Neuromuscular Mechanisms of Movement Variability: Implications for Rehabilitation and Augmentation

Although speed-accuracy trade-offs and planning and execution of rapid goaldirected movements have garnered significant research interest, far fewer studies have reported results on the lower end of the movement speed spectrum. Not only do very interesting observations exist that are unique to slow movements, but an explanation of these observations is highly relevant to motor function recovery and motor skill learning, where movements are typically slow at the initiation of therapy or learning, and movement speed increases through practice, exercise or therapy. In the first part of this thesis, based on data from nine stroke patients who underwent a month-long hybrid traditional and robotic therapy protocol, a correlation analysis shows that measures of movement quality based on minimum jerk theory for movement planning correlates significantly and strongly with clinical measures of motor impairment. In contrast, measures of movement speed lack statistical significance and show only weak to moderate correlations with clinical measures. These results constitute an important step towards establishing a much-needed bridge between clinical and robotic rehabilitation research communities. In the second part, the origins of movement intermittency or variability in slow movements are explored. A study with five healthy subjects who completed a manual circular tracking task shows that movement intermittency increases in distal direction along the arm during multi-joint movements. This result suggests that a neuromuscular noise option is favored against a submovement-based central planning alternative, as the source of variability in slow movements. An additional experimental study with eight healthy subjects who completed slow elbow flexion movements at a constant slow speed target under varying resistive torque levels demonstrates that resistive torques can significantly decrease movement speed variability. The relationship between resistive torque levels and speed variability, however, is not monotonic. This finding may constitute a basis for proper design of novel human skill augmentation methods for delicate tasks and improve motor rehabilitation and learning protocols. Finally, a neuro-musculoskeletal model of the elbow suggests that movement speed variability in slow movements cannot be solely attributed to variability in the mechanics of muscle force generation. Together, these analyses, simulations, and experiments shed light on variability in slow movements, and will inform the development of novel paradigms for robotic rehabilitation, motor skill learning and augmentation

