Hadrien Ceyte

Developmental changes of static standing balance in children

The purpose of the present experiment was to investigate the time course by which children aged 6-, 8- and 10-year-old adapt and maintain their static balance. Participants (N=30) were required to stand on a force platform with their eyes closed. Ten adult subjects served as a reference group. We analyzed moment-to-moment modifications of quiet stance equilibrium by measuring the range and speed of the center of foot pressure (COP) displacements over time (i.e., periods of 2 s). Results showed that: (1) with age, the range of the COP decreased non-monotonically, with a maximum at 8 years of age, whereas the speed of the COP decreased linearly from 6 to 10 years of age, and (2) over time, both parameters decreased and stabilized, similarly for all age groups, suggesting that the processes underlying the maintenance of an optimal postural stability are mature at least as soon as 6 years of age.

Effect of Achilles tendon vibration on postural orientation

Vibration applied to the Achilles tendon is well known to induce in freely standing subjects a backward body displacement and in restrained subjects an illusory forward body tilt. The purpose of the present experiment was to evaluate the effect of Achilles tendon vibration (90Hz) on postural orientation in subjects free of equilibrium constraints. Subjects (n=12) were strapped on a backboard that could be rotated in the antero-posterior direction with the axis of rotation at the level of the ankles. They stood on a rigid horizontal floor with the soles of their feet parallel to the ground. They were initially positioned 7 degrees backward or forward or vertical and were required to adjust their body (the backboard) to the vertical orientation via a joystick. Firstly, results showed that in response to Achilles tendon vibration, subjects adjusted their body backward compared to the condition without vibration. This backward body adjustment likely cancel the appearance of an illusory forward body tilt. It was also observed that the vibratory stimulus applied to the Achilles tendon elicited in restrained standing subjects an increased EMG activity in both the gastrocnemius lateralis and the soleus muscles. Secondly, this vibration effect was more pronounced when passive displacement during the adjustment phase was congruent with the simulated elongation of calf muscles. These results indicated that the perception of body orientation is coherent with the postural response classically observed in freely standing subjects although the relationship between these two responses remains to be elucidated.

Effects of temporal and/or spatial instructions on the speed–accuracy trade-off of pointing movements in children

The present experiment examined in a visuo-manual task the effects of verbal instructions on the speed/accuracy trade-off across children aged 6, 8 and 10 years and adults. Three different verbal instructions (speed, accuracy and speed-accuracy) had to be respected to perform a pointing task. Analysis of reaction time (RT), movement time (MT) and percentage of targets reach showed that: (1) whatever the age, children were able to comply with the verbal instructions to adapt the velocity and/or the precision of their response (initiation and movement execution); (2) the main age-related difference of the speed-accuracy trade-off concerned the temporal (MT) but not the accuracy (targets reach) characteristics of the pointing movements; and (3) in the older children and even more precisely in adults, a temporal deficit was observed when the accuracy of aiming was required. This deficit increased as accuracy increased. These results were discussed within the theoretical frameworks of the developmental speed processing model proposed by Kail [Psychol. Bull., 109(3) (1991) 490-501] for RT data, and the speed-accuracy trade-off model proposed by Pachella [Pachella, R.G., The interpretation of reaction time in information-processing research, in, Kantowitz, B. (ed) Human Information Processing: Tutorial in Performance and Recognition, Erlbaum, (1974) 41-82] for MT and targets reach data.

Effects of neck muscles vibration on the perception of the head and trunk midline position

The present study focused on the influence of neck vibration on the perception of the head and trunk midline position (orientation and localization). The orientation of the head and trunk was investigated by the rolling adjustment of a rod on their midline while their localization was investigated by the adjustment of the position of a visual dot as being straight-ahead the eyes or the sternum. The first experiment investigated whether a head–trunk dissociation was induced by the unilateral vibration of neck muscles in upright and restrained subjects. Results showed that the subjective orientation and localization of whole-body midline were shifted toward the vibrated side. The second experiment determined the effect of the neck muscles vibration when the subjects were lying on their side. The effect of vibration disappeared when the side of vibration was opposed to the side of postural inclination and it was stronger than in the upright position when the side of vibration and the side of postural inclination were congruent. Whereas, results suggested that the input from neck muscle proprioceptors participates directly to the elaboration of the egocentric space, the question may be raised as to how the sensory cues interacted in their contribution to the neural generation of an egocentric, body centred coordinate system.

