Differential influence of vision and proprioception on control of movement distance

Evaluation of proprioceptive sense of the elbow joint with RehabRoby

In recent years, robot-assisted rehabilitation systems have been an active research area that can quantitatively monitor and adapt to patient progress, and ensure consistency during rehabilitation. In this work, an exoskeleton type robot-assisted rehabilitation system called RehabRoby is developed. A control architecture, which contains a high level controller and a low level controller, is designed for RehabRoby. Proprioceptive sense of healthy subjects has been evaluated during the execution of a task with RehabRoby. Additionally, usability of RehabRoby has been evaluated using a questionnaire.

Visuomotor feedback gains upregulate during the learning of novel dynamics

At an early stage of learning novel dynamics, changes in muscle activity are mainly due to corrective feedback responses. These feedback contributions to the overall motor command are gradually reduced as feedforward control is learned. The temporary increased use of feedback could arise simply from the large errors in early learning with either unaltered gains or even slightly downregulated gains, or from an upregulation of the feedback gains when feedforward prediction is insufficient. We therefore investigated whether the sensorimotor control system alters feedback gains during adaptation to a novel force field generated by a robotic manipulandum. To probe the feedback gains throughout learning, we measured the magnitude of involuntary rapid visuomotor responses to rapid shifts in the visual location of the hand during reaching movements. We found large increases in the magnitude of the rapid visuomotor response whenever the dynamics changed: both when the force field was first presented, and when it was removed. We confirmed that these changes in feedback gain are not simply a byproduct of the change in background load, by demonstrating that this rapid visuomotor response is not load sensitive. Our results suggest that when the sensorimotor control system experiences errors, it increases the gain of the visuomotor feedback pathways to deal with the unexpected disturbances until the feedforward controller learns the appropriate dynamics. We suggest that these feedback gains are upregulated with increased uncertainty in the knowledge of the dynamics to counteract any errors or disturbances and ensure accurate and skillful movements.

Haptic interaction of touch and proprioception: implications for neuroprosthetics

Somatosensation is divided into multiple discrete modalities that we think of separably: e.g., tactile, proprioceptive, and temperature sensation. However, in processes such as haptics,those modalities all interact. If one intended to artificially generate a sensation that could be used for stereognosis, for example, it would be crucial to understand these interactions. We are presently examining the relationship between tactile and proprioceptive modalities in this context. In this overview of some of our recent work, we show that signals that would normally be attributed to two of these systems separately, tactile contact and self-movement, interact both perceptually and physiologically in ways that complicate the understanding of haptic processing. In the first study described here, we show that a tactile illusion on the fingertips, the cutaneous rabbit effect, can be abolished by changing the posture of the fingers. We then discuss activity in primary somatosensory cortical neurons illustrating the interrelationship of tactile and postural signals. In this study, we used a robot-enhanced virtual environment to show that many neurons in primary somatosensory cortex with cutaneous receptive fields encode elements both of tactile contact and self-motion. We then show the results of studies examining the structure of the process which extracts the spatial location of the hand from proprioceptive signals. The structure of the spatial errors in these maps indicates that the proprioceptive-spatial map is stable but individually constructed.These seemingly disparate studies lead us to suggest that tactile sensation is encoded in a 2-D map, but one which undergoes continual dynamic modification by an underlying proprioceptive map. Understanding how the disparate signals that comprise the somatosensory system are processed to produce sensation is an important step in realizing the kind of seamless integration aspired to in neuroprosthetics.

