The effects of sub-threshold vibratory noise on visuomotor entrainment during human walking and standing in a virtual reality environment

Humans will naturally synchronize their posture to the motion of a visual surround, but it is unclear if this visuomotor entrainment can be attenuated with an increased sensitivity to somatosensory information. Sub-threshold vibratory noise applied to the Achilles tendons has proven to enhance ankle proprioception through the phenomenon of stochastic resonance. Our purpose was to compare visuomotor entrainment during walking and standing, and to understand how this entrainment might be attenuated by applying sub-threshold vibratory noise over the Achilles tendons. We induced visuomotor entrainment during standing and treadmill walking for ten subjects (24.5 ± 2.9 years) using a speed-matched virtual hallway with continuous mediolateral perturbations at three different frequencies. Vibrotactile motors over the Achilles tendons provided noise (0–400 Hz) with an amplitude set to 90% of each participant’s sensory threshold. Mediolateral sacrum, C7, and head motion was greatly amplified (4–8× on average) at the perturbation frequencies during walking, but was much less pronounced during standing. During walking, individuals with greater mediolateral head motion at the fastest perturbation frequency saw the greatest attenuation of that motion with applied noise. Similarly, during standing, individuals who exhibited greater postural sway (as measured by the center of pressure) also saw the greatest reductions in sway with sub-threshold noise applied in three of our summary metrics. Our results suggest that, at least for healthy young adults, sub-threshold vibratory noise over the Achilles tendons can slightly improve postural control during disruptive mediolateral visual perturbations, but the applied noise does not substantially attenuate visuomotor entrainment during walking or standing.     

Age-related differences in postural reaction time and coordination during voluntary sway movements

The elderly are known to exhibit declines in postural control during standing and walking, however little is known about how the elderly react under time-critical and challenging postural situations. The purpose of this study was to examine age-related differences in reaction time (RT) and the pattern of temporal coordination between center of pressure (COP), trunk and head motion during voluntary postural sway movements. Healthy young (n=10; mean=24 years; SD=5 years) and elderly men (n=8; mean=75 years; SD=2 years) stood on a force plate with tri-axial accelerometers attached to the head and lower trunk. Participants were required to generate sway in the anterior-posterior (AP) or medial-lateral (ML) direction in response to an auditory cue during two different testing conditions called Static reaction and Dynamic reaction. Static reactions involved the initiation of voluntary sway in either the AP or ML direction from quiet stance. Dynamic reactions involved an orthogonal switch of voluntary sway between the AP and ML directions. Compared to the young, elderly individuals exhibited slower RT during both Static and Dynamic reaction, and smaller differences in RT and phasing between COP, trunk, and head motion. The results of this study suggest that the elderly adopted more rigid coordination strategies compared to the young when executing a rapid change in direction of whole body motion. The rigid movement strategy of the elderly was presumably generated in an effort to compensate for increased challenge to the maintenance of stability.     

Spatial and Temporal Adaptations That Accompany Increasing Catching Performance During Learning

During stance, head extension increases postural sway, possibly due to interference with sensory feedback. The sit-to-stand movement is potentially destabilizing due to the development of momentum as the trunk flexes forward and the body transitions to a smaller base of support. It is unclear what role head orientation plays in the postural and movement characteristics of the sit-to-stand transition. The authors assessed how moving from sitting to standing with head-on-trunk extension compared with moving with the head neutral or flexed, or with moving with the head facing forward in space (which would involve head-on-trunk extension, but not head-in-space extension) in healthy, young participants. Head-on-trunk extension increased center of pressure variability, but decreased movement velocities, movement duration, and trunk flexion compared with flexed and neutral head-on-trunk orientations. Similarities in movement characteristics between head-on-trunk extension and the forward head-in-space orientation suggest that stabilizing the head in space does not fully counteract the postural and movement changes due to head-on-trunk extension. Findings suggest that proprioceptive feedback from the neck muscles contributes to the regulation of posture and movement, and therefore should not be overlooked in research on the role of sensory feedback in postural control.     

