An acoustic measure of lexical stress differentiates aphasia and aphasia plus apraxia of speech after stroke

Background: Apraxia of Speech (AOS) is partly characterised by impaired production of prosody in words and sentences. Identification of dysprosody is based on perceptual judgements of clinicians, with limited literature on potential quantitative objective measures. Aims: This study investigated whether an acoustic measure quantifying degree of lexical stress contrastiveness in three syllable words, produced in isolation and in a carrier sentence, differentiated individuals with AOS with/without aphasia (AOS), aphasia only (APH), and healthy controls (CTL). Methods & Procedures: Eight individuals with aphasia, nine with AOS plus aphasia and 8 age-matched control participants named pictures of strong-weak and weak-strong polysyllabic words in isolation and in a declarative carrier sentence. Pairwise Variability Indices (PVI) were used to measure the normalised relative vowel duration and peak intensity over the first two syllables of the polysyllabic words. Outcomes & Results: Individuals with aphasia performed similarly to control participants in all conditions. AOS participants demonstrated significantly lower PVI_vowel duration values for words with weak-strong stress produced in the sentence condition only, compared to controls and individuals with aphasia. This was primarily due to disproportionately long vowels in the word-initial weak syllable for AOS participants. There was no difference among groups on PVI_intensity. Conclusions: The finding of reduced lexical stress contrastiveness for weak-strong words in sentences for individuals with mild to moderate-severe AOS is consistent with the perceptual diagnostic feature of equal stress in AOS. Findings provide support for use of the objective PVI_vowel duration measure to help differentiate individuals with AOS (with/without aphasia), from those with aphasia only. Future research is warranted to explore the utility of this acoustic measure, and others, for reliable diagnosis of AOS.