An optimal velocity for online limb-target regulation processes?

This study clarifies the effects of follower–leader information asymmetry, which is due to a target position and a transport cycle, on brain activities during human–human 1-DOF parallel reciprocating transport work. Elucidating the effects of information asymmetry on leader–follower brain activities by analyzing brain activity and hand movements in humans with visual information and humans without visual information will reveal the behavioral characteristics of humans for cooperative work. Therefore, in this study, we simultaneously measured brain activities, the forces applied to a 1-DOF transportation mechanism, and the hand positions of two people during human–human 1-DOF parallel reciprocating transport work using a transport cycle of 3 s. Three levels were set for the follower side to antisymmetrize the information of a follower with respect to a leader: i) the follower was asked to wear an eye mask and not be informed of a target position and a transport cycle (Level 1), ii) the follower was informed of a target position but not a transport cycle (Level 2), and iii) the follower was informed of a transport cycle and a target position by a metronome sound (Level 3). In the evaluation of brain activity, brain activation was assessed by the amount of change in the concentration of oxygenated hemoglobin (Oxy–Hb), which had the highest correlation with changes in cerebral blood flow. The results showed that Level 2, which corresponded to cooperative work between humans with visual information, demonstrated significant synchronization changes in both cerebral blood flow in mirror neurons and hand inertia/stiffness compared to Level 1, which corresponded to cooperative work between humans with visual information and humans without visual information, suggesting a relationship between them. In addition, the evaluation of brain activation and impedance change suggests that cooperative work among humans with visual information did not involve voluntary movements but smoother cooperative work from both neuroscientific and kinematic perspectives.

A brief glimpse at a haptic target is sufficient for multisensory integration in reaching movements

The rapid-chase theory does not extend to movement execution.

A comparison of the dorsomedial and the dorsolateral reach-to-grasp pathways' roles in response to virtual visual perturbations of object goal.

Vestibular information is available to the brain during navigation, as are the other self-generated (idiothetic) and external (allothetic) sensorimotor cues that contribute to central estimates of position and motion. Rodent studies provide strong evidence that vestibular information contributes to navigation but human studies have been less conclusive. Furthermore, sex-based differences have been described in human navigation studies performed with the head stationary, a situation where dynamic vestibular (and other idiothetic) information is absent, but sex differences in the utilization of vestibular information have not been described. Here, we studied men and women with severe bilateral vestibular damage as they navigated through a visually barren virtual reality environment and compared their performance to normal men and women. Two navigation protocols were employed, which either activated dynamic idiothetic cues ('dynamic task', navigate by turning, walking in place) or eliminated them ('static task', navigate with key presses, head stationary). For both protocols, we employed a standard 'triangle completion task' in which subjects moved to two visual targets in series and then were required to return to their perceived starting position without localizing visual information. The angular and linear 'accuracy' (derived from response error) and 'precision' (derived from response variability) were calculated. Comparing performance 'within tasks', navigation on the dynamic paradigm was worse in male vestibular-deficient patients than in normal men but vestibular-deficient and normal women were equivalent; on the static paradigm, vestibular-deficient men (but not women) performed better than normal subjects. Comparing performance 'between tasks', normal men performed better on the dynamic than the static paradigm while vestibular-deficient men and both normal and vestibular-deficient women were equivalent on both tasks. Statistical analysis demonstrated that for the angular precision metric, sex had a significant effect on the interaction between vestibular status and the test paradigm. These results provide evidence that humans use vestibular information when they navigate in a virtual visual environment and that men and women may utilize vestibular (and visual) information differently. On our navigation paradigm, men used vestibular information to improve navigation performance, and in the presence of severe vestibular damage, they utilized visual information more effectively. In contrast, we did not find evidence that women used vestibular information while navigating on our virtual task, nor did we find evidence that they improved their utilization of visual information in the presence of severe vestibular damage.

