Investigating the origin of nonfluency in aphasia: A path modeling approach to neuropsychology

Connected Speech Fluency in Poststroke and Progressive Aphasia: A Scoping Review of Quantitative Approaches and Features

PURPOSE

Speech fluency has important diagnostic implications for individuals with poststroke aphasia (PSA) as well as primary progressive aphasia (PPA), and quantitative assessment of connected speech has emerged as a widely used approach across both etiologies. The purpose of this review was to provide a clearer picture on the range, nature, and utility of individual quantitative speech/language measures and methods used to assess connected speech fluency in PSA and PPA, and to compare approaches across etiologies.

METHOD

We conducted a scoping review of literature published between 2012 and 2022 following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews guidelines. Forty-five studies were included in the review. Literature was charted and summarized by etiology and characteristics of included patient populations and method(s) used for derivation and analysis of speech/language features. For a subset of included articles, we also charted the individual quantitative speech/language features reported and the level of significance of reported results.

RESULTS

Results showed that similar methodological approaches have been used to quantify connected speech fluency in both PSA and PPA. Two hundred nine individual speech-language features were analyzed in total, with low levels of convergence across etiology on specific features but greater agreement on the most salient features. The most useful features for differentiating fluent from nonfluent aphasia in both PSA and PPA were features related to overall speech quantity, speech rate, or grammatical competence.

CONCLUSIONS

Data from this review demonstrate the feasibility and utility of quantitative approaches to index connected speech fluency in PSA and PPA. We identified emergent trends toward automated analysis methods and data-driven approaches, which offer promising avenues for clinical translation of quantitative approaches. There is a further need for improved consensus on which subset of individual features might be most clinically useful for assessment and monitoring of fluency.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.25537237.

A narrative review of the anatomy and function of the white matter tracts in language production and comprehension

Much is known about the role of cortical areas in language processing. The shift towards network approaches in recent years has highlighted the importance of uncovering the role of white matter in connecting these areas. However, despite a large body of research, many of these tracts’ functions are not well-understood. We present a comprehensive review of the empirical evidence on the role of eight major tracts that are hypothesized to be involved in language processing (inferior longitudinal fasciculus, inferior fronto-occipital fasciculus, uncinate fasciculus, extreme capsule, middle longitudinal fasciculus, superior longitudinal fasciculus, arcuate fasciculus, and frontal aslant tract). For each tract, we hypothesize its role based on the function of the cortical regions it connects. We then evaluate these hypotheses with data from three sources: studies in neurotypical individuals, neuropsychological data, and intraoperative stimulation studies. Finally, we summarize the conclusions supported by the data and highlight the areas needing further investigation.

Behavioural and neural structure of fluent speech production deficits in aphasia

Deficits in fluent speech production following left hemisphere stroke are a central concern because of their impact on patients' lives and the insight they provide about the neural organization of language processing. Fluent speech production requires the rapid coordination of phonological, semantic, and syntactic processing, so this study examined how deficits in connected speech relate to these language sub-systems. Behavioural data (N = 69 participants with aphasia following left hemisphere stroke) consisted of a diverse and comprehensive set of narrative speech production measures and measures of overall severity, semantic deficits, and phonological deficits. These measures were entered into a principal component analysis with bifactor rotation-a latent structure model where each item loads on a general factor that reflects what is common among the items, and orthogonal factors that explain variance not accounted for by the general factor. Lesion data were available for 58 of the participants, and each factor score was analysed with multivariate lesion-symptom mapping. Effects of connectivity disruption were evaluated using robust regression with tract disconnection or graph theoretic measures of connectivity as predictors. The principal component analysis produced a four-factor solution that accounted for 70.6% of the variance in the data, with a general factor corresponding to the overall severity and length and complexity of speech output (complexity factor), a lexical syntax factor, and independent factors for Semantics and Phonology. Deficits in the complexity of speech output were associated with a large temporo-parietal region, similar to overall aphasia severity. The lexical syntax factor was associated with damage in a relatively small set of fronto-parietal regions which may reflect the recruitment of control systems to support retrieval and correct usage of lexical items that primarily serve a syntactic rather than semantic function. Tract-based measures of connectivity disruption were not statistically associated with the deficit scores after controlling for overall lesion volume. Language network efficiency and average clustering coefficient within the language network were significantly associated with deficit scores after controlling for overall lesion volume. These results highlight overall severity as the critical contributor to fluent speech in post-stroke aphasia, with a dissociable factor corresponding to lexical syntax.

