[The influence of certain contextual factors on voice onset time, vowel duration and utterance duration in verbal apraxia].
The concept of context sensitivity as borrowed from the coalition model has important implications for the interpretation of the symptoms of apraxia of speech and for a better understanding of the nature of the disorder. In this study which was part of the wider investigation into the effect of variation in contextual factors on apraxia of speech (van der Merwe, Uys, Loots and Grimbeek, 1987; 1988), the effect of two contextual factors namely sound structure and articulatory features on voice onset time, vowel duration and utterance duration was acoustically analysed. Four subjects with acquired apraxia of speech and one with developmental apraxia of speech were tested. Findings indicated that voice onset time is not context sensitive. Errors in voice onset time were not true substitutions with voiceless sounds but were inconsistent distortions due to an inability to keep interarticulator synchronization within the critical temporal boundaries of motor equivalence. Vowel and utterance duration were found to be context sensitive. The deviation in duration increased with increased complexity of the utterance. As was the case with the auditorily perceived symptoms, the temporal characteristics of apraxia of speech can also be classified into core symptoms and associated symptoms based on their context sensitivity.