Biases in the perception of self-motion during whole-body acceleration and deceleration

International audienceSeveral studies have investigated whether vestibular signals can be processed to determine the magnitude of passive body motions. Many of them required subjects to report their perceived displacements offline, i.e., after being submitted to passive displacements. Here, we used a protocol that allowed us to complement these results by asking subjects to report their introspective estimation of their displacement continuously, i.e., during the ongoing body rotation. To this end, participants rotated the handle of a manipulandum around a vertical axis to indicate their perceived change of angular position in space at the same time as they were passively rotated in the dark. The rotation acceleration (Acc) and deceleration (Dec) lasted either 1.5 s (peak of 60 • /s 2 , referred to as being " High ") or 3 s (peak of 33 • /s 2 , referred to as being " Low "). The participants were rotated either counterclockwise or clockwise, and all combinations of acceleration and deceleration were tested (i.e., AccLow-DecLow; AccLow-DecHigh; AccHigh-DecLow; AccHigh-DecHigh). The participants' perception of body rotation was assessed by computing the gain, i.e., ratio between the amplitude of the perceived rotations (as measured by the rotating manipulandum's handle) and the amplitude of the actual chair rotations. The gain was measured at the end of the rotations, and was also computed separately for the acceleration and deceleration phases. Three salient findings resulted from this experiment: (i) the gain was much greater during body acceleration than during body deceleration, (ii) the gain was greater during High compared to Low accelerations and (iii) the gain measured during the deceleration was influenced by the preceding acceleration (i.e., Low or High). These different effects of the angular stimuli on the perception of body motion can be interpreted in relation to the consequences of body acceleration and deceleration on the vestibular system and on higher-order cognitive processes

Cortical facilitation of tactile afferents during the preparation of a body weight transfer when standing on a biomimetic surface

Self-generated movement shapes tactile perception, but few studies have investigated the brain mechanisms involved in the processing of the mechanical signals related to the static and transient skin deformations generated by forces and pressures exerted between the foot skin and the standing surface. We recently found that standing on a biomimetic surface (i.e., inspired by the characteristics of mechanoreceptors and skin dermatoglyphics), that magnified skin–surface interaction, increased the sensory flow to the somatosensory cortex and improved balance control compared to standing on control (e.g., smooth) surfaces. In this study, we tested whether the well-known sensory suppression that occurs during movements is alleviated when the tactile afferent signal becomes relevant with the use of a biomimetic surface. Eyes-closed participants (n = 25) self-stimulated their foot cutaneous receptors by shifting their body weight toward one of their legs while standing on either a biomimetic or a control (smooth) surface. In a control task, similar forces were exerted on the surfaces (i.e., similar skin–surface interaction) by passive translations of the surfaces. Sensory gating was assessed by measuring the amplitude of the somatosensory-evoked potential over the vertex (SEP, recorded by EEG). Significantly larger and shorter SEPs were found when participants stood on the biomimetic surface. This was observed whether the forces exerted on the surface were self-generated or passively generated. Contrary to our prediction, we found that the sensory attenuation related to the self-generated movement did not significantly differ between the biomimetic and control surfaces. However, we observed an increase in gamma activity (30–50 Hz) over centroparietal regions during the preparation phase of the weight shift only when participants stood on the biomimetic surface. This result might suggest that gamma-band oscillations play an important functional role in processing behaviorally relevant stimuli during the early stages of body weight transfer

Two Neural Circuits to Point Towards Home Position After Passive Body Displacements

International audienceA challenge in motor control research is to understand the mechanisms underlying the transformation of sensory information into arm motor commands. Here, we investigated these transformation mechanisms for movements whose targets were defined by information issued from body rotations in the dark (i.e., idiothetic information). Immediately after being rotated, participants reproduced the amplitude of their perceived rotation using their arm (Experiment 1). The cortical activation during movement planning was analyzed using electroencephalography and source analyses. Task-related activities were found in regions of interest (ROIs) located in the prefrontal cortex (PFC), dorsal premotor cortex, dorsal region of the anterior cingulate cortex (ACC) and the sensorimotor cortex. Importantly, critical regions for the cognitive encoding of space did not show significant task-related activities. These results suggest that arm movements were planned using a sensorimotor-type of spatial representation. However, when a 8 s delay was introduced between body rotation and the arm movement (Experiment 2), we found that areas involved in the cognitive encoding of space [e.g., ventral premotor cortex (vPM), rostral ACC, inferior and superior posterior parietal cortex (PPC)] showed task-related activities. Overall, our results suggest that the use of a cognitive-type of representation for planning arm movement after body motion is necessary when relevant spatial information must be stored before triggering the movement

Do Gravity-Related Sensory Information Enable the Enhancement of Cortical Proprioceptive Inputs When Planning a Step in Microgravity?

