Laurence Mouchnino

Voluntary head stabilization in space during oscillatory trunk movements in the frontal plane performed in weightlessness

Abstract The ability voluntarily to stabilize the head in space during lateral rhythmic oscillations (0.59±0.09 Hz) of the trunk has been investigated during microgravity (μG) and normal gravity (nG) conditions (parabolic flights). Five healthy young subjects, who gave informed consent, were examined. The movements were performed with eyes open or eyes closed, during phases of either μG or nG. The main result was that head orientation with respect to vertical may be stabilized about the roll axis under μG with, as well as without vision, despite the reduction in vestibular afferent and muscle proprioceptive inputs. Moreover, the absence of head stabilization about the yaw axis confirms that the degrees of freedom of the neck can be independently controlled, as was previously reported. These results seem to indicate that voluntary head stabilization does not depend crucially upon static vestibular afferents. Head stabilization in space may in fact be organized on the basis of either dynamic vestibular afferents or a short-term memorized postural body schema.

CAN PREPARED ANTICIPATORY POSTURAL ADJUSTMENTS BE UPDATED BY PROPRIOCEPTION

Stepping over an obstacle is preceded by a center of pressure (CoP) shift, termed anticipatory postural adjustments (APAs). It provides an acceleration of the center of mass forward and laterally prior to step initiation. The APAs are characterized in the lateral direction by a force exerted by the moving leg onto the ground, followed by an unloading of the stepping leg and completed by an adjustment corresponding to a slow CoP shift toward the supporting foot. While the importance of sensory information in the setting of the APAs is undisputed, it is currently unknown whether sensory information can also be used online to modify the feedforward command of the APAs. The purpose of this study was to investigate how the CNS modulates the APAs when a modification of proprioceptive information (Ia) occurs before or during the initiation of the stepping movement. We used the vibration of ankle muscles acting in the lateral direction to induce modification of the afferent inflow. Subjects learned to step over an obstacle, eyes closed, in synchrony to a tone signal. When vibration was applied during the initiation of the APAs, no change in the early APAs was observed except in the case of a cutaneous stimulation (low frequency vibration); it is thus possible that the CNS relies less on proprioceptive information during this early phase. Only the final adjustment of the CoP seems to take into account the biased proprioceptive information. When vibration was applied well before the APAs onset, a postural reaction toward the side of the vibration was produced. When subjects voluntarily initiated a step after the postural reaction, the thrust amplitude was set according to the direction of the postural reaction. This suggests that the planned motor command of the APAs can be updated online before they are triggered.

Postural reorganization of weight-shifting in below-knee amputees during leg raising

Abstract The position of the center of gravity (CG) is a reference value that is controlled by the nervous system during the performance of movements. In order to maintain equilibrium, leg movement is preceded by a shift of the CG towards the supporting side. This CG shift is initiated by an early displacement of the center of pressure (CP) towards the moving leg. This characteristic CP thrust partly results from the activity of a distal muscle in the leg to be moved: the gastrocnemius medialis (GM). The aim of this study was to determine how this weight-shifting is initiated when the distal muscles are missing, as in amputees, and to identify any change in the central command. Experiments were performed on ten subjects: five below-knee amputees with no pathology and five control subjects. While standing, the subjects were instructed to raise one leg laterally as fast as possible to an angle of 45° and to maintain the final position. The same weight-shifting strategy was used by both groups, whereas local adaptations associated with the behavior occurred. When the GM is lacking, an early tensor-fasciae-latae (TFL) burst is observed just prior to and associated with the onset of the lateral CP change. This moving-leg abductor may be responsible for initiating the thrust at a proximal level when that leg is still on the ground. In addition, upon analyzing the lateral displacement of the CP, two modes of CP shift were detected. The first CP-shift mode has been previously described and the second mode (which we term here the pre-pushing mode) was used by both amputees and controls. The pre-pushing mode consisted of two thrusts: an early thrust onto the ground was exerted by the leg about to become the supporting leg followed by the previously described thrust exerted by the leg about to be raised. The early thrust, which could be exerted by either the sound or prosthetic leg, may have increased the efficiency of the second, classical thrust by initiating a swing.