Fast adjustments of ongoing movements in hemiparetic cerebral palsy

Three experiments were conducted to determine if a representation of the movement environment is functional in the organization and control of limb movements, when direct visual contact with the environment is prevented. In Experiment 1, a visual rearrangement procedure was employed to show that a representation of the environment that provides inaccurate information about the spatial location of a target can disrupt manual target aiming. In Experiment 2, we demonstrated that spatial information about the position of a target can be destroyed by a visual pattern mask, supporting our claim that the representation is visual. A target-cuing procedure was used in Experiment 3 to show that representation of target position can be useful for premovement organization in a target-aiming task. Together our findings suggest that a short-lived visual representation of the movement environment may serve a useful role in the organization and control of limb movements.

The ability to gain voluntary control over agonist premotor silence through electromyographic (EMG) feedback was examined in healthy subjects performing maximal horizontal elbow flexions. Subjects exhibiting premotor silence on at least 50% of the pretest trials showed significantly greater peak angular velocity than subjects who produced the silent period on fewer than 20% of the trials during the pretest. The latter subjects acquired control of agonist premotor silence with practice and graphic feedback regarding their EMG patterns. The subjects who were the most successful in learning to produce the silent period increased their angular velocity to the level of the subjects who naturally exhibited the inhibition. The less successful subjects showed smaller increases in velocity. The data provide further evidence that premotor silence is primarily under central influence, that its control can be acquired, and that it may be functionally related to contractile rate.

Control of voluntary limb movement is predominantly attributed to the contralateral motor cortex. However, increasing evidence suggests the involvement of ipsilateral cortical networks in this process, especially in motor tasks requiring bilateral coordination, such as locomotion. In this study, we combined a unilateral thoracic spinal cord injury (SCI) with a cortical neuroprosthetic approach to investigate the functional role of the ipsilateral motor cortex in rat movement through spared contralesional pathways. Our findings reveal that in all SCI rats, stimulation of the ipsilesional motor cortex promoted a bilateral synergy. This synergy involved the elevation of the contralateral foot along with ipsilateral hindlimb extension. Additionally, in two out of seven animals, stimulation of a sub-region of the hindlimb motor cortex modulated ipsilateral hindlimb flexion. Importantly, ipsilateral cortical stimulation delivered after SCI immediately alleviated multiple locomotor and postural deficits, and this effect persisted after ablation of the homologous motor cortex. These results provide strong evidence of a causal link between cortical activation and precise ipsilateral control of hindlimb movement. This study has significant implications for the development of future neuroprosthetic technology and our understanding of motor control in the context of SCI.

Three experiments were conducted to determine how variables other than movement time influence the speed of visual feedback utilization in a target-pointing task. In Experiment 1, subjects moved a stylus to a target 20 cm away with movement times of approximately 225 msec. Visual feedback was manipulated by leaving the room lights on over the whole course of the movement or extinguishing the lights upon movement initiation, while prior knowledge about feedback availability was manipulated by blocking or randomizing feedback. Subjects exhibited less radial error in the lights-on/blocked condition than in the other three conditions. In Experiment 2, when subjects were forced to use vision by a laterally displacing prism, it was found that they benefited from the presence of visual feedback regardless of feedback uncertainty even when moving very rapidly (e.g. less than 190 msec). In Experiment 3, subjects pointed with and without a prism over a wide variety of movement times. Subjects benefited from vision much earlier in the prism condition. Subjects seem able to use vision rapidly to modify aiming movements but may do so only when the visual information is predictably available and/or yields an error large enough to detect early enough to correct.

The directional control of smooth pursuit eye movements was studied by presenting human subjects with targets that moved in a straight line at a constant speed and then changed direction abruptly and unpredictably. To minimize the probability of saccadic responses in the interval following the target's change in direction, target position was offset so as to eliminate position error after the reaction time. Smooth pursuit speed declined at a latency of 90 ms, whereas the direction of smooth pursuit began to change later (130 ms). The amplitude of the offset in target position did not affect the subsequent smooth pursuit response. In other experiments, the target's speed or acceleration was changed abruptly at the time of the change in direction. Step changes in speed elicited short-latency responses in smooth pursuit tracking but step changes in acceleration did not. In all instances, the earliest component of the response did not depend on the parameters of the stimulus. The data were fit with a model in which smooth pursuit resulted from the vector addition of two components, one representing a response to the arrest of the initial target motion and the other the response to the onset of target motion in the new direction. This model gave an excellent fit but further analysis revealed nonlinear interactions between the two vector components. These interactions represented directional anisotropies both in terms of the initial tracking direction (which was either vertical or 45 degrees ) and in terms of the cardinal directions (vertical and horizontal).

