The Effects of Word Frequency and Word Probability on Speech Rhythm in Dysarthria

Lexical Characteristics of the Speech Intelligibility Test: Effects on Transcription Intelligibility for Speakers With Multiple Sclerosis and Parkinson's Disease

PURPOSE

Lexical characteristics of speech stimuli can significantly impact intelligibility. However, lexical characteristics of the widely used Speech Intelligibility Test (SIT) are unknown. We aimed to (a) define variation in neighborhood density, word frequency, grammatical word class, and type-token ratio across a large corpus of SIT sentences and tests and (b) determine the relationship of lexical characteristics to speech intelligibility in speakers with multiple sclerosis (MS), Parkinson's disease (PD), and neurologically healthy controls.

METHOD

Using an extant database of 92 speakers (32 controls, 30 speakers with MS, and 30 speakers with PD), percent correct intelligibility scores were obtained for the SIT. Neighborhood density, word frequency, word class, and type-token ratio were calculated and summed for each of the 11 sentences of each SIT test. The distribution of each characteristic across SIT sentences and tests was examined. Linear mixed-effects models were performed to assess the relationship between intelligibility and the lexical characteristics.

RESULTS

There was large variability in the distribution of lexical characteristics across this large corpus of SIT sentences and tests. Modeling revealed a relationship between intelligibility and the lexical characteristics, with word frequency and word class significantly contributing to the model.

CONCLUSIONS

Three primary findings emerged: (a) There was considerable variability in lexical characteristics both within and across the large corpus of SIT tests; (b) there was not a robust association between intelligibility and the lexical characteristics; and (c) findings from a study demonstrating an effect of neighborhood density and word frequency on intelligibility were replicated. Clinical and research implications of the findings are discussed, and three exemplar SIT tests systematically controlling for neighborhood density and word frequency are provided.

Intelligibility Across a Reading Passage: The Effect of Dysarthria and Cued Speaking Styles

OBJECTIVE

Reading a passage out loud is a commonly used task in the perceptual assessment of dysarthria. The extent to which perceptual characteristics remain unchanged or stable over the time course of a passage is largely unknown. This study investigated crowdsourced visual analogue scale (VAS) judgments of intelligibility across a reading passage as a function of cued speaking styles commonly used in treatment to maximize intelligibility.

PATIENTS AND METHOD

The Hunter passage was read aloud in habitual, slow, loud, and clear speaking styles by 16 speakers with Parkinson's disease (PD), 30 speakers with multiple sclerosis (MS), and 32 control speakers. VAS judgments of intelligibility from three fragments representing the beginning, middle, and end of the reading passage were obtained from 540 crowdsourced online listeners.

RESULTS

Overall passage intelligibility was reduced for the two clinical groups relative to the control group. All speaker groups exhibited intelligibility variation across the reading passage, with trends of increased intelligibility toward the end of the reading passage. For control speakers and speakers with PD, patterns of intelligibility variation across passage reading did not differ with speaking style. For the MS group, intelligibility variation across the passage was dependent on speaking style.

CONCLUSIONS

The presence of intelligibility variation within a reading passage warrants careful selection of speech materials in research and clinical practice. Results further indicate that the crowdsourced VAS rating paradigm is useful to document intelligibility in a reading passage for different cued speaking styles commonly used in treatment for dysarthria.

Investigating Acoustic Correlates of Intelligibility Gains and Losses During Slowed Speech: A Hybridization Approach

Purpose This exploratory study sought to identify acoustic variables explaining rate-related variation in intelligibility for speakers with dysarthria secondary to multiple sclerosis. Method Seven speakers with dysarthria due to multiple sclerosis produced the same set of Harvard sentences at habitual and slow rates. Speakers were selected from a larger corpus on the basis of rate-related intelligibility characteristics. Four speakers demonstrated improved intelligibility and three speakers demonstrated reduced intelligibility when rate was slowed. A speech analysis resynthesis paradigm termed hybridization was used to create stimuli in which segmental (i.e., short-term spectral) and suprasegmental variables (i.e., sentence-level fundamental frequency, energy characteristics, and duration) of sentences produced at the slow rate were donated individually or in combination to habitually produced sentences. Online crowdsourced orthographic transcription was used to quantify intelligibility for six hybridized sentence types and the original habitual and slow productions. Results Sentence duration alone was not a contributing factor to improved intelligibility associated with slowed rate. Speakers whose intelligibility improved with slowed rate showed higher intelligibility scores for duration spectrum hybrids and energy hybrids compared to the original habitual rate sentences, suggesting these acoustic cues contributed to improved intelligibility for sentences produced with a slowed rate. Energy contour characteristics were also found to play a role in intelligibility losses for speakers with decreased intelligibility at slowed rate. The relative contribution of speech acoustic variables to intelligibility gains and losses varied considerably between speakers. Conclusions Hybridization can be used to identify acoustic correlates of intelligibility variation associated with slowed rate. This approach has further elucidated speaker-specific and individualized speech production adjustments when slowing rate.