Speech Production as State Feedback Control

Background : Ziegler (2003) reviews neural and behavioural evidence to support a task-dependent model of motor control, whereby motor speech disorders represent impairment of a sensory-minor system specialised for speech. Based on this approach, Ziegler argues against the examination of volitional or novel nonspeech motor activities to gain insight into motor control for speech. Aims : The primary objective of this paper is to discuss Ziegler's (2003) conceptualisation of the task-dependent model and present an alternative integrative model. In the latter, speech and volitional nonspeech motor control are integrated into the functioning of a more general motor system where neural and behavioural systems demonstrate areas of overlap. Studies of the nervous system, the evolutionary foundations of speech in great apes, behaviour, and motor learning are presented to support an integrative model of motor control. Main Contribution : Neurological and evolutionary evidence strongly suggest that neural networks are flexible, multifaceted, multifunctional and overlapping in function. It is highly likely that a higher-level behaviour like speech involves networks that are similarly multifunctional and overlapping with other motor functions. Ziegler's concept of task-dependent motor control may relate as much to parameters such as complexity, familiarity, and automaticity of task performance as to the nonpeech-speech distinction. Studies are reviewed that support the inclusion of nonspeech motor tasks in assessment of the disordered speech motor control system. Specific nonspeech tasks clearly facilitate differential diagnosis and provide insight into the functioning breakdown of the motor system for related but more complex speech behaviours. Finally, motor learing studies are discussed with particular reference to how these might inform models of motor control. Conclusions : This response cautions against the seemingly premature acceptance of a model assuming separate sensory motor systems for volitional nonspeech motor activities and speech. Continued research, without the limitations imposed by such an assumption, is required to enhance understanding of component parts of complex behaviours and how those components interact in normal and damaged systems at different levels of a performance. These data have the potential to form the basis for new, more effective approaches to treatment of motor speech disorders.

A treatment system for reducing stuttering uses an auditory feedback modification technique to train the stutterer's speech motor control system to be more stable. The auditory feedback modification is based on a model of speech motor control in the human brain that incorporates a variation of observer-based control and Smith prediction. In addition, the Kalman gain of the model is set by comparing the speech muscular control signals sent to the person's vocal tract to the corresponding auditory speech sounds the person actually hears. It is believed that the speech motor control system of a stutterer has set the Kalman gain too high, thereby creating an unstable control system that in turn causes stuttering. A feedback modifier feeds back the stutterer's speech to the stutterer with a perturbation that is small enough to pass a validator function that is believed to be part of the speech motor control system. The small perturbations increase the difference between the target speech and the fed back speech, which is believed to cause the speech motor control system to decrease the Kalman gain. Over a treatment program, it is believed that the perturbations "train" or adapt the stutterer's speech motor control system so as to reduce the Kalman gain, which is further believed to persist when the auditory feedback is discontinued.

Many voice disorders are the result of intricate neural and/or biomechanical impairments that are poorly understood. The limited knowledge of their etiological and pathophysiological mechanisms hampers effective clinical management. Behavioral studies have been used concurrently with computational models to better understand typical and pathological laryngeal motor control. Thus far, however, a unified computational framework that quantitatively integrates physiologically relevant models of phonation with the neural control of speech has not been developed. Here, we introduce LaDIVA, a novel neurocomputational model with physiologically based laryngeal motor control. We combined the DIVA model (an established neural network model of speech motor control) with the extended body-cover model (a physics-based vocal fold model). The resulting integrated model, LaDIVA, was validated by comparing its model simulations with behavioral responses to perturbations of auditory vocal fundamental frequency (fo) feedback in adults with typical speech. LaDIVA demonstrated capability to simulate different modes of laryngeal motor control, ranging from short-term (i.e., reflexive) and long-term (i.e., adaptive) auditory feedback paradigms, to generating prosodic contours in speech. Simulations showed that LaDIVA’s laryngeal motor control displays properties of motor equivalence, i.e., LaDIVA could robustly generate compensatory responses to reflexive vocal fo perturbations with varying initial laryngeal muscle activation levels leading to the same output. The model can also generate prosodic contours for studying laryngeal motor control in running speech. LaDIVA can expand the understanding of the physiology of human phonation to enable, for the first time, the investigation of causal effects of neural motor control in the fine structure of the vocal signal.

