Quantifying articulatory impairments in neurodegenerative motor diseases: A scoping review and meta-analysis of interpretable acoustic features

Abstract

Purpose: Neurodegenerative motor diseases (NMDs) have devastating effects on the lives of patients and their loved ones, in part due to the impact of neurologic abnormalities on speech, which significantly limits functional communication. Clinical speech researchers have thus spent decades investigating speech features in populations suffering from NMDs. Features of impaired articulatory function are of particular interest given their detrimental impact on intelligibility, their ability to encode a variety of distinct movement disorders, and their potential as diagnostic indicators of neurodegenerative diseases. The objectives of this scoping review were to identify (1) which components of articulation (i.e. coordination, consistency, speed, precision, and repetition rate) are the most represented in the acoustic literature on NMDs; (2) which acoustic articulatory features demonstrate the most potential for detecting speech motor dysfunction in NMDs; and (3) which articulatory components are the most impaired within each NMD.

Method: This review examined literature published between 1976 and 2020. Studies were identified from six electronic databases using predefined key search terms. The first research objective was addressed using a frequency count of studies investigating each articulatory component, while the second and third objectives were addressed using meta-analyses.

Result: Findings from 126 studies revealed a considerable emphasis on articulatory precision. Of the 24 features included in the meta-analyses, vowel dispersion/distance and stop gap duration exhibited the largest effects when comparing the NMD population to controls. The meta-analyses also revealed divergent patterns of articulatory performance across disease types, providing evidence of unique profiles of articulatory impairment.

Conclusion: This review illustrates the current state of the literature on acoustic articulatory features in NMDs. By highlighting the areas of need within each articulatory component and disease group, this work provides a foundation on which clinical researchers, speech scientists, neurologists, and computer science engineers can develop research questions that will both broaden and deepen the understanding of articulatory impairments in NMDs.

Keywords: acoustics; articulation; articulatory impairment; domain knowledge; dysarthria; neurodegenerative diseases; speech acoustics.

Figure 1.

Dysarthria subtypes for each neurodegenerative motor diseases in the current review, their perceptual articulatory characteristics based on a widely used taxonomy of speech motor disorders (ordered from most severe to least severe, with * indicating more severely impaired than other dysarthria subtypes), and the corresponding areas of impairment based on our framework of articulatory components.

Figure 2.

Definitions of the five framework components of articulatory motor and examples of acoustic features that represent each component.

Figure 3.

PRISMA chart illustrating the article search procedures and selection of included studies.

Figure 4.

Pie chart illustrating the number of studies investigating each neurodegenerative motor disease. Studies were counted more than once if they investigated more than one disease. The number in parentheses represents the percentage of the 126 studies that investigated that specific disease.

Figure 5.

Bar graph illustrating the articulatory components investigated across all studies in individuals with neurodegenerative motor diseases. The frequency of features investigated in each disease is displayed in the corresponding box.

Figure 6.

Forest plot illustrating effect sizes (disease group compared to healthy controls) for each articulatory feature. The number of participants investigated in each disease is displayed above the corresponding dot. The total number of participants across all diseases is displayed in brackets to the right of the features. Positive effect sizes indicate that the disease group exhibited poorer performance compared to controls, while negative effect sizes indicate that the disease group performed better than controls.

Figure 7.

Forest plot illustrating divergent patterns of articulatory performance across neurodegenerative motor diseases. Each dot represents the mean effect size (disease group compared to healthy controls) for all acoustic features within each articulatory component.