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

Signs of spatial neglect in unilateral peripheral vestibulopathy

Background and purpose In this study, the question of whether egocentric representation of space is impaired in chronic unilateral vestibulopathies was examined. The objective was to test current theories attributing a predominant role to vestibular afferents in spatial cognition and to assess whether representational neglect signs are common in peripheral vestibular loss. Methods The subjective straight-ahead (SSA) direction was investigated using a horizontal rod allowing the translation and rotation components of the body midline representation to be dissociated in 21 patients with unilateral vestibular loss (right, 13; left, eight) and in 12 healthy controls. Results Compared to the controls, the patients with unilateral vestibulopathy showed a translation bias of their SSA, without rotation bias. The translation bias was not lateralized towards the lesioned side as typically found for biases reported after unilateral vestibular loss. Rather, the SSA bias was rightward whatever the side of the vestibular loss. The translation bias correlated with the vestibular loss, as measured by caloric response and vestibulo-ocular reflex gain, but not with the subjective visual vertical or the residual spontaneous nystagmus. Conclusion The present data suggest that the dysfunctions of neural networks involved in egocentred and allocentred representations of space are differentially compensated for in unilateral vestibular defective patients. In particular, they suggest that asymmetrical vestibular inputs to cortical regions lead to representational spatial disturbances as does defective cortical processing of vestibular inputs in spatial neglect after right hemisphere stroke. They also highlight the predominant role of symmetrical and unaltered vestibular inputs in spatial cognition

Age-based stereotype threat effects on dynamic balance in healthy older adults

IntroductionStereotype threat can lead older adults to perceive their experiences in a biased manner, giving rise to interfering thoughts and negative emotions that generate stress and anxiety. Negative beliefs about aging may serve as an additional factor that increases the need for attentional demand, potentially resulting in a performance level below their actual capabilities. In the present study, we asked whether negative aging stereotypes influence a dynamic balance task and explored the means to counteract them in healthy elderly participants.MethodsThe performance of balance was compared in two groups of participants aged 65 to 75 years (n = 22) under stereotype threat or reduced-threat situation. Balance abilities were tested under dynamic conditions, requiring participants to maintain balance on a moving platform and using a gradient of difficulty (with eyes open or closed, without or with foam). Postural performance was evaluated by means of posturographic evaluation of the center of pressure displacement and motion analysis. Additionally, we investigated the effects of stereotype threat on a preferred walking speed task and on the Timed Up and Go (TUG) test.ResultsParticipants under stereotype threat showed poorer balance, particularly in challenging conditions (eyes closed, on foam), with less effective body segments stabilization. Their postural stabilization on foam was worse compared to a solid surface. Conversely, those in the reduced threat condition maintained better body segment stabilization across all conditions, indicating consistent postural control regardless of the presence of foam. Stereotype threat did not affect preferred walking speed or the time to complete the “Time Up and Go” test.Discussion-conclusionThis study provides the first description of age-based stereotype threat effects on a dynamic balance task and how to counteract them in healthy older adults. We suggest that the decrease in postural performance observed in participants exposed to stereotype threat can be attributed to a split in attentional focus between negative intrusive thoughts and the attention needed for maintaining balance. These findings open new perspectives on how to overcome negative expectations when evaluating and training physical abilities, thereby contributing to fall prevention among older adults

Functional Neuroanatomy of Vertical Visual Perception in Humans

Vertical representation is central to posture control, as well as to spatial perception and navigation. This representation has been studied for a long time in patients with vestibular disorders and more recently in patients with hemispheric damage, in particular in those with right lesions causing spatial or postural deficits. The aim of the study was to determine the brain areas involved in the visual perception of the vertical. Sixteen right-handed healthy participants were evaluated using fMRI while they were judging the verticality of lines or, in a control task, the color of the same lines. The brain bases of the vertical perception proved to involve a bilateral temporo-occipital and parieto-occipital cortical network, with a right dominance tendency, associated with cerebellar and brainstem areas. Consistent with the outcomes of neuroanatomical studies in stroke patients, The data of this original fMRI study in healthy subjects provides new insights into brain networks associated with vertical perception which is typically impaired in both vestibular and spatial neglect patients. Interestingly, these networks include not only brain areas associated with postural control but also areas implied in body representation

Unilateral and bilateral fatiguing contractions similarly alter postural stability but differently modify postural position on bipedal stance

