Mechanisms of interpersonal sway synchrony and stability

Effects of Altered Haptic Feedback Gain Upon Balance Are Explained by Sensory Conflict Estimation

Lightly touching a solid object reduces postural sway. Here, we determine the effect of artificially modifying haptic feedback for balance. Participants stood with their eyes closed, lightly gripping a manipulandum that moved synchronously with body sway to systematically enhance or attenuate feedback gain between +2 and -2, corresponding to motion in the same or opposite direction to the body, respectively. This intervention had a systematic effect on postural sway, which exhibited an asymmetric u-shape function with respect to haptic feedback gain. Sway was minimal around zero gain, corresponding to a static object. Sway increased slightly at gains below -0.25 but increased greatly at gains above +0.25. At +2, it was approximately double that of a no-touch condition. Mean interaction force between the hand and manipulandum remained < 0.9 N throughout, although it increased slightly at extreme gains. Cross-correlations between hand force and trunk position were highest during conditions of least sway, suggesting that higher quality haptic feedback is associated with greater sway reduction. We successfully replicated the sway behaviour using a feedback control model that attenuated haptic feedback signals when the discrepancy between haptic and proprioceptive signals reached a threshold. Our findings suggests the CNS can utilise augmented haptic feedback for balance, but only with relatively small changes to natural feedback gain. In healthy volunteers, it offers minimal benefit over a static object. Haptic feedback is therefore optimal when motion is physiologically realistic and subtle enough to be misinterpreted as self-motion.

Moving beyond the lab: investigating empathy through the Empirical 5E approach

Empathy is a complex and multifaceted phenomenon that plays a crucial role in human social interactions. Recent developments in social neuroscience have provided valuable insights into the neural underpinnings and bodily mechanisms underlying empathy. This methodology often prioritizes precision, replicability, internal validity, and confound control. However, fully understanding the complexity of empathy seems unattainable by solely relying on artificial and controlled laboratory settings, while overlooking a comprehensive view of empathy through an ecological experimental approach. In this article, we propose articulating an integrative theoretical and methodological framework based on the 5E approach (the "E"s stand for embodied, embedded, enacted, emotional, and extended perspectives of empathy), highlighting the relevance of studying empathy as an active interaction between embodied agents, embedded in a shared real-world environment. In addition, we illustrate how a novel multimodal approach including mobile brain and body imaging (MoBi) combined with phenomenological methods, and the implementation of interactive paradigms in a natural context, are adequate procedures to study empathy from the 5E approach. In doing so, we present the Empirical 5E approach (E5E) as an integrative scientific framework to bridge brain/body and phenomenological attributes in an interbody interactive setting. Progressing toward an E5E approach can be crucial to understanding empathy in accordance with the complexity of how it is experienced in the real world.

Coordination tending towards an anti-phase relationship determines greater sway reduction during entrainment with a simulated partner

The increased risk of falls in the older aged population demands the development of assistive robotic devices capable of effective balance support. For the development and increased user acceptance of such devices, which provide balance support in a human-like way, it is important to understand the simultaneous occurrence of entrainment and sway reduction in human-human interaction. However, sway reduction has not been observed yet during a human touching an external, continuously moving reference, which rather increased human body sway. Therefore, we investigated in 15 healthy young adults (27.20±3.55 years, 6 females) how different simulated sway-responsive interaction partners with different coupling modes affect sway entrainment, sway reduction and relative interpersonal coordination, as well as how these human behaviours differ depending on the individual body schema accuracy. For this, participants were lightly touching a haptic device that either played back an average pre-recorded sway trajectory ("Playback") or moved based on the sway trajectory simulated by a single-inverted pendulum model with either a positive (Attractor) or negative (Repulsor) coupling to participant's body sway. We found that body sway reduced not only during the Repulsor-interaction, but also during the Playback-interaction. These interactions also showed a relative interpersonal coordination tending more towards an anti-phase relationship, especially the Repulsor. Moreover, the Repulsor led to the strongest sway entrainment. Finally, a better body schema contributed to a reduced body sway in both the "reliable" Repulsor and the "less reliable" Attractor mode. Consequently, a relative interpersonal coordination tending more towards an anti-phase relationship and an accurate body schema are important to facilitate sway reduction.

