Motor Control Studies Research Articles

Human Spinal Motor Control.

Human studies in the past three decades have provided us with an emerging understanding of how cortical and spinal networks collaborate to ensure the vast repertoire of human behaviors. Humans have direct cortical connections to spinal motoneurons, which bypass spinal interneurons and exert a direct (willful) muscle control with the aid of a context-dependent integration of somatosensory and visual information at cortical level. However, spinal networks also play an important role. Sensory feedback through spinal circuitries is integrated with central motor commands and contributes importantly to the muscle activity underlying voluntary movements. Regulation of spinal interneurons is used to switch between motor states such as locomotion (reciprocal innervation) and stance (coactivation pattern). Cortical regulation of presynaptic inhibition of sensory afferents may focus the central motor command by opening or closing sensory feedback pathways. In the future, human studies of spinal motor control, in close collaboration with animal studies on the molecular biology of the spinal cord, will continue to document the neural basis for human behavior.

How Variability and Effort Determine Coordination at Large Forces.

Motor control is a challenging task for the central nervous system, since it involves redundant degrees of freedom, nonlinear dynamics of actuators and limbs, as well as noise. When an action is carried out, which factors does your nervous system consider to determine the appropriate set of muscle forces between redundant degrees-of-freedom? Important factors determining motor output likely encompass effort and the resulting motor noise. However, the tasks used in many previous motor control studies could not identify these two factors uniquely, as signal-dependent noise monotonically increases as a function of the effort. To address this, a recent paper introduced a force control paradigm involving one finger in each hand that can disambiguate these two factors. It showed that the central nervous system considers both force noise and amplitude, with a larger weight on the absolute force and lower weights on both noise and normalized force. While these results are valid for the relatively low force range considered in that paper, the magnitude of the force shared between the fingers for large forces is not known. This paper investigates this question experimentally, and develops an appropriate Markov chain Monte Carlo method in order to estimate the weightings given to these factors. Our results demonstrate that the force sharing strongly depends on the force level required, so that for higher force levels the normalized force is considered as much as the absolute force, whereas the role of noise minimization becomes negligible.

Combining Multiple Data Acquisition Systems to Study Corticospinal Output and Multi-segment Biomechanics

Transcranial magnetic stimulation techniques allow for an in-depth investigation into the neural mechanisms that underpin human behavior. To date, the use of TMS to study human movement, has been limited by the challenges related to precisely timing the delivery of TMS to features of the unfolding movement and, also, by accurately characterizing kinematics and kinetics. To overcome these technical challenges, TMS delivery and acquisition systems should be integrated with an online motion tracking system. The present manuscript details technical innovations that integrate multiple acquisition systems to facilitate and advance the use of TMS to study human movement. Using commercially available software and hardware systems, a step-by-step approach to both the hardware assembly and the software scripts necessary to perform TMS studies triggered by specific features of a movement is provided. The approach is focused on the study of upper limb, planar, multi-joint reaching movements. However, the same integrative system is amenable to a multitude of sophisticated studies of human motor control.

Combining Multiple Data Acquisition Systems to Study Corticospinal Output and Multi-segment Biomechanics.

Transcranial magnetic stimulation techniques allow for an in-depth investigation into the neural mechanisms that underpin human behavior. To date, the use of TMS to study human movement, has been limited by the challenges related to precisely timing the delivery of TMS to features of the unfolding movement and, also, by accurately characterizing kinematics and kinetics. To overcome these technical challenges, TMS delivery and acquisition systems should be integrated with an online motion tracking system. The present manuscript details technical innovations that integrate multiple acquisition systems to facilitate and advance the use of TMS to study human movement. Using commercially available software and hardware systems, a step-by-step approach to both the hardware assembly and the software scripts necessary to perform TMS studies triggered by specific features of a movement is provided. The approach is focused on the study of upper limb, planar, multi-joint reaching movements. However, the same integrative system is amenable to a multitude of sophisticated studies of human motor control.

