Tuning in to real-time social interactions in macaques.

Movement synchronization between individuals has been implicated in reinforcing their cohesion. How might such interindividual motor entrainment be controlled by the social brain? The answer remains elusive owing largely to the lack of suitable animal models in which direct neural recordings are available. Here, we show that macaque monkeys exhibit social motor entrainment without human prompting. We found that repetitive arm movements for horizontal bar sliding were phase coherent between two monkeys. The nature of motor entrainment was specific to animal pairs, consistent across days, dependent on visual inputs, and affected by social hierarchy. Notably, the entrainment was diminished when paired with prerecorded movies of a monkey making the same movements or bar motion alone. These findings demonstrate that motor entrainment is facilitated by real-time social exchanges, providing a behavioral platform to study the neural basis of potentially evolutionarily conserved mechanisms that support group cohesion.

Impact of decision and action outcomes on subsequent decision and action behaviours in humans.

Speed-accuracy trade-off adjustments in decision-making have been mainly studied separately from those in motor control. In the wild, however, animals coordinate their decision and action, often deciding while acting. Recent behavioural studies support this view, indicating that animals, including humans, trade decision time for movement time to maximize their global rate of reward during experimental sessions. Besides, it is well established that choice outcomes impact subsequent decisions. Crucially though, whether and how a decision outcome also influences the subsequent motor performance, and whether and how the outcome of a movement influences the next decision, is unclear. Here, we address these questions by analysing trial-to-trial changes of choice and motor behaviours in healthy human participants instructed to perform successive perceptual decisions expressed with reaching movements whose duration was either weakly or strongly constrained in separate tasks. Results indicate that after a wrong decision, subjects who were weakly constrained in their action duration decided more slowly and more accurately. Interestingly, they also shortened their subsequent movement duration by moving faster. Conversely, we found that errors of constrained movements influenced not only the speed and the amplitude of the following movement but those of the decision too. If the movement had to be slowed down, the decision that precedes that movement was accelerated and vice versa. Together, these results indicate that from one trial to the next, humans seek to determine a behavioural duration as a whole instead of optimizing each of the decision and action speed-accuracy trade-offs independently of each other.

A methodological exploration to study 2D arm kinematics in Ophiuroidea (Echinodermata).

Brittle stars, unlike most other echinoderms, do not use their small tube feet for locomotion but instead use their flexible arms to produce a rowing or reverse rowing movement. They are among the fastest-moving echinoderms with the ability of complex locomotory behaviors. Considering the high species diversity and variability in morphotypes, a proper understanding of intra- and interspecies variation in arm flexibility and movement is lacking. This study focuses on the exploration of the methods to investigate the variability in brittle star locomotion and individual arm use. We performed a two-dimensional (2D) image processing on horizontal movement only. The result indicated that sinuosity, disc displacement and arm angle are important parameters to interpret ophiuroid locomotion. A dedicated Python script to calculate the studied movement parameters and visualize the results applicable to all 5-armed brittle stars was developed. These results can serve as the basis for further research in robotics inspired by brittle star locomotion.

Age-related changes in mobility assessments correlate with repetitive goal-directed arm-movement performance.