Anterior connectivity critical for recovery of connected speech after stroke

Connected speech recovers to different degrees across people after left hemisphere stroke, but white matter predictors of differential recovery from the acute stage of stroke are unknown. We assessed changes in lexical-syntactic aspects of connected speech in a longitudinal analysis of 40 individuals (18 females) from the acute stage of left hemisphere stroke (within an average of 4 days post-stroke) to subacute (within 2 months) and chronic stages (early: 6 months, late: 1 year) while measuring the extent of acute lesions on white matter tracts to identify tracts predictive of recovery. We found that acute damage to the frontal aslant tract led to a decreased recovery of the fluency and structural complexity of connected speech during the year following left hemisphere stroke. The results were independent of baseline performance, overall lesion volume and the proportion of damage to tract-adjacent grey matter. This longitudinal analysis from acute to chronic stroke provides the first evidence that recovery of fluent and structurally complex spontaneous connected speech requires intact left frontal connectivity via the frontal aslant tract. That the frontal aslant tract was critical for recovery at early as well as later stages of stroke demonstrates that anterior connectivity plays a lasting and important role for the reorganization of function related to the successful production of connected speech.

How Do Clinicians Judge Fluency in Aphasia?

PURPOSE

Aphasia fluency is multiply determined by underlying impairments in lexical retrieval, grammatical formulation, and speech production. This poses challenges for establishing a reliable and feasible tool to measure fluency in the clinic. We examine the reliability and validity of perceptual ratings and clinical perspectives on the utility and relevance of methods used to assess fluency.

METHOD

In an online survey, 112 speech-language pathologists rated spontaneous speech samples from 181 people with aphasia (PwA) on eight perceptual rating scales (overall fluency, speech rate, pausing, effort, melody, phrase length, grammaticality, and lexical retrieval) and answered questions about their current practices for assessing fluency in the clinic.

RESULTS

Interrater reliability for the eight perceptual rating scales ranged from fair to good. The most reliable scales were speech rate, pausing, and phrase length. Similarly, clinicians' perceived fluency ratings were most strongly correlated to objective measures of speech rate and utterance length but were also related to grammatical complexity, lexical diversity, and phonological errors. Clinicians' ratings reflected expected aphasia subtype patterns: Individuals with Broca's and transcortical motor aphasia were rated below average on fluency, whereas those with anomic, conduction, and Wernicke's aphasia were rated above average. Most respondents reported using multiple methods in the clinic to measure fluency but relying most frequently on subjective judgments.

CONCLUSIONS

This study lends support for the use of perceptual rating scales as valid assessments of speech-language production but highlights the need for a more reliable method for clinical use. We describe next steps for developing such a tool that is clinically feasible and helps to identify the underlying deficits disrupting fluency to inform treatment targets.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.19326419.

Neural Correlates of Letter and Semantic Fluency in Primary Progressive Aphasia

Verbal fluency (VF) is an informative cognitive task. Lesion and functional imaging studies implicate distinct cerebral areas that support letter versus semantic fluency and the understanding of neural and cognitive mechanisms underlying task performance. Most lesion studies include chronic stroke patients. People with primary progressive aphasia (PPA) provide complementary evidence for lesion-deficit associations, as different brain areas are affected in stroke versus PPA. In the present study we sought to determine imaging, clinical and demographic correlates of VF in PPA. Thirty-five patients with PPA underwent an assessment with letter and category VF tasks, evaluation of clinical features and an MRI scan for volumetric analysis. We used stepwise regression models to determine which brain areas are associated with VF performance while acknowledging the independent contribution of clinical and demographic factors. Letter fluency was predominantly associated with language severity (R2 = 38%), and correlated with the volume of the left superior temporal regions (R2 = 12%) and the right dorsolateral prefrontal area (R2 = 5%). Semantic fluency was predominantly associated with dementia severity (R2 = 47%) and correlated with the volume of the left inferior temporal gyrus (R2 = 7%). No other variables were significantly associated with performance in the two VF tasks. We concluded that, independently of disease severity, letter fluency is significantly associated with the volume of frontal and temporal areas whereas semantic fluency is associated mainly with the volume of temporal areas. Furthermore, our findings indicated that clinical severity plays a critical role in explaining VF performance in PPA, compared to the other clinical and demographic factors.

Development of a measure of function word use in narrative discourse: core lexicon analysis in aphasia

BACKGROUND

Although discourse-level assessments contribute to predicting real-world performance in persons with aphasia (PWA), the use of discourse measures is uncommon in clinical settings due to resource-heavy procedures. Moreover, assessing function word use in discourse requires the arduous procedure of defining grammatical categories for each word in language transcripts.

AIMS

The purpose of this exploratory study was twofold: (1) to develop core function word lists as a clinician-friendly means of evaluating function word use in discourse; and (2) to examine the ability of the core function word measure to differentiate PWA from cognitively healthy adults and persons with fluent aphasia from non-fluent aphasia.

