shifts the probabilities of error outcome in lexical retrieval. In the present study, Martin and Saffran′s account was tested and supported in a series of simulations that reproduce essential features of NC′s lexical error pattern in naming and repetition. Also described here are the results of a longitudinal study of NC′s naming and repetition, which revealed a shift in relative lexical error rates toward a qualitatively normal pattern. This change in error pattern was simulated by assuming that recovery reflects resolution of the rapid decay rate toward normal levels. The patient data and computational studies are discussed in terms of their significance for the understanding of aphasic impairments and their implications for models of lexical retrieval.