Keys for reporting an fMRI in motor control study

Keys for reporting an fMRI in motor control study

Quantifying the Ebbinghaus figure effect: target size, context size, and target-context distance determine the presence and direction of the illusion

Over the last 20 years, visual illusions, like the Ebbinghaus figure, have become widespread to investigate functional segregation of the visual system. This segregation reveals itself, so it is claimed, in the insensitivity of movement to optical illusions. This claim, however, faces contradictory results (and interpretations) in the literature. These contradictions may be due to methodological weaknesses in, and differences across studies, some of which may hide a lack of perceptual illusion effects. Indeed, despite the long history of research with the Ebbinghaus figure, standardized configurations to predict the illusion effect are missing. Here, we present a complete geometrical description of the Ebbinghaus figure with three target sizes compatible with Fitts' task. Each trial consisted of a stimulus and an isolated probe. The probe was controlled by the participant's response through a staircase procedure. The participant was asked whether the probe or target appeared bigger. The factors target size, context size, target-context distance, and a control condition resulted in a 3 × 3 × 3+3 factorial design. The results indicate that the illusion magnitude, the perceptual distinctiveness, and the response time depend on the context size, distance, and especially, target size. In 33% of the factor combinations there was no illusion effect. The illusion magnitude ranged from zero to (exceptionally) 10% of the target size. The small (or absent) illusion effects on perception and its possible influence on motor tasks might have been overlooked or misinterpreted in previous studies. Our results provide a basis for the application of the Ebbinghaus figure in psychophysical and motor control studies.

Strength of syllabic influences on articulation in Mandarin Chinese and French: Insights from a motor control approach

This paper investigates how strong the influences of syllables are on the organization of speech movements in Mandarin Chinese and French. More specifically, we evaluate the strength of these influences by measuring the extent of anticipatory coarticulation in V1CV2 sequences, within and across the boundaries of the CV2 syllable. In line with motor control studies that have investigated serial-order motor tasks such as speech production, we consider that the extent of anticipatory motor planning reveals information about the units of planning, their boundaries and their strengths. For V1/t/V2 and V1/k/V2 sequences, where V1 and V2 are either /i/, /a/ or /u/, we used an electromagnetic magnetometer to record articulatory data from three native speakers of Mandarin Chinese and three native speakers of French. We labeled key articulatory configurations for V1, C and V2 based on general articulatory criteria. Our two major findings are:(1)Within CV2 boundaries, coarticulation is strong for both groups of speakers; in French, the coarticulation patterns can be fully explained by anticipatory behavior, while in Mandarin Chinese other strategies might also be involved.(2)Across CV2 boundaries, anticipatory coarticulation is observed in numerous cases in French, while no significant anticipation can be noted in Mandarin Chinese. We conclude that these measures provide evidence for the hypothesis that the strength of the syllable as a unit of speech planning is more important in Mandarin Chinese than in French. Finally, we propose potential explanations for these differences.

Effects of Force Requirements on Pinch Force Production in Healthy Adults.

Previous studies of fine motor control have focused on the ability of participants to match their grip force production to a visually provided template. We investigated differences exhibited in pinch force control during variable force production templates, including sine-, sawtooth-, and square-wave templates. Our results indicate that increased force requirements are associated with increased error rates and a noisier frequency spectrum, consistent with previous studies. Our results also indicate that visual feedback, in the form of template shape, directly affect pinch force production features and motor unit firing patterns, despite the use of consistent baseline force requirements, amplitude changes, and visual signal frequency. This suggests that CNS modulation of motor unit responses can be triggered by basic changes in visual feedback unrelated to force requirements. The potential implications of error compensation based on this study due to aging are also discussed.

Remapping residual coordination for controlling assistive devices and recovering motor functions