Is head-on-trunk extension a proprioceptive mediator of postural control and sit-to-stand movement characteristics?

During stance, head extension increases postural sway, possibly due to interference with sensory feedback. The sit-to-stand movement is potentially destabilizing due to the development of momentum as the trunk flexes forward and the body transitions to a smaller base of support. It is unclear what role head orientation plays in the postural and movement characteristics of the sit-to-stand transition. The authors assessed how moving from sitting to standing with head-on-trunk extension compared with moving with the head neutral or flexed, or with moving with the head facing forward in space (which would involve head-on-trunk extension, but not head-in-space extension) in healthy, young participants. Head-on-trunk extension increased center of pressure variability, but decreased movement velocities, movement duration, and trunk flexion compared with flexed and neutral head-on-trunk orientations. Similarities in movement characteristics between head-on-trunk extension and the forward head-in-space orientation suggest that stabilizing the head in space does not fully counteract the postural and movement changes due to head-on-trunk extension. Findings suggest that proprioceptive feedback from the neck muscles contributes to the regulation of posture and movement, and therefore should not be overlooked in research on the role of sensory feedback in postural control.     

Effect of visual surrounding motion on body sway in a three-dimensional environment

Unidirectional motion of a uniplanar background induces a codirectional postural sway. It has been shown recently that fixation of a stationary foreground object induces a sway response in the opposite direction (Bronstein & Buckwell, 1997) when the background moves transiently. The present study investigated factors determining this contradirectional postural response. In the experiments presented, center of foot pressure and head displacements were recorded from normal subjects. The subjects faced a visual background of 2 x 3 m, at a distance of 1.5 m, which could be moved parallel to the interaural axis. Results showed that when the visual scene consisted solely of a moving background, the conventional codirectional postural response was elicited. When subjects were asked to fixate an earth-fixed foreground (window frame) placed between them and the moving background, a consistent postural response in the opposite direction to background motion was observed. In addition, we showed that this contradirectional postural response was not transient but was sustained for the 11 sec of background motion. We investigated whether this contradirectional postural response was the consequence of the induced movement of the foreground by background motion. Although induced movement was verbally reported by subjects when viewing an earth-fixed target projected onto the moving background, the contradirectional sway did not occur. These results indicate that foreground-background separation in depth was necessary for the contradirectional postural response to occur rather than induced movement. Another experiment showed that, when the fixated foreground was attached to the head of the observer, the contradirectional sway was not observed and was therefore unrelated to vergence. Finally, results showed that the contradirectional postural response was, in the main, monocularly mediated. We conclude that the direction of the postural sway produced by a moving background in a three-dimensional environment is determined primarily by motion parallax.     

Horizontal plane head stabilization during locomotor tasks

Frequency characteristics of head stabilization were examined during locomotor tasks in healthy young adults(N = 8) who performed normal walking and 3 walking tasks designed to produce perturbations primarily in the horizontal plane. In the 3 walking tasks, the arms moved in phase with leg movement, with abnormally large amplitude, and at twice the frequency of leg movement. Head-in-space angular velocity was examined at the predominant frequencies of trunk motion. Head movements in space occurred at low frequencies (< 4.0 Hz) in all conditions and at higher frequencies (> 4.0 Hz) when the arms moved at twice the frequency of the legs. Head stabilization strategies were determined from head-on-trunk with respect to trunk frequency profiles derived from angular velocity data. During natural walking at low frequencies (< 3.0 Hz), head-on-trunk movement was less than trunk movement. At frequencies 3.0 Hz or greater, equal and opposite compensatory movement ensured head stability. When arm swing was altered, compensatory movement guaranteed head stability at all frequencies. Head stabilization was successful for frequencies up to 10.0 Hz during locomotor tasks. Maintaining head stability at high frequencies during voluntary tasks suggests that participants used feedforward mechanisms to coordinate head and trunk movements. Maintenance of head stability during dynamic tasks allows optimal conditions for vestibulo-ocular reflex function.     

