Influence of Normal Aging and Multisensory Data Fusion on Cybersickness and Postural Adaptation in Immersive Virtual Reality.

Immersive Virtual Reality (VR) systems are expanding as sensorimotor readaptation tools for older adults. However, this purpose may be challenged by cybersickness occurrences possibly caused by sensory conflicts. This study aims to analyze the effects of aging and multisensory data fusion processes in the brain on cybersickness and the adaptation of postural responses when exposed to immersive VR. METHODS: We repeatedly exposed 75 participants, aged 21 to 86, to immersive VR while recording the trajectory of their Center of Pressure (CoP). Participants rated their cybersickness after the first and fifth exposure. RESULTS: The repeated exposures increased cybersickness and allowed for a decrease in postural responses from the second repetition, i.e., increased stability. We did not find any significant correlation between biological age and cybersickness scores. On the contrary, even if some postural responses are age-dependent, a significant postural adaptation occurred independently of age. The CoP trajectory length in the anteroposterior axis and mean velocity were the postural parameters the most affected by age and repetition. CONCLUSIONS: This study suggests that cybersickness and postural adaptation to immersive VR are not age-dependent and that cybersickness is unrelated to a deficit in postural adaptation or age. Age does not seem to influence the properties of multisensory data fusion.

Modulation of ellipses drawing by sonification.

Most studies on the regulation of speed and trajectory during ellipse drawing have used visual feedback. We used online auditory feedback (sonification) to induce implicit movement changes independently from vision. The sound was produced by filtering a pink noise with a band-pass filter proportional to movement speed. The first experiment was performed in 2D. Healthy participants were asked to repetitively draw ellipses during 45 s trials whilst maintaining a constant sonification pattern (involving pitch variations during the cycle). Perturbations were produced by modifying the slope of the mapping without informing the participants. All participants adapted spontaneously their speed: they went faster if the slope decreased and slower if it increased. Higher velocities were achieved by increasing both the frequency of the movements and the perimeter of the ellipses, but slower velocities were achieved mainly by decreasing the perimeter of the ellipses. The shape and the orientation of the ellipses were not significantly altered. The analysis of the speed-curvature power law parameters showed consistent modulations of the speed gain factor, while the exponent remained stable. The second experiment was performed in 3D and showed similar results, except that the main orientation of the ellipse also varied with the changes in speed. In conclusion, this study demonstrated implicit modulation of movement speed by sonification and robust stability of the ellipse geometry. Participants appeared to limit the decrease in movement frequency during slowing down to maintain a rhythmic and not discrete motor regimen.

Investigating three types of continuous auditory feedback in visuo-manual tracking.

The use of continuous auditory feedback for motor control and learning is still understudied and deserves more attention regarding fundamental mechanisms and applications. This paper presents the results of three experiments studying the contribution of task-, error-, and user-related sonification to visuo-manual tracking and assessing its benefits on sensorimotor learning. First results show that sonification can help decreasing the tracking error, as well as increasing the energy in participant's movement. In the second experiment, when alternating feedback presence, the user-related sonification did not show feedback dependency effects, contrary to the error and task-related feedback. In the third experiment, a reduced exposure of 50% diminished the positive effect of sonification on performance, whereas the increase of the average energy with sound was still significant. In a retention test performed on the next day without auditory feedback, movement energy was still superior for the groups previously trained with the feedback. Although performance was not affected by sound, a learning effect was measurable in both sessions and the user-related group improved its performance also in the retention test. These results confirm that a continuous auditory feedback can be beneficial for movement training and also show an interesting effect of sonification on movement energy. User-related sonification can prevent feedback dependency and increase retention. Consequently, sonification of the user's own motion appears as a promising solution to support movement learning with interactive feedback.

Relationships between empathy and creativity in collective games: a comparison between handball and sitting ball.