Synergistic control of joint angle variability: influence of target shape

Reaching movements are often used to study the effectiveness of motor control processes with respect to the final position of arm and hand. Empirical evidence shows that different targets can be grasped with similar final position accuracy. However, movements that achieve similar accuracy at their final position may nevertheless be controlled differently. In particular, control strategies may differ in the control of the abundant degrees of freedom with respect to the task-specific costs. The objective of the present study was to investigate whether the applied control strategy was influenced by the shape of the target to be grasped. It was investigated whether mechanical constraints, imposed on final hand orientation or final hand position by the shape of the targets, affected the synergistic coordination of the kinematic degrees of freedom of the arm. Subjects were asked to grasp either a cylindrical or a spherical target, which imposed different constraints on final hand orientation and position. Besides temporal movement aspects, variability of the joint angles of the arm, as well as variability of hand orientation and hand position was analyzed over the whole time course of movement execution, using the uncontrolled manifold method. Overall movement duration differed between cylindrical and spherical target condition, due to differences in deceleration duration. Reaching movements towards the cylindrical target, which was more constraint in final hand orientation and position, took longer than movements towards the spherical target. Analysis further revealed that the degrees of freedom of the arm were synergistically coordinated to stabilize both hand orientation and hand position, when grasping either the spherical or the cylindrical target. This suggests that the applied control strategy in natural reaching movements can simultaneously account for multiple task constraints. The analysis further revealed that stabilization of hand orientation was stronger when reaching towards a cylindrical target, which imposed more constraints on final hand orientation. In contrast, hand position was more strongly stabilized in the spherical target shape condition, where stronger constraints on final hand position were applied. This suggests that different target shapes do influence the control strategy of reaching movements even though variability at movement end was not affected.

The proprioceptive map of the arm is systematic and stable, but idiosyncratic

Visual and somatosensory signals participate together in providing an estimate of the hand's spatial location. While the ability of subjects to identify the spatial location of their hand based on visual and proprioceptive signals has previously been characterized, relatively few studies have examined in detail the spatial structure of the proprioceptive map of the arm. Here, we reconstructed and analyzed the spatial structure of the estimation errors that resulted when subjects reported the location of their unseen hand across a 2D horizontal workspace. Hand position estimation was mapped under four conditions: with and without tactile feedback, and with the right and left hands. In the task, we moved each subject's hand to one of 100 targets in the workspace while their eyes were closed. Then, we either a) applied tactile stimulation to the fingertip by allowing the index finger to touch the target or b) as a control, hovered the fingertip 2 cm above the target. After returning the hand to a neutral position, subjects opened their eyes to verbally report where their fingertip had been. We measured and analyzed both the direction and magnitude of the resulting estimation errors. Tactile feedback reduced the magnitude of these estimation errors, but did not change their overall structure. In addition, the spatial structure of these errors was idiosyncratic: each subject had a unique pattern of errors that was stable between hands and over time. Finally, we found that at the population level the magnitude of the estimation errors had a characteristic distribution over the workspace: errors were smallest closer to the body. The stability of estimation errors across conditions and time suggests the brain constructs a proprioceptive map that is reliable, even if it is not necessarily accurate. The idiosyncrasy across subjects emphasizes that each individual constructs a map that is unique to their own experiences.

Bilateral hand representations in human primary proprioceptive areas

Sensory representations in the postcentral gyrus are supposed to be strictly lateralised and to provide spatially unbiased representations of limb positions. However, electrophysiological and behavioural measurements in humans and non-human primates tentatively suggested some degree of bilateral processing even in early somatosensory areas. We report a patient who suffered a small and confined lesion of the hand area in the postcentral gyrus that resulted in a proprioceptive deficit without any concomitant primary motor impairment. We performed a finger position-matching task with target locations being defined proprioceptively. Without visual feedback of either hand, the patient demonstrated a significant leftward shift of perceived locations when reaching with the ipsilesional right hand to her contralesional left hand and an opposite rightward shift when reaching with the left hand to the position of the right hand. Although these directional errors improved when vision of the active hand was allowed, errors were still significantly larger than those of age-matched healthy controls with unconstrained view of the active contralesional hand. Reaching to visual targets without visual online feedback the patient revealed comparable errors with both hands. Reaching to visual targets with full visual feedback, she was as accurate as controls with either hand. In summary, our data demonstrate an effect of the right postcentral lesion on proprioceptive information processing for both hands. The results suggest an integration of contralateral and ipsilateral proprioceptive information already at this early processing stage possibly mediated by callosal connections.

