Abstract
The last decades of research have gradually elucidated the complex functions of the dopamine system in the vertebrate brain. The multiple roles of dopamine in motor function, learning, attention, motivation, and the emotions have been difficult to reconcile. A broad and detailed understanding of the physiology of cerebral dopamine is of importance in understanding a range of human disorders. One of the core functions of dopamine involves the basal ganglia and the learning and execution of automatized sequences of movements. Speech is one of the most complex and highly automatized sequential motor behaviors, though the exact roles that the basal ganglia and dopamine play in speech have been difficult to determine. Stuttering is a speech disorder that has been hypothesized to be related to the functions of the basal ganglia and dopamine. The aim of this review was to provide an overview of the current understanding of the cerebral dopamine system, in particular the mechanisms related to motor learning and the execution of movement sequences. The primary aim was not to review research on speech and stuttering, but to provide a platform of neurophysiological mechanisms, which may be utilized for further research and theoretical development on speech, speech disorders, and other behavioral disorders. Stuttering and speech are discussed here only briefly. The review indicates that a primary mechanism for the automatization of movement sequences is the merging of isolated movements into chunks that can be executed as units. In turn, chunks can be utilized hierarchically, as building blocks of longer chunks. It is likely that these mechanisms apply also to speech, so that frequent syllables and words are produced as motor chunks. It is further indicated that the main learning principle for sequence learning is reinforcement learning, with the phasic release of dopamine as the primary teaching signal indicating successful sequences. It is proposed that the dynamics of the dopamine system constitute the main neural basis underlying the situational variability of stuttering.
Highlights
1.1 Background and AimStuttering is a speech disorder, which core symptoms manifest as an intermittent loss of volitional control of the speech movements, resulting in various forms of speech disruptions and speech motor abnormalities (Perkins, 1990; Bloodstein and Bernstein-Ratner, 2008)
The motivation came from indications that the dopamine system and the neural mechanisms for automatization are likely to be fundamental for the childhood acquisition of speech, and are likely to be involved in the mechanisms of stuttering in some ways
The aim of this article was to provide a brief general overview of the current understanding of the cerebral dopamine system, and to focus on mechanisms related to the automatization of motor sequences, which may be of importance for the understanding of speech and stuttering
Summary
Stuttering is a speech disorder, which core symptoms manifest as an intermittent loss of volitional control of the speech movements, resulting in various forms of speech disruptions and speech motor abnormalities (Perkins, 1990; Bloodstein and Bernstein-Ratner, 2008). This means that stuttering is displayed as a disorder of motor execution. The present review on dopamine and motor automatization was written as part of a research program on stuttering and speech, within a series of theoretical articles. Speech and stuttering are discussed briefly in appropriate contexts, and some possible implications of the reviewed research are included
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