Characteristics of postural control during fixed light-touch and interpersonal light-touch contact and the involvement of interpersonal postural coordination

Haptic feedback by light touch with a fingertip influences the postural control of the human body by postural orientation. Postural control might therefore differ depending on the characteristics of the contacting object. The main experimental targets of contact have been a fixed object (fixed light touch: FLT) and an individual (interpersonal light touch: ILT), but the postural control characteristics of FLT and ILT have not been directly compared within the same study. Nor has there been a study comparing frequency characteristics in these conditions. We hypothesized that (1) the frequency of postural sway would be higher in FLT and that no such change would be observed in ILT, and (2) the interpersonal postural coordination that is specific to ILT, i.e., sway that resembles the other person's sway, would be observed in the low-frequency component (≤0.4 Hz) rather than the high-frequency component (>0.4 Hz). We applied a closed-eyes tandem stance by adult subjects as the standard condition, and the center of pressure was measured when they performed four standing conditions: no-touch, FLT, stable ILT with a bipedal partner, and unstable ILT with a tandem partner. The results demonstrated that the FLT condition and both the stable and unstable ILT conditions also stabilized the posture, but the stability was superior in the FLT condition. Further, the difference in postural stability depending on the axis is not clear in any conditions for velocity, whereas for amplitude, stabilization by contact is more easily captured in the medio-lateral (ML) axis than in the anterior-posterior (AP) axis. The mean power frequency (MPF) in the FLT condition was higher than the no-touch condition, and the stable ILT condition in the ML axis and was higher than any other conditions in the AP axis. Moreover, the stable ILT condition in both axes was not significantly different from the no-touch condition. The unstable ILT condition in the AP axis was also not significantly different, though the ML axis was higher than the no-touch condition. The interpersonal postural coordination in both the stable and unstable ILT conditions was observed in the low-frequency component (except for the ML axis of the unstable ILT condition) and not in the high-frequency component. These results support our hypotheses and suggest that although FLT and ILT exert effects on reducing postural sway to some certain extent, in actuality, these conditions result in different postural controls in the frequency domain due to postural coordination based on the low-frequency component.

Human-Human Hand Interactions Aid Balance During Walking by Haptic Communication

Principles from human-human physical interaction may be necessary to design more intuitive and seamless robotic devices to aid human movement. Previous studies have shown that light touch can aid balance and that haptic communication can improve performance of physical tasks, but the effects of touch between two humans on walking balance has not been previously characterized. This study examines physical interaction between two persons when one person aids another in performing a beam-walking task. 12 pairs of healthy young adults held a force sensor with one hand while one person walked on a narrow balance beam (2 cm wide x 3.7 m long) and the other person walked overground by their side. We compare balance performance during partnered vs. solo beam-walking to examine the effects of haptic interaction, and we compare hand interaction mechanics during partnered beam-walking vs. overground walking to examine how the interaction aided balance. While holding the hand of a partner, participants were able to walk further on the beam without falling, reduce lateral sway, and decrease angular momentum in the frontal plane. We measured small hand force magnitudes (mean of 2.2 N laterally and 3.4 N vertically) that created opposing torque components about the beam axis and calculated the interaction torque, the overlapping opposing torque that does not contribute to motion of the beam-walker’s body. We found higher interaction torque magnitudes during partnered beam-walking vs. partnered overground walking, and correlation between interaction torque magnitude and reductions in lateral sway. To gain insight into feasible controller designs to emulate human-human physical interactions for aiding walking balance, we modeled the relationship between each torque component and motion of the beam-walker’s body as a mass-spring-damper system. Our model results show opposite types of mechanical elements (active vs. passive) for the two torque components. Our results demonstrate that hand interactions aid balance during partnered beam-walking by creating opposing torques that primarily serve haptic communication, and our model of the torques suggest control parameters for implementing human-human balance aid in human-robot interactions.