The influence of visual perception of self-motion on locomotor adaptation to unilateral limb loading

Self-perception of motion through visual stimulation may be important for adapting to locomotor conditions. Unilateral limb loading is a locomotor condition that can improve stability and reduce abnormal limb movement. In the present study, the authors investigated the effect of self-perception of motion through virtual reality (VR) on adaptation to unilateral limb loading. Healthy young adults, assigned to either a VR or a non-VR group, walked on a treadmill in the following 3 locomotor task periods--no load, loaded, and load removed. Subjects in the VR group viewed a virtual corridor during treadmill walking. Exposure to VR reduced cadence and muscle activity. During the loaded period, the swing time of the unloaded limb showed a larger increase in the VR group. When the load was removed, the swing time of the previously loaded limb and the stance time of the previously unloaded limb showed larger decrease and the swing time of the previously unloaded limb showed a smaller increase in the VR group. Lack of visual cues may cause the adoption of cautious strategies (higher muscle activity, shorter and more frequent steps, changes in the swing and stance times) when faced with situations that require adaptations. VR technology, providing such perceptual cues, has an important role in enhancing locomotor adaptation.

Coordination deficits in ideomotor apraxia during visually targeted reaching reflect impaired visuomotor transformations

Ideomotor limb apraxia, commonly defined as a disorder of skilled, purposeful movement, is characterized by spatiotemporal deficits during a variety of actions. These deficits have been attributed to damage to, or impaired retrieval of, stored representations of learned actions, especially object-related movements. However, such deficits might also arise from impaired visuomotor transformation mechanisms that operate in parallel to or downstream from mechanisms for storage of action representations. These transformation processes convert extrinsic visual information into intrinsic neural commands appropriate for the desired motion. These processes are a key part of the movement planning process and performance errors due to inadequate transformations have been shown to increase with the dynamic complexity of the movement. This hypothesis predicts that apraxic patients should show planning deficits when reaching to visual targets, especially when the coordination and/or dynamic requirements of the task increase. Three groups (18 healthy controls, 9 non-apraxic and 9 apraxic left hemisphere damaged patients) performed reaching movements to visual targets that varied in the degree of interjoint coordination required. Relative to the other two groups, apraxic patients made larger initial direction errors and showed higher variability during their movements, especially when reaching to the target with the highest intersegmental coordination requirement. These problems were associated with poor coordination of shoulder and elbow torques early in the movement, consistent with poor movement planning. These findings suggest that the requirement to transform extrinsic visual information into intrinsic motor commands impedes the ability to accurately plan a visually targeted movement in ideomotor limb apraxia.

Illusory force perception following a voluntary limb movement

We present a novel illusion in which participants report constant forces on their hand as steadily increasing. Participants made discrete reaching movements perturbed by a lateral force that increased with the distance moved; when stationary at the end of the movement, a true constant force was perceived to increase. We tested perceived subjective equality by increasing or decreasing the force. The illusion was significantly stronger when the perturbation was applied during active movement. We conclude that the unusual context of moving against lateral spring forces results in participants failing to predict steady lateral forces at the end of their movement, and causes an illusion of increasing forces even after movement termination. This result further emphasizes the role of action prediction in sensory perception.

Brain activity in goal-directed movements in a real compared to a virtual environment using the Nintendo Wii

Low budget virtual environments like the Nintendo Wii increased in popularity and may play a role in motor learning related to sports and exercise. But nothing was known about the comparability of cortical activity of motor tasks in real and virtual environments. The aim of the study was to examine cortical differences between real and Wii based virtual sports performances using the golf putt as a model. Ten male golfers (26.0 +/- 0.7 years; 81.8 +/- 5.6 kg; 184.5 +/- 6.0 cm; handicap 30.0+/-10.0; 2.9+/-1.0 years of golf experience) were asked to putt for 3 min in random order in the real and the virtual Wii condition. A rest in sitting position (3 min) followed each performance. The score and cortical activity (EEG) were recorded continuously. The participants performed with a significant better score in the real condition (p < or = 0.01). Compared to virtual putting Theta spectral power showed a significant increase during real performance at F3 and F4 (p < or = 0.05). Significantly increased Alpha-2 power was demonstrated during real putting compared to the virtual putting performance at P3 (p < or = 0.05). The findings suggested that putting performance and brain activity was influenced by the choice of a real or virtual environment. The results were discussed based on the concept of the working memory where increased frontal Theta power indicated higher focused attention and higher Alpha-2 power was inversely related to the quantity of sensory information processing in the real putting compared to the virtual condition.

Hand-assisted laparoscopic surgery is associated with enhanced depth perception in novices

BACKGROUND

The visual information obtained in hand-assisted laparoscopic surgery (HALS) and total laparoscopy is based on two-dimensional depth cues. This study was designed to test the hypothesis that intracorporeal hand improves depth perception in HALS.

