The nonlinear gestural model of speech apraxia: clinical implications and applications

ABSTRACT

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.

KEYWORDS:

• Apraxia of speech
• error patterns
• influencing factors
• computational model
• articulatory gestures

Acknowledgments

We thank ReHa-Hilfe e.V. for their continuing support and Anja Staiger for her substantial input in numerous discussions. We are also grateful to our collaborators in clinics and private practices, and to the many persons with AoS who were willing to cooperate as participants in the studies reviewed here.

Disclosure statement

The authors declare no conflict of interest.

Notes

1. There are several other, non-phonological influencing factors which are not listed in Table 1, such as word or syllable frequency, lexicality, elicitation mode, etc. (Wertz et al., 1984).

2. Phonological word (or prosodic word) is a prosodic category. It describes constituents above the level of the metrical foot, but below the level of the intonation phrase.

3. In the AP framework, specific assumptions are made regarding the dynamics of the articulator movements constituting a gesture and the phase relationships of inter-gestural coupling (Goldstein et al., 2006). The NLG model is not specified in any particular way for such assumptions, even though they may be relevant to the pathomechanism of AoS. See for instance the discussion in Hagedorn et al. (2017).

4. This index is not universal in the sense of Jakobson (1941), since it is (i) language specific and (ii) derived from the speech of a population of adult speakers who we consider specifically impaired in the phonetic planning of words, but not in other learned, sub-cortical aspects of motor speech.

5. All calculations were performed using the eRm-package in R (Mair, Hatzinger, & Maier, 2015).

Additional information

Funding

The research reviewed in this article was supported by the Deutsche Forschungsgemeinschaft [DFG, ZI 469/6-1/8-1/14-1,2]. The second author was granted a PhD fellowship of the German National Academic Foundation, the third author was supported by the Bayerische Sparkassenstiftung.