Moving in synchrony with an avatar – presenting a novel and unbiased body sway synchronization paradigm

Moving in synchrony with one another is a fundamental mechanism that maintains human social bonds. Yet, not all individuals are equally likely to coordinate their behaviors with others. The degree of interpersonal coordination is greatly influenced by pre-existing characteristics of the interacting partners, like the cultural homogeneity of a group, shared goals, and the likability of the other person. Considering that most research questions necessitate an experimental set-up without such uncontrolled biases, we created a novel, unbiased paradigm: a human-avatar body sway synchronization paradigm. Participants’ body sway was measured by a force plate while being exposed to a medio-laterally moving avatar. Forty-nine participants were tested in a social condition (motionless vs. moving avatar) and a non-social control condition (motionless vs. moving column). The results revealed that participants increased their body sway on their medio-lateral axis while the avatar was moving. The participants did not increase their body sway in the non-social control condition, indicating that the participant’s movement was not simply caused by a basal motion perception process. The current study builds a methodological fundament that can help to reduce biases due to pre-existing rapport between interaction partners and serves as a valuable experimental paradigm for future synchrony studies.

The Untapped Potential of Virtual Reality in Rehabilitation of Balance and Gait in Neurological Disorders

Dynamic systems theory transformed our understanding of motor control by recognizing the continual interaction between the organism and the environment. Movement could no longer be visualized simply as a response to a pattern of stimuli or as a demonstration of prior intent; movement is context dependent and is continuously reshaped by the ongoing dynamics of the world around us. Virtual reality is one methodological variable that allows us to control and manipulate that environmental context. A large body of literature exists to support the impact of visual flow, visual conditions, and visual perception on the planning and execution of movement. In rehabilitative practice, however, this technology has been employed mostly as a tool for motivation and enjoyment of physical exercise. The opportunity to modulate motor behavior through the parameters of the virtual world is often ignored in practice. In this article we present the results of experiments from our laboratories and from others demonstrating that presenting particular characteristics of the virtual world through different sensory modalities will modify balance and locomotor behavior. We will discuss how movement in the virtual world opens a window into the motor planning processes and informs us about the relative weighting of visual and somatosensory signals. Finally, we discuss how these findings should influence future treatment design.

Perception of haptic motion is enhanced during conditions of increased postural stability

BACKGROUND

Coupling between postural sway and fingertip displacement has been observed in individuals lightly touching a moving surface. This can be attributed to the central nervous system (CNS) misinterpreting surface motion as self-motion, evoking a compensatory sway response.

RESEARCH QUESTION

Does baseline postural state influence the correct perception of haptic object motion?

METHODS

Motion perception detection thresholds of index finger displacement at 1 mm s^-1 velocity during light touch were determined for three postural conditions: standing with eyes open (EO) and closed (EC), and sitting with eyes closed. For the standing condition with eyes shut, displacement thresholds were measured using three velocities (1, 2 and 4 mm s^-1).

RESULTS

Postural condition had a large influence on motion perception, with a reduction in displacement threshold from 12 → 6 → 2 mm during the transition from standing EC → standing EO → sitting EC. A systematic decrease in displacement perception threshold was observed with increasing velocity. This tends to suggest that the increase of the touched object velocity may help overcoming the misinterpretation.

SIGNIFICANCE

These results suggest that the ability to disambiguate self motion from haptic motion is enhanced during stable postures, and when stimulus velocity is high. Our findings may help to understand the mechanisms underlying the coupling between surface movements and postural sway, reported in the literature.