METHODS

Ten subjects were divided into two groups using HALS or total laparoscopy. The first task was passing a 10-cm thread through four small metallic loops five times; the second was tying 30 square three-throw intracorporeal laparoscopic knots. End points were (1) number of past pointing as an index of movement accuracy and reflection of depth perception, (2) execution time, and (3) knot quality score.

RESULTS

The HALS group was significantly faster than total laparoscopy with a median (interquartile range (IQR)) of 117 s (75.5) vs. 179 s (235.5; P = 0.001) for the thread passing task and 97 s (43.3) vs. 130 s (68.3; P < 0.0005) for knot tying. The number of past-pointing events was significantly lower with HALS compared with total laparoscopy with a median (IQR) of 12 (8) vs. 23 (24; P = 0.001) for the thread passing task and 7 (5) vs. 13 (10; P < 0.0005) for knot tying. There was a significant main effect of whether the targeted object was in contact with the other instrument in knot tying task when past-pointing occurred (P = 0.031). No difference in a knot quality score was found between the HALS and total laparoscopy.

CONCLUSIONS

The accuracy of instrument movement was significantly better in the HALS group compared with total laparoscopy. The study confirmed the hypothesis that intracorporeal hand improves depth perception in HALS. Also, the touch of the assisting instrument with the target object enhances spatial orientation in the laparoscopic operative field. However, further study is needed to test the hypothesis in experts.

Learning a visuomotor rotation: simultaneous visual and proprioceptive information is crucial for visuomotor remapping

Visuomotor adaptation is mediated by errors between intended and sensory-detected arm positions. However, it is not clear whether visual-based errors that are shown during the course of motion lead to qualitatively different or more efficient adaptation than errors shown after movement. For instance, continuous visual feedback mediates online error corrections, which may facilitate or inhibit the adaptation process. We addressed this question by manipulating the timing of visual error information and task instructions during a visuomotor adaptation task. Subjects were exposed to a visuomotor rotation, during which they received continuous visual feedback (CF) of hand position with instructions to correct or not correct online errors, or knowledge-of-results (KR), provided as a static hand-path at the end of each trial. Our results showed that all groups improved performance with practice, and that online error corrections were inconsequential to the adaptation process. However, in contrast to the CF groups, the KR group showed relatively small reductions in mean error with practice, increased inter-trial variability during rotation exposure, and more limited generalization across target distances and workspace. Further, although the KR group showed improved performance with practice, after-effects were minimal when the rotation was removed. These findings suggest that simultaneous visual and proprioceptive information is critical in altering neural representations of visuomotor maps, although delayed error information may elicit compensatory strategies to offset perturbations.

Generalization of visuomotor learning between bilateral and unilateral conditions

A long history of behavioral and physiological research has suggested that bilateral coordination invokes unique neural processes that are not involved in unilateral movements. This hypothesis predicts that motor learning should show limited transfer between unilateral and bilateral conditions, which is consistent with a recent finding that indicated partial, but not complete, transfer of learning between the two conditions. However, during learning of new motor skills, transformations must also be made between visual and proprioceptive coordinate systems, a process that may occur upstream to the processes that differentiate bilateral from unilateral movements. We now investigate whether visuomotor adaptations are shared between unilateral and bilateral movement conditions. Our results indicate substantial transfer from bilateral to subsequent unilateral conditions for both arms. Interestingly, whereas the nondominant arm never showed complete adaptation to visual rotation under bilateral conditions, this interference, or lack of improvement, in bilateral performance did not disturb the visuomotor adaptation process or transfer, as reflected by superb unilateral performances immediately following the bilateral conditions. These findings unambiguously indicate that visuomotor adaptation can extensively generalize between bilateral and unilateral conditions, thus suggesting a substantial overlap in the neural processes underlying visuomotor transformations between the two movement conditions. Our findings provide support for a two-stage model of motor planning, in which the visuomotor transformation process precedes the processes that convert the visuomotor plan into effector-specific commands that incorporate bilateral synergies and that result in the forces that determine motion.