Vestibular signal processing in a subject with somatosensory deafferentation: The case of sitting posture

BackgroundThe vestibular system of the inner ear provides information about head translation/rotation in space and about the orientation of the head with respect to the gravitoinertial vector. It also largely contributes to the control of posture through vestibulospinal pathways. Testing an individual severely deprived of somatosensory information below the nose, we investigated if equilibrium can be maintained while seated on the sole basis of this information.ResultsAlthough she was unstable, the deafferented subject (DS) was able to remain seated with the eyes closed in the absence of feet, arm and back supports. However, with the head unconsciously rotated towards the left or right shoulder, the DSs instability markedly increased. Small electrical stimulations of the vestibular apparatus produced large body tilts in the DS contrary to control subjects who did not show clear postural responses to the stimulations.ConclusionThe results of the present experiment show that in the lack of vision and somatosensory information, vestibular signal processing allows the maintenance of an active sitting posture (i.e. without back or side rests). When head orientation changes with respect to the trunk, in the absence of vision, the lack of cervical information prevents the transformation of the head-centered vestibular information into a trunk-centered frame of reference of body motion. For the normal subjects, this latter frame of reference enables proper postural adjustments through vestibular signal processing, irrespectively of the orientation of the head with respect to the trunk.

Coordination between postural and movement controls : effect of changes in body mass distribution on postural and focal component characteristics

Whole-body reaching movements are accomplished through a combination of anticipatory postural adjustments (APAs) and focal movements. Two different modes of central organization is usually proposed for this coordination: first, a single-process control, where the APAs and the focal movements would share a common command; second, where the APAs and the focal movements would be independently controlled through parallel commands (dual-process control). Here, we investigated which one of these modes of control could better explain the coordination between the trunk and the upper limb for standing subjects reaching for a target located beyond arm’s length. This was done evaluating the effect of changing the APAs settings on the arm movement. The APAs modification was achieved by shifting the subject’s centre of mass prior to the focal movement onset; this was done by adding an asymmetric load on either side of the head (a control condition with the load fixed centrally at the top of the head was also performed). As it changed the body mass distribution, the muscular torques and the orientation of the head inertia tensor, it is assumed that the addition of the asymmetric load led to a change in the APAs. Analyses indeed showed that both the initial head and trunk displacement towards the supporting side (during the unloading of the moving leg) were smaller when the load was fixed on the side of the supporting leg than when it was fixed on the side of the moving leg. However, changing the initial conditions, and therefore the APAs settings, had no significant effect on the path and kinematics of the focal hand movement. Therefore, subjects cancelled out the effect of the trunk motion on the hand-in-space motion through compensatory arm movements. These results support the dual-process control hypothesis for the postural and the focal components.

Cortical facilitation of proprioceptive inputs related to gravitational balance constraints during step preparation

Several studies have shown that the transmission of afferent inputs from the periphery to the somatosensory cortex is attenuated during the preparation of voluntary movements. In the present study, we tested whether sensory attenuation is also observed during the preparation of a voluntary step. It would appear dysfunctional to suppress somatosensory information, which is considered to be of the utmost importance for gait preparation. In this context, we predict that the somatosensory information is facilitated during gait preparation. To test this prediction, we recorded cortical somatosensory potentials (SEPs) evoked by bilateral lower limb vibration (i.e., proprioceptive inputs) during the preparation phase of a voluntary right-foot stepping movement (i.e., stepping condition). The subjects were also asked to remain still during and after the vibration as a control condition (i.e., static condition). The amplitude and timing of the early arrival of afferent inflow to the somatosensory cortices (i.e., P1-N1) were not significantly different between the static and stepping conditions. However, a large sustained negativity (i.e., late SEP) developed after the P1-N1 component, which was larger when subjects were preparing a step compared with standing. To determine whether this facilitation of proprioceptive inputs was related to gravitational equilibrium constraints, we performed the same experiment in microgravity. In the absence of equilibrium constraints, both the P1-N1 and late SEPs did not significantly differ between the static and stepping conditions. These observations provide neurophysiological evidence that the brain exerts a dynamic control over the transmission of the afferent signal according to their current relevance during movement preparation.

Online control of anticipated postural adjustments in step initiation: evidence from behavioral and computational approaches.

Anticipatory postural adjustments (APAs) prior to step execution are thought to be immutable once released. Here we challenge this assumption by testing whether APAs can be modified online if a body perturbation occurs during execution. Two directions of perturbation (resisting and assisting) relative to the body weight transfer were used during the execution of APAs. We found that APAs are modified online (increase in both ground pressure and muscle activity) to compensate for resisting perturbations. The outcomes of a biomechanical model confirmed that the early changes in the APAs resulted from an active control of the APAs and were not merely mechanical consequences of the perturbation. However, no modification of the initial feedforward command was observed for assisting perturbations. The motor command changes for the resisting perturbation may originate from the mismatch between passively originated forces and those actively specified by the central command when acting in the opposite direction. The absence of a mismatch in the assisting perturbation might explain why the central nervous system was not prompted to modify the APAs in this condition.