Sensorimotor Manipulations of the Balance Control Loop-Beyond Imposed External Perturbations

Standing balance relies on the integration of multiple sensory inputs to generate the motor commands required to stand. Mechanical and sensory perturbations elicit compensatory postural responses that are interpreted as a window into the sensorimotor processing involved in balance control. Popular methods involve imposed external perturbations that disrupt the control of quiet stance. Although these approaches provide critical information on how the balance system responds to external disturbances, the control mechanisms involved in correcting for these errors may differ from those responsible for the regulation of quiet standing. Alternative approaches use manipulations of the balance control loop to alter the relationship between sensory and motor cues. Coupled with imposed perturbations, these manipulations of the balance control loop provide unique opportunities to reveal how sensory and motor signals are integrated to control the upright body. In this review, we first explore imposed perturbation approaches that have been used to investigate the neural control of standing balance. We emphasize imposed perturbations that only elicit balance responses when the disturbing stimuli are relevant to the balance task. Next, we highlight manipulations of the balance control loop that, when carefully implemented, replicate and/or alter the sensorimotor dynamics of quiet standing. We further describe how manipulations of the balance control loop can be used in combination with imposed perturbations to characterize mechanistic principles underlying the control of standing balance. We propose that recent developments in the use of robotics and sensory manipulations will continue to enable new possibilities for simulating and/or altering the sensorimotor control of standing beyond compensatory responses to imposed external perturbations.

Interpersonal interactions for haptic guidance during balance exercises

BACKGROUND

Caregiver-patient interaction relies on interpersonal coordination during support provided by a therapist to a patient with impaired control of body balance.

RESEARCH QUESTION

The purpose of this study was to investigate in a therapeutic context active and passive participant involvement during interpersonal support in balancing tasks of increasing sensorimotor difficulty.

METHODS

Ten older adults stood in semi-tandem stance and received support from a physical therapist (PT) in two support conditions: 1) physical support provided by the PT to the participant's back via an instrumented handle affixed to a harness worn by the participant ("passive" interpersonal touch; IPT) or 2) support by PT and participant jointly holding a handle instrumented with a force-torque transducer while facing each other ("active" IPT). The postural stability of both support conditions was measured using the root-mean-square (RMS) of the Centre-of-Pressure velocity (RMS dCOP) in the antero-posterior (AP) and medio-lateral (ML) directions. Interpersonal postural coordination (IPC) was characterized in terms of cross-correlations between both individuals' sway fluctuations as well as the measured interaction forces.

RESULTS

Active involvement of the participant decreased the participant's postural variability to a greater extent, especially under challenging stance conditions, than receiving support passively. In the passive support condition, however, stronger in-phase IPC between both partners was observed in the antero-posterior direction, possibly caused by a more critical (visual or tactile) observation of participants' body sway dynamics by the therapist. In-phase cross-correlation time lags indicated that the therapist tended to respond to participants' body sway fluctuations in a reactive follower mode, which could indicate visual dominance affecting the therapist during the provision of haptic support.

SIGNIFICANCE

Our paradigm implies that in balance rehabilitation more partnership-based methods promote greater postural steadiness. The implications of this finding with regard to motor learning and rehabilitation need to be investigated.

Sensory integration of a light touch reference in human standing balance

In upright stance, light touch of a space-stationary touch reference reduces spontaneous sway. Moving the reference evokes sway responses which exhibit non-linear behavior that has been attributed to sensory reweighting. Reweighting refers to a change in the relative contribution of sensory cues signaling body sway in space and light touch cues signaling finger position with respect to the body. Here we test the hypothesis that the sensory fusion process involves a transformation of light touch signals into the same reference frame as other sensory inputs encoding body sway in space, or vice versa. Eight subjects lightly gripped a robotic manipulandum which moved in a circular arc around the ankle joint. A pseudo-randomized motion sequence with broad spectral characteristics was applied at three amplitudes. The stimulus was presented at two different heights and therefore different radial distances, which were matched in terms of angular motion. However, the higher stimulus evoked a significantly larger sway response, indicating that the response was not matched to stimulus angular motion. Instead, the body sway response was strongly related to the horizontal translation of the manipulandum. The results suggest that light touch is integrated as the horizontal distance between body COM and the finger. The data were well explained by a model with one feedback loop minimizing changes in horizontal COM-finger distance. The model further includes a second feedback loop estimating the horizontal finger motion and correcting the first loop when the touch reference is moving. The second loop includes the predicted transformation of sensory signals into the same reference frame and a non-linear threshold element that reproduces the non-linear sway responses, thus providing a mechanism that can explain reweighting.

