Anticipatory coarticulation of the Spanish alveolar fricative /s/ in adults with apraxia versus dysarthria

Abstract

This acoustic study compares anticipatory coarticulation characteristics of the Spanish alveolar sibilant fricative /s/ when in utterance-initial position followed by a vowel in adults with dysarthria and apraxia of speech. Three groups of participants (28 individuals with no speech disorder, 20 with dysarthria, and 8 with apraxia of speech) produced 12 monosyllabic words that included the five vowel sounds of central-peninsular Spanish. The acoustic measurements compared within and between groups were frequency of the spectral intensity peak (Freq_Mid) in different zones of fricative execution, magnitude of the change in frequency of the spectral intensity peak (ΔFreq) in the end zone compared to the average of the initial and middle zones, first three spectral moments, and the difference in spectral center of gravity between the middle and end zones (Diff_M-E CoG). Several of these measures were able to differentiate between dysarthric and healthy speech, especially when /s/ was followed by an unrounded vowel, and the same occurred for apraxia, but this time when the adjacent vowel was rounded. While both disorders showed similar spectral patterns, the two motor speech disorders differed in terms of the measures Freq_Mid and Diff_M-E CoG. Possible explanations for these differences are here discussed within the framework of motor control models.