On the Causal Mechanisms of Stuttering
Abstract: Stuttering is one of the most common speech disorders. However, the etiology is poorly understood, and is likely to be heterogeneous. The aim of this thesis was to clarify causal mechanisms, focusing the brain. The project included theoretical development based on published data, and a broad approach of explorative studies and testing of hypotheses. The theoretical work focused the basal ganglia, leading to a model based on the dual premotor systems hypothesis (G. Goldberg, 1985, 1991), which defines two parallel premotor systems: the medial (the basal ganglia and the SMA), and the lateral (the lateral premotor cortex and the cerebellum). Stuttering is suggested to be caused by a disturbance of the medial system, in most cases in the basal ganglia. The core dysfunction is proposed to be impaired "go-signals" from the medial system, supposed to trigger the next motor segment in speech. According to this model, under some conditions speech control is shifted from the medial to the lateral system, thereby bypassing the dysfunction and resulting in fluent speech. The lateral system is suggested to be active when speech is combined with sensory input, like chorus speech or metronome. Also the effect of altered auditory feedback in reducing stuttering is proposed to be based on this mechanism. It seems as the lateral system is able control speech timing without sensory input, but that this demands increased attention to some particular aspect of speech, as occurs in imitation of dialects, exaggerated rhythm, reduced speech rate, or role play. Also singing is suggested to be based on the lateral system. Superfluous muscular activation accompanying stuttering may be a type of dystonia: involuntary contractions related to the basal ganglia disturbance. The high prevalence of stuttering at age 2.5 to 3 years is proposed to reflect a normally occurring peak in the number of dopamine receptors at this age. A total of 35 stuttering adults participated. Two studies, of copper metabolism and "startle prepulse inhibition", did not indicate any significant differences in comparison with matched controls. It has previously been reported that stuttering may be associated with increased neuromuscular reactivity, measured as exaggerated eye-blink in response to noise. This aspect was investigated. The stuttering group showed somewhat stronger eye-blink, though not statistically significant. Strong eye-blink was not related to anxiety, but was clearly related to low calcium, which is known to increase the excitability of the nervous system. The stuttering group showed somewhat lower calcium, and a weak tendency towards more severe stuttering in case of low calcium. It is possible that low calcium can increase the severity of stuttering in some cases. A subgroup reported some traits of childhood ADHD, and this group typically also reported neurological incidents before the onset of stuttering. The subgroup without traits of ADHD typically reported having stuttering relatives but no neurological incident.
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