Neurobiology of speaking and stuttering

Abstract

A multi-level model of speech motor control including the word, syllable and phonem level is proposed, which takes into account linguistic, technical, biological, physiological and developmental aspects. Crucial point is the double function of the vowel onset, which serves to control vowel length and also to synchronize the serialization of words into temporal strings of syllables. In (long) stressed syllables this is done via auditory feedback (re-afferent control), while the utterance of unstressed syllables is automatized using internal signals (efferent control). Young children's problem when crossing from infantil to adultlike speech is to learn to inhibit re-afferent and to apply efferent control of unstressed syllables. If this task is learned incompletely, the model predicts stuttering, e.g. speech blocks, or repetitions and prolongations of syllables or phonems. Because all these control and learning processes refer to low level motor control, the model also contributes to unburden persons who stutterer and their parents from personal responsibility for the occurence of this communication disorder. FACTS, THEORIES AND BELIEVES CONCERNING STUTTERING Basic symptoms of stuttering are repetitions of words or syllables or parts of them, prolongations of speech sounds (phonems), and blockades. All of young children aged between two and five years use to have normal dysfluencies (Yairi 1981, 1997, Froschels 1952), but about 5% of the children show the basic symptoms of stuttering mentioned above. Many of the children lose these dysfluencies until puberty. In the end, about 1% of the adult population exhibits, regardless of culture and language, persistent speech dysfluencies, now three times more male than female people. Stuttering often is detrimental for personal achievement, social contacts, carreer, and self-esteeme, therefore, almost all people who stutter participated in at least one therapy, whereby, however, stuttering often cannot be cured completely, or re-appears later on. Explanations of the symptoms of stuttering usually start from investigations referring to sensory feedback of self produced speech, especially to the effects of delayed auditory feedback (Lee 1951). Usually, nonstuttering persons exhibit repetitions of syllables and words under this experimental manipulation, whereby delay times between 100 and 300 ms are most effective. Paradoxically, symptoms diminish in stutterers under delayed auditory feedback, at least at the beginning of application (Bergmann 1987). Occasionally, also auditory masking by loud noise applied through headphones facilitates fluent in stutterers (Bergmann 1987). Fluency enhancement in stutterers is also reported when using frequency shifted auditory feedback (Howell et al 1987, Natke & Kalveram, in press). Because of such studies, it is concluded (e.g. Van Riper 1982, Perkins et al. 1976, Adams 1974) that during speech development the auditory feedback channel gradually loses influence on speech control, until, in adults, only tactile-proprioceptive feedback is used for purposes of control. Speech dysfluencies, then, originate from auditory feedback not sufficiently being suppressed. This may lead to interferences and/or discoordinations between different feedback channels, which disturbes the auto-regulation of speaking (Fiedler & Standop 1992). However, experiments of Bauer et al. (1997) do not support interference models of stuttering. In these experiments, irregular tactile and proprioceptive feedback was artificially generated through mechanical disturbations of the jaw movement during speaking. These disturbations neither led to significant changes of speech relevant parameters of the acoustical speech signal, nor differentiated between stutterers and non-stutterers. Furthermore, experiments with manipulated auditory feedback (see also the chapter concerning audiophonatory coupling) show that the auditory feedback channel is not at all switched off during speaking, neither in stutterers nor in fluent speakers. Therefore, it can be argued that auditory, not proprioceptive, feedback is of particular importance for speech flow control, 1 Supported by grants Ka417/13-1 to -3 from Deutsche Forschungsgemeinschaft (DFG)