Horizontal Plane Head Stabilization During Locomotor Tasks

Frequency characteristics of head stabilization were examined during locomotor tasks in healthy young adults (N = 8) who performed normal walking and 3 walking tasks designed to produce perturbations primarily in the horizontal plane. In the 3 walking tasks, the arms moved in phase with leg movement, with abnormally large amplitude, and at twice the frequency of leg movement. Head-in-space angular velocity was examined at the predominant frequencies of trunk motion. Head movements in space occurred at low frequencies (< 4.0 Hz) in all conditions and at higher frequencies (> 4.0 Hz) when the arms moved at twice the frequency of the legs. Head stabilization strategies were determined from head-on-trunk with respect to trunk frequency profiles derived from angular velocity data. During natural walking at low frequencies (< 3.0 Hz), head-on-trunk movement was less than trunk movement. At frequencies 3.0 Hz or greater, equal and opposite compensatory movement ensured head stability. When arm swing was altered, compensatory movement guaranteed head stability at all frequencies. Head stabilization was successful for frequencies up to 10.0 Hz during locomotor tasks Maintaining head stability at high frequencies during voluntary tasks suggests that participants used feedforward mechanisms to coordinate head and trunk movements. Maintenance of head stability during dynamic tasks allows optimal conditions for vestibulo-ocular reflex function.     

Is voluntary control of natural postural sway possible?

The authors explored whether standing human participants could voluntarily decrease the amplitude of their natural postural sway when presented with explicit visual feedback and a target. Participants (N = 9) stood quietly, without any feedback and with feedback on the center of pressure coordinate or the head orientation. They were unable to decrease sway amplitude when presented with visual feedback and a target. Decreasing target size led to contrasting effects on the 2 fractions of sway: rambling and trembling. The smaller target was associated with a decrease in rambling and an increase in trembling. Those observations suggest that sway represents a superposition of at least 2 independent processes. They also suggest that providing visual feedback on a variable tied to body sway may not be an effective way to decrease postural sway in young healthy people.     

Broad stance conditions change postural control and postural sway

Intuitively, a broad stance (i.e., standing with the feet farther apart than usual) should significantly improve postural stability. However, this intuition was not confirmed in quiet stance. Hence, a motion analysis system (markers attached to the trunk and head) and a force platform were used to investigate 13 healthy, young adults who performed 8 trials in standard and broad stances. In broad stance, the medialateral center of pressure (COP) sway mean power frequency was expected to be greater, whereas the variability (standard deviation) of COP, head, and trunk sway and the mean velocity of head and trunk sway was expected to be significantly lower. Accordingly, adoption of a broad stance significantly increased the medialateral mean power frequency of COP sway; decreased the standard deviation of medialateral COP, trunk, and head sway; and decreased the medialateral mean velocity of head sway. A broad stance was also associated with lower variability for head and COP sways in the anteroposterior axis. Unexpectedly, an effect of trial repetition was found for the variability of medialateral trunk sway. This was probably due to the break halfway through the study. In practical terms, broad stance conditions can improve postural control in the medialateral and anteroposterior axes.     

The role of central and peripheral vision in postural control during walking

Three hypotheses have been proposed for the roles of central and peripheral vision in the perception and control of self-motion: (1) peripheral dominance, (2) retinal invariance, and (3) differential sensitivity to radial flow. We investigated postural responses to optic flow patterns presented at different retinal eccentricities during walking in two experiments. Oscillating displays of radial flow (0 degree driver direction), lamellar flow (90 degrees), and intermediate flow (30 degrees, 45 degrees) patterns were presented at retinal eccentricities of 0 degree, 30 degrees, 45 degrees, 60 degrees, or 90 degrees to participants walking on a treadmill, while compensatory body sway was measured. In general, postural responses were directionally specific, of comparable amplitude, and strongly coupled to the display for all flow patterns at all retinal eccentricities. One intermediate flow pattern (45 degrees) yielded a bias in sway direction that was consistent with triangulation errors in locating the focus of expansion from visible flow vectors. The results demonstrate functionally specific postural responses of both central and peripheral vision, contrary to the peripheral dominance and differential sensitivity hypotheses, but consistent with retinal invariance. This finding emphasizes the importance of optic flow structure for postural control regardless of the retinal locus of stimulation.     