Modulation of proprioceptive inflow when initiating a step influences postural adjustments

A synergistic inclination of the whole body towards the supporting leg is required when producing a stepping movement. It serves to shift the centre of mass towards the stance foot. While the importance of sensory information in the setting of this postural adjustment is undisputed, it is currently unknown the extent to which proprioceptive afferences (Ia) give rise to postural regulation during stepping movement when the availability of other sensory information relying on static linear acceleration (gravity) is no longer sensed in microgravity. We tested this possibility asking subjects to step forward with their eyes closed in normo- and microgravity environments. At the onset of the stepping movement, we vibrated the ankle muscles acting in the lateral direction to induce modification of the afferent inflow (Ia fibres). Vibration-evoked movement (perceived movement) was in the same direction as the forthcoming body shift towards the supporting side (current movement). A control condition was performed without vibration. In both environments, when vibration was applied, the hip shift towards the supporting side decreased. These postural modifications occurred, however, earlier in normogravity before initiating the stepping movement than in microgravity (i.e. during the completion of the stepping movement). Our results suggest that proprioceptive information induced by vibration and afferent inflow related to body movement exaggerated sense of movement. This biased perception led to the postural adjustment decrease. We propose that in both environments, proprioceptive inflow enables the subject to scale the postural adjustments, provided that body motion-induced afferences are present to activate this postural control.

When Standing on a Moving Support, Cutaneous Inputs Provide Sufficient Information to Plan the Anticipatory Postural Adjustments for Gait Initiation

Gait initiation is preceded by initial postural adjustments whose goal is to set up the condition required for the execution of the focal stepping movement. For instance, the step is preceded by a shift of the body’s center of mass towards the stance foot unloading the stepping leg. This displacement is produced by exerting forces on the ground (i.e., thrust) while the body is still motionless. The purpose of this study was to identify whether the mere cutaneous inputs from the feet soles evoked by a lateral translation of the support could be used to scale the initial postural adjustments. Participants stood with their eyes closed on a force platform that could be moved laterally with a low acceleration (between 0.14 m/s2 and 0.30 m/s2) to reach a constant velocity of 0.02 m/s. This translation resulted in a change in the somatosensory cues from the feet soles without modifying vestibular inputs. Participants were instructed to produce a step with the right foot as soon as they felt the platform start to move (on either side) or heard an auditory cue. In the latter case, the platform stayed stationary. We found that the thrust duration was lengthened when the platform moved towards the supporting foot. In this condition, the cutaneous stimulation provided information related to a body shift towards the stepping leg. This increased thrust duration likely helped overcoming the non-functional body shift perceived towards the stepping leg. This result highlights the accuracy with which the actual standing position can be determined from foot sole cutaneous cues in the absence of visual and vestibular or proprioceptive inputs.

Effect of gravity-like torque on goal-directed arm movements in microgravity.

Gravitational force level is well-known to influence arm motor control. Specifically, hyper- or microgravity environments drastically change pointing accuracy and kinematics, particularly during initial exposure. These modifications are thought to partly reflect impairment in arm position sense. Here we investigated whether applying normogravitational constraints at joint level during microgravity episodes of parabolic flights could restore movement accuracy equivalent to that observed on Earth. Subjects with eyes closed performed arm reaching movements toward predefined sagittal angular positions in four environment conditions: normogravity, hypergravity, microgravity, and microgravity with elastic bands attached to the arm to mimic gravity-like torque at the shoulder joint. We found that subjects overshot and undershot the target orientations in hypergravity and microgravity, respectively, relative to a normogravity baseline. Strikingly, adding gravity-like torque prior to and during movements performed in microgravity allowed subjects to be as accurate as in normogravity. In the former condition, arm movement kinematics, as notably illustrated by the relative time to peak velocity, were also unchanged relative to normogravity, whereas significant modifications were found in hyper- and microgravity. Overall, these results suggest that arm motor planning and control are tuned with respect to gravitational information issued from joint torque, which presumably enhances arm position sense and activates internal models optimally adapted to the gravitoinertial environment.