In collective motor situations, creativity and empathy are central and strongly connected to cognitive and affective processes. Indeed, in the environment of high social uncertainty of games and sports, empathy would allow the player to anticipate motor behaviors in order to promote creative decision-making, i.e., to destabilize his opponents. On this basis, this study pursues two objectives. The first is to propose indicators to question the links between sociomotor empathy and motor creativity in an ecological situation. The second is to investigate the potential influence of the internal logic of two very different collective games (handball and Sitting ball) on the type of links that are woven between empathy and creativity. Two groups of students were recruited (n = 22 and 23) and participated in each of the games mentioned. The games were video recorded. The praxical communications made by each player were recorded and sorted by two trained observers. The results revealed major differences between the two studied collective games. In handball, there was a correlation between instrumental empathy (valuing cognitive aspects) and indicators of motor creativity (p < 0.05). The more creative the players are (quantity, diversity and quality of performance), the more they manage to accurately anticipate the behavior of other players. In Sitting Ball, there was no correlation between creativity indicators and instrumental empathy. On the other hand, it is noticed that instrumental empathy was correlated with socio-affective empathy (p < 0.001). To make their motor decisions, the players do not rely exclusively on the decoding of behaviors but significantly mobilize the feelings that they ascribe to the other co-participants. The results of this work invite reflection on the diversity of playful reading grids to be offered to students in order to develop their motor adaptability.

Tolerance of immersive head-mounted virtual reality among older nursing home residents.

Introduction: Virtual Reality (VR) is a tool that is increasingly used in the aging population. Head-Mounted Displays (HMDs) are stereoscopic vision devices used for immersive VR. Cybersickness is sometimes reported after head-mounted display (HMD) VR exposure. Cybersickness severity and anxiety state reflect VR low tolerance. We aimed to evaluate HMD VR tolerance among older nursing home residents through cybersickness and anxiety state. Methods: A total of 36 participants were included in this preliminary study, 33 of whom (mean age: 89.33 ± 5.48) underwent three individual HMD VR sessions with three different contents. Cybersickness occurrence and severity were scored by the Simulator Sickness Questionnaire (SSQ) after each session. Anxiety state was assessed by the State-Trait Anxiety Inventory form Y-A before and after each session. Anxiety trait (using State-Trait Anxiety Inventory form Y-B) was also evaluated before and after the experiment. In total, 92% (33/36) of patients completed all three sessions, of which 61% (20/33) did not report any cybersickness symptoms (SSQ = 0). Six participants reported significant cybersickness (defined by an SSQ score ⩾10) in at least one session. Discussion: Only two participants stopped the study after the first exposure because of cybersickness. Age, cognitive function, anxiety trait, and well-being were not associated with cybersickness. The mean anxiety state decreased significantly from pre- to post-session. This immersive HMD VR experience was well tolerated among nursing home dwellers. Further larger studies in this population aiming to identify CS determinants are needed in order to use HMD VR on a standard basis.

Improving upper-limb and trunk kinematics by interactive gaming in individuals with chronic stroke: A single-blinded RCT.