Reaching in depth: hand position dominates over binocular eye position in the rostral superior parietal lobule

Neural activity was recorded in area PE (dorsorostral part of Brodmann's area 5) of the posterior parietal cortex while monkeys performed arm reaching toward memorized targets located at different distances from the body. For any given distance, arm movements were performed while the animal kept binocular eye fixation constant. Under these conditions, the activity of a large proportion (36%) of neurons was modulated by reach distance during the memory period. By varying binocular eye position (vergence angle) and initial hand position, we found that the reaching-related activity of most neurons (61%) was influenced by changing the starting position of the hand, whereas that of a smaller, although substantial, population (13%) was influenced by changes of binocular eye position (i.e., by the angle of vergence). Furthermore, the modulation of the neural activity was better explained expressing the reach movement end-point, corresponding to the memorized target location, in terms of distance from the initial hand position, rather than from the body. These results suggest that the activity of neurons in area PE combines information about eye and hand position to encode target distance for reaching in depth predominantly in hand coordinates. This encoding mechanism is consistent with the position of PE in the functional gradient that characterizes the parieto-frontal network underlying reaching.

The influence of target sensory modality on motor planning may reflect errors in sensori-motor transformations

Multi-sensory integration studies have shown that combining heterogeneous signals can optimize motor performance by reducing errors inherent to any single modality. However, it has also been suggested that errors could arise from erroneous transformations between heterogeneous coordinate systems. Here we investigated the effect of visuo-proprioceptive integration on the control of multi-joint arm movements by manipulating target modality. When the target was visual, movement control required the integration of visual target signals with proprioceptive signals about limb configuration. In contrast, when the target was the unseen fingertip, movement control relied solely on proprioceptive signals since visual feedback of hand position was precluded. We hypothesized that a faulty integration of visual target signals with proprioceptive arm signals would result in a less accurate planning of visually-targeted movements with respect to proprioceptively-targeted movements. Different inter-joint coordinations patterns were tested by varying starting hand position. Results showed larger initial trajectory deviations from target direction for visually-targeted movements involving substantial shoulder and elbow motions. Inverse dynamic analysis revealed that these deviations were associated with less efficient intersegmental coordination. The control of visually-targeted movements thus appeared sub-optimal compared to proprioceptively-targeted movements when considering theoretical models of motor planning assuming kinematic or dynamic optimizations. Additional experiments further highlighted the effect of target position, and visual feedback of starting hand position, on motor planning for proprioceptively- and visually-targeted movements. Our findings suggest that the integration of heterogeneous sensory signals related to hand and target positions introduces errors in motor planning.

On-line corrections for visuomotor errors

This study was designed to determine how visual feedback mediates error corrections during reaching. We used visuomotor rotations to dissociate a cursor, representing finger position, from the actual finger location. We then extinguished cursor feedback at different distances from the start location to determine whether corrections were based on error extrapolation from prior cursor information. Results indicated that correction amplitude varied with the extent of cursor feedback. A second experiment tested specific aspects of error information that might mediate corrections to visuomotor rotations: rotation angle, distance between the finger and cursor positions and the duration of cursor exposure. Results showed that corrections did not depend on the amplitude of the rotation angle or the amount of time the cursor was shown. Instead, participants corrected for the cursor-finger distance, at the point where cursor feedback was last-seen. These findings suggest that within-trial corrections and inter-trial adaptation might employ different mechanisms.

The roles of vision and proprioception in the planning of reaching movements

While vision and proprioception can both provide information about arm configuration prior to movement, substantial evidence suggests that each modality is used for different stages of the planning process. In this chapter, we provide support for the idea that vision is mainly used to define the trajectory and the kinematics of reaching movements. Proprioception appears to be critical in the transformation of this plan into the motor commands sent to the arm muscles.

