Special Issue: Mechanisms of regulation in speech, eds. Mücke, Hermes & Cho When words don't come easily: A latent trait analysis of impaired speech motor planning in patients with apraxia of speech $ Wolfram Ziegler n , Ingrid Aichert, Anja Staiger EKN, Clinical Neuropsychology Research Group, Institute of Phonetics and Speech Processing Ludwig-Maximilians University Munich, Germany ARTICLE INFO Article history: Received 8 May 2016 Received in revised form 7 October 2016 Accepted 29 October 2016 Keywords: Apraxia of speech Articulation Item-response-theory Word production Speech errors Model ABSTRACT Apraxia of speech (AOS), a speech impairment caused by left hemisphere stroke, is considered as a clinical model to study the organization of speech motor plans. An earlier nonlinear gestural (NLG) model of word production based on AOS speech error data informs about the structure of phonetic plans of words (Ziegler & Aichert, 2015), but with the disadvantage that it does not allow for single case testing. The latent trait approach presented here complements this model by permitting assessment of the extent to which an individual non- standard word production pattern conforms to the NLG framework. A Rasch model of speech planning ability was calculated using accuracy data from 2 examinations each of 33 clinical cases on 114 German words. Case fit parameters of the patient sample revealed high model conformity of all cases. In simulation studies, simulated cases with response patterns reflecting five different error mechanisms were examined: random, word frequency based, syllable number based, consonant number based, and NLG based. Inclusion of simulated cases in the Rasch model revealed that only the NLG simulation was compatible with the latent trait reflecting speech motor planning ability. Our approach provides a powerful tool to test whether an observed pattern of sound production impairment conforms to the AOS pattern. Under the premise that AOS is considered as a clinical model of acquired speech planning abilities, this framework allows for the testing of specific hypotheses concerning the phonetic basis of ease-of-articulation. & 2016 Elsevier Ltd. All rights reserved. Words don’t come easily Tracy Chapman 1. Introduction Speaking is easy, – at least for healthy adults when they talk in their native language. Except for some tongue twisters or some rarely used foreign words there is nothing in our daily speaking that would ever call for any particular attention to our articulator movements. However, articulatory virtuosity is not a universal trait, since it is restricted to the motor patterns of the language we have acquired during childhood and adolescence. For a German speaker, the Czech word “prst” (finger) may be challenging for its consonant clusters, or the name of the Kerala city of “Tiruvananthapuram” may be difficult due to its length and its unfamiliar consonant articulations, but for speakers of Czech or Malayalam, respectively, these words will not cause any motor problems at all. Hence, “ease of articulation” is a language-dependent property which is hardly measurable in speakers as soon as they have reached a saturation point in the acquisition of their first language (cf. Ladefoged, 1990). This faculty is assumed to be anchored in a neural network of the human brain that becomes specialized, through extensive motor learning, to master the articulatory patterns of a speaker's native language. More specifically, the basal ganglia, a formation of subcortical nuclei in the brain, are considered to play a major role in the coordinative patterning of articulator movements during the stage of speech acquisition. With growing maturation, the acquired motor capacity is shifted to frontal cortical areas of the left hemisphere, i.e., left posterior inferior frontal cortex including the opercular part of Broca's area and adjacent ventral premotor cortex, Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/phonetics Journal of Phonetics 0095-4470/$ - see front matter & 2016 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.wocn.2016.10.002 ☆ Acknowledgment: This research was supported by the German Research Foundation (Grants Zi 469/9-1/14-1/14-2) and by ReHa-Hilfe e.V. n Correspondence to: Institute of Phonetics and Speech Processing Ludwig-Maximilians-Universität, Schellingstr. 3, 80799 München, Germany. E-mail address: wolfram.ziegler@ekn-muenchen.de (W. Ziegler). Please cite this article as: Ziegler, W., et al. When words don't come easily: A latent trait analysis of impaired speech motor planning in patients with apraxia of speech. Journal of Phonetics (2016), http://dx.doi.org/10.1016/j.wocn.2016.10.002 Journal of Phonetics ] (]]]]) ]]]–]]]