Movement Intermittency in Social Coordination

Coordination of movements in humans has been extensively studied at a macroscopic level, such as the pacing of movements, particularly in tasks of interpersonal and bimanual coordination. However, by examining the fine structure of movement, another form of rhythmicity becomes apparent at a microscopic level. Movement is never completely smooth, but rather is organized into smaller units known as submovements, which appear as recurrent speed breaks occurring at faster timescales (2-3 Hz). These submovements may reflect intermittent feedback-based motor adjustments. To better understand the relationship between submovements in different coordination contexts, we characterized the timing of submovements emission in a series of rhythmic motor coordination task by asking participants to coordinate their index fingers either in-phase or anti-phase with themselves or with a real/virtual partner. In Study 1, we analysed the temporal relationship between submovements emitted by both hands of a single participant during a bimanual coordination task. We also manipulated the availability of visual feedback to understand its impact on the emission of submovements, which are believed to reflect a vision based movement correction mechanism. In Study 2, we explored the dynamics of submovements during interpersonal coordination, and thus with the goal of moving beyond their temporal emission in single individuals. In Study 3, we combined interpersonal and bimanual coordination into a single task by asking participants to coordinate with each other using both their hands. In Study 4, we tested the validity of our results on mutual adaptation of submovements during interpersonal coordination by replacing one member of the pair with an unresponsive virtual partner. Finally, in Study 5, building on the ease of transferability of the previous task to clinical settings, we investigated the pattern of submovements emission in individuals with Parkinson's disease and cerebellar disorders to identify potentially new diagnostic markers and gain novel insights into the neural substrates underlying movement intermittency. Overall, our results suggest that the mechanism responsible for the organization of movement into submovements is at least partly shared across different effectors, such as the two hands, and might be modulated by the availability and usability of visual and proprioceptive feedback. Moreover, the identification of different temporal patterns of submovements emission leads us to conclude that the mechanisms controlling submovements production are highly flexible and tunable depending on the coordinative context. Submovements control can thus provide valuable insights into the low-level motor control mechanisms involved in achieving intra- and interpersonal motor coordination. Finally, submovement-level control may serve as a novel objective marker of individual and social motor coordination capabilities that may be selectively impaired in some neurological and psychiatric conditions.La coordinazione dei movimenti negli esseri umani è stata ampiamente studiata a livello macroscopico, ad es. il ritmo dei movimenti, in particolare in compiti di coordinazione interpersonale e bimanuale. Tuttavia, esaminando la struttura fine del movimento, un'altra forma di ritmicità appare evidente a livello microscopico. Il movimento non è mai completamente fluido, ma è organizzato in unità più piccole note come sottomovimenti, che si manifestano come interruzioni di velocità ricorrenti su una scala temporale più veloce (2-3 Hz). Questi sottomovimenti possono riflettere aggiustamenti motori intermittenti basati sul feedback. Per comprendere meglio la relazione tra i sottomovimenti in contesti di coordinazione diversi, abbiamo caratterizzato i pattern di emissione temporale dei sottomovimenti in una serie di compiti di coordinazione motoria ritmica, chiedendo ai partecipanti di coordinare i loro indici in-fase o in anti-fase con se stessi o con un partner reale/virtuale. Nello Studio 1, abbiamo analizzato la relazione temporale tra i sottomovimenti emessi da entrambe le mani di un singolo partecipante durante un compito di coordinazione bimanuale. Abbiamo anche manipolato la disponibilità del feedback visivo per comprendere il suo impatto sull'emissione dei sottomovimenti, che si ritiene riflettano un meccanismo di correzione dei movimenti basato sulla visione. Nello Studio 2, abbiamo esplorato la dinamica dei sottomovimenti durante la coordinazione interpersonale, con l’obiettivo di indagare i loro pattern di emissione temporale in coppie di individui. Nello Studio 3, abbiamo combinato la coordinazione interpersonale e bimanuale in un unico compito, chiedendo ai partecipanti di coordinarsi reciprocamente utilizzando entrambe le mani. Nello Studio 4, abbiamo testato la validità dei nostri risultati sull'adattamento reciproco dei sottomovimenti durante la coordinazione interpersonale sostituendo uno dei membri della coppia con un partner virtuale non reattivo. Infine, nello Studio 5, considerata la facile trasferibilità del compito precedente in contesti clinici, abbiamo indagato il modello di emissione dei sottomovimenti in individui con malattia di Parkinson e disturbi cerebellari per identificare potenziali nuovi marker diagnostici e acquisire nuove informazioni sui substrati neurali alla base dell'intermittenza del movimento. Complessivamente, i nostri risultati suggeriscono che il meccanismo responsabile dell'organizzazione del movimento in sottomovimenti è almeno in parte condiviso tra differenti effettori, come le due mani, e potrebbe essere modulato dalla disponibilità e utilizzabilità del feedback visivo e propriocettivo. Inoltre, l'identificazione di diversi modelli temporali di emissione dei sottomovimenti ci porta a concludere che i meccanismi che controllano la produzione dei sottomovimenti sono altamente flessibili e adattabili in base al contesto coordinativo. Il controllo dei sottomovimenti può quindi fornire preziose informazioni sui meccanismi di controllo motorio di basso livello coinvolti nel raggiungimento della coordinazione motoria intra- e interpersonale. Infine, il controllo motorio a livello dei sottomovimenti potrebbe fungere da nuovo marker oggettivo delle capacità individuali e sociali di coordinazione motoria che potrebbero essere selettivamente compromesse in alcune condizioni neurologiche e psichiatriche

Modeling cerebrocerebellar control in horizontal planar arm movements of humans and the monkey

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2007.Includes bibliographical references (leaves 215-236).In daily life, animals including humans make a wide repertoire of limb movements effortlessly without consciously thinking about joint trajectories or muscle contractions. These movements are the outcome of a series of processes and computations carried out by multiple subsystems within the central nervous system. In particular, the cerebrocerebellar system is central to motor control and has been modeled by many investigators. The bulk of cerebrocerebellar control involves both forward command and sensory feedback information inextricably combined. However, it is not yet clear how these types of signals are reflected in spiking activity in cerebellar cells in vivo. Segmentation of apparently continuous movements was first observed more than a century ago. Since then, submovements, which have been identified by non-smooth speed profiles, have been described in many types of movements. However, physiological origins of submovement have not been well understood. This thesis demonstrates that a currently proposed recurrent integrator PID (RIPID) cerebellar limb control model (Massaquoi 2006a) is consistent with average neural activity recorded in a monkey by developing the Recurrent Integrator-based Cerebellar Simple Spike (RICSS) model.(cont.) The RICSS formulation is consistent with known or plausible cerebrocerebellar and spinocerebellar neurocircuitry, including hypothetical classification of mossy fiber signals. The RICSS model accounts well for variety of cerebellar simple spike activity recorded from the monkey and outperforms any other existing models. The RIPID model is extended to include a simplified cortico-basal ganglionic loop to capture statistical characterization of intermittency observed in individual trials of the monkey. In order to extend the capability of the RIPID model to a larger workspace and faster movements, the model needs to be gainscheduled based on the local state information. A linear parameter varying (LPV) formulation, which shares a similar structure to that suggested by the RICSS model, is performed and its applicability was tested on human subjects performing double step tasks which requires rapid change in movement directions.by Kazutaka Takahashi.Ph.D