BACKGROUND: Commercial gaming systems are increasingly being used for stroke rehabilitation; however, their effect on upper-limb recovery versus compensation is unknown. OBJECTIVES: We aimed to compare the effect of upper-limb rehabilitation using interactive gaming (Nintendo Wii) with dose-matched conventional therapy on elbow extension (recovery) and forward trunk motion (compensation) in individuals with chronic stroke. Secondary aims were to compare the effect on (1) clinical tests of impairment and activity, pain and effort, and (2) trajectory kinematics. We also explored arm and trunk motion (acceleration) during Wii sessions to understand how participants performed movements during Wii gaming. METHODS: This single-centre, randomized controlled trial compared 12 hourly sessions over 4 weeks of upper-limb Wii therapy to conventional therapy. Outcomes were evaluated at baseline and 4 weeks. The change in elbow extension and trunk motion during a reaching task was evaluated by electromagnetic sensors. Secondary outcomes were change in Fugl-Meyer assessment, Box and Block test, Action Research Arm Test, Motor Activity Log, and Stroke Impact Scale scores. Arm and trunk acceleration during Wii therapy was evaluated by using inertial sensors. A healthy control group was included for reference data. RESULTS: Nineteen participants completed Wii therapy and 21 conventional therapy (mean [SD] time post-stroke 66.4 [57.2] months). The intervention and control groups did not differ in mean change in elbow extension angle (Wii: +4.5°, 95% confidence interval [CI] 0.1; 9.1; conventional therapy: +6.4°, 95%CI 0.6; 12.2) and forward trunk position (Wii: -3.3 cm, 95%CI -6.2;-0.4]; conventional therapy: -4.1 cm, 95%CI -6.6; -1.6) (effect size: elbow, d = 0.16, p = 0.61; trunk, d = 0.13, p = 0.65). Clinical scores improved similarly but to a small extent in both groups. The amount of arm but not trunk acceleration produced during Wii sessions increased with training. CONCLUSIONS: Supervised upper-limb gaming therapy induced similar recovery of elbow extension as conventional therapy and did not enhance the development of compensatory forward trunk movement in individuals with chronic stroke. More sessions may be necessary to induce greater improvements. CLINICALTRIALS: GOV: NCT01806883.

Influence of Traditional Sporting Games on the Development of Creative Skills in Team Sports. The Case of Football.

The aim of this present study is to investigate the influence of three learning contexts on the development of motor creativity of young footballers (8-9 years old). In team sport, creativity is a fundamental issue because it allows players to adapt in an environment of high social uncertainty. To carry out this work, we suggest a method for assessing motor creativity into ecological situations based on the analysis of praxical communications. Creativity originates from an interaction between divergence and convergence. In our case, the number of communications (fluidity) and the diversity of updated communications (flexibility) are our divergence indicators. Convergence, understood as the ability to make good decisions, is assessed by two expert judges (R > 0.90). Sixty boys' football players (M = 8.67; SD = 0.3) coming from three football clubs participated in this research. The study lasted 2 years. Each year, a team of 10 players from each club participated in the research twice a week for 32 weeks (8 months), these groups attended different training sessions: (a) the control group (n = 20) followed a classical learning; (b) the decoding group (n = 20) attended training focused on learning the praxemes of football; (c) the traditional sporting games group (n = 20) followed a training session that was jointly focused on praxemes and the practice of traditional sporting games. The motor creativity of players and groups was assessed both at the beginning and at the end of the year during football matches. Compared to the control group, in the post-test, the group with the highest fluidity is the decoding group (p < 0.001) and the one with the highest fluidity is the traditional sporting games group. The latter group is also the one with the best convergence (p < 0.001). The results showed that traditional games can help develop players' creative abilities. This research invites us to investigate the complementarity between the different offered training.

Effect of auditory feedback differs according to side of hemiparesis: a comparative pilot study.

BACKGROUND: Following stroke, patients frequently demonstrate loss of motor control and function and altered kinematic parameters of reaching movements. Feedback is an essential component of rehabilitation and auditory feedback of kinematic parameters may be a useful tool for rehabilitation of reaching movements at the impairment level. The aim of this study was to investigate the effect of 2 types of auditory feedback on the kinematics of reaching movements in hemiparetic stroke patients and to compare differences between patients with right (RHD) and left hemisphere damage (LHD). METHODS: 10 healthy controls, 8 stroke patients with LHD and 8 with RHD were included. Patient groups had similar levels of upper limb function. Two types of auditory feedback (spatial and simple) were developed and provided online during reaching movements to 9 targets in the workspace. Kinematics of the upper limb were recorded with an electromagnetic system. Kinematics were compared between groups (Mann Whitney test) and the effect of auditory feedback on kinematics was tested within each patient group (Friedman test). RESULTS: In the patient groups, peak hand velocity was lower, the number of velocity peaks was higher and movements were more curved than in the healthy group. Despite having a similar clinical level, kinematics differed between LHD and RHD groups. Peak velocity was similar but LHD patients had fewer velocity peaks and less curved movements than RHD patients. The addition of auditory feedback improved the curvature index in patients with RHD and deteriorated peak velocity, the number of velocity peaks and curvature index in LHD patients. No difference between types of feedback was found in either patient group. CONCLUSION: In stroke patients, side of lesion should be considered when examining arm reaching kinematics. Further studies are necessary to evaluate differences in responses to auditory feedback between patients with lesions in opposite cerebral hemispheres.

