A Vowel-Centric View Toward Characterizing Temporal Organization of Motor Speech Activities in Neurologically Impaired and Healthy Speakers

Purpose: This study tested the hypotheses that (a) motor speech activities are temporally organized around the nuclei into vowel-centric units that hold both stability and flexibility and (b) such temporal organization is impacted by motor speech impairment. Method: Thirteen individuals with amyotrophic lateral sclerosis and 10 healthy controls read a sentence 3 times at each of the following rates: habitual, fast, and slow. Articulatory gestures and phonatory event were assessed in two vowel-centric units, as operationally defined within and across the boundaries of two target words-cat and must-to accommodate common coda omission and coarticulation. Twelve absolute and relative timing measures centering on the nucleus were derived to characterize the temporal organization of each unit. These measures were evaluated in terms of (a) their relations with global duration across rate conditions and (b) between-groups differences for the habitual rate condition.Results: Both vowel-centric units remained stable in relative timing between the articulatory gestures approaching and moving away from the nucleus across rate conditions. Relative timing between the articulatory gestures and phonatory event at smaller temporal granularities varied with global duration, but in different ways for neurologically impaired and healthy speakers. Disease impacts on relative timing were only detected across word boundaries. All absolute timing measures revealed consistent temporal scaling effects and disease-related prolongations.Conclusions: The findings provide preliminary support for vowel-centric temporal organization of motor speech activities. Such temporal organization holds some extent of both stability and flexibility, which may facilitate the parsing of syllabic events during auditory processing, while accommodating task-specific suprasegmental variations. The timing impairments in amyotrophic lateral sclerosis are likely attributed to the disease-imposed dynamic constraints, reducing the entrainment of the related motor speech activities to the underlying linguistic elements. These findings have potential implications in guiding the assessment and management of temporal speech deficits in ALS.