Postural Instabilities and the Maintenance of Bi-manual Rhythmic Movement

Most research on bimanual rhythmic coordination has occurred with the participants in a seated posture. Many activities of daily living, however, require the interaction of standing postural and manual tasks. A population of individuals that are ideal for studying the integration of a manual task into the ongoing control of posture are expert marching percussionists; they have learned to produce rhythmic movements accurately under a variety of temporal and postural constraints. The purpose of the current study was to investigate the integration of bimanual rhythmic movements and posture in expert marching percussionists. Participants (N=11) were recruited from the University of Massachusetts Drumline, and were asked to perform three rhythmic tasks [1:1, 2:3, and 2:3-F (2:3 rhythm played faster at a self-selected tempo)] in one of three postures: sitting, standing on one foot, and standing on two feet. Discrete relative phase, postural time-to-contact, and coherence analysis, were used to analyze the performance of the manual task, postural control, and the integration between postural and manual performance. Across all three rhythms, discrete relative phase mean and variability (SD) results showed no effects of posture on rhythmic performance. The complexity of the manual task (1:1 vs 2:3) had no effect on postural time-to-contact. However, increasing the tempo of the manual task (2:3 vs. 2:3=F) did result in a decreased postural time-to-contact in the two-footed posture). Coherence analysis revealed that the coupling between the postural and manual task significantly decreased as a function of posture (going from a two footed to a one footed posture) and rhythmic complexity (1:1 vs. 2:3). Taken together, these results demonstrate that expert marching percussionists systematically decouple postural and manual fluctuations in order to preserve the performance of the rhythmic movement task

Systems Identification of Sensorimotor Control for Visually Guided Wrist Movements

The sensorimotor control system is a complicated system in which the neural controller uses the feedback information from sensory modalities (visual, proprioceptive, vestibular, auditory, etc.) to actuate the musculo-skeletal system in order to execute intended movements. It has been an ongoing research to decode this sensorimotor integration. The current study utilized a systems identification approach in conjunction with a one-degree-of-freedom robotic manipulandum to quantify (delays, noises, wrist dynamics and controller parameters) a simplified (linear time-invariant) model of sensorimotor control for visually guided wrist stabilization movements. Four sensorimotor tasks were used to characterize the parameters of the sensorimotor control model. Open loop visual and proprioceptive delays along with effective feedforward delay (associated with motor processing and feedforward conduction) were estimated from subject\u27s response to perturbation (Exp. 1) using cross-correlation analysis. Multiplicative feedforward (motor) noise was estimated by measuring the force variability in isometric torque contractions at 5 different torque levels (Exp. 2). Frequency response analysis (Exp.3 and 4) was used to obtain estimates of wrist dynamics (inertia, viscosity and stiffness), the feedback (visual and proprioceptive) gains, the controller gains (proportional, integral and derivative) and an additive sensory noise. The experimental paradigms were validated by simulating and testing the experimental task along with the sensorimotor control model in SIMULINK®. The ability of the experiments to characterize the model was tested over a range of parameter values to determine the robustness of the approach. Model performance was measured by characterizing the sensorimotor control system in 11 subjects. Variance Accounted For (VAF) by the model was used as a performance metric to compare model\u27s response (obtained using the parameters measured for each subject in the model) with subject\u27s performance (Exp. 5). The proposed model of sensorimotor control contained 13 parameters, which were measured successively to study their interaction during wrist stabilization in 11 neurologically-intact subjects. The model parameters estimated for human subjects resulted in accurate predictions of hand position, with a high percentage of variance accounted for (VAF) across all subjects (78.3±3.3 %). Future studies will use these techniques to quantify how the sensorimotor control changes across tasks (tracking vs. stabilization), age and neuro-motor disabilities

MACOP modular architecture with control primitives

Walking, catching a ball and reaching are all tasks in which humans and animals exhibit advanced motor skills. Findings in biological research concerning motor control suggest a modular control hierarchy which combines movement/motor primitives into complex and natural movements. Engineers inspire their research on these findings in the quest for adaptive and skillful control for robots. In this work we propose a modular architecture with control primitives (MACOP) which uses a set of controllers, where each controller becomes specialized in a subregion of its joint and task-space. Instead of having a single controller being used in this subregion (such as MOSAIC on which MACOP is inspired), MACOP relates more to the idea of continuously mixing a limited set of primitive controllers. By enforcing a set of desired properties on the mixing mechanism, a mixture of primitives emerges unsupervised which successfully solves the control task. We evaluate MACOP on a numerical model of a robot arm by training it to generate desired trajectories. We investigate how the tracking performance is affected by the number of controllers in MACOP and examine how the individual controllers and their generated control primitives contribute to solving the task. Furthermore, we show how MACOP compensates for the dynamic effects caused by a fixed control rate and the inertia of the robot

