Vowel targeting and perception in apraxia of speech
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Vowel production and perception were examined as a window on speech motor control processes in five adults with acquired apraxia of speech (AOS) and five nonbrain-damaged (NBD) control participants. Articulatory targeting for vowels was assessed acoustically for the three front vowels [ɪ], [ɛ], and [æ] in four stimulus conditions varying in word length. Vowels were produced in a normal, unconstrained context and in two bite block conditions, where the mouth opening was controlled by placing a small or large piece of plastic between the teeth. Vowel perception for front vowels was tested using standard categorical tests of identification and same-different discrimination. Processing of vowel information also was assessed in tests of rhyme generation and judgment. Acoustic analysis of apraxic vowel production revealed formant frequencies within normal ranges. Introduction of the bite block constraint destabilized vowel targeting for both apraxic and normal participants, resulting in greater targeting error and reduced distinctiveness between adjacent vowels in the vowel space. Vowel formants in multisyllabic words varied from those produced in monosyllables for both groups, although these deviations were perceived as normal variation by listeners. Although apraxic vowel formants generally conformed to normal ranges, perceptual goodness ratings indicated poorer perceived quality of apraxic vowels compared to NBD controls, and measures of vowel targeting accuracy and vowel distinctiveness also were consistently inferior for apraxic speakers. Perceptual testing revealed normal vowel discrimination in all AOS participants, while four of five apraxic listeners had inconsistent ability to identify vowels. Comparison of production and perception measures indicated no significant relationship between abilities in the two domains for AOS or NBD participants. Distinctiveness of produced vowels was significantly related to clinical measures of speech deficits in apraxia, while perceptual deficits were correlated with auditory comprehension scores. Findings suggest that vowel production in AOS is characteristic of a motor targeting deficit, although variability of vowel formants has a minor effect on the overall communicative impairment in people with the disorder. The lack of correspondence between production and perception abilities indicates that perceptual processing is not a major factor in the motor targeting abilities of individual with AOS.