The association between longitudinal declines in speech sound accuracy and speech intelligibility in speakers with amyotrophic lateral sclerosis.
Hannah P Rowe, Kaila L Stipancic, Thomas F Campbell and 2 others
PMID 37122073WHAT IT FOUND
In 21 speakers with ALS, intelligibility dropped sharply when phoneme accuracy fell below 75%.
Consonant errors hurt intelligibility more than vowel errors, especially for fricatives and affricates. For those already below 75% accuracy, improving phoneme scores may not help intelligibility.
Key findings
01The relationship between phoneme accuracy and intelligibility is non-linear: small declines in accuracy cause large drops in intelligibility when accuracy is above 75%, but further declines have little effect when accuracy is already below 75%.
02Consonant accuracy was significantly associated with intelligibility, whereas vowel accuracy was not.
03Fricatives, affricates, and stops declined significantly more over time than nasals, and labiodentals declined significantly more than bilabials and palatals.
STILL TO COME
How it was doneWhat they foundWhat it means for SLPs
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What it does not show
The sample size was small (21 participants), which limited statistical power and required separate models for different predictor groups. The time between disease onset and the first data collection session was unknown for all participants, so decline was measured from the first session rather than diagnosis. Participants read the same passage multiple times, which may have caused practice effects. The Bamboo Passage is not phonetically balanced and is loaded with voiced sounds, which may bias findings related to voicing. Transcribers were familiar with the passage, which might have slightly inflated phoneme accuracy scores. The study did not analyze specific error types (e.g., substitutions vs. distortions) or word position effects.
The easy way to misread this
Do not assume that improving phoneme accuracy will always improve intelligibility. The study found that for speakers whose phoneme accuracy is already below 75%, further declines in accuracy did not lead to noticeable additional drops in intelligibility, suggesting a limited working range for this outcome.