A study of motor control in healthy subjects and in Parkinson's disease patients

Thesis (Ph. D.)--Massachusetts Institute of Technology, Biological Engineering Division, 2008.Includes bibliographical references.Parkinson's disease (PD) is a primarily motor disorder which affects at least half a million people in the US alone. Deep brain stimulation (DBS) is a neurosurgical intervention by which neural structures are stimulated electrically by an implanted pacemaker. It has become the treatment of choice for PD, when not adequately controlled by drug therapy. We introduced a novel robotic platform for the study of the effects of DBS on motor control in PD. Subjects performed discrete wrist movements with and without a force field. We found preliminary indication that motor learning may be taking place with stimulation, and demonstrated how robotic testing can augment existing clinical tools in evaluation of the disease. To study the effect of stimulation on movement frequency, we employed a rhythmic task that required movements of the elbow to remain within a closed shape on a phase plane. Three closed shapes required varying frequency/amplitude combinations of elbow movement. The task was performed with and without visual feedback. Analysis of data from the healthy control subjects revealed a non-monotonic relation between accuracy on the phase plane and movement speed. Further kinematic analyses, including movement intermittency and harmonicity, number and type of submovements (movement primitives) fit per movement cycle, and the effects of vision on intermittency were used to support the model we propose, whereby there exist two subtypes of rhythmic movement; small-amplitude, high-frequency movements are nearly maximally harmonic, and harness the elastic properties of the limb to achieve smoothness and accuracy, and large-amplitude, low-frequency movements share characteristics with a string of discrete movements, and make use of visual feedback to achieve smoothness and accuracy.(cont.) Bradykinesia (slowness of movement) is one of the hallmarks of PD. We examined the effects of visual feedback on bradykinesia. PD patients off dopaminergic medication and healthy age-matched controls performed significantly faster movements when visual feedback was withdrawn. For the bradykinetic subjects, this increase in movement speed meant either a mitigation or an elimination of bradykinesia. Our results support a role of the basal ganglia in sensorimotor integration, and argue for the integration of nonvision exercises into patients' physical therapy regime.by Shelly Levy-Tzedek.Ph.D

On the psychological origins of tool use

The ubiquity of tool use in human life has generated multiple lines of scientific and philosophical investigation to understand the development and expression of humans' engagement with tools and its relation to other dimensions of human experience. However, existing literature on tool use faces several epistemological challenges in which the same set of questions generate many different answers. At least four critical questions can be identified, which are intimately intertwined-(1) What constitutes tool use? (2) What psychological processes underlie tool use in humans and nonhuman animals? (3) Which of these psychological processes are exclusive to tool use? (4) Which psychological processes involved in tool use are exclusive to Homo sapiens? To help advance a multidisciplinary scientific understanding of tool use, six author groups representing different academic disciplines (e.g., anthropology, psychology, neuroscience) and different theoretical perspectives respond to each of these questions, and then point to the direction of future work on tool use. We find that while there are marked differences among the responses of the respective author groups to each question, there is a surprising degree of agreement about many essential concepts and questions. We believe that this interdisciplinary and intertheoretical discussion will foster a more comprehensive understanding of tool use than any one of these perspectives (or any one of these author groups) would (or could) on their own

A Comparative Analysis of Speed Profile Models for Ankle Pointing Movements: Evidence that Lower and Upper Extremity Discrete Movements are Controlled by a Single Invariant Strategy

Little is known about whether our knowledge of how the central nervous system controls the upper extremities (UE), can generalize, and to what extent to the lower limbs. Our continuous efforts to design the ideal adaptive robotic therapy for the lower limbs of stroke patients and children with cerebral palsy highlighted the importance of analyzing and modeling the kinematics of the lower limbs, in general, and those of the ankle joints, in particular. We recruited 15 young healthy adults that performed in total 1,386 visually evoked, visually guided, and target-directed discrete pointing movements with their ankle in dorsal–plantar and inversion–eversion directions. Using a non-linear, least-squares error-minimization procedure, we estimated the parameters for 19 models, which were initially designed to capture the dynamics of upper limb movements of various complexity. We validated our models based on their ability to reconstruct the experimental data. Our results suggest a remarkable similarity between the top-performing models that described the speed profiles of ankle pointing movements and the ones previously found for the UE both during arm reaching and wrist pointing movements. Among the top performers were the support-bounded lognormal and the beta models that have a neurophysiological basis and have been successfully used in upper extremity studies with normal subjects and patients. Our findings suggest that the same model can be applied to different “human” hardware, perhaps revealing a key invariant in human motor control. These findings have a great potential to enhance our rehabilitation efforts in any population with lower extremity deficits by, for example, assessing the level of motor impairment and improvement as well as informing the design of control algorithms for therapeutic ankle robots