The cortical computations underlying feedback control in vocal production

Traditionally, psycholinguistic models and models of speech motor control have approached the word production process from different angles and have remained, to a large extent, separate from one another. Ultimately, however, the process entails the entire pathway from semantic processing to articulation: producing a word from meaning requires activating a concept, retrieving the word, selecting its segments, ordering those segments into the right sequence, and finally articulating them with the correct timing. This chapter reviews the neural basis of word production from the complementary perspectives of the psycholinguistic literature and the speech motor control literature.

Neurobiology of speaking and stuttering

What Is Optimal about Motor Control?

The scientific literature on poststroke rehabilitation is remarkably vast. Over the last decades, dozens of rehabilitation approaches have been investigated. However, sometimes it is challenging to trace new experimental interventions back to some of the known models of motor control and sensorimotor learning. This scoping review aimed to investigate motor control models' diffusion among the literature on motor recovery after stroke. We performed a literature search on Medline, Cochrane, Web of Science, Embase, and Scopus databases. The last search was conducted in September 2023. This scoping review included full-text articles published in English in peer-reviewed journals that provided rehabilitation interventions based on motor control or motor learning frameworks for at least one individual with stroke. For each study, we identified the theoretical framework the authors used to design the experimental treatment. To this aim, we used a previously proposed classification of the known models of motor control, dividing them into the following categories: neuroanatomy, robotics, self-organization, and ecological context. In total, 2,185 studies were originally considered in this scoping review. After the screening process, we included and analyzed 45 studies: 20 studies were randomized controlled trials, 12 were case series, 4 were case reports, 8 were observational longitudinal pilot studies, and 1 was an uncontrolled trial. Only 10 studies explicitly declared the reference theoretical model. Considering their classification, 21 studies referred to the robotics motor control model, 12 to the self-organization model, 8 to the neuroanatomy model, and 4 to the ecological model. Our results showed that most of the rehabilitative interventions purposed in stroke rehabilitation have no clear theoretical bases on motor control and motor learning models. We suggest this is an issue that deserves attention when designing new experimental interventions in stroke rehabilitation.

The primary objective of this article is to study whether an assessment instrument specifically designed to assess speech motor control on word level productions would be able to add differential diagnostic speech characteristics between people who clutter and people who stutter. It was hypothesized that cluttering is a fluency disorder in which speech motor control on word level is disturbed in high speech rate, resulting in errors in flow of speech and sequencing. An assessment instrument on speech motor coordination on word level was developed and validated. In an elicitation procedure, repetitions of complex multi-syllabic words at a fast speech rate were obtained from 47 dysfluent participants (mean age 24.3; SD 10.25, range 14.2–47.4 yrs) and 327 controls (mean age 25.56 yrs; SD 8.49; age range 14.3–50.1). Speech production was judged on articulatory accuracy, smooth-flow (coarticulation, flow and sequencing) and articulatory rate. Results from people who clutter (PWC) and people who stutter (PWS) were compared to normative data based on control group data. PWC produced significantly more flow and sequencing errors compared to PWS. Further research is needed in order to study speech motor control in spontaneous speech of people who clutter.

This paper reviews current research and contemporary theories of subcortical participation in the motor control of speech production and language processing. As a necessary precursor to the discussion of the functional roles of the basal ganglia and thalamus, the neuroanatomy of the basal ganglial-thalamocortical circuitry is described. Contemporary models of hypokinetic and hyperkinetic movement disorders based on recent neuroanatomical descriptions of the multi-segmented circuits that characterise basal ganglion anatomy are described. Reported effects of surgically induced lesions in the globus pallidus and thalamus on speech production are reviewed. In addition, contemporary models proposed to explain the possible contribution of various subcortical structures to language processing are described and discussed in the context of evidence gained from observation of the effects of circumscribed surgically induced lesions in the basal ganglia and thalamus on language function. The potential of studies based on examination of the speech/language outcomes of patients undergoing pallidotomy and thalamotomy to further inform the debate relating to the role of subcortical structures in speech motor control and language processing is highlighted.

