Haptic stabilization of posture: changes in arm proprioception and cutaneous feedback for different arm orientations

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.

Balance Adaptation While Standing on a Compliant Base Depends on the Current Sensory Condition in Healthy Young Adults

Background

Several investigations have addressed the process of balance adaptation to external perturbations. The adaptation during unperturbed stance has received little attention. Further, whether the current sensory conditions affect the adaptation rate has not been established. We have addressed the role of vision and haptic feedback on adaptation while standing on foam.

Methods

In 22 young subjects, the analysis of geometric (path length and sway area) and spectral variables (median frequency and mean level of both total spectrum and selected frequency windows) of the oscillation of the centre of feet pressure (CoP) identified the effects of vision, light-touch (LT) or both in the anteroposterior (AP) and mediolateral (ML) direction over 8 consecutive 90 s standing trials.

Results

Adaptation was obvious without vision (eyes closed; EC) and tenuous with vision (eyes open; EO). With trial repetition, path length and median frequency diminished with EC (p < 0.001) while sway area and mean level of the spectrum increased (p < 0.001). The low- and high-frequency range of the spectrum increased and decreased in AP and ML directions, respectively. Touch compared to no-touch enhanced the rate of increase of the low-frequency power (p < 0.05). Spectral differences in distinct sensory conditions persisted after adaptation.

Conclusion

Balance adaptation occurs during standing on foam. Adaptation leads to a progressive increase in the amplitude of the lowest frequencies of the spectrum and a concurrent decrease in the high-frequency range. Within this common behaviour, touch adds to its stabilising action a modest effect on the adaptation rate. Stabilisation is improved by favouring slow oscillations at the expense of sway minimisation. These findings are preliminary to investigations of balance problems in persons with sensory deficits, ageing, and peripheral or central nervous lesion.

Specific Posture-Stabilising Effects of Vision and Touch Are Revealed by Distinct Changes of Body Oscillation Frequencies

We addressed postural instability during stance with eyes closed (EC) on a compliant surface in healthy young people. Spectral analysis of the centre of foot pressure oscillations was used to identify the effects of haptic information (light-touch, EC-LT), or vision (eyes open, EO), or both (EO-LT). Spectral median frequency was strongly reduced by EO and EO-LT, while spectral amplitude was reduced by all "stabilising" sensory conditions. Reduction in spectrum level by EO mainly appeared in the high-frequency range. Reduction by LT was much larger than that induced by the vision in the low-frequency range, less so in the high-frequency range. Touch and vision together produced a fall in spectral amplitude across all windows, more so in anteroposterior (AP) direction. Lowermost frequencies contributed poorly to geometric measures (sway path and area) for all sensory conditions. The same subjects participated in control experiments on a solid base of support. Median frequency and amplitude of the spectrum and geometric measures were largely smaller when standing on solid than on foam base but poorly affected by the sensory conditions. Frequency analysis but not geometric measures allowed to disclose unique tuning of the postural control mode by haptic and visual information. During standing on foam, the vision did not reduce low-frequency oscillations, while touch diminished the entire spectrum, except for the medium-high frequencies, as if sway reduction by touch would rely on rapid balance corrections. The combination of frequency analysis with sensory conditions is a promising approach to explore altered postural mechanisms and prospective interventions in subjects with central or peripheral nervous system disorders.

Analysis of Light Grip Influence on Standing Posture

Upright posture control and gait are essential for achieving autonomous daily living activities. Postural control of upright posture relies, among others, on the integration of various sensory information. In this context, light touch (LT) and light grip (LG) of a stationary object provide an additional haptic sensory input that helps to reduce postural sway. When LG was studied through