Association between Unintentional Interpersonal Postural Coordination Produced by Interpersonal Light Touch and the Intensity of Social Relationship

Interpersonal postural coordination (IPC) produced by interpersonal light touch (ILT), whereby time-series variations in the postural sway between two people unintentionally resemble each other, may be a possible social interaction. From a sociopsychological standpoint, close mutual behavioral coordination is recognized as "social glue," which represents the closeness of relationships and contributes to the building of a good rapport. Therefore, we hypothesized that if IPC functions as social glue, then IPC produced by ILT also represents a social relationship. Participants were dyadic pairs with a preexisting social relationship (acquaintance, friend, or best-friend), and we assessed the closeness between the partners. Postural sway in two quiet standing conditions-no touch (NT) and ILT (a mutual light touch with <1 N) condition-was concurrently measured with the side-by-side standing position, and the association of IPC with intradyadic closeness (rapport) was analyzed using hierarchical linear modeling. The results showed that unintentional IPC was higher in both axes of the ILT condition than in NT condition. Additionally, IPC in the mediolateral axis (the partner side) of the ILT condition was positively correlated with intradyadic closeness, whereas that in the anteroposterior axis (the non-partner side) showed a negative association. As expected, IPC represented intradyadic closeness (rapport). Results indicate that, in unintentional IPC produced by ILT, the priority of processing sensory feedback for postural control, which is received from the individual and a partner, is modulated depending on the rapport in interactional coupled feedback loops between the two individuals (i.e., good rapport increases the degree of taking in feedback from a partner). Thus, unintentional IPC produced by ILT functions as social glue, and it provides an understanding of the sociopsychological aspect in the human-to-human postural coordination mechanism.

Co-ordination of the upper and lower limbs for vestibular control of balance

Key points

When standing and holding an earth‐fixed object, galvanic vestibular stimulation (GVS) can evoke upper limb responses to maintain balance.

In the present study, we determined how these responses are affected by grip context (no contact, light grip and firm grip), as well as how they are co‐ordinated with the lower limbs to maintain balance.

When GVS was applied during firm grip, hand and ground reaction forces were generated.

The directions of these force vectors were co‐ordinated such that the overall body sway response was always aligned with the inter‐aural axis (i.e. craniocentric).

When GVS was applied during light grip (< 1 N), hand forces were secondary to body movement, suggesting that the arm performed a mostly passive role.

These results demonstrate that a minimum level of grip is required before the upper limb becomes active in balance control and also that the upper and lower limbs co‐ordinate for an appropriate whole‐body sway response.

Abstract

Vestibular stimulation can evoke responses in the arm when it is used for balance. In the present study, we determined how these responses are affected by grip context, as well as how they are co‐ordinated with the rest of the body. Galvanic vestibular stimulation (GVS) was used to evoke balance responses under three conditions of manual contact with an earth‐fixed object: no contact, light grip (< 1 N) (LG) and firm grip (FG). As grip progressed along this continuum, we observed an increase in GVS‐evoked hand force, with a simultaneous reduction in ground reaction force (GRF) through the feet. During LG, hand force was secondary to the GVS‐evoked body sway response, indicating that the arm performed a mostly passive role. By contrast, during FG, the arm became actively involved in driving body sway, as revealed by an early force impulse in the opposite direction to that seen in LG. We then examined how the direction of this active hand vector was co‐ordinated with the lower limbs. Consistent with previous findings on sway anisotropy, FG skewed the direction of the GVS‐evoked GRF vector towards the axis of baseline postural instability. However, this was effectively cancelled by the hand force vector, such that the whole‐body sway response remained aligned with the inter‐aural axis, maintaining the craniocentric principle. These results show that a minimum level of grip is necessary before the upper limb plays an active role in vestibular‐evoked balance responses. Furthermore, they demonstrate that upper and lower‐limb forces are co‐ordinated to produce an appropriate whole‐body sway response.