METHODS & PROCEDURES

The 25 most commonly used function words (core function words) were extracted from narrative language samples from 470 cognitively healthy adults, which were divided into seven age groups (20s, 30s, 40s, 50s, 60s, 70s and 80s). The percent agreement of core function words for 11 PWA (fluent aphasia = 5; non-fluent aphasia = 6) and 11 age- and education-matched controls were then calculated. Percent agreement for the core function words produced was compared between the controls and the PWA group, and between participants with fluent aphasia and non-fluent aphasia.

OUTCOMES & RESULTS

The results indicated that PWA produced fewer core function words from the lists than the control group, and that core function word use was strongly correlated with aphasia severity. Persons with non-fluent aphasia produced fewer core function words than those with fluent aphasia, although this could be a confound of aphasia classification from the use of the Western Aphasia Battery (WAB)-Revised.

CONCLUSIONS & IMPLICATIONS

Core function word lists consisting of a limited number of items for quantifying function word use in discourse remain in a nascent stage of development. However, the findings are consistent with previous studies analysing the total production of function words in language samples produced by PWA. Therefore, core function words may potentially serve as a clinician-friendly manner of quantifying function words produced in discourse. What this paper adds What is already known on the subject Function word analysis in discourse requires arduous processes of identifying the error production and grammatical category of function words in discourse. Previous studies have demonstrated that core lexicon measures are an efficient, simple means of quantifying discourse in PWA. However, function words have never been considered for generating an independent core lexicon list. What this paper adds to existing knowledge As an exploratory study, we focused primarily on developing a clinician-friendly measure to evaluate function word production in discourse, motivated by the idea of an adaptation strategy within the core lexicon framework. Our findings demonstrated that by using a simple scoring system that the core lexicon measure provides, we differentiated the control group from the PWA group, and persons with fluent aphasia from persons with non-fluent aphasia. Additionally, we found significant correlations between function word production and aphasia severity determined by WAB Aphasia Quotient (AQ). What are the potential or actual clinical implications of this work? The results add empirical evidence for the utility of core function word lists for quantifying function word usage in discourse in PWA. Counting the presence and absence of function words in discourse will allow clinicians to avoid labour-intensive preparatory work, and to obtain useful diagnostic information in a less time-consuming way.

Factor Analysis of Spontaneous Speech in Aphasia

Purpose Spontaneous speech tasks are critically important for characterizing spoken language production deficits in aphasia and for assessing the impact of therapy. The utility of such tasks arises from the complex interaction of linguistic demands (word retrieval, sentence formulation, articulation). However, this complexity also makes spontaneous speech hugely variable and difficult to assess. The current study aimed to simplify the problem by identifying latent factors underlying performance in spontaneous speech in aphasia. The ecological validity of the factors was examined by examining how well the factor structures corresponded to traditionally defined aphasia subtypes. Method A factor analysis was conducted on 17 microlinguistic measures of narratives from 274 individuals with aphasia in AphasiaBank. The resulting factor scores were compared across aphasia subtypes. Supervised (linear discriminant analysis) and unsupervised (latent profile analysis) classification techniques were then conducted on the factor scores and the solutions compared to traditional aphasia subtypes. Results Six factors were identified. Two reflected aspects of fluency, one at the phrase level (Phrase Building) and one at the narrative level (Narrative Productivity). Two other factors reflected the accuracy of productions, one at the word level (Semantic Anomaly) and one at the utterance level (Grammatical Error). The other two factors reflected the complexity of sentence structures (Grammatical Complexity) and the use of repair behaviors (Repair), respectively. Linear discriminant analyses showed that only about two thirds of speakers were classified correctly and that misclassifications were similar to disagreements between clinical diagnoses. The most accurately diagnosed syndromes were the largest groups-Broca's and anomic aphasia. The latent profile analysis also generated profiles similar to Broca's and anomic aphasia but separated some subtypes according to severity. Conclusions The factor solution and the classification analyses reflected broad patterns of spontaneous speech performance in a large and representative sample of individuals with aphasia. However, such data-driven approaches present a simplified picture of aphasia patterns, much as traditional syndrome categories do. To ensure ecological validity, a hybrid approach is recommended, balancing population-level analyses with examination of performance at the level of theoretically specified subgroups or individuals. Supplemental Material https://doi.org/10.23641/asha.13232354.

Dissociation between frontal and temporal-parietal contributions to connected speech in acute stroke

Humans are uniquely able to retrieve and combine words into syntactic structure to produce connected speech. Previous identification of focal brain regions necessary for production focused primarily on associations with the content produced by speakers with chronic stroke, where function may have shifted to other regions after reorganization occurred. Here, we relate patterns of brain damage with deficits to the content and structure of spontaneous connected speech in 52 speakers during the acute stage of a left hemisphere stroke. Multivariate lesion behaviour mapping demonstrated that damage to temporal-parietal regions impacted the ability to retrieve words and produce them within increasingly complex combinations. Damage primarily to inferior frontal cortex affected the production of syntactically accurate structure. In contrast to previous work, functional-anatomical dissociations did not depend on lesion size likely because acute lesions were smaller than typically found in chronic stroke. These results are consistent with predictions from theoretical models based primarily on evidence from language comprehension and highlight the importance of investigating individual differences in brain-language relationships in speakers with acute stroke.

