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Ziegler, Wolfram; Lehner, Katharina; Pfab, Jakob; Aichert, Ingrid (2020): The nonlinear gestural model of speech apraxia: clinical implications and applications. In: Aphasiology, Vol. 35, No. 4: pp. 462-484
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Rationale: In the approach presented here, adult apraxia of speech (AoS) is understood as a disorder affecting the language-specific motor patterns a speaker acquires during childhood and adolescence. The phonological factors that influence articulatory accuracy in AoS reflect the relative vulnerability of their accompanying motor planning variables to apraxic impairment, depending on the extent to which they have been consolidated in speech motor learning. Since these factors interact strongly, the Nonlinear Gestural (NLG) model combines them into an integrative computational framework. We thereby aim at explaining word production accuracy in AOS from a word's structural properties across multiple hierarchical layers, - from articulatory gestures to metrical patterns. The NLG model: In a review of earlier articles, the NLG-model is first introduced by describing five core principles underlying its design, whereupon the computational and empirical underpinnings of the model are explained. Finally, the model shape is delineated by specifying its empirically derived coefficients, with a focus on clinical implications. Clinical applications: A web-based NLG calculator is described that allows clinicians to compute NLG scores for German words or pseudowords and create displays of their gestural scores, as a prerequisite of incorporating the NLG approach in their diagnostic and therapeutic work. Furthermore, we describe how the structural properties of words, as represented by their NLG scores, can be linked to other clinically relevant lexical variables. Potential applications of these tools involve (i) the design of assessment and treatment materials following a hierarchy of articulatory difficulty, (ii) the interpretation of error patterns following the NLG hierarchy as a signature of the apraxic pathomechanism, with possible differential diagnostic applications, and (iii) computation of a distance metric to describe the relationship between target words and their realisations by patients with AoS. Expansions to other types of speech sound impairment in adults and children are discussed.