BACKGROUND: There is ample evidence that mobility abilities between healthy young and elderly people differ. However, we do not know whether these differences are based on different lower leg motor capacity or instead reveal a general motor condition that could be detected by monitoring upper-limb motor behavior. We therefore captured body movements during a standard mobility task, namely the Timed Up and Go test (TUG) with subjects following different instructions while performing a rapid, repetitive goal-directed arm-movement test (arm-movement test). We hypothesized that we would be able to predict gait-related parameters from arm motor behavior, even regardless of age. METHODS: Sixty healthy individuals were assigned to three groups (young: mean 26 ± 3 years, middle-aged 48 ± 9, old 68 ± 7). They performed the arm-movement and TUG test under three conditions: preferred (at preferred movement speed), dual-task (while counting backwards), and fast (at fast movement speed). We recorded the number of contacts within 20 s and the TUG duration. We also extracted TUG walking sequences to analyze spatiotemporal gait parameters and evaluated the correlation between arm-movement and TUG results. RESULTS: The TUG condition at preferred speed revealed differences in gait speed and step length only between young and old, while dual-task and fast execution increased performance differences significantly among all 3 groups. Our old group's gait speed decreased the most doing the dual-task, while the young group's gait speed increased the most during the fast condition. As in our TUG results, arm-movements were significant faster in young than in middle-aged and old. We observed significant correlations between arm movements and the fast TUG condition, and that the number of contacts closely predicts TUG timefast and gait speedfast. This prediction is more accurate when including age. CONCLUSION: We found that the age-related decline in mobility performance that TUG reveals strongly depends on the test instruction: the dual-task and fast condition clearly strengthened group contrasts. Interestingly, a fast TUG performance was predictable by the performance in a fast repetitive goal-directed arm-movements test, even beyond the age effect. We assume that arm movements and the fast TUG condition reflect similarly reduced motor function. TRIAL REGISTRATION: German Clinical Trials Register (DRKS) number: DRKS00016999, prospectively registered on March, 26, 2019.

Can we use peripheral vision to create a visuospatial map for compensatory reach-to-grasp reactions?

Perturbation-induced reach-to-grasp reactions are dependent on vision to capture environmental features of potential support surfaces. Previous research proposed the use of an intrinsic visuospatial map of the environment to reduce delays in motor responses (e.g., stepping, grasping a handrail). Forming such a map from foveal vision would be challenging during movement as it would require constant foveal scanning. The objective of this study was to determine if compensatory reach-to-grasp reactions could be successfully executed while relying on a visuospatial map acquired using peripheral vision. Subjects were instructed to respond to a perturbation by grasping a handle randomly located at 0°, 20° or 40° in their field of view under three visual conditions: full vision throughout the entire trial (FV), vision available prior to perturbation only (MAP), and vision available post-perturbation only (ONLINE). Electromyography was used to determine reaction time and kinematic data were collected to determine initial reach angle. Overall, participants were successful in arresting whole-body motion across all visual conditions and handle locations. Initial reach angles were target specific when vision was available prior to perturbation onset (FV and MAP). However, the 40° handle location produced a greater initial reach angle in MAP, suggesting some limitations for mapping in the further visual periphery. These findings suggest that peripheral vision contributes to the ability to spatially locate targets by building an a priori visuospatial map, which benefits the control of rapid compensatory reach-to-grasp reactions evoked in the response to unpredictable events of instability.

Distributed processing of movement signaling.

Basic neurophysiological research with monkeys has shown how neurons in the motor cortex have firing rates tuned to movement direction. This original finding would have been difficult to uncover without the use of a behaving primate paradigm in which subjects grasped a handle and moved purposefully to targets in different directions. Subsequent research, again using behaving primate models, extended these findings to continuous drawing and to arm and hand movements encompassing action across multiple joints. This research also led to robust extraction algorithms in which information from neuronal populations is used to decode movement intent. The ability to decode intended movement provided the foundation for neural prosthetics in which brain-controlled interfaces are used by paralyzed human subjects to control computer cursors or high-performance motorized prosthetic arms and hands. This translation of neurophysiological laboratory findings to therapy is a clear example of why using nonhuman primates for basic research is valuable for advancing treatment of neurological disorders. Recent research emphasizes the distribution of intention signaling through neuronal populations and shows how many movement parameters are encoded simultaneously. In addition to direction and velocity, the arm's impedance has now been found to be encoded as well. The ability to decode motion and force from neural populations will make it possible to extend neural prosthetic paradigms to precise interaction with objects, enabling paralyzed individuals to perform many tasks of daily living.

Spatial eye-hand coordination during bimanual reaching is not systematically coded in either LIP or PRR.