The perception of an optical flow projected on the ground surface

In most experiments in which the importance of visual control on postural stability is studied, optical stimuli attached to vertical surfaces are used. Analyses of long-term standing readjustments generally involve back-and-forth movements of a visual scene or its projection on vertical circular screens. In a natural environment, however, visual information is largely available from the ground. The aim of the experiment reported was to assess the effect of a flow pattern simulating an open outdoor setting on motion perception and postural control. Subjects were presented with an optical texture projected onto the ground. Periods of motionless texture alternated with equivalent durations of unidirectional flows. The change of position of the subject's centre of gravity over time was recorded on a force platform. Results show that the direction of body sway corresponded with that of texture motion. Important aftereffects, as shown in linear vection experiments, were also observed. However, the latency of postural responses was much shorter than with prolonged unidirectional flows produced in other locations of the visual environment. The hypothesis of an ecological specificity of the flows perceived on the ground during terrestrial displacements is discussed.     

The role of central and peripheral vision in postural control duringwalking

Three hypotheses have been proposed for the roles of central and peripheral vision in the perception and control of self-motion: (1) peripheral dominance, (2) retinal invariance, and (3) differential sensitivity to radial flow. We investigated postural responses to optic flow patterns presented at different retinal eccentricities during walking in two experiments. Oscillating displays of radial flow (0° driver direction), lamellar flow (90°), and intermediate flow (30°, 45°) patterns were presented at retinal eccentricities of 0°, 30°, 45°, 60°, or 90° to participants walking on a treadmill, while compensatory body sway was measured. In general, postural responses were directionally specific, of comparable amplitude, and strongly coupled to the display for all flow patterns at all retinal eccentricities. One intermediate flow pattern (45°) yielded a bias in sway direction that was consistent with triangulation errors in locating the focus of expansion from visible flow vectors. The results demonstrate functionally specific postural responses in both central and peripheral vision, contrary to the peripheral dominance and differential sensitivity hypotheses, but consistent with retinal invariance. This finding emphasizes the importance of optic flow structure for postural control regardless of the retinal locus of stimulation.     

Analysis of the Movement-Inducing Effects of Music through the Fractality of Head Sway during Standstill

Abstract

The links between music and human movement have been shown to provide insight into crucial aspects of human’s perception, cognition, and sensorimotor systems. In this study, we examined the influence of music on movement during standstill, aiming at further characterizing the correspondences between movement, music, and perception, by analyzing head sway fractality. Eighty seven participants were asked to stand as still as possible for 500?seconds while being presented with alternating silence and audio stimuli. The audio stimuli were all rhythmic in nature, ranging from a metronome track to complex electronic dance music. The head position of each participant was captured with an optical motion capture system. Long-range correlations of head movement were estimated by detrended fluctuation analysis (DFA). Results agree with previous work on the movement-inducing effect of music, showing significantly greater head sway and lower head sway fractality during the music stimuli. In addition, patterns across stimuli suggest a two-way adaptation process to the effects of music, with musical stimuli influencing head sway while at the same time fractality modulated movement responses. Results indicate that fluctuations in head movement in both conditions exhibit long-range correlations, suggesting that the effects of music on head movement depended not only on the value of the most recent measured intervals, but also on the values of those intervals at distant times.     