Role of gravity-based information on the orientation and localization of the perceived body midline

The present study focused on the influence of gravity-based information on the orientation and localization of the perceived body midline. The orientation was investigated by the rolling adjustment of a rod on the subjects’ Z-axis and the localization by the horizontal adjustment of a visual dot as being straight ahead. Experiment 1 investigated the effect of the dissociation between the Z-axis and the direction of gravity by placing subjects in roll tilt and supine postures. In roll tilt, the perception of the body midline orientation was deviated in the direction of body tilt and the perception of its localization was deviated in the opposite direction. In the supine body orientation, estimates of the Z-axis and straight-ahead remained veridical as when the body was upright. Experiment 2 highlighted the relative importance of the otolithic and tactile information using diffuse pressure stimulation. The estimation of body midline orientation was modified contrarily to the estimation of its localization. Thus, subjects had no absolute representation of their egocentric space. The main hypothesis regarding the dissociation between the orientation and localization of the body midline may be related to a difference in the integration of sensory information. It can be suggested that the horizontal component of the vestibulo-ocular reflex (VOR) contributed to the perceived localization of the body midline, whereas its orientation was mainly influenced by tactile information.

Does calculating impair postural stabilization allowed by visual cues

AbstractIn many daily situations, balance control is associated with a cognitive activity such as reading or a simple calculation. The objective of this study was to investigate the relationship between these two specific human activities, especially the influence of visual cues and support surface stability on body sway during a calculation task. A Sensory Organization Test, which can disrupt or suppress sensory inputs, was performed on 71 healthy young adults. The evaluations were performed both with and without mental arithmetic tasks which consisted of backward counting by three or thirteen. Our results showed that the addition of a calculation task induced an increase in body sway only when visual cues were available. They also showed the same instability effect of the support surface on the amount of body sway no matter what the associated cognitive task was. Moreover, no difference in body sway was observed between the two calculation tasks no matter what the visual context and/or the stability of the support surface were. We suggest that focusing on fulfilling the requirements of the mental calculation challenge may be responsible for the increase in body sway. This increase may be related to the use of oculomotor activity as unintentional attempts to increase arousal by self-generated body movement. Thus, this activity facilitates information processing rather than minimizing unbalance by a visual anchor point.

Visuo-proprioceptive interactions in degenerative cervical spine diseases requiring surgery.

Cervical proprioception plays a key role in postural control, but its specific contribution is controversial. Postural impairment was shown in whiplash injuries without demonstrating the sole involvement of the cervical spine. The consequences of degenerative cervical spine diseases are underreported in posture-related scientific literature in spite of their high prevalence. No report has focused on the two different mechanisms underlying cervicobrachial pain: herniated discs and spondylosis. This study aimed to evaluate postural control of two groups of patients with degenerative cervical spine diseases with or without optokinetic stimulation before and after surgical treatment. Seventeen patients with radiculopathy were recruited and divided into two groups according to the spondylotic or discal origin of the nerve compression. All patients and a control population of 31 healthy individuals underwent a static posturographic test with 12 recordings; the first four recordings with the head in 0° position: eyes closed, eyes open without optokinetic stimulation, with clockwise and counter clockwise optokinetic stimulations. These four sensorial situations were repeated with the head rotated 30° to the left and to the right. Patients repeated these 12 recordings 6weeks postoperatively. None of the patients reported vertigo or balance disorders before or after surgery. Prior to surgery, in the eyes closed condition, the herniated disc group was more stable than the spondylosis group. After surgery, the contribution of visual input to postural control in a dynamic visual environment was reduced in both cervical spine diseases whereas in a stable visual environment visual contribution was reduced only in the spondylosis group. The relative importance of visual and proprioceptive inputs to postural control varies according to the type of pathology and surgery tends to reduce visual contribution mostly in the spondylosis group.

Perceived head-trunk angle during microgravity produced by parabolic flight.

