Cortical and subcortical mechanisms in persistent stuttering ...

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addition I conducted an fMRI study of the neuroanatomical correlates of continuous performance in stuttering in cooperation with the Biomedical NMR Research GmbH at the Max Planck Institute for Biophysical Chemistry. An internship at the speech Laboratory of Purdue University allowed me to study the consolidation of speech motor learning in children who stutter by using the Optotrak, a system that delivers a 3D tracking and measuring of speech kinematics. These ongoing studies are not included here. As the drafts of the studies themselves give only limited space to introduction of the topic, a more detailed introduction is given in sections 1.1 to 1.5. 1.1 Components of the process of fluent articulation Stuttering is a disorder with intermittent interruptions of fluent articulation. Fluent articulation is one of human‘s most complex motor skills. It comprises the coordinated use of approximately 100 muscles (Ackermann, 2008) and it is fascinating how effortless this skill is managed by almost every human being. Rapid, complex movements are essential to articulate the sounds of speech. Here I briefly summarize the structures involved in articulation, which is the ultimate readout of language planning and speech motor control processes. Subsequently, I refer to influential theories on language planning and speech motor control because all these aspects are implied in different approaches to explain stuttering. Articulation involves three anatomically distinct subsystems: the respiratory, laryngeal and supralaryngeal system. The respiratory system regulates the outflow of air during speech and thus provides the energy for the acoustic targets of speech. The core structures of the laryngeal system are the vocal folds, controlling voicing and loudness of speech. During voicing the oscillation of the vowel folds generates the fundamental frequency on which resonation builds. The larynx provides the quasiperiodic and tone-like sound fundamental for vowels and voiced consonants (e.g. [b], [z] and [m]). The supralaryngeal system contains the pharyngeal, oral and nasal cavities whose architecture and configuration shape the timbre and the sound of the generated acoustic signal. The supralaryngeal system also called the vocal tract can be constricted at different places for example via lip closure, lip protrusion, tongue tip or body elevation or retraction, and velum elevation. Characteristic sound features of speech vowels are generated by overlapping vocal tract actions such as jaw lowering, tongue body elevation, and lip protrusion. In contrast, the striking acoustic features of consonants are generated by the magnitude of obstruction, resulting in bursts due to closure and friction-like noise due to fine-tuned constriction. 10
Chapter 1 Introduction The respiratory, laryngeal and supralaryngeal systems recruit distributed neural networks to channel the muscle activation into organized spatio-temporal speech movement patterns. The following neural structures are central for this function: (1) Orofacial and laryngeal sensorimotor cortex and the corticobulbar and the corticospinal tracts (2) Premotor cortex, insula, supplementary motor area, and cingulate motor area (3) Motoneurons in the brainstem (nucleus trigeminus, nucleus facialis, nucleus glossopharyngeus, nucleus vagus, nucleus accessories, nucleus hypoglossus) (4) Motoneurons and associated spinal interneurons which control the respiratory musculature are found distributed across cervical segments (C1-C8) and thoracic segments (T1-T12) of the spinal cord (5) Peripheral nerve fibers of the mentioned motoneurons and their neuromuscular junctions (6) Extrapyramidal tracts, basal ganglia-thalamocortical pathway (7) Cerebellum with its efferent and afferent fibers, cerebello-thalamocortical pathway Anatomy and physiology of speech production is comprehensively described by Steven M. Barlow or Kenneth N. Stevens (Barlow et al., 1999; Stevens, 2000). In normal conversation, a speaker produces 3 to 5 intelligible syllables per second (Smith, 1992); thus, the nervous system manages to simultaneously control and coordinate the overlapping articulatory gestures to produce rapidly altering configurations of the multilevel executing speech organs. Preceding and simultaneously, a message that is intended to be transferred to a communication partner has to be created and transformed into the verbal code. This cognitive process is detailed by Levelt in his influential model of speech production (Levelt, 1989c). A brief summary of this psycholinguistic model which shaped several theories on stuttering is given in Appendix A. Speech motor control is a further aspect that needs to be considered for the complex process of speech production. A current model of speech motor control is the Directions into Velocities of Articulators model (DIVA; Golfinopoulos et al., 2010; Guenther, 1994, 1995). In this model speech motor control is based on a feed forward and a feedback control subsystem. The feedforward process is supposed to control the execution of speech movements. Additionally, the feedforward subsystem activates predictive internal models (efference copies) in the feedback subsystem. These internal models represent the expectation of the incoming somatosensory and auditory feedback resulting from current speech 11
Page 1 and 2: Cortical and subcortical mechanisms
Page 3: Statement of Originality I hereby d
Page 6 and 7: Appendix B - Stuttering and acquire
Page 8 and 9: PM premotor cortex PMd dorsolateral
Page 12 and 13: movements which enable a fast detec
Page 14 and 15: aforementioned description of the s
Page 16 and 17: encoding lexical, grammatical, phon
Page 18 and 19: 1.5.1 The cerebral dominance hypoth
Page 20 and 21: motor hand area in adults who stutt
Page 22 and 23: Figure 1-1 from Mink, 1996: Schemat
Page 24 and 25: dystonia (Berardelli et al., 2008).
Page 26 and 27: Table 1-2 Results from diffusion te
Page 29: Chapter 2 Original articles 2 Origi
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Chapter 2 Study 2 Excitability of t
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Chapter 2 Study 3 Phoneme Categoriz
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Chapter 4 Conclusions and future pr
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include the conceptualizer, the for
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ω σ´ σ σ st� t� r�� Fi
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Figure A-5 Directions into velociti
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associated with dysarthria, aphasia
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Table B-2 Tuning of fluency by Deep
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A Appendix C Appendix C - Psycholin
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are useful tools to investigate int
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Benabid AL, Pollak P, Louveau A, He
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De Nil LF, Kroll RM, and Houle S. F
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Hickok G and Poeppel D. The cortica
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Levelt WJM. Speaking: from intentio
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Neef N, Paulus W, and Sommer M. Dem
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Smith A and Kelly E. Stuttering: A
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160 unilateral left subthalamic bra
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I thank Tibor Auer for his constant
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