We often orient to where we are about to reach. Spatial and temporal correlations in eye and arm movements may depend on the posterior parietal cortex (PPC). Spatial representations of saccade and reach goals preferentially activate cells in the lateral intraparietal area (LIP) and the parietal reach region (PRR), respectively. With unimanual reaches, eye and arm movement patterns are highly stereotyped. This makes it difficult to study the neural circuits involved in coordination. Here, we employ bimanual reaching to two different targets. Animals naturally make a saccade first to one target and then the other, resulting in different patterns of limb-gaze coordination on different trials. Remarkably, neither LIP nor PRR cells code which target the eyes will move to first. These results suggest that the parietal cortex plays at best only a permissive role in some aspects of eye-hand coordination and makes the role of LIP in saccade generation unclear.

Coupling of head and hand movements during eye-head-hand coordination: there is more to reaching than meets eye.

Does arm reaching affect eye-head shifts? Does the head alter eye-hand coordinated movements? Sensorimotor research has focused on either eye-head or eye-hand coordination, with only occasional works studying all these effectors together. Arora et al. (Arora HK, Bharmauria V, Yan X, Sun S, Wang H, Crawford JD. J Neurophysiol 122: 1946-1961, 2019) examined eye-head-hand coordination for the first time in nonhuman primates and provide evidence suggesting that head and hand movements are more coupled than traditionally considered.

Early-stage Parkinson's patients show selective impairment in reactive but not proactive inhibition.

BACKGROUND: It is well known that a deficit in inhibitory control is a hallmark of Parkinson's disease (PD). However, inhibition is not a unitary construct, and it is unclear whether patients in the early stage of the disease (Hoehn and Yahr stage 1) exhibit a deficit in outright stopping (reactive inhibition), a deficit in the ability to shape their response strategies according to the context (proactive inhibition), or both. OBJECTIVE: We assessed whether PD patients at Hoehn and Yahr stage 1 show a global or selective impairment in inhibitory control. As it has been suggested that inhibition relies upon a right-lateralized pathway, we tested whether left-dominant PD patients suffered from a more severe deficit in this executive function than right-dominant PD patients. METHODS: Via a reaching stop-signal task, we assessed both proactive and reactive inhibition in 17 left-dominant PD and 17 right-dominant PD patients and in 24 age-matched participants. RESULTS: We found that reactive inhibition was more impaired in PD patients than in healthy participants. However, proactive inhibition was not affected. Furthermore, we found no differences between left-dominant PD and right-dominant PD patients. CONCLUSIONS: For the first time, we found evidence for a deficit of reactive inhibition in the early-stage PD patients in the absence of evidence for deficits in proactive inhibition. These findings have clinical relevance as they provide critical insights on the time course of the disease. In addition, we confirmed, on a population of PD patients at Hoehn and Yahr stage 1, previous results showing that the onset of the disease does not affect inhibition. © 2019 International Parkinson and Movement Disorder Society.

Experimental and theoretical study of velocity fluctuations during slow movements in humans.

Moving smoothly is generally considered as a higher-order goal of motor control and moving jerkily as a witness of clumsiness or pathology, yet many common and well-controlled movements (e.g., tracking movements) have irregular velocity profiles with widespread fluctuations. The origin and nature of these fluctuations have been associated with the operation of an intermittent process but in fact remain poorly understood. Here we studied velocity fluctuations during slow movements, using combined experimental and theoretical tools. We recorded arm movement trajectories in a group of healthy participants performing back-and-forth movements at different speeds, and we analyzed velocity profiles in terms of series of segments (portions of velocity between 2 minima). We found that most of the segments were smooth (i.e., corresponding to a biphasic acceleration) and had constant duration irrespective of movement speed and linearly increasing amplitude with movement speed. We accounted for these observations with an optimal feedback control model driven by a staircase goal position signal in the presence of sensory noise. Our study suggests that one and the same control process can explain the production of fast and slow movements, i.e., fast movements emerge from the immediate tracking of a global goal position and slow movements from the successive tracking of intermittently updated intermediate goal positions. NEW & NOTEWORTHY We show in experiments and modeling that slow movements could result from the brain tracking a sequence of via points regularly distributed in time and space. Accordingly, slow movements would differ from fast movement by the nature of the guidance and not by the nature of control. This result could help in understanding the origin and nature of slow and segmented movements frequently observed in brain disorders.