Grasping versus pointing and the differential use of visual feedback

This work presents an intention decoding algorithm that can be used to control a 4 degrees-of-freedom shoulder-elbow exoskeleton in reaching tasks. The algorithm was designed to assist the movement of users with upper-limb impairments who can initiate the movement by themselves. It relies on the observation of the initial part of the user's movement through joint angle measures and aims to estimate in real-time the phase of the movement and predict the goal position of the hand in the reaching task. The algorithm is based on adaptive Dynamic Movement Primitives and Gaussian Mixture Models. The performance of the algorithm was verified in robot-assisted planar reaching movements performed by one healthy subject wearing the exoskeleton. Tests included movements of different amplitudes and orientations. Results showed that the algorithm could predict the hand's final position with an error lower than 5 cm after 0.25 s from the movement onset, and that the final position reached during the tests was on average less than 4 cm far from the target position. Finally, the effects of the assistance were observed in a reduction of the activation of the Biceps Brachii and of the time to execute the reaching tasks.

In this study, we examine whether corrections made during an ongoing movement are differentially affected by left hemisphere damage (LHD) and right hemisphere damage (RHD). Our hypothesis of motor lateralization proposes that control mechanisms specialized to the right hemisphere rely largely on online processes, while the left hemisphere primarily utilizes predictive mechanisms to specify optimal coordination patterns. We therefore predict that RHD, but not LHD, should impair online correction when task goals are unexpectedly changed. Fourteen stroke subjects (7 LHD, 7 RHD) and 14 healthy controls reached to 1 of the 3 targets that unexpectedly "jumped" during movement onset. RHD subjects showed a considerable delay in initiating the corrective response relative to controls and LHD subjects. However, both stroke groups made large final position errors on the target jump trials. Position deficits following LHD were associated with poor intersegmental coordination, while RHD subjects had difficulty terminating their movements appropriately. These findings confirm that RHD, but not LHD, produces a deficit in the timing of online corrections and also indicate that both stroke groups show position deficits that are related to the specialization of their damaged hemisphere. Further research is needed to identify specific neural circuits within each hemisphere critical for these processes.

Introduction: Recent views posit that precise control of the interceptive timing can be achieved by combining on-line processing of visual information with predictions based on prior experience. Indeed, for interception of free-falling objects under gravity's effects, experimental evidence shows that time-to-contact predictions can be derived from an internal gravity representation in the vestibular cortex. However, whether the internal gravity model is fully engaged at the target motion outset or reinforced by visual motion processing at later stages of motion is not yet clear. Moreover, there is no conclusive evidence about the relative contribution of internalized gravity and optical information in determining the time-to-contact estimates. Methods: We sought to gain insight on this issue by asking 32 participants to intercept free falling objects approaching directly from above in virtual reality. Object motion had durations comprised between 800 and 1100ms and it could be either congruent with gravity (1g accelerated motion) or not (constant velocity or -1g decelerated motion). We analyzed accuracy and precision of the interceptive responses, and fitted them to Bayesian regression models, which included predictors related to the recruitment of a priori gravity information at different times during the target motion, as well as based on available optical information. Results: Consistent with the use of internalized gravity information, interception accuracy and precision were significantly higher with 1g motion. Moreover, Bayesian regression indicated that interceptive responses were predicted very closely by assuming engagement of the gravity prior 450ms after the motion onset, and that adding a predictor related to on-line processing of optical information improved only slightly the model predictive power. Discussion: Thus, engagement of a priori gravity information depended critically on the processing of the first 450 ms of visual motion information, exerting a predominant influence on the interceptive timing, compared to continuously available optical information. Finally, these results may support a parallel processing scheme for the control of interceptive timing.