The concept of human motor redundancy attracted much attention since the early studies of motor control, as it highlights the ability of the motor system to generate a great variety of movements to achieve any well-defined goal. The abundance of degrees of freedom in the human body may be a fundamental resource in the learning and remapping problems that are encountered in human–machine interfaces (HMIs) developments. The HMI can act at different levels decoding brain signals or body signals to control an external device. The transformation from neural signals to device commands is the core of research on brain–machine interfaces (BMIs). However, while BMIs bypass completely the final path of the motor system, body–machine interfaces (BoMIs) take advantage of motor skills that are still available to the user and have the potential to enhance these skills through their consistent use. BoMIs empower people with severe motor disabilities with the possibility to control external devices, and they concurrently offer the opportunity to focus on achieving rehabilitative goals. In this study we describe a theoretical paradigm for the use of a BoMI in rehabilitation. The proposed BoMI remaps the user’s residual upper body mobility to the two coordinates of a cursor on a computer screen. This mapping is obtained by principal component analysis (PCA). We hypothesize that the BoMI can be specifically programmed to engage the users in functional exercises aimed at partial recovery of motor skills, while simultaneously controlling the cursor and carrying out functional tasks, e.g. playing games. Specifically, PCA allows us to select not only the subspace that is most comfortable for the user to act upon, but also the degrees of freedom and coordination patterns that the user has more difficulty engaging. In this article, we describe a family of map modifications that can be made to change the motor behavior of the user. Depending on the characteristics of the impairment of each high-level spinal cord injury (SCI) survivor, we can make modifications to restore a higher level of symmetric mobility (left versus right), or to increase the strength and range of motion of the upper body that was spared by the injury. Results showed that this approach restored symmetry between left and right side of the body, with an increase of mobility and strength of all the degrees of freedom in the participants involved in the control of the interface. This is a proof of concept that our BoMI may be used concurrently to control assistive devices and reach specific rehabilitative goals. Engaging the users in functional and entertaining tasks while practicing the interface and changing the map in the proposed ways is a novel approach to rehabilitation treatments facilitated by portable and low-cost technologies.

Repetitive transcranial magnetic stimulation and transcranial direct current stimulation in motor rehabilitation after stroke: An update

Stroke is a leading cause of adult motor disability. The number of stroke survivors is increasing in industrialized countries, and despite available treatments used in rehabilitation, the recovery of motor functions after stroke is often incomplete. Studies in the 1980s showed that non-invasive brain stimulation (mainly repetitive transcranial magnetic stimulation [rTMS] and transcranial direct current stimulation [tDCS]) could modulate cortical excitability and induce plasticity in healthy humans. These findings have opened the way to the therapeutic use of the 2 techniques for stroke. The mechanisms underlying the cortical effect of rTMS and tDCS differ. This paper summarizes data obtained in healthy subjects and gives a general review of the use of rTMS and tDCS in stroke patients with altered motor functions. From 1988 to 2012, approximately 1400 publications were devoted to the study of non-invasive brain stimulation in humans. However, for stroke patients with limb motor deficit, only 141 publications have been devoted to the effects of rTMS and 132 to those of tDCS. The Cochrane review devoted to the effects of rTMS found 19 randomized controlled trials involving 588 patients, and that devoted to tDCS found 18 randomized controlled trials involving 450 patients. Without doubt, rTMS and tDCS contribute to physiological and pathophysiological studies in motor control. However, despite the increasing number of studies devoted to the possible therapeutic use of non-invasive brain stimulation to improve motor recovery after stroke, further studies will be necessary to specify their use in rehabilitation.

Technology Enhanced Functioning

Virtual Reality on Stroke Rehabilitation Paolo Tonin About six hundred million people live with disabilities of various types all around the world. The majority of these disabilities originate from neurological lesions and the process of recovery emerges as one of the most relevant topic in the World Health Organization. A new or recurrent stroke is the main cause of disability in the industrial world, affecting the ability of a wide population to perform daily activities independently. Rehabilitation interventions are of utmost importance for this population. Recent studies in motor control, motor learning, and in the mechanisms of recovery after stroke produced a major impact in the field of neurorehabilitation. These acquisitions may lead to the development of scientifically based therapies, hopefully, more effective than current ones. Although, little is known about the mechanisms by which therapeutic measures influence motor recovery, some neuro-physiological studies on healthy subjects have reported that appropriate feedback on the nature of the movement pattern (knowledge of performance) and some variables of outcome (knowledge of results) may help to temporarily or permanently improve motor performance. Virtual Reality (VR) is a computer simulation which gives the user the impression of being and interacting in a real three-dimensional environment. Virtual Environments are a three-dimensional data set describing an artificial environment based on real world that the user interacts with. VR could be immersive or not immersive. The immersive VR systems may produce negative effects (Motion sickness syndrome, Photic seizures, Migraines, Hearing loss, Trauma, Eyestrain), not found during not immersive experiences. Virtual Reality (VR) systems have been proposed as useful tools in post stroke rehabilitation for both, motor rehabilitation, of upper limb particularly and cognitive rehabilitation. To create a virtual motor task, the therapist moves the handling object (for instance an envelope, a hammer, or a glass) connected to a receiver, inside the VE. The raw data are recorded by the tracking system. A dedicate software transforms the raw data in the movement of a virtual object that changes position corresponding to the real object motion. The motor task is automatically adapted to the patient's anthropomorphic characteristics. According to the patient's arm motor deficit, the therapist may orient the target. The location of the starting position, the target and the other objects, virtually represented in the arm workspace, determines the type and the difficulty of movement requested; the therapist could add virtual obstacles to increase the task complexity. VR may be exploited to provide the CNS with artificially generated information about the arm movement and the context, particularly showing: the patient's motor performance in the VE in real time (in the same frame of reference); the representation of the whole arm; the correct movement to the patients; a real-time feedback of the mismatch between desired and actual movement; the result of patient's performance, supplied via a reward based method with a numeric score; trace of the patient trajectory and repeated animation of trajectory after the task. Using treatment interventions created in virtual environments, practice conditions can be manipulated to explicitly engage motivation, cognitive processes, motor control, and sensory feedback- based learning mechanisms. The presentation will be focused on principles of motor learning underlying the VR systems for post-stroke patients; on the results of recent meta-analysis on the effects of VR; on the efficacy of systems specifically created for rehabilitation and of the commercial gaming systems.

