Perceptual, acoustic, aerodynamic, kinematic, videostroboscopic, electroglotto-graphic (EGG), and electromyographic (EMG) studies have documented disorders of laryngeal, respiratory, articulatory, and velopharyngeal function in individuals with PD (19-21). The neural mechanisms underlying these voice and speech disorders are unclear (22-25). Traditionally, these abnormalities have been attributed to rigidity, bradykinesia, hypokinesia, and tremor secondary to dopamine deficiency, yet there is little evidence in support of these etiologic factors. Alternative explanations for the speech and voice disorders have been proposed, such as deficits in internal cueing, sensory gating, scaling of movement amplitude, and self-regulation of vocal effort (4,26-28). These deficits have been hypothesized to be related to nondopaminergic or special dopaminergic mechanisms (29,30).
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