Cortical and subcortical areas involved in the regulation of reach movement speed in the human brain: An fMRI study.

Reach movements are characterized by multiple kinematic variables that can change with age or due to medical conditions such as movement disorders. While the neural control of reach direction is well investigated, the elements of the neural network regulating speed (the nondirectional component of velocity) remain uncertain. Here, we used a custom made magnetic resonance (MR)-compatible arm movement tracking system to capture the real kinematics of the arm movements while measuring brain activation with functional magnetic resonance imaging to reveal areas in the human brain in which BOLD-activation covaries with the speed of arm movements. We found significant activation in multiple cortical and subcortical brain regions positively correlated with endpoint (wrist) speed (speed-related activation), including contralateral premotor cortex (PMC), supplementary motor area (SMA), thalamus (putative VL/VA nuclei), and bilateral putamen. The hand and arm regions of primary sensorimotor cortex (SMC) and a posterior region of thalamus were significantly activated by reach movements but showed a more binary response characteristics (movement present or absent) than with continuously varying speed. Moreover, a subregion of contralateral SMA also showed binary movement activation but no speed-related BOLD-activation. Effect size analysis revealed bilateral putamen as the most speed-specific region among the speed-related clusters whereas primary SMC showed the strongest specificity for movement versus non-movement discrimination, independent of speed variations. The results reveal a network of multiple cortical and subcortical brain regions that are involved in speed regulation among which putamen, anterior thalamus, and PMC show highest specificity to speed, suggesting a basal-ganglia-thalamo-cortical loop for speed regulation.

Effects of non-paretic arm movements during bridge exercises on trunk muscle activity in stroke patients.

[Purpose] The purpose of this study was to determine the effects of non-paretic arm movement during the bridge exercise on trunk muscle activity in stroke patients. [Participants and Methods] In total, 18 stroke patients were recruited. Surface EMG electrodes were attached over the trunk muscles (rectus abdominis, RA; internal oblique, IO; erector spinae, ES), and three kinds of bridge exercises were performed: 1) 'standard' bridge, 2) bridge with unilateral isometric arm flexion, and 3) bridge with unilateral isometric arm horizontal abduction. [Results] According to the activity of the trunk muscles measured during bridge exercises, only the IO and ES showed significantly greater muscle activity during bridges with isometric arm horizontal abduction and flexion than during the standard bridge. Additionally, comparison of the paretic and non-paretic sides showed that muscle activity was higher on the paretic side. [Conclusion] This study showed that, as an exercise to heighten the activity of the trunk muscles in stroke patients, bridge exercises with accompanying non-paretic arm flexion and horizontal abduction were more effective clinically than a standard bridge.

Interplay of Gravicentric, Egocentric, and Visual Cues About the Vertical in the Control of Arm Movement Direction.

Our perception of the vertical corresponds to the weighted sum of gravicentric, egocentric, and visual cues. Here we evaluate the interplay of those cues not for the perceived but rather for the motor vertical. Participants were asked to flip an omnidirectional switch down while their egocentric vertical was dissociated from their visual-gravicentric vertical. Responses were directed mid-between the two verticals; specifically, the data suggest that the relative weight of congruent visual-gravicentric cues averages 0.62, and correspondingly, the relative weight of egocentric cues averages 0.38. We conclude that the interplay of visual-gravicentric cues with egocentric cues is similar for the motor and for the perceived vertical. Unexpectedly, we observed a consistent dependence of the motor vertical on hand position, possibly mediated by hand orientation or by spatial selective attention.

On the neural basis of sensory weighting: Alpha, beta and gamma modulations during complex movements.