Ears on the Hand: Reaching Three-Dimensional Targets With an Audio-Motor Device.

Understanding the processes underlying sensorimotor coupling with the environment is crucial for sensorimotor rehabilitation and sensory substitution. In doing so, devices which provide novel sensory feedback consequent to body movement may be optimized in order to enhance motor performance for particular tasks. The aim of the study reported here was to investigate audio-motor coupling when the auditory experience is linked to movements of the head or the hands. The participants had to localize and reach a virtual source with the dominant hand in response to sounds. An electromagnetic system recorded the position and orientation of the participants' head and hands. This system was connected to a 3D audio system that provided binaural auditory feedback on the position of the virtual listener located on the participants' body. The listener's position was computed either from the hands or from the head. For the hand condition, the virtual listener was placed on the dominant hand (the one used to reach the target) in Experiment 1 and on the non-dominant hand, which was constrained in order to have similar amplitude and degrees of freedom as that of the head, in Experiment 2. The results revealed that, in the two experiments, the participants were able to localize a source within the 3D auditory environment. Performance varied as a function of the effector's degrees of freedom and the spatial coincidence between sensor and effector. The results also allowed characterizing the kinematics of the hand and head and how they change with audio-motor coupling condition and practice.

The trunk as a part of the kinematic chain for arm elevation in healthy subjects and in patients with frozen shoulder.

During arm elevation, the trunk may have both a postural and synergic role, but few 3-D experimental studies exist of the phenomenon, and the contribution of trunk rotations to arm elevation has not been studied in patients with frozen shoulder. Thirty healthy volunteers performed maximal dominant arm elevation in 2 planes, sagittal (anteflexion) and frontal (abduction), and 13 patients with unilateral frozen shoulder performed arm elevation on the unaffected then affected side. Trunk rotations and humeral elevation were measured with use of an electromagnetic system (Polhemus Fastrak). Flexion/extension, inclination (lateral bending) and torsion (rotation around the main axis) of the trunk were measured at intermediate (45 degrees and 60 degrees ) and maximal levels of arm elevation. For patients, trunk rotations were also measured during elevation on the unaffected side at a level corresponding to maximal arm elevation of the contralateral affected side. Healthy volunteers made a small (4 degrees -9 degrees ) but consistent pattern of trunk rotations characterized mainly by extension during anteflexion and torsion during abduction associated with biphasic inclination (ipsilateral then contralateral). As expected, patients showed restricted arm elevation of the affected shoulder but performed larger trunk extension and torsion at intermediate levels of elevation with a similar pattern as above. Inclination range was limited during elevation of the affected shoulder, with no initial ipsilateral inclination on any side. Our results suggest that the trunk contributes to the kinematic chain for arm elevation in both groups. Trunk extension and torsion may compensate for impaired arm elevation. Conversely, the irregularities in trunk inclination may contribute to the impairment and be a target for rehabilitative management.

Three-dimensional scapular kinematics and scapulohumeral rhythm in patients with glenohumeral osteoarthritis or frozen shoulder.

