The domain of phonetic encoding in apraxia of speech: Which sub‐lexical units count?

Abstract

Background: Apraxia of speech (AOS) is generally viewed as a disorder of phonetic encoding. According to current theories, the phonetic encoding process is considered to be based on linear strings of holistic speech motor representations of the size of sub‐lexical units (e.g., phonemes or syllables). This type of model predicts that error rate in apraxia of speech is proportional to the number of phonetic encoding units in an utterance and is not systematically influenced by structural factors originating above or below the level of the critical unit. Aims: In the present study we tested this prediction for five models based on different units of phonetic encoding. Methods and Procedures: Ten patients with AOS were examined with a list of words of varying phonological complexity. Phoneme errors in word repetition were counted in different ways, according to five models postulating different domains of phonetic encoding. Linear regression analyses were used to examine the scores obtained for these models for influences of structural factors. Outcomes & Results: No sublexical unit could be identified that was not systematically influenced by one of the structural factors describing the materials. Major influencing factors were (1) the complexity of syllable constituents and (2) the number of metrical feet in a word. Conclusions: A model of phonetic encoding in apraxia of speech is proposed that is based on a hierarchy of structural relationships rather than on linearly ordered, holistic programming units. This model may also contain clues for a theory of normal processing mechanisms. This study was supported by a grant from the German research foundation (DFG; ZI 469/10‐2). We would like to thank ReHa‐Hilfe e.V. for additional support. We are also grateful to our colleagues from the Neuropsychological Clinic, Bogenhausen Hospital, München, for their collaboration on clinical issues, and to Katrin Lindner and Patrizia Noël for discussions of phonological issues. Moreover, we would like to express our gratitude to the participants of this study.