Cortical and subcortical mechanisms in persistent stuttering ...

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encoding lexical, grammatical, phonological or suprasegmental (e.g. word stress) targets in speech production (Bloodstein and Ratner, 2008). Phonological encoding is the linguistic process that is most often considered to be disturbed in stuttering (Smith et al., 2010). Prominent theories are the neuropsycholinguistic theory (Perkins et al., 1991), the Covert Repair Hypothesis (Postma and Kolk, 1993) and the EXPLAN theory (Howell, 2004). These theoretical accounts posit that motor breakdowns result from slowed or faulty phonological encoding, a linguistic processing stage that precedes motor planning and execution as detailed in Appendix A. These accounts suggest primary a deficit in language competence and language performance. An elaborated review on psycholinguistic accounts is given in (Bloodstein and Ratner, 2008) and a brief summary, focusing on accounts of a phonological encoding deficit in stuttering, is given in Appendix C. 1.4.2 Motor deficit perspective Persons who stutter exhibit difficulties in initiating and controlling speech movements (Peters et al., 2000). Mechanisms governing a precise adjustment of the respiratory, laryngeal and articulatory system are operating less efficiently or are disrupted in timing or coordination and thus interfere with the smooth course of articulatory movements (Adams, 1974; Kent, 1984; Van Riper, 1971; Zimmermann, 1980b). Difficulties in initiating speech movements have been extensively examined by means of acoustic reaction time studies. Compared to control subjects, persons who stutter were slower in initiating speech movements unequivocally for the initiation of complex utterances (Peters et al., 1989). Because reaction time is a cumulative measure of linguistic and motor processes a general conclusion regarding the initiation of speech movements in stuttering is pending (Smits-Bandstra, 2010). The control of timing is one important aspect of speech motor control and in several attempts it has been hypothesized that stuttering is a disorder of timing (Kent, 1984; Ludlow and Loucks, 2003; Olander et al., 2010). Several studies of perceptually fluent speech of persons who stutter reveal deviations in variability, speed and relative timing of speech movements (Kleinow and Smith, 2000; Max et al., 2003; Zimmermann, 1980a). Comparisons of non-speech oral movements and finger movements between persons who stutter and control subjects suggest a general neuromotor deficit in stuttering (Cooper and Allen, 1977; Max et al., 2003; Zelaznik et al., 1997). Examinations of unimanual and bimanual rhythmic finger tapping or finger sequencing studies reveal unequivocal results and differences between persons who stutter and control subjects manifest mainly in complex conditions (Olander et al., 2010). To conclude, the complex spatial-temporal coordination 16
Chapter 1 Introduction independent of the executing organs (orofacial /limb) is constrained in the system of persons who stutter. Aberrant production-perception-interaction Current theories of speech production integrate perceptive processes and productionperception-interactions. Several researchers consider an aberrant sensory feedback system as potential cause of stuttering. Civier and Guenther (2010) distinguish three views: (1) persons who stutter differ from control subjects by relying too heavily on sensory feedback (Tourville et al., 2008; van Lieshout et al., 1993); (2) persons who stutter benefit from reliance on sensory feedback (Max et al., 2004; Namasivayam et al., 2008; van Lieshout et al., 1996); (3) due to an impaired feedforward control system, persons who stutter rely more heavily on a feedback-based motor control strategy (Civier et al., 2010; De Nil et al., 2001; Kalveram and Jancke, 1989; Zimmermann, 1980b). This suggests that an over-reliance towards an auditory feedback control strategy increases the systems’ vulnerability to produce errors. Those errors might cause the motor system to “reset” and repeat the current syllable (Civier et al., 2010). Repetitions would then result from the attempts to repair large sensorimotor errors as simulated in the computer model and proven in one person who stutters. 1.5 Neurophysiological approaches to explain stuttering Since the formulation of the cerebral dominance theory (Orton, 1928) researchers have speculated about potential involvement of aberrant neural processes in the onset and development of stuttering (De Nil, 2004). Early research into the nature of these deviations was mainly based on behavioural observations and electromyographic measurements. With advances in neuroimaging techniques such as positron emissions tomography (PET) and magnetic resonance imaging (MRI) manifold findings about the neural differences between persons who stutter and control subjects has been aggregated, motivating the emergence of different hypothesis of brain function in stuttering. The following section targets to introduce three of these hypotheses leading to the motivation of the studies presented in this dissertation. 17
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 10 and 11: addition I conducted an fMRI study
Page 12 and 13: movements which enable a fast detec
Page 14 and 15: aforementioned description of the s
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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