Vowel quality in aphasia and apraxia of speech: Phonetic transcription and formant analyses

We examined acoustic and perceptual features of vowel quality in aphasia and apraxia of speech. Twenty aphasic speakers with and without apraxia of speech and ten normal speakers produced the words "hid'' and "head'' approximately 24 times. Each production was transcribed with broad phonetic transcription, and the first and second formant frequencies were measured at the midpoint of the vowel steady state. According to the phonetic transcription, some aphasic and apraxic speakers displayed a large number of vowel substitutions, whereas others were indistinguishable from normal speakers. Perceived substitutions were generally close to the target and affected almost exclusively vowel height rather than vowel frontness. Acoustically, several speakers in both aphasic groups displayed a formant pattern that deviated from normal. The nature of the deviation pattern varied across individual aphasic and apraxic speakers. For some, formant frequencies were abnormally variable, whereas others displayed a pattern of only occasional deviations, and yet others demonstrated a collapsing of phonetic categories. The results are consistent with previous reports that articulatory positioning for vowels is impaired in many aphasic and apraxic speakers. The existence of individual articulatory patterns is emphasised, and the limitations of a static approach to formant analysis are noted.