We aimed to describe 3D scapular kinematics and scapulohumeral rhythm (SHR) in glenohumeral (GH) osteoarthritis shoulders compared to unaffected shoulders and to compare the abnormal scapular kinematic schema for GH osteoarthritis with that for frozen shoulder. Thirty-two patients with stiff shoulder (16 with GH osteoarthritis and 16 with frozen shoulder) performed maximal arm elevation in two planes, sagittal and frontal. Scapular rotations and humeral elevation of the affected and unaffected shoulders were measured by the Polhemus Fastrak electromagnetic system. Patients with GH osteoarthritis were older, had longer disease duration (p<0.001) and less restricted humeral elevation in the frontal plane (p=0.01). Protraction was significantly lower for the affected shoulders except for arm elevation in the frontal plane in the GH osteoarthritis group. Furthermore, protraction was lower with frozen shoulder than GH osteoarthritis during arm elevation in the frontal plane. Scapular lateral rotation and SHR were significantly higher for the affected shoulders in both groups whatever the plane of elevation. SHR showed a fair to moderate negative correlation with maximal humeral elevation in both groups and appears to be higher with frozen shoulder than GH osteoarthritis. In addition, SHR of the affected shoulder showed a fair to moderate correlation with disease duration only with GH osteoarthritis. Scapular tilt did not differ between affected and unaffected sides and was not influenced by type of disease. In conclusion, the increased scapular lateral rotation described in frozen shoulder is also observed in GH osteoarthritis. SHR of the affected shoulder is inversely related to severity of limitation of shoulder range of motion, which suggests a compensatory pattern.

Relationship of glenohumeral elevation and 3-dimensional scapular kinematics with disability in patients with shoulder disorders.

OBJECTIVE: To determine variables among glenohumeral elevation and 3-dimensional scapular rotations that are related to shoulder function as assessed by the Disability of the Arm, Shoulder and Hand-Disability/Symptom (DASH-D/S) scale. DESIGN: Prospective, cross-sectional study. PATIENTS: Eighty-eight patients with shoulder pain and limited range of motion were included. METHODS: Each patient performed 2 full active range of motion activities, forward flexion and abduction, and 2 activities of daily living, combing hair and simulating washing the back. Glenohumeral elevation and scapular rotations were measured by the Polhemus Fastrak electromagnetic system. RESULTS: On multiple regression analysis, glenohumeral elevation in combing hair and scapular lateral rotation in both abduction and simulating washing the back were the best predictors of shoulder function and explained 39.7% of the variance of the DASH-D/S score. CONCLUSION: These findings support the classical rehabilitation of the shoulder based on glenohumeral elevation and suggest the importance of attention paid to scapular lateral rotation.

A strategy of faster movements used by elderly humans to lift objects of increasing weight in ecological context.

It is not known whether, during the course of aging, changes occur in the motor strategies used by the CNS for lifting objects of different weights. Here, we analyzed the kinematics of object-lifting in two different healthy groups (young and elderly people) plus one well-known deafferented patient (GL). The task was to reach and lift onto a shelf an opaque cylindrical object with changing weight. The movements of the hand and object were recorded with electromagnetic sensors. In an ecological context (i.e. no instruction was given about movement speed), we found that younger participants, elderly people and GL did not all move at the same speed and that, surprisingly, elder people are faster. We also observed that the lifting trajectories were constant for both the elderly and the deafferented patient while younger participants raised their hand higher when the object weighed more. It appears that, depending on age and on available proprioceptive information, the CNS uses different strategies of lifting. We suggest that elder people tend to optimize their feedforward control in order to compensate for less functional afferent feedback, perhaps to optimize movement time and energy expenditure at the expense of high precision. In the case of complete loss of proprioceptive input, however, compensation follows a different strategy as suggested by GL's behavior who moved more slowly compared to both our younger and older participants.

Shoulder movements during the initial phase of learning manual wheelchair propulsion in able-bodied subjects.