INTRODUCTION Neck proprioceptors are essential for orienting the head relative to the trunk. However, it has been shown that the available information about the relationship of gravity to different body parts would augment the clues about their relative orientation. In weightlessness, the absence of relevant body position signals from the otoliths and other inertial graviceptors requires the substitution of other sensory information. The aim of the present study was to investigate the ability of humans to accurately locate the head relative to the trunk in microgravity. METHODS Experiments were conducted during two separate sessions: on Earth and during parabolic flights. Volunteers were asked to adjust a visual rod until it looked parallel to their head or trunk axis in two different segmental configurations: head and trunk aligned or head tilted. RESULTS There was no effect of microgravity when the head and trunk were aligned. However, when the head was tilted with respect to the trunk, the orientation of the visual rod relative to the head or the trunk (visual egocentric coordinates) was deviated toward the head tilt, although the orientation between the body parts themselves (head-trunk angle) was correctly estimated. DISCUSSION These results suggested that, in microgravity, the proprioceptive signals from neck muscles seem sufficient to provide accurate head on trunk information. However, the representation of orientation in visual space was modified. This experiment provides evidence for the role of gravity on the visual perception of head- and trunk-based egocentric coordinates.

Perception of body movement when real and simulated displacements are combined

Muscle-tendon vibration has often been used to study the contribution of proprioception to kinesthesia and postural control. This technique is known to simulate the lengthening of the vibrated muscle and, in the presence of balance constraints, evoke compensatory postural responses. The objective of the present study was to clarify the consequences of this stimulation on the dynamic features of whole-body movement perception in upright stance and in the absence of balance constraints. Eleven participants were restrained in a dark room on a motorized backboard that was able to tilt the upright body around the ankle joints. The participants were passively tilted backwards or forwards with a maximum amplitude of four degrees and at very low acceleration (thus preventing the semicircular canals from contributing to movement perception). In half the trials, the body displacement was combined with continuous vibration of the Achilles tendons, which simulates a forward tilt. Participants used a joystick to report when and in which direction they perceived their own whole-body movement. Our results showed that during backward whole-body displacement, the movement detection threshold (i.e. the minimum angular velocity required to accurately perceive passive displacement) was higher in the presence of vibration, whereas the accuracy rate (i.e. the proportion of the overall trial duration during which the movement was correctly indicated) was lower. Conversely, the accuracy rate for forward displacements was higher in the presence of vibration. In the absence of vibration, forward movement was detected earlier than backward movement. The simulated whole-body displacement evoked by Achilles tendon vibration was therefore able to either enhance or disrupt the perception of real, slow, whole-body tilt movements, depending on the congruence between the direction of real and simulated displacements.

Lasting improvements in left spatial neglect following a protocol combining neck-muscle vibration and voluntary arm movements: a case-study

Abstract Purpose: Beyond promising experimental results of sensory passive stimulations in spatial cognition disorders, some questions still remain regarding interests of these stimulations during the daily activities in neglect. The aim of this case-study was to evaluate the effects of a protocol combining left neck-muscle vibration with daily simple movements, like arm pointing movements, on perceptivo-locomotor deficits in a left spatial neglect patient. Materials and methods: Two neuropsychological tests, one subjective straight-ahead pointing (SSA) test and one wheelchair navigation test were carried out before the combination protocol, immediately after, 1 h later, and 24 h later. Results: The results showed a reduction of neglect spatial bias following the protocol lasted at least 24 h in all the tests (except for the SSA test due to the unavailability of the pointing device). Conclusions: The range of improvements in the symptoms of spatial neglect suggests that this therapeutic intervention based on the combining neck-muscle vibration to voluntary arm movements could be a useful treatment for this condition. One of future investigation axes should be the development of a vibratory tool in order to facilitate the combining this proprioceptive stimulation to daily activities. Implications for rehabilitation Spatial neglect is a perplexing neuropsychological syndrome, affecting different domains of spatial cognition and impacting also the functional domain. The treatments based on neck-muscle vibration are simple to use, non-invasive and requires none active participation of patient. A therapeutic intervention based on the combining left neck-muscle vibration and voluntary arm movements in a left-spatial-neglect show a lasting reduction of symptoms especially in daily activities. The combination of treatments based on the Bottom–Up approach opens innovative perspectives in rehabilitation.