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Perturbation du contrôle postural monopodal controlatéral après des contractions musculaires volontaires et électro-induites du membre ipsilatéral

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Interactions équilibration et cognition

International audienceDans ce chapitre, nous rappelons le rôle des systèmes sensoriels dans les performances de posture, d’équilibration et de marche, nous précisons les fonctions cognitives susceptibles d’avoir un impact sur ces régulations et proposons un modèle théorique d'interactions posture-cognition. L'implication cognitive dans le contrôle postural est classiquement étudiée dans des situations de double-tâches qui consistent le plus souvent en la réalisation simultanée d’une tâche posturale et d’une tâche cognitive. Cette situation de doubles-tâches présente un effet délétère du fait du surcoût attentionnel. Le système cognitif humain ayant des capacités limitées, nous proposons que la réalisation simultanée de deux tâches dépend de la capacité de chaque sujet de réaliser ces tâches selon un continuum compris entre une réalisation automatique jusqu’à une réalisation hautement contrôlée. Un niveau de contrôle maximal dépasse les capacités attentionnelles du sujet rendant la réalisation simultanée de ces deux tâches impossibles. Le sujet priorise alors une des deux tâches. Ces situations de double-tâches fournissent également une base théorique pour expliquer certains changements posturo-locomoteurs observés chez le sujet âgé, de même que les difficultés d'équilibration chez les sujets âgés dits "chuteurs", ou encore diverses modifications du contrôle postural intervenant après atteinte sensorielle périphérique. Parmi la vaste littérature sur les double-tâches, nous rapportons des travaux représentatifs des relations entre différentes composantes cognitives et leurs conséquences sur le contrôle de la posture et de la marche, chez l'adulte jeune, au cours du vieillissement, chez des sujets chuteurs ou dont les capacités sensorimotrices sont altérées, et chez des personnes dont les capacités cognitives sont altérées. Récemment, cette relation posture-cognition a été envisagée de façon nouvelle. Nous décrivons ici comment un entraînement cognitif est capable de favoriser une meilleure gestion de la double-tâche et précisons les mécanismes cognitifs qui seraient responsables d’un meilleur équilibre postural

Improving postural control by applying mechanical noise to ankle muscle tendons

International audienceThe application of subthreshold mechanical vibrations with random frequencies (white mechanical noise) to ankle muscle tendons is known to increase muscle proprioceptive information and to improve the detection of ankle movements. The aim of the present study was to analyze the effect of this mechanical noise on postural control, its possible modulation according to the sensory strategies used for postural control, and the consequences of increasing postural difficulty. The upright stance of 20 healthy young participants tested with their eyes closed was analyzed during the application of four different levels of noise and compared to that in the absence of noise (control) in three conditions: static, static on foam, and dynamic (sinusoidal translation). The quiet standing condition was conducted with the eyes open and closed to determine the subjects' visual dependency to maintain postural stability. Postural performance was assessed using posturographic and motion analysis evaluations. The results in the static condition showed that the spectral power density of body sway significantly decreased with an optimal level of noise and that the higher the spectral power density without noise, the greater the noise effect, irrespective of visual dependency. Finally, noise application was ineffective in the foam and dynamic conditions. We conclude that the application of mechanical noise to ankle muscle tendons is a means to improve quiet standing only. These results suggest that mechanical noise stimulation may be more effective in more impaired populations

Introduction : Amélioration du contrôle postural par une stimulation mécanique imperceptible

National audienceL’application de vibrations mécaniques de fréquences aléatoires et faibles amplitudes (appelées « bruit ») sur les tendons des muscles posturaux de la cheville augmente la sensibilité proprioceptive et améliore la détection de mouvements de l’articulation de la cheville. L’objectif de cette étude a été d’analyser les conséquences de l’application de ce bruit sur la stabilisation posturale chez le sujet adulte jeune et en bonne santé

Introduction : Amélioration du contrôle postural par une stimulation mécanique imperceptible

National audienceL’application de vibrations mécaniques de fréquences aléatoires et faibles amplitudes (appelées « bruit ») sur les tendons des muscles posturaux de la cheville augmente la sensibilité proprioceptive et améliore la détection de mouvements de l’articulation de la cheville. L’objectif de cette étude a été d’analyser les conséquences de l’application de ce bruit sur la stabilisation posturale chez le sujet adulte jeune et en bonne santé