Stuttering, a neurodevelopmental speech fluency disorder, is associated with intermittent disruptions of speech-motor control. Behavioural treatments for adults who stutter (AWS) concentrate on adopting speech patterns that enhance fluency, such as speaking rhythmically or prolonging speech sounds. However, maintaining these treatment benefits can be challenging. Neuroimaging studies suggest that supplementary motor area (SMA) which play a crucial role in speech initiation, planning and internal timing shows aberrant activation in speech production of AWS and may contribute to stuttering. Preliminary evidence suggests that brain stimulation may impact responsiveness to behavioural treatments. The present study aims to investigate whether excitatory repetitive transcranial magnetic stimulation (rTMS) of the SMA and rhythmic speech can consistently reduce stuttering severity across various measures. Ten self-identified Cantonese-speaking AWS participated in this double-blinded, sham-controlled clinical trial study (NCT05472181). The participants underwent 10 sessions of rhythmic speech training across two phases, combined with either neuronavigated rTMS or sham, with a 2-week washout period between phases. The stuttering severity was assessed through various outcome measures, including the percentage of syllables stuttered, self-perceived stuttering severity, and the brief version of Unhelpful Thoughts and Beliefs About Stuttering before and after each treatment phase. Results demonstrated improved speech fluency in various speaking contexts, with no significant difference between rTMS and sham conditions immediately and 1 week post-treatment. Notably, rTMS specifically led to less stuttering in tongue twister production (d = -0.70). Both treatment conditions effectively reduced self-perceived stuttering severity and negative thoughts and beliefs about stuttering. The findings of this study indicate that stimulating the SMA reduced stuttering, only in the production of tongue twisters that may require greater motor control and coordination. Furthermore, it indicates that rhythmic speech might help alleviate negative beliefs and anxiety related to stuttering. This research contributes to our understanding of neuromodulation in stuttering treatment and the role of the SMA in speech motor control and emphasises the need for more research on the potential benefits and limitations of applying rTMS in this condition. What is already known on the subject Behavioural treatments for adults who stutter concentrate on adopting speech patterns that enhance fluency, such as speaking rhythmically or prolonging speech sounds. However, maintaining these treatment benefits can be challenging. Neuroimaging studies indicate that aberrant neural activation in speech production regions, like the supplementary motor area (SMA), is involved in stuttering. The SMA plays a crucial role in initiating, planning, and sequencing motor behaviours. Preliminary evidence suggests that brain stimulation (e.g., transcranial direct current stimulation or transcranial magnetic stimulation) may impact responsiveness to behavioural treatments. What this paper adds to existing knowledge There is limited knowledge regarding the potential effects of stimulating the SMA to enhance speech fluency in people who stutter. Existing research primarily consists of single case studies that lack proper control conditions or involve only a single stimulation session. Due to their limited scope and power, these studies may not provide sufficient evidence. The current study expands upon existing research by investigating whether multiple sessions of repetitive transcranial magnetic stimulation over the SMA, combined with rhythmic speech, improve speech fluency in adults who stutter. Furthermore, it addresses the limitations of brain stimulation methods and proposes directions for future research. What are the potential or actual clinical implications of this work? This study implies that the stimulation of SMA reduced stuttering only in speaking contexts that may require greater motor control and coordination such as tongue twisters. Additionally, the research suggests that using rhythmic speech could potentially alleviate negative beliefs and anxiety associated with stuttering.