BACKGROUND: The kinetics and kinematics of the glenohumeral joints, and shoulder muscles activities during the initial phase of learning wheelchair propulsion by individuals with paraplegia have been partially described. But no studies carry out the kinematics of the upper-limb during the learning of non-constrained open field locomotion with a wheelchair. METHODS: Ten subjects with no manual wheelchair experience propelled themselves repetitively straight ahead along an 11m linear path. Upper-limb segment orientations were measured with an electromagnetic system. Angular displacements of joints were determined using a standardized protocol [Wu, G., Van der Helm, F.C.T., Veeger, H.E.J., Makhsous, M., Van Roy, P., Anglin, C., Nagels, J., Karduna, A.R., McQuade, K., Wang, X., Wernerl, F.W., Buchholzm, B., 2005. ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion--Part II: shoulder, elbow, wrist and hand. J. Biomech. 38(5), 981-992] and a modeling method [Biryukova, E.V., Roby-Brami, A., Frolov, A.A., Mokhtari, M., 2000. Kinematics of human arm reconstructed from spatial tracking system recordings. J. Biomech. 33, 985-995]. FINDINGS: A full 3-D scapular motion description was obtained that allowed us to measure the mean amplitudes of rotation for the nine degrees of freedom of our model. The change in the propulsion pace and the repetition of trials induced a general increase of amplitudes of joint rotations that can explain the increase of velocity among trials. INTERPRETATION: We did not observe a freezing of any DOF as predicted by Bernstein's theory of motor learning, but the increase in joint movement amplitudes could correspond to his "unfreezing of DOF" stage. This non-respect of Bernstein's theory could originate in the relative simplicity of the task.

Sensori-Motor Learning with Movement Sonification: Perspectives from Recent Interdisciplinary Studies.

This article reports on an interdisciplinary research project on movement sonification for sensori-motor learning. First, we describe different research fields which have contributed to movement sonification, from music technology including gesture-controlled sound synthesis, sonic interaction design, to research on sensori-motor learning with auditory-feedback. In particular, we propose to distinguish between sound-oriented tasks and movement-oriented tasks in experiments involving interactive sound feedback. We describe several research questions and recently published results on movement control, learning and perception. In particular, we studied the effect of the auditory feedback on movements considering several cases: from experiments on pointing and visuo-motor tracking to more complex tasks where interactive sound feedback can guide movements, or cases of sensory substitution where the auditory feedback can inform on object shapes. We also developed specific methodologies and technologies for designing the sonic feedback and movement sonification. We conclude with a discussion on key future research challenges in sensori-motor learning with movement sonification. We also point out toward promising applications such as rehabilitation, sport training or product design.

There is something out there: distal attribution in sensory substitution, twenty years later.

Sensory substitution constitutes an interesting domain of study to consider the philosopher's classical question of distal attribution: how we can distinguish between a sensation and the perception of an object that causes this sensation. We tested the hypothesis that distal attribution consists of three distinct components: an object, a perceptual space, and a coupling between subjects' movements and stimulation. We equipped sixty participants with a visual-to-auditory substitution device, without any information about it. The device converts the video stream produced by a head-mounted camera into a sound stream. We investigated several experimental conditions: the existence or not of a correlation between movements and resulting stimulation, the direct or indirect manipulation of an object, and the presence of a background environment. Participants were asked to describe their impressions by rating their experiences in terms of seven possible "scenarios". These scenarios were carefully chosen to distinguish the degree to which the participants attributed their sensations to a distal cause. Participants rated the scenarios both before and after they were given the possibility to interrupt the stimulation with an obstacle. We were interested in several questions. Did participants extract laws of co-variation between their movements and resulting stimulation? Did they deduce the existence of a perceptual space originating from this coupling? Did they individuate objects that caused the sensations? Whatever the experimental conditions, participants were able to establish that there was a link between their movements and the resulting auditory stimulation. Detection of the existence of a coupling was more frequent than the inferences of distal space and object.

Speed-accuracy tradeoff during performance of a tracking task without visual feedback.