When standing and holding an earth‐fixed object, galvanic vestibular stimulation (GVS) can evoke upper limb responses to maintain balance.

In the present study, we determined how these responses are affected by grip context (no contact, light grip and firm grip), as well as how they are co‐ordinated with the lower limbs to maintain balance.

When GVS was applied during firm grip, hand and ground reaction forces were generated.

The directions of these force vectors were co‐ordinated such that the overall body sway response was always aligned with the inter‐aural axis (i.e. craniocentric).

When GVS was applied during light grip (< 1 N), hand forces were secondary to body movement, suggesting that the arm performed a mostly passive role.

These results demonstrate that a minimum level of grip is required before the upper limb becomes active in balance control and also that the upper and lower limbs co‐ordinate for an appropriate whole‐body sway response.

Interpersonal interactions for haptic guidance during maximum forward reaching

Caregiver-patient interactions rely on interpersonal coordination (IPC) involving the haptic and visual modalities. We investigated in healthy individuals spontaneous IPC during joint maximum forward reaching. A 'contact-provider' (CP; n=2) kept light interpersonal touch (IPT) laterally with the wrist of the extended arm of a forward reaching, blind-folded 'contact-receiver' (CR; n=22). Due to the stance configuration, CP was intrinsically more stable. CR received haptic feedback during forward reaching in two ways: (1) presence of a light object (OBT) at the fingertips, (2) provision of IPT. CP delivered IPT with or without vision or tracked manually with vision but without IPT. CR's variabilities of Centre-of-Pressure velocity (CoP) and wrist velocity, interpersonal cross-correlations and time lags served as outcome variables. OBT presence increased CR's reaching amplitude and reduced postural variability in the reach end-state. CR's variability was lowest when CP applied IPT without vision. OBT decreased the strength of IPC. Correlation time lags indicated that CP retained a predominantly reactive mode with CR taking the lead. When CP had no vision, presumably preventing an effect of visual dominance, OBT presence made a qualitative difference: with OBT absent, CP was leading CR. This observation might indicate a switch in CR's coordinative strategy by attending mainly to CP's haptic 'anchor'. Our paradigm implies that in clinical settings the sensorimotor states of both interacting partners need to be considered. We speculate that haptic guidance by a caregiver is more effective when IPT resembles the only link between both partners.

Anisotropy and Antagonism in the Coupling of Two Oscillators: Concepts and Applications for Between-Person Coordination

Coupled oscillators provide a pertinent model approach to study between-person movement dynamics. While ample literature in this respect has considered the influence of external/environmental constraints and/or effects of a difference between the two agents' individual component dynamics (e.g., mismatch in natural frequency), recent studies also started to more directly consider the interaction per-se. The current perspective paper sets forth that while movement coordination dynamics has mainly been studied alongside a model in which the coupling is considered isotropic (i.e., symmetrical; both oscillators coupled to same degree) or strictly unidirectional (e.g., for moving to a given external rhythm), between-agent coupling involves a natural anisotropy: components influence each other bidirectionally to different degrees. Furthermore, recent research from different areas has considered so-called antagonistic or "competitive" coupling, which refers to the idea that one component is positively coupled to the other (attractive interaction), while the coupling in the other direction is negative (repulsive interaction). Although the latter would be rather tricky to address in within-person coordination, it does have strong applications and implications for between-person dynamics, for instance in the study of competitive interactions in sports situations (e.g., attacker-defender) and conflicting social (movement) interactions. The paper concludes by offering a conceptual framework and perspectives for future studies on the dynamic anisotropic nature of the interaction in between-person contexts.