Evidence for multiple routes of speech production in a case of fluent aphasia

Neural organization of speech production: A lesion-based study of error patterns in connected speech

While numerous studies have explored single-word naming, few have evaluated the behavioral and neural correlates of more naturalistic language, like connected speech, which we produce every day. Here, in a retrospective analysis of 120 participants at least six months following left hemisphere stroke, we evaluated the distribution of word errors (paraphasias) and associated brain damage during connected speech (picture description) and object naming. While paraphasias in connected speech and naming shared underlying neural substrates, analysis of the distribution of paraphasias suggested that lexical-semantic load is likely reduced during connected speech. Using voxelwise lesion-symptom mapping (VLSM), we demonstrated that verbal (real word: semantically related and unrelated) and sound (phonemic and neologistic) paraphasias during both connected speech and naming loaded onto the left hemisphere ventral and dorsal streams of language, respectively. Furthermore, for the first time using both connected speech and naming data, we localized semantically related paraphasias to more anterior left hemisphere temporal cortex and unrelated paraphasias to more posterior left temporal and temporoparietal cortex. The connected speech results, in particular, highlight a gradient of specificity as one translates visual recognition from left temporo-occipital cortex to posterior and subsequently anterior temporal cortex. The robustness of VLSM results for sound paraphasias derived during connected speech was notable, in that analyses performed on sound paraphasias from the connected speech task, and not the naming task, demonstrated significant results following removal of lesion volume variance and related apraxia of speech variance. Therefore, connected speech may be a particularly sensitive task on which to evaluate further lexical-phonological processing in the brain. The results presented here demonstrate the related, though different, distribution of paraphasias during connected speech, confirm that paraphasias arising in connected speech and single-word naming likely share neural origins, and endorse the need for continued evaluation of the neural substrates of connected speech processes.

Language mixing patterns in a bilingual individual with non-fluent aphasia

BACKGROUND

Language mixing in bilingual speakers with aphasia has been reported in a number of research studies, but the reasons for the mixing and whether it reflects typical or atypical behaviour has been a matter of debate.

AIMS

In this study we tested the hypothesis that language mixing behaviour in bilingual aphasia reflects lexical retrieval difficulty.

METHODS & PROCEDURES

We recruited a Hebrew-English bilingual participant with mild-moderate non-fluent agrammatic aphasia and assessed his languages at three timepoints. We analysed the participant's Hebrew and English production for retrieval during single-word naming, sentences, and discourse, and identified all instances of language mixing.

OUTCOMES & RESULTS

We found that there was a greater frequency of language mixing during production of more difficult lexical items, namely the post-morbidly less proficient language (compared to the more proficient language), function words (compared to content words), and single-word naming (compared to retrieval in the context of connected speech tasks), but not for verbs (compared to nouns).

CONCLUSIONS

In this bilingual participant with non-fluent aphasia, language mixing behaviour closely resembles lexical retrieval difficulty. Thus, we suggest that bilingual speakers with aphasia may mix their languages as a strategy to maximise communication.

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.

Modality-specific naming deficit: cognitive and neural mechanisms implicated in naming to definition

We report the case of an anomic patient who had a significantly greater impairment in naming to definition than in picture naming. His difficulty did not depend on the number of semantic attributes (two vs. four) or type of information (visual vs. non-visual) carried in the definition. When asked to visualize the perceptual characteristics of the stimulus by means of drawing, his performance on the naming to definition task improved significantly. We first discuss this performance pattern in terms of the debate between the modality access deficit hypothesis and the multiple semantics position. The patient's more severe impairment in naming to definition suggests that his dissociation may be due to a more selective involvement of verbal semantics. In line with recent PET findings indicating that top-down semantic-to-visual sensory neural feedback improves the retrieval of semantic information in naming to definition tasks, we hypothesize that the patient's difficulty in retrieving the name was due to his inability to automatically activate this pathway. Picture drawing facilitated his naming because it activated the visual system directly (through the structure description level), implementing the retrieval of semantic information.

Innovations in neuropsychological assessment using event-related brain potentials

Historically, clinicians have utilized evoked potentials for evaluating sensory functions and neuropsychological tests for evaluating cognitive functions. However, the clinical implementation of event-related brain potentials (ERPs), an on-line index of cognitive processing, remains to be developed fully. We describe a new method for assessing language functions using neuropsychological tests that are formatted for computer presentation with simultaneous ERP recordings. From its inception, there have been two major objectives of this ERP language assessment research. Practically, we have sought to develop assessment techniques that would enable clinicians to evaluate the language functions of individuals with limited behavioral and communicative abilities. Conceptually, we have endeavored to increase the precision of neuropsychological testing through the development of measures that are sensitive to readily identifiable and objective neural responses. This article summarizes the issues central to the development of ERP assessment techniques, reviews recent normative studies with healthy individuals, and suggests some future avenues of research in this area.

An event-related brain potential study of receptive speech comprehension using a modified Token Test

There is a long and distinguished history of evaluating sensory functions with evoked potentials (EPs). However, the clinical utility of event-related brain potentials (ERPs) in the assessment of cognitive functions remains to be established. We present evidence for the efficacy of ERPs in the assessment of speech comprehension. In this normative study, a version of the Token Test (Boller and Vignolo, 1966, Brain, 89, 815-831: De Renzi and Vignolo, 1962, Brain, 85, 665-678) was modified for computer presentation with simultaneous ERP recordings and then administered to a group of healthy individuals (n = 27). A differential neural response pattern to incorrect and correct test items was obtained for the group and, more importantly, this result was also observed reliably at the individual subject level. This research complements on-going efforts in our laboratory to develop innovative measures for patients that are difficult or impossible to assess due to motoric and/or communicative limitations.

Global aphasia: an innovative assessment approach

OBJECTIVE

To provide an alternative language comprehension assessment strategy for patients unable to be tested with traditional verbally/behaviorally based methods.

DESIGN

Event-related brain potentials were recorded from three midline scalp locations to visually and aurally computer-presented sentences, 50% of which were semantically appropriate and 50% semantically incongruous.

SETTING

A rehabilitation hospital.

PATIENT

A 21-year-old man with a traumatic brain injury.

RESULTS

The patient exhibited brain response patterns to aurally presented congruous and incongruous sentences indicative of intact semantic processing capabilities. These findings resulted in reinstatement of individualized rehabilitative intervention, with a successful outcome.

CONCLUSIONS

This innovative technique provides new opportunities for assessing intellectual function in noncommunicative patients who were patients previously unable to be tested.