The functional role of central and peripheral vision in the control of posture

Three experiments were conducted to investigate the role of central and peripheral vision (CV and PV) in postural control. In Experiment 1, either the central or peripheral visual field were selectively stimulated using a circular random dot pattern that was either static or alternated at 5 Hz. Center of foot pressure (CoP) was used to examine postural sway during quiet standing under both CV and PV conditions. The results showed that, when the visual stimulus was presented in the periphery, the CoP area decreased and more so in the anterior-posterior (AP) than in the medio-lateral (ML) direction, indicating a characteristic directional specificity. There was no significant difference between the static and dynamic (alternating) conditions. Experiment 2 investigated the directional specificity of body sway found in Experiment 1 by having the trunk either be faced toward the stimulus display or perpendicularly to it, with the head always facing the display. The results showed that the stabilizing effect of peripheral vision was present in the direction of stimulus observation (i.e., the head/gaze direction), irrespective of trunk orientation. This suggested that head/gaze direction toward the stimulus presentation, rather than a biomechanical factor like greater mobility of the ankle joint in AP direction than in ML direction, was essential to postural stability. Experiment 3 further examined whether the stabilizing effect of peripheral vision found in Experiments 1 and 2 was caused because more dots (500) were presented as visual cues to the peripheral visual field than to the central visual field (20 dots) by presenting the same number of dots (20) in both conditions. It was found that, in spite of the equal number of dots, the postural sway amplitudes were larger for the central vision conditions than for the peripheral vision conditions. In conclusion, the present study showed that peripheral rather than central vision contributes to maintaining a stable standing posture, with postural sway being influenced more in the direction of stimulus observation, or head/gaze direction, than in the direction of trunk orientation, which suggests that peripheral vision operates primarily in a viewer-centered frame of reference characterized by the head/gaze direction rather than in a body-centered frame of reference characterized by the anatomical planes of the body.     

Age-related differences in head and trunk coordination during walking

The purpose of this study was to examine the effect of ageing on the pattern and structure of head and trunk accelerations during walking. Head and trunk accelerations of young (n=8; mean=23 years, SD=4 years) and elderly (n=8; mean=74 years, SD=3 years) individuals were measured using triaxial accelerometers while performing preferred speed walking. Accelerations were examined using power-spectral analysis and measures of signal smoothness, regularity and coupling. No differences in walking speed or signal regularity were detected between age groups. Compared to the young participants, the elderly had (1) a greater proportion of signal power above 6 Hz for the trunk, (2) a smaller difference in signal smoothness between the trunk and head, (3) less signal smoothness in the mediolateral direction, and (4) a greater degree of directional coupling for the head compared to the trunk. Overall these results suggest that the pattern of head accelerations was relatively unaffected by age, and that both age groups achieved similar levels of head stability despite differences in trunk acceleration characteristics. The manner in which head stability was achieved differed between age groups, with the elderly employing an upper body coordination strategy that enhanced coupling between acceleration directions of the head compared to the trunk. The findings of this study also suggest that an absence of age-related differences in signal complexity at one level of postural system, combined with differences at another level, may provide information about the way in which the motor system prioritises postural control during gait.     

How trunk turns affect locomotion when you are not looking where you go

How well do we maintain heading direction during walking while we look at objects beside our path by rotating our eyes, head, or trunk? Common experience indicates that it may be fairly hazardous not to look where you are going. In the present study, 12 young adults walked on a treadmill while they followed a moving dot along a horizontal line with their gaze by rotating primarily either their eyes, head, or trunk for amplitudes of up to 25 degrees . During walking the movement of the center of pressure (COP) was monitored using force transducers under a treadmill. Under normal light conditions, the participants showed little lateral deviation of the COP from the heading direction when they performed the eye or head movement task during walking, even when optic flow information was limited. In contrast, trunk rotations led to a doubling of the COP deviation in the mediolateral direction. Some of this deviation was attributed to foot rotation. Participants tended to point their feet in the gaze direction when making trunk turns. The tendency of the feet to be aligned with the trunk is likely to be due to a preference to have feet and body in the same orientation. Such alignment is weaker for the feet with respect to head position and it is absent with respect to eye position. It is argued that feet and trunk orientation are normally tightly coupled during gait and that it requires special abilities to move both segments independently when walking.     

Spatial orientation from optic flow in the central visual field

Previous research has shown that stimulation of the central visual field with radial flow patterns (produced by forward motion) can induce perceived self-motion, but has failed to demonstrate effects on postural stability of either radial flow patterns or lamellar flow patterns (produced by horizontal translation) in the central visual field. The present study examined the effects of lamellar and radial flow on postural stability when stimulation was restricted to the central visual field. Displays simulating observer motion through a volume of randomly positioned points were observed binocularly through a window that limited the field of view to 15 degrees. The velocity of each display varied according to the sum of four sine functions of prime frequencies. Changes in posture were used to measure changes in perceived spatial orientation. A frequency analysis of postural sway indicated that increased sway occurred at the frequencies of motion simulated in the display for both lamellar and radial flow. These results suggest that both radial and lamellar optic flow are effective for determining spatial orientation when stimulation is limited to the central visual field.     