Upper Limb Asymmetries in the Matching of Proprioceptive Versus Visual Targets

The purpose of the current study was to determine the extent to which “sensory dominance” exists in right-handers with respect to the utilization of proprioceptive versus visual feedback. Thirteen right-handed adults performed two target-matching tasks using instrumented manipulanda. In the proprioceptive matching task, the left or right elbow of blindfolded subjects was passively extended by a torque motor system to a target position and held for 3 s before being returned to the start position. The target angle was then matched with either the ipsilateral or contralateral arm. In the second task, visual matching, circular targets were briefly projected to either side of a visual fixation point located in front of the subject. Subjects then matched the target positions with a laser pointer by moving either the ipsilateral or contralateral arm. Overall, marked arm differences in accuracy were seen based on the type of sensory feedback used for target presentation. For the proprioceptive matching task errors were smaller for the nonpreferred left arm, whereas during the visual matching task smaller errors were found for the preferred right arm. These results suggest a left arm/right hemisphere advantage for proprioceptive feedback processing and a right arm/left hemisphere advantage for visual information processing. Such asymmetries may reflect fundamental differences between the two arm/hemisphere systems during the performance of bimanual tasks where the preferred arm requires visual guidance to manipulate an object, whereas the nonpreferred stabilizes that object on the basis of proprioceptive feedback.

Evidence for Automatic On-Line Adjustments of Hand Orientation During Natural Reaching Movements to Stationary Targets

Control of the spatial orientation of the hand is an important component of reaching and grasping movements. We studied the contribution of vision and proprioception to the perception and control of hand orientation in orientation-matching and letter-posting tasks. In the orientation-matching task, subjects aligned a “match” handle to a “target” handle that was fixed in different orientations. In letter-posting task 1, subjects simultaneously reached and rotated the right hand to insert a match handle into a target slot fixed in the same orientations. Similar sensory conditions produced different error patterns in the two tasks. Furthermore, without vision of the hand, final hand-orientation errors were smaller overall in letter-posting task 1 than in the orientation-matching task. In letter-posting task 2, subjects first aligned their hand to the angle of the target and then reached to it with the instruction not to change their initial hand orientation. Nevertheless, hand orientation changed during reaching in a way that reduced the initial orientation errors. This did not occur when there was no explicitly defined target toward which the subjects reached (letter-posting task 3). The reduction in hand-orientation errors during reach, even when told not to change it, suggests the engagement of an automatic error correction mechanism for hand orientation during reaching movements toward stationary targets. The correction mechanism was engaged when the task involved transitive actions directed at the target object. The on-line adjustments can occur without vision of the hand and even when target orientation is defined only by proprioceptive inputs.

Flexibility and individual differences in visuo-proprioceptive integration: evidence from the analysis of a morphokinetic control task

Conflicting theories of visuo-proprioceptive integration in movement control suggest that each modality can be weighted according to either its statistical reliability or the computations in which the integrated estimates will be used. However, the psychophysical experiments on which most studies are based use sensory conflicts and are therefore likely to reflect particular rather than normal behavior. In this paper, we: (1) propose a method avoiding the use of sensory conflicts (delayed recall task), (2) restrict our interest to spontaneous rather than adapted behavior, and (3) focus on a complex task requiring fine online control in order for hand movements to fit a precise path during execution. Subjects were provided with either visual, proprioceptive or both cues while their right hand was passively moved to fit a precise three segment pathway. As soon as this encoding phase ended, they were instructed to reproduce actively the trajectory, either with vision, proprioception or both cues. Results provide evidence that vision and proprioception may be used very differently, (1) not only according to the relative resolution of the sensory systems in the actual context, (2) not only according to the processes involved in the task, but also (3) according to subjects. They also suggest that visual and proprioceptive cues are not fused to provide a weighted average position, but that the non-dominant cue could simply be ignored when subjects are provided with multiple sensory cues. We conclude that each of these observations illustrates the same fundamental property of flexibility of integrative mechanisms.

The effect of target modality on visual and proprioceptive contributions to the control of movement distance

Abstract The goal of this study was to determine whether the sensory nature of a target influences the roles of vision and proprioception in the planning of movement distance. Two groups of subjects made rapid, elbow extension movements, either toward a visual target or toward the index fingertip of the unseen opposite hand. Visual feedback of the reaching index fingertip was only available before movement onset. Using a virtual reality display, we randomly introduced a discrepancy between actual and virtual (cursor) fingertip location. When subjects reached toward the visual target, movement distance varied with changes in visual information about initial hand position. For the proprioceptive target, movement distance varied mostly with changes in proprioceptive information about initial position. The effect of target modality was already present at the time of peak acceleration, indicating that this effect include feedforward processes. Our results suggest that the relative contributions of vision and proprioception to motor planning can change, depending on the modality in which task relevant information is represented.