Mapping psycholinguistic features to the neuropsychological and lesion profiles in aphasia

Naming and word retrieval deficits are two of the most persistent symptoms in chronic post-stroke aphasia. Naming success or failure on specific words can sometimes be predicted by the psycholinguistic properties of the word. Despite a wealth of literature investigating the influence of psycholinguistic properties in neuro-typical and clinical language processing, the underlying structure of these properties and their relation to the fundamental language components and neural correlates are unexplored. In this study, a multivariate data-decomposition approach was used to identify the underlying structure within a collection of psycholinguistic properties (word imageability, frequency, age-of-acquisition, familiarity, length, semantic diversity and phonological neighbourhood density) and their influence on naming accuracy was explored in a cohort of 42 participants with a diverse range of chronic post-stroke aphasia classifications and severities. The results extracted three principal psycholinguistic factors, which were best described as 'lexical usage', 'semantic clarity' and 'phonological complexity'. Furthermore, a novel approach was used to systematically relate the influence of these psycholinguistic properties to participants' neuropsychological and lesion profiles. The findings did not show a one-to-one mapping between psycholinguistic features and core language components. 'Lexical usage' was the only factor that showed a significant difference between fluent versus non-fluent aphasia groups in terms of the influence of this lexical factor on successful naming, and it was the only factor that was related to the pattern of patients' brain lesions. Voxel-wise whole brain lesion-symptom mapping identified left frontal regions, aligning with previous evidence that these regions are related to language production functions, including word retrieval and repetition. The evidence from the current study suggests that the functional locus of psycholinguistic properties is distributed across multiple language components rather than being localised to a single language element.

Apraxia of speech involves lesions of dorsal arcuate fasciculus and insula in patients with aphasia

Objective

To determine the contributions of apraxia of speech (AOS) and anomia to conversational dysfluency.

Methods

In this observational study of 52 patients with chronic aphasia, 47 with concomitant AOS, fluency was quantified using correct information units per minute (CIUs/min) from propositional speech tasks. Videos of patients performing conversational, how-to and picture-description tasks, word and sentence repetition, and diadochokinetic tasks were used to diagnose AOS using the Apraxia of Speech Rating Scale (ASRS). Anomia was quantified by patients' scores on the 30 even-numbered items from the Boston Naming Test (BNT).

Results

Together, ASRS and BNT scores accounted for 51.4% of the total variance in CIUs/min; the ASRS score accounted for the majority of that variance. The BNT score was associated with lesions in the left superior temporal gyrus, left inferior frontal gyrus, and large parts of the insula. The global ASRS score was associated with lesions in the left dorsal arcuate fasciculus (AF), pre- and post-central gyri, and both banks of the central sulcus of the insula. The ASRS score for the primary distinguishing features of AOS (no overlap with features of aphasia) was associated with less AF and more insular involvement. Only ∼27% of this apraxia-specific lesion overlapped with lesions associated with the BNT score. Lesions associated with AOS had minimal overlap with the frontal aslant tract (FAT) (<1%) or the extreme capsule fiber tract (1.4%). Finally, ASRS scores correlated significantly with damage to the insula but not to the AF, extreme capsule, or FAT.

Conclusions

Results are consistent with previous findings identifying lesions of the insula and AF in patients with AOS, damage to both of which may create dysfluency in patients with aphasia.

Uncovering the Neuroanatomy of Core Language Systems Using Lesion-Symptom Mapping

Recent studies have integrated noninvasive brain-imaging methods and advanced analysis techniques to study associations between the location of brain damage and cognitive deficits. By applying data-driven analysis methods to large sets of data on language deficits after stroke (aphasia), these studies have identified the cognitive systems that support language processing—phonology, semantics, fluency, and executive functioning—and their neural basis. Phonological processing is supported by dual pathways around the Sylvian fissure, a ventral speech-recognition component and a dorsal speech-production component; fluent sentence-level speech production relies on a more anterior frontal component, and the semantic system relies on a hub in the anterior temporal lobe and frontotemporal white-matter tracts. The executive function system was less consistently localized, possibly because of the kinds of brain damage tested in these studies. This review synthesizes the results of these studies, showing how they converge with contemporary models of primary systems that support perception, action, and conceptual knowledge across domains, and highlights some divergent findings and directions for future research.