International audienceWe recently found that the cortical response to proprioceptive stimulation was greater when participants were planning a step than when they stood still, and that this sensory facilitation was suppressed in microgravity. The aim of the present study was to test whether the absence of gravity-related sensory afferents during movement planning in microgravity prevented the proprioceptive cortical processing to be enhanced. We reestablished a reference frame in microgravity by providing and translating a horizontal support on which the participants were standing and verified whether this procedure restored the proprioceptive facilitation. The slight translation of the base of support (lateral direction), which occurred prior to step initiation, stimulated at least cutaneous and vestibular receptors. The sensitivity to proprioceptive stimulation was assessed by measuring the amplitude of the cortical somatosensory-evoked potential (SEP, over the Cz electrode) following the vibration of the leg muscle. The vibration lasted 1 s and the participants were asked to either initiate a step at the vibration offset or to remain still. We found that the early SEP (90–160 ms) was smaller when the platform was translated than when it remained stationary, revealing the existence of an interference phenomenon (i.e., when proprioceptive stimulation is preceded by the stimulation of different sensory modalities evoked by the platform translation). By contrast, the late SEP (550 ms post proprioceptive stimulation onset) was greater when the translation preceded the vibration compared to a condition without pre-stimulation (i.e., no translation). This suggests that restoring a body reference system which is impaired in microgravity allowed a greater proprioceptive cortical processing. Importantly, however, the late SEP was similarly increased when participants either produced a step or remained still. We propose that the absence of step-induced facilitation of proprioceptive cortical processing results from a decreased weight of proprioception in the absence of balance constraints in microgravity

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

<p>Abstract</p> <p>Background</p> <p>The 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.</p> <p>Results</p> <p>Although 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 DS's 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.</p> <p>Conclusion</p> <p>The 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.</p

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/s(2) and 0.30 m/s(2)) 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

Contrôle en ligne des ajustements posturaux anticipés (modèle d'intégration des informations proprioceptives)

Lors d un enjambement d obstacle nous devons d abord déplacer notre centre de masse vers la jambe support et vers l avant, de manière à délester la jambe qui va initier le pas et à projeter l ensemble du corps vers l avant. Alors que l importance des informations sensorielles pour la mise en place de ces Ajustements Posturaux Anticipés (APAs) est incontestée, il est encore actuellement peu connu si ces informations sensorielles peuvent être utilisées en ligne pour modifier la commande des APAs. Nous avons tout d abord étudié comment le SNC module les APAs lorsqu une modification des afférences proprioceptives (Ia) apparaît avant ou pendant l initiation du mouvement. Pour ce faire nous avons utilisé un protocole de vibration musculo tendineuse appliquée dans la direction latérale, au niveau des chevilles. Les sujets ont appris à enjamber un obstacle, yeux fermés, en synchronisant leur pas avec un signal sonore. Lorsque la vibration était appliquée pendant l initiation des APAs, aucune modification de la phase précoce des APAs n était observée, sauf lors d une stimulation cutanée (vibration basse fréquence). Il est donc possible que le SNC se base peu sur les informations proprioceptives pendant cette phase précoce. Uniquement l ajustement final de la phase de délestage semblerait prendre en compte l information proprioceptive erronée. Lorsque la vibration est appliquée bien avant le début des APAs, une réaction posturale se produit du côté de la vibration. Lorsque les sujets initient le mouvement après la réaction posturale, l amplitude de la poussée est calibrée en accord avec la direction de la réaction posturale. Ceci suggère que la commande motrice planifiée des APAs peut être mise à jour en ligne avant qu ils soient déclenchés. Enfin, pour comprendre l étendue de la contribution des afférences proprioceptives au contrôle postural, nous avons utilisé un environnement micro gravitaire nous permettant de minimiser la contribution des autres entrées sensorielles. En accord avec le signal proprioceptif évoqué par la vibration, la diminution significative du déplacement de la hanche lors de la phase tardive des ajustements suggère une facilitation sensori-sensorielle pour le contrôle de la phase dynamique tardive des ajustements posturaux.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 work was to investigate how the central nervous system (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 unloading phase seems to take into account the erroneous 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. Under micro gravity environment the decrease of the hip displacement during the later phase of the postural adjustments suggests a sensor-sensory facilitation for the control of the dynamic phase.AIX-MARSEILLE2-BU Sci.Luminy (130552106) / SudocSudocFranceF

CoP lateral displacement.

<p>Lateral CoP recorded during stepping for a representative trial (left panel). Mean amplitude and duration of both the thrust and the subsequent unloading component of the APAs (right panel). ns: p>0.05, *p<0.05, **p<0.01, ***p<0.001.</p

Enhancing the internal representation of the body through sensorimotor training in sports and dance improves balance control

International audiencePostural or balance control is of paramount importance for motor actions such as standing or walking. Interestingly preserving balance or body orientation while a voluntary movement is performed implies prediction of the postural disturbances provoked by the movement itself [1]. Indeed, limb or trunk movements induce a shift of body’s center of mass that can disturb balance if not compensated prior to their execution [2]. Such anticipated control of balance is grounded on the capacity of the brain to use a body internal representation (BIR) in space [3,4]. The BIR is built up and updated from multisensory integration involving proprioceptive, tactile, vestibular, visual inputs and is referred relative to a stable reference frame such as gravity. For example, the updating of the BIR through labyrinthine and muscle proprioceptive Ia inputs allows the fine tuning of the postural reactions following body disturbance [5]. It is worth noting that despite being important for calibrating proprioceptive inputs, visual information appears to be less involved than somatosensory for the fast updating of the body parts or whole-body position in space (i.e. BIR) required for enabling appropriate postural reactions. In this review, we will explore how the accuracy of the BIR can be improved by sensorimotor experience enabled by the practice of physical activities. We will particularly focus on activities that involve knowledge of the body parts’ relative motion and that require keeping or disrupting the vertical alignment of the body

Shear forces and head acceleration.

<p>(A) lateral forces recorded during quiet standing position for a representative trial. The amplitude of the force was computed from the translation onset to the peak of the first shear force. (B) mean amplitude of lateral (shear) forces measured at the maximal peak force evoked by the platform translation when the participants were standing still or preparing to step forward. (C) head acceleration recorded during quiet standing position for the same represenative trial as above. (D) mean head accelerations measured at the time to peak force. ns: p>0.05, **p<0.01, ***p<0.001.</p