To help people with visual impairment, especially people with severely impaired vision, access graphic information on a computer screen, we have carried out fundamental research on the effect of increasing the number of detection fields. In general, application of the parallelism concept enables information to be accessed more precisely and easily when the number of sensors is high. We have developed a "Braille Box" by modifying Braille cells to form a tactile stimulator array which is compatible with the fingertip. Each pin can be controlled independently so that we can change the size and type of array in order to study the tactile perception of both simple and complex graphical forms. Our results show that by applying the parallelism concept to the detection field, people with visual impairment can increase the speed of exploration of geometric forms without decreasing the level of accuracy: thus, avoiding a speed-accuracy tradeoff. Further experiments need to be done with this Braille Box in order to improve the device and help people with visual impairment access graphic information.

From ear to hand: the role of the auditory-motor loop in pointing to an auditory source.

Studies of the nature of the neural mechanisms involved in goal-directed movements tend to concentrate on the role of vision. We present here an attempt to address the mechanisms whereby an auditory input is transformed into a motor command. The spatial and temporal organization of hand movements were studied in normal human subjects as they pointed toward unseen auditory targets located in a horizontal plane in front of them. Positions and movements of the hand were measured by a six infrared camera tracking system. In one condition, we assessed the role of auditory information about target position in correcting the trajectory of the hand. To accomplish this, the duration of the target presentation was varied. In another condition, subjects received continuous auditory feedback of their hand movement while pointing to the auditory targets. Online auditory control of the direction of pointing movements was assessed by evaluating how subjects reacted to shifts in heard hand position. Localization errors were exacerbated by short duration of target presentation but not modified by auditory feedback of hand position. Long duration of target presentation gave rise to a higher level of accuracy and was accompanied by early automatic head orienting movements consistently related to target direction. These results highlight the efficiency of auditory feedback processing in online motor control and suggest that the auditory system takes advantages of dynamic changes of the acoustic cues due to changes in head orientation in order to process online motor control. How to design an informative acoustic feedback needs to be carefully studied to demonstrate that auditory feedback of the hand could assist the monitoring of movements directed at objects in auditory space.

Direct kinematic modeling of the upper limb during trunk-assisted reaching.

The study proposes a rigid-body biomechanical model of the trunk and whole upper limb including scapula and the test of this model with a kinematic method using a six-dimensional (6-D) electromagnetic motion capture (mocap) device. Large unconstrained natural trunk-assisted reaching movements were recorded in 7 healthy subjects. The 3-D positions of anatomical landmarks were measured and then compared to their estimation given by the biomechanical chain fed with joint angles (the direct kinematics). Thus, the prediction errors was attributed to the different joints and to the different simplifications introduced in the model. Large (approx. 4 cm) end-point prediction errors at the level of the hand were reduced (to approx. 2 cm) if translations of the scapula were taken into account. As a whole, the 6-D mocap seems to give accurate results, except for prono-supination. The direct kinematic model could be used as a virtual mannequin for other applications, such as computer animation or clinical and ergonomical evaluations.

Perceptual weight judgments when viewing one's own and others' movements under minimalist conditions of visual presentation.

Across five experiments, we investigated the parameters involved in the observation and in the execution of the action of lifting an object. The observers were shown minimal information on movements, consisting of either the working-point displacement only (ie two points representing the hand and object) or additional configural information on the kinematics of the trunk, shoulder, arm, forearm, and hand, joined by a stick diagram. Furthermore, displays showed either a participant's own movements or those of another person, when different weights were lifted. The participants' task was to estimate the weight of the lifted objects. The results revealed that, although overall performance was not dependent on the visual conditions (working point versus stick diagram) or ownership conditions (self versus other), the kinematic cues used to perform the task differed as a function of these conditions. In addition, the kinematic parameters relevant for action observation did not match those relevant for action execution. This was confirmed in experiments by using artificially altered movement samples, where the variations in critical kinematic variables were manipulated separately or in combination. We discuss the implications of these results for the roles of motor simulation and visual analysis in action observation.