Previous studies have revealed that visual and somatosensory information is processed as a function of its relevance during movement execution. We thus performed spectral decompositions of ongoing neural activities within the somatosensory and visual areas while human participants performed a complex visuomotor task. In this task, participants followed the outline of irregular polygons with a pen-controlled cursor. At unpredictable times, the motion of the cursor deviated 120° with respect to the actual pen position creating an incongruence between visual and somatosensory inputs, thus increasing the importance of visual feedback to control the movement as suggested in previous studies. We found that alpha and beta power significantly decreased in the visual cortex during sensory incongruence when compared to unperturbed conditions. This result is in line with an increased gain of visual inputs during sensory incongruence. In parallel, we also found a simultaneous decrease of gamma and beta power in sensorimotor areas which has not been reported previously. The gamma desynchronization suggests a reduced integration of somatosensory inputs for controlling movements with sensory incongruence while beta ERD could be more specifically linked to sensorimotor adaptation processes.

Referent control and motor equivalence of reaching from standing.

Motor actions may result from central changes in the referent body configuration, defined as the body posture at which muscles begin to be activated or deactivated. The actual body configuration deviates from the referent configuration, particularly because of body inertia and environmental forces. Within these constraints, the system tends to minimize the difference between these configurations. For pointing movement, this strategy can be expressed as the tendency to minimize the difference between the referent trajectory (RT) and actual trajectory (QT) of the effector (hand). This process may underlie motor equivalent behavior that maintains the pointing trajectory regardless of the number of body segments involved. We tested the hypothesis that the minimization process is used to produce pointing in standing subjects. With eyes closed, 10 subjects reached from a standing position to a remembered target located beyond arm length. In randomly chosen trials, hip flexion was unexpectedly prevented, forcing subjects to take a step during pointing to prevent falling. The task was repeated when subjects were instructed to intentionally take a step during pointing. In most cases, reaching accuracy and trajectory curvature were preserved due to adaptive condition-specific changes in interjoint coordination. Results suggest that referent control and the minimization process associated with it may underlie motor equivalence in pointing. NEW & NOTEWORTHY: Motor actions may result from minimization of the deflection of the actual body configuration from the centrally specified referent body configuration, in the limits of neuromuscular and environmental constraints. The minimization process may maintain reaching trajectory and accuracy regardless of the number of body segments involved (motor equivalence), as confirmed in this study of reaching from standing in young healthy individuals. Results suggest that the referent control process may underlie motor equivalence in reaching.

Vector and position coding in goal-directed movements.

Two different ways to code a goal-directed movement have been proposed in the literature: vector coding and position coding. Assuming that the code is fine-tuned if a movement is immediately repeated, one can predict that repeating movements to the same endpoint will increase precision if movements are coded in terms of the position of the endpoint. Repeating the same movement vector at slightly different positions will increase precision if movements are coded in terms of vectors. Following this reasoning, Hudson and Landy (J Neurophys 108(10):2708-2716, 2012) found evidence for both types of coding when participants moved their hand over a table while the target and feedback were provided on a vertical screen. Do we also see evidence for both types of coding if participants repeat movements within a more natural visuo-motor mapping? To find out, we repeated the study of Hudson and Landy (J Neurophys 108(10):2708-2716, 2012), but our participants made movements directly to the targets. We compared the same movements embedded in blocks of repetitions of endpoints and blocks of repetitions of movement vectors. Within blocks, the movements were presented in a random order. We found no benefit of repeating either a position or a vector. We subsequently repeated the experiment with a similar mapping between movements and images to those used by Hudson and Landy and found that participants only clearly benefit from repeating a position. We conclude that repeating a position is particularly useful when dealing with unusual visuo-motor mappings.

Initial information prior to movement onset influences kinematics of upward arm pointing movements.

To elaborate a motor plan and perform online control in the gravity field, the brain relies on priors and multisensory integration of information. In particular, afferent and efferent inputs related to the initial state are thought to convey sensorimotor information to plan the upcoming action. Yet it is still unclear to what extent these cues impact motor planning. Here we examined the role of initial information on the planning and execution of arm movements. Participants performed upward arm movements around the shoulder at three speeds and in two arm conditions. In the first condition, the arm was outstretched horizontally and required a significant muscular command to compensate for the gravitational shoulder torque before movement onset. In contrast, in the second condition the arm was passively maintained in the same position with a cushioned support and did not require any muscle contraction before movement execution. We quantified differences in motor performance by comparing shoulder velocity profiles. Previous studies showed that asymmetric velocity profiles reflect an optimal integration of the effects of gravity on upward movements. Consistent with this, we found decreased acceleration durations in both arm conditions. However, early differences in kinematic asymmetries and EMG patterns between the two conditions signaled a change of the motor plan. This different behavior carried on through trials when the arm was at rest before movement onset and may reveal a distinct motor strategy chosen in the context of uncertainty. Altogether, we suggest that the information available online must be complemented by accurate initial information.