Adaptive coding of orofacial and speech actions in motor and somatosensory spaces with and without overt motor behavior.

Studies of speech motor control suggest that articulatory and phonemic goals are defined in multidimensional motor, somatosensory, and auditory spaces. To test whether motor simulation might rely on sensory-motor coding common with those for motor execution, we used a repetition suppression (RS) paradigm while measuring neural activity with sparse sampling fMRI during repeated overt and covert orofacial and speech actions. RS refers to the phenomenon that repeated stimuli or motor acts lead to decreased activity in specific neural populations and are associated with enhanced adaptive learning related to the repeated stimulus attributes. Common suppressed neural responses were observed in motor and posterior parietal regions in the achievement of both repeated overt and covert orofacial and speech actions, including the left premotor cortex and inferior frontal gyrus, the superior parietal cortex and adjacent intraprietal sulcus, and the left IC and the SMA. Interestingly, reduced activity of the auditory cortex was observed during overt but not covert speech production, a finding likely reflecting a motor rather an auditory imagery strategy by the participants. By providing evidence for adaptive changes in premotor and associative somatosensory brain areas, the observed RS suggests online state coding of both orofacial and speech actions in somatosensory and motor spaces with and without motor behavior and sensory feedback.

Does our brain use the same policy for interacting with people and manipulating different objects?

Our first impression of other people is greatly affected by our previous experiences. Schematic processing, proposed in social psychology, explains our behavior in interacting with other people. It suggests existence of different schemas in our brain for different groups of people, e.g., extroverts, introverts, shy, women, men, etc. and also schemas related to special people like our parents, close friends, supervisor, and even ourselves. Each schema is recalled when we meet the corresponding person/personality (Atkinson, 1996). On the other hand there is a relatively well accepted theory–model based theory- in motor control and learning studies (Daw and Dayan, 2014; Dayan and Berridge, 2014). It suggests existence of some internal models (forward and/or inverse) in the brain which help us for planning and execution of the actions. Although these two viewpoints may seem very distinct, there are some interesting similarities between them, which are explained in the following section. I hypothesize that these correspondences may suggest that the brain employs same algorithms in dealing with both situations. Understanding the brain function is a great challenge for many scientists. Further evaluation of the proposed hypothesis may be helpful to achieve better understanding of the brain function, as advances in each field may encourage new ideas in the other one. In the following sections each of the two viewpoints and then their similarities are explained.

Modeling Stability in On-line Signatures

A novel definition of stability regions and a new method for detecting them from on-line signatures is introduced in this paper. Building upon handwriting generation and motor control studies, the stability regions is defined as the longest similar sequences of strokes between a pair of genuine signatures. The stability regions are then used to select the most stable signatures, as well as to estimate the extent to which these stability regions are encountered in both genuine and simulated (forged) signatures, thus modeling the signing habit of a subject. Experimental results on the SUSig database show that the proposed model can be effectively used for signature verification.
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Millisecond-scale motor encoding in a cortical vocal area.

