On the achievement of accuracy in limb movement and speech production

Movements are inherently variable. This property of behaviour is so glaring it can be proved with little more than a piece of paper and a pen. Attempting to repeatedly sign one's name in the same spot quickly yields a blob of ink instead of a legible, ever darkening signature. Even so, we rarely stumble over our words or mistakenly step off of cliffs. The nervous system has strategies for controlling unpredictable disturbances and countering predictable ones so that behavioural goals are, more or less, achieved. What causes movement variability in the first place and how does the nervous system achieve movement accuracy when confronted with it? This doctoral thesis presents a series of studies that examine the problem of making accurate limb and speech movements when faced with unpredictable and predictable disturbances of movement. Through a literature review, Chapter 1 explains where movement variability comes from by looking at the neural mechanisms that drive movement and the environmental perturbations that can disrupt it. Strategies for dealing with these unpredictable and predictable perturbations are then reviewed. These strategies are explored directly in later chapters using two model systems. In Chapter 2, it is hypothesized that patterns of reaching variability at the end of movements are closely related to patterns of limb "stiffness"—or a limb's resistance to displacement. Stiffness is easily manipulated in the arm by changing posture; this makes reaching an ideal system to manipulate stiffness and test this hypothesis. Over two experiments, patterns of limb stiffness are observed to predict patterns of movement variability at the end of reaching movements into circular targets. This relationship does not depend on the direction of movement, and is maintained across different postures and patterns of limb stiffness. In Chapter 4, it is hypothesized that individuals show differences in how they use sensory information to counter predictable environment perturbations of movement. The sensory systems that maintain accurate speech—auditory feedback from the sound of the voice and somatosensory feedback from the movement of the articulators—can be simultaneously and independently altered. This makes the speech production system ideal for testing this hypothesis. Over three experiments, the sound of the voice and the movements of speech are manipulated; individual differences are observed in response to these auditory and somatosensory perturbations. The more individuals counter one type of perturbation the less they counter the other.In light of these findings, Chapters 3 and 5 discuss the similarities and differences in the control of movement variability between the limb motor control and speech motor control systems. There are large individual differences in response to predictable perturbations of speech that are not observed in similar perturbations of limb movement. It is hypothesized that these speech-related individual differences are caused by differences in linguistic experience.Les mouvements sont fondamentalement variables. Cette propriété de comportement est si flagrante elle pourrait être prouvée avec peu plus d'un morceau de papier et un style. Une de signer à plusieurs reprises son tentative descend pas des falaises nerveux a des stratégies ceux nom dans le même endroit produit rapidement une goutte d'encre au lieu d'une signature qui devient plus foncé et lisible. Cependant, on ne bute que rarement sur ses mots et on ne par mégarde non plus. Le système à contrôler des désordres imprévisibles et à s'opposer à qui sont prévisibles pour que les objectifs comportementaux soient plus ou moins achevés. Tout d'abord, qu'est-ce que la cause de la variabilité du mouvement et comment est-ce que le système nerveux achève l'exactitude quand il est confronté avec lui? Cette thèse de doctorat présent une série des étudies qui examinent le problème de faire des mouvements précis des membres et d'expression orale quand on est affronté avec les désordres du mouvement imprévisibles et prévisibles. À travers une analyse de la littérature, le premier chapitre explique d'où vient la variabilité du mouvement en observant les mécanismes neuraux qui actionnent le mouvement et les perturbations environnementales qui pourraient l'interrompre. Les stratégies pour faire face à ces perturbations imprévisibles et prévisibles sont ensuite examinées. Ces stratégies sont explorées directement dans les chapitres suivants en utilisant deux systèmes modèles. Dans le deuxième chapitre, on fait l'hypothèse que des tendances de la variabilité d'atteindre à la fin des mouvements sont intimement liées aux tendances de l'impédance (rigidité) musculaire – ou la résistance d'un muscle au déplacement. L'impédance est facilement manipulée dans le bras en changeant de posture ; par conséquent l'acte d'atteindre est un système idéal à manipuler l'impédance musculaire et tester cette hypothèse. Au cours de deux expériences, les tendances de l'impédance de membre sont observées pour prédire les tendances de la variabilité du mouvement à la fin des mouvements d'atteindre dans les cibles circulaires.Dans le quatrième chapitre, on fait l'hypothèse que les individus montrent des différences dans la façon dont ils utilisent de l'information sensorielle pour contrer des perturbations prévisibles environnementales du mouvement. Les systèmes sensoriels qui soutiennent l'expression orale précis – la rétroaction auditive du son de la voix et la rétroaction somatosensoriel du mouvement des articulateurs – pourraient être modifiés simultanément et indépendamment. À la suite, le système de production de la parole est idéal pour tester cette hypothèse. Au cours de trois expériences, le son de la voix et les mouvements de l'expression orale sont manipulés ; des différences individuelles sont observées en réponse à ces perturbations auditives et somatosensorielles. Le plus que les individus contrent un type de perturbation, ils contrent l'autre type moins. À la lumière de ces découvertes, les chapitres trois et cinq discutent des similarités et des différences dans le contrôle de la variabilité du mouvement entre les systèmes du contrôle moteur des membres et du contrôle moteur de l'expression orale. Il y a de grandes différences individuelles en réponse à des perturbations de l'expression orale qui ne sont pas observés dans les perturbations similaires du mouvement des membres. On fait l'hypothèse que ces différences individuelles liées à la parole sont causées par les différences dans l'expérience linguistique