Primate superior colliculus neurons activated by unexpected sensation.

Midbrain superior colliculus (SC) contains a variety of neuronal types, influencing a rich spectrum of functions beyond gaze orienting. Here, we report on a novel class of SC neurons in the rhesus monkey (Macaca mulatta) that are activated by an unexpected perturbation in a goal-directed arm-movement task. One monkey subject reached for and pressed an illuminated target on a working panel upon a visual go-signal, while maintaining visual fixation elsewhere. On 50 % of trials, a task perturbation occurred-the working panel abruptly and unexpectedly moved against the subject's hand after he pressed the target. During the performance, we recorded single SC neurons and found neurons activated exclusively for the task perturbation. These perturbation neurons were localized in the deep lateral zone of the SC, were silent during non-perturbed trials, did not appear to respond to task-irrelevant stimuli, and they had intriguingly long neuronal latencies. If the perturbation neurons' activity relates to the hand-target contact, it may reflect the saliency of an unexpected sensation, i.e. a sensation that is not self-induced and thus cannot be predicted on a basis of the monkey's motor program.

Modifying Kinect placement to improve upper limb joint angle measurement accuracy.

STUDY DESIGN: Repeated measures. INTRODUCTION: The Kinect (Microsoft, Redmond, WA) is widely used for telerehabilitation applications including rehabilitation games and assessment. PURPOSE OF THE STUDY: To determine effects of the Kinect location relative to a person on measurement accuracy of upper limb joint angles. METHODS: Kinect error was computed as difference in the upper limb joint range of motion (ROM) during target reaching motion, from the Kinect vs 3D Investigator Motion Capture System (NDI, Waterloo, Ontario, Canada), and compared across 9 Kinect locations. RESULTS: The ROM error was the least when the Kinect was elevated 45° in front of the subject, tilted toward the subject. This error was 54% less than the conventional location in front of a person without elevation and tilting. The ROM error was the largest when the Kinect was located 60° contralateral to the moving arm, at the shoulder height, facing the subject. The ROM error was the least for the shoulder elevation and largest for the wrist angle. DISCUSSION: Accuracy of the Kinect sensor for detecting upper limb joint ROM depends on its location relative to a person. CONCLUSION: This information facilitates implementation of Kinect-based upper limb rehabilitation applications with adequate accuracy. LEVEL OF EVIDENCE: 3b.

The effects of spinal mobilization with arm movements on shoulder muscle strengthening.

[Purpose] This study was performed to examine the immediate effects of spinal mobilization with arm movements (SMWAMs) for strengthening the shoulder muscles. [Subjects] The subjects of this study were 12 healthy male students who were studying at S University, Busan City. They had no musculoskeletal disease, or upper congenital malformation, and had no history of surgery or neurological disease. [Methods] The SMWAMs were performed on the 4th cervical vertebra (C4). For stimulation of the 5th cervical nerve, the spinous process of the C4 was pushed to the left and held in place. Then the arm was abducted to the point where spinous process was felt, and the range of abduction was examined. Subsequently, the abduction movement was repeated 10 times to the same point, and the gliding of C4 was held until the arm returned to the starting position. During the treatment, the head and the trunk were held in the correct posture. [Results] After SMWAMs had been performed, the muscular strength of shoulder flexion, extension and adduction significantly increased. [Conclusion] In our opinion, the SMWAMs are a very useful method for correcting spinal malalignment, and for stimulating the joint mechanoreceptors without the risks of manipulation. SMWAM is a valuable therapy method that can complement the demerits of mobilization.