Dysarthria and Apraxia of Speech (AoS) are motor speech disorders in which neurological lesions differentially affect motor control, possibly leading to noticeable differences in articulation and consequently sound production. Among the sounds requiring greater motor capacity because of its articulatory complexity is the voiceless alveolar sibilant fricative /s/. The aim of this study was to identify acoustic variables able to distinguish between dysarthria and AoS, and between these disorders and normal speech in Spanish speakers. The production of this fricative was acoustically examined in 28 individuals with motor neurological disorders (20 with dysarthria, 8 with AoS) and in 28 neurologically healthy persons. Participants repeated 12 monosyllabic words containing the fricative plus one of the five Spanish vowels. The variables measured were absolute durations of the fricative, vowel, and fricative+vowel sequence, along with the vowel-to-fricative duration ratio. Findings indicate that duration of the fricative can distinguish between controls and speakers with dysarthria, but not between controls and speakers with AoS. Measures related to vowel duration served to distinguish between speakers with dysarthria and speakers with AoS and between each of them and controls. Further, speakers with dysarthria and those with AoS differed from each other and from controls in terms of articulatory variability; speakers with dysarthria showing most variability. In the latter participants, articulatory variability was higher for unrounded segments, vowels and fricatives, while in speakers with AoS this variability was higher for rounded segments. These observations are discussed within a framework of motor control models.
Keywords: Apraxia of speech; differential diagnosis; dysarthria; fricative; segmental duration.