On the control of movement variability through the regulation of limb impedance

Humans routinely make movements to targets that have different accuracy requirements in different directions. Examples extend from everyday occurrences such as grasping the handle of a coffee cup to the more refined instance of a surgeon positioning a scalpel. The attainment of accuracy in situations such as these might rest upon the nervous system's capacity to regulate the limb's resistance to displacement, or impedance. To test this idea, subjects made movements from random starting locations to targets that had shape dependant accuracy requirements. A robotic device was used to assess both limb impedance and patterns of movement variability just as the subject reached the target. Impedance was seen to increase in directions where required accuracy was high. Furthermore, independent of target shape patterns of limb stiffness were seen to predict spatial patterns of movement variability. The nervous system was thus seen to modulate limb impedance in wholly predictable environments to shape movement variability and achieve reaching accuracy

Language Enables the Acquisition of Distinct Sensorimotor Memories for Speech

This project examines the acquisition of new sensorimotor memories for speech in bilingual

Sensorimotor learning during synchronous speech is modulated by the acoustics of the other voice

This study tested the hypothesis that speaking with other voices can influence sensorimotor predictions of one’s own voice. Real-time manipulations of auditory feedback were used to drive sensorimotor adaptation in speech, while participants spoke sentences in synchrony with another voice, a task known to induce implicit imitation (phonetic convergence). The acoustic-phonetic properties of the other voice were manipulated between-groups, such that convergence with it would either oppose (incongruent group, n = 15) or align with speech motor adaptation (congruent group, n = 16). As predicted, significantly greater adaptation was seen in the congruent compared to the incongruent group. This suggests the use of shared sensory targets in speech for predicting the sensory outcomes of both the actions of others (speech perception) and the actions of the self (speech production). This finding has important implications for wider theories of shared predictive mechanisms across perception and action, such as active inference.This work was funded by a Leverhulme Trust Early Career Fellowship (awarded to Abigail Bradshaw, ECF-2021-207), a Leverhulme Trust Research Leadership Award (awarded to Carolyn McGettigan, RL-2016-013), a Natural Sciences and Engineering Research Council of Canada (NSERC) Undergraduate Student Research Award (awarded to Emma Wheeler), and an NSERC Discovery Grant (awarded to Daniel Lametti, 2019-05236)

Immediate Cross-Language Transfer of Novel Articulatory Plans in Bilingual Speech

Data and an analysis script associated with the paper, Immediate Cross-Language Transfer of Novel Articulatory Plans in Bilingual Speec

Impedance Control and Its Relation to Precision in Orofacial Movement

Speech production involves some of the most precise and finely timed patterns of human movement. Here, in the context of jaw movement in speech, we show that spatial precision in speech production is systematically associated with the regulation of impedance and in particular, with jaw stiffness—a measure of resistance to displacement. We estimated stiffness and also variability during movement using a robotic device to apply brief force pulses to the jaw. Estimates of stiffness were obtained using the perturbed position and force trajectory and an estimate of what the trajectory would be in the absence of load. We estimated this “reference trajectory” using a new technique based on Fourier analysis. A moving-average (MA) procedure was used to estimate stiffness by modeling restoring force as the moving average of previous jaw displacements. The stiffness matrix was obtained from the steady state of the MA model. We applied this technique to data from 31 subjects whose jaw movements were perturbed during speech utterances and kinematically matched nonspeech movements. We observed systematic differences in stiffness over the course of jaw-lowering and jaw-raising movements that were correlated with measures of kinematic variability. Jaw stiffness was high and variability was low early and late in the movement when the jaw was elevated. Stiffness was low and variability was high in the middle of movement when the jaw was lowered. Similar patterns were observed for speech and nonspeech conditions. The systematic relationship between stiffness and variability points to the idea that stiffness regulation is integral to the control of orofacial movement variability