Author SummaryA central question in neuroscience is how neurons use patterns of electrical events to represent sensory information and control behavior. Neurons might use two different codes to transmit information. First, signals might be conveyed by the total number of electrical events (called “action potentials”) that a neuron produces. Alternately, the timing pattern of action potentials, as distinct from the total number of action potentials produced, might be used to transmit information. Although many studies have shown that timing can convey information about sensory inputs, such as visual scenery or sound waveforms, the role of action potential timing in the control of complex, learned behaviors is largely unknown. Here, by analyzing the pattern of action potentials produced in a songbird's brain as it precisely controls vocal behavior, we demonstrate that far more information about upcoming behavior is present in spike timing than in the total number of spikes fired. This work suggests that timing can be equally (or more) important in motor systems as in sensory systems.

The biophysics of bird flight: functional relationships integrate aerodynamics, morphology, kinematics, muscles, and sensors

Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier.

Distributions in the error space: goal-directed movements described in time and state-space representations.

Manipulation of error feedback has been of great interest to recent studies in motor control and rehabilitation. Typically, motor adaptation is shown as a change in performance with a single scalar metric for each trial, yet such an approach might overlook details about how error evolves through the movement. We believe that statistical distributions of movement error through the extent of the trajectory can reveal unique patterns of adaption and possibly reveal clues to how the motor system processes information about error. This paper describes different possible ordinate domains, focusing on representations in time and state-space, used to quantify reaching errors. We hypothesized that the domain with the lowest amount of variability would lead to a predictive model of reaching error with the highest accuracy. Here we showed that errors represented in a time domain demonstrate the least variance and allow for the highest predictive model of reaching errors. These predictive models will give rise to more specialized methods of robotic feedback and improve previous techniques of error augmentation.

Modular control of movement and posture by the corticospinal alpha-gamma motor systems.

It is widely assumed that neural control of movement is carried out by the a motor system sufficiently. The role of the γ motor system in movement and posture has not been adequately addressed in motor control studies. Here, we propose a modular control model for movement and posture based on propriospinal neuronal (PN) network and spinal α-γ motor system. In the modular control model, the a and γ motor commands are divided into static and dynamic functions. The static commands are specified by the higher center of brain for posture control, and the dynamic commands for movement generation, respectively. Centrally planned kinematics based on the minimal jerk criterion is conveyed to the periphery via the γ motor system, while centrally programmed bi-phasic burst pattern of muscle activation is relayed to a pair of antagonistic muscles through the a motor system via the PN. Results of simulation showed that elbow kinematics and biceps and triceps activations displayed the similar kinematic and EMG features of fast reaching movement in human. This suggests a hypothesis that the α-γ motor systems can achieve modular control of movement and posture in parallel.

Modular organization of reaching and grasping movements investigated using EEG microstates.

How movements are generated and controlled by the central nervous system (CNS) is still not well understood. In this work, we tested the hypothesis of a modular organization of the brain activity during the execution of voluntary movements. In particular, we extracted meta-stable topographies as a measure for global brain state, so-called microstates, from electroencephalography (EEG) data during pure planar reaching movements as well as reaching and grasping of different objects, and we compared them with those extracted during resting-state. The results showed the emergence of specific EEG microstates related to movement execution. Our results provide evidence about the benefits of EEG microstate analysis for motor control studies and their importance to better understand brain reorganization in neurological pathologies.

Uncontrolled manifold analysis of arm joint angle variability during robotic teleoperation and freehand movement of surgeons and novices.

Teleoperated robot-assisted surgery (RAS) is used to perform a wide variety of minimally invasive procedures. However, current understanding of the effect of robotic manipulation on the motor coordination of surgeons is limited. Recent studies in human motor control suggest that we optimize hand movement stability and task performance while minimizing control effort and improving robustness to unpredicted disturbances. To achieve this, the variability of joint angles and muscle activations is structured to reduce task-relevant variability and increase task-irrelevant variability. In this study, we determine whether teleoperation of a da Vinci Si surgical system in a nonclinical task of simple planar movements changes this structure of variability in experienced surgeons and novices. To answer this question, we employ the UnControlled manifold analysis that partitions users' joint angle variability into task-irrelevant and task-relevant manifolds. We show that experienced surgeons coordinate their joint angles to stabilize hand movements more than novices, and that the effect of teleoperation depends on experience--experts increase teleoperated stabilization relative to freehand whereas novices decrease it. We suggest that examining users' exploitation of the task-irrelevant manifold for stabilization of hand movements may be applied to: (1) evaluation and optimization of teleoperator design and control parameters, and (2) skill assessment and optimization of training in RAS.