Postural control in otolith disorders

It was the aim of the present paper to investigate the influence of otolith disorders on human postural control by different methods. The 33 patients of our study had undergone a minor head injury and suffered subsequently from an utricular or sacculo-utricular disorder as evidenced by vestibular evoked myogenic potential recordings and eccentric rotation recordings of the otolith-ocular responses. Postural control was assessed by performing stance/gait tests (standard balance deficit test, SBDT) and by evaluating trunk sway (using angular velocity sensors). Moreover, classical tests of the posterior column of the spinal tract (Romberg/Unterberger) and the dynamic posturography (sensory organization test, SOT) were included. It could be shown that SBDT tasks with reduced proprioceptive and visual cues (e.g. standing on foam, eyes closed) are most sensitive for an otolith disorder. The patients showed an increased trunk sway in the pitch plane (i.e. linear motion as adequate utricular stimulus) and an increase in sway velocities (i.e. tilting movements as adequate saccular stimulus) compared to controls. The SOT was most sensitive for combined (sacculo-utricular) otolith disorders (78%) while vestibulospinal tests are not enough sensitive. In essence, otolith disorders evidently impair human postural control and have been possibly underestimated as a source of posttraumatic postural imbalance as yet.     

Haptic cues for orientation and postural control

Haptic cues from fingertip contact with a stable surface attenuate body sway in subjects even when the contact forces are too small to provide physical support of the body. We investigated how haptic cues derived from contact of a cane with a stationary surface at low force levels aids postural control in sighted and congenitally blind individuals. Five sighted (eyes closed) and five congenitally blind subjects maintained a tandem Romberg stance in five conditions: (1) no cane; (2, 3) touch contact (<2 N of applied force) while holding the cane in a vertical or slanted orientation; and (4, 5) force contact (as much force as desired) in the vertical and slanted orientations. Touch contact of a cane at force levels below those necessary to provide significant physical stabilization was as effective as force contact in reducing postural sway in all subjects, compared to the no-cane condition. A slanted cane was far more effective in reducing postural sway than was a perpendicular cane. Cane use also decreased head displacement of sighted subjects far more than that of blind subjects. These results suggest that head movement control is linked to postural control through gaze stabilization reflexes in sighted subjects; such reflexes are absent in congenitally blind individuals and may account for their higher levels of head displacement.     

Influence of vision and static stretch of the calf muscles on postural sway during quiet standing

The purpose of this experimental study was to evaluate the effects of vision and stretching of the calf muscles on postural sway during quiet standing. Under pre-stretch conditions, participants stood on a force plate for 30s and the sway of the ground reaction force center of pressure was recorded. The following postural sway variables were calculated off-line: sweep speed, sway speed, standard deviation, maximal mediolateral range, maximal anteroposterior range, mean mediolateral position and mean anteroposterior position. For post-stretch conditions, participants stood quietly on a device that was utilized to impose a static 3 min ankle joint dorsiflexion stretch. Immediately thereafter, participants moved onto the force platform where postural sway parameters were again recorded. Randomized eyes-open and eyes-closed conditions were tested in both cases. Results showed that postural sway significantly increased due to stretch (sweep speed, sway speed, standard deviation, maximal anteroposterior range, mean anteroposterior position), as well as eye closure (sweep speed, sway speed, standard deviation, maximal mediolateral range, maximal anteroposterior range). The interaction between stretch and eye closure was also significant (sweep speed, sway speed, standard deviation, maximal mediolateral range), suggesting that there were only minor increases in postural sway after stretch under the eyes-open condition. It was suggested that stretching of the calf muscles has the effect of increasing postural sway, although this effect can be greatly compensated for when vision is included.