The cerebellum has been hypothesized to form a crucial part of the speech motor control network. Evidence for this comes from patients with cerebellar damage, who exhibit a variety of speech deficits, as well as imaging studies showing cerebellar activation during speech production in healthy individuals. To date, the precise role of the cerebellum in speech motor control remains unclear, as it has been implicated in both anticipatory (feedforward) and reactive (feedback) control. Here, we assess both anticipatory and reactive aspects of speech motor control, comparing the performance of patients with cerebellar degeneration and matched controls. Experiment 1 tested feedforward control by examining speech adaptation across trials in response to a consistent perturbation of auditory feedback. Experiment 2 tested feedback control, examining online corrections in response to inconsistent perturbations of auditory feedback. Both male and female patients and controls were tested. The patients were impaired in adapting their feedforward control system relative to controls, exhibiting an attenuated anticipatory response to the perturbation. In contrast, the patients produced even larger compensatory responses than controls, suggesting an increased reliance on sensory feedback to guide speech articulation in this population. Together, these results suggest that the cerebellum is crucial for maintaining accurate feedforward control of speech, but relatively uninvolved in feedback control.SIGNIFICANCE STATEMENT Speech motor control is a complex activity that is thought to rely on both predictive, feedforward control as well as reactive, feedback control. While the cerebellum has been shown to be part of the speech motor control network, its functional contribution to feedback and feedforward control remains controversial. Here, we use real-time auditory perturbations of speech to show that patients with cerebellar degeneration are impaired in adapting feedforward control of speech but retain the ability to make online feedback corrections; indeed, the patients show an increased sensitivity to feedback. These results indicate that the cerebellum forms a crucial part of the feedforward control system for speech but is not essential for online, feedback control.

The author presents a view of research in speech motor control over the past 5 decades, as observed from within Ken Stevens's Speech Communication Group (SCG) in the Research Laboratory of Electronics at MIT. The author presents a limited overview of some important developments and discoveries. The perspective is based largely on the research interests of the Speech Motor Control Group (SMCG) within the SCG; thus, it is selective, focusing on normal motor control of the vocal tract in the production of sound segments and syllables. It also covers the particular theories and models that drove the research. Following a brief introduction, there are sections on methodological advances, scientific advances, and conclusions. Scientific and methodological advances have been closely interrelated. Advances in instrumentation and computer hardware and software have made it possible to record and process increasingly large, multifaceted data sets; introduce new paradigms for feedback perturbation; image brain activity; and develop more sophisticated, computational physiological and neural models. Such approaches have led to increased understanding of the widespread variability in speech, motor-equivalent trading relations, sensory goals, and the nature of feedback and feedforward neural control mechanisms. Some ideas about important future directions for speech research are presented.

A comprehensive and unified account of the neural computations underlying speech production, offering a theoretical framework bridging the behavioral and the neurological literatures. In this book, Frank Guenther offers a comprehensive, unified account of the neural computations underlying speech production, with an emphasis on speech motor control rather than linguistic content. Guenther focuses on the brain mechanisms responsible for commanding the musculature of the vocal tract to produce articulations that result in an acoustic signal conveying a desired string of syllables. Guenther provides neuroanatomical and neurophysiological descriptions of the primary brain structures involved in speech production, looking particularly at the cerebral cortex and its interactions with the cerebellum and basal ganglia, using basic concepts of control theory (accompanied by nontechnical explanations) to explore the computations performed by these brain regions. Guenther offers a detailed theoretical framework to account for a broad range of both behavioral and neurological data on the production of speech. He discusses such topics as the goals of the neural controller of speech; neural mechanisms involved in producing both short and long utterances; and disorders of the speech system, including apraxia of speech and stuttering. Offering a bridge between the neurological and behavioral literatures on speech production, the book will be a valuable resource for researchers in both fields.

The oral structures such as the tongue and lips have remarkable somatosensory capacities, but understanding the roles of somatosensation in speech production requires a more comprehensive knowledge of somatosensation in the speech production system in its entirety, including the respiratory, laryngeal, and supralaryngeal subsystems. This review was conducted to summarize the system-wide somatosensory information available for speech production. The search was conducted with PubMed/Medline and Google Scholar for articles published until November 2023. Numerous search terms were used in conducting the review, which covered the topics of psychophysics, basic and clinical behavioral research, neuroanatomy, and neuroscience. The current understanding of speech somatosensation rests primarily on the two pillars of psychophysics and neuroscience. The confluence of polymodal afferent streams supports the development, maintenance, and refinement of speech production. Receptors are both canonical and noncanonical, with the latter occurring especially in the muscles innervated by the facial nerve. Somatosensory representation in the cortex is disproportionately large and provides for sensory interactions. Speech somatosensory function is robust over the lifespan, with possible declines in advanced aging. The understanding of somatosensation in speech disorders is largely disconnected from research and theory on speech production. A speech somatoscape is proposed as the generalized, system-wide sensation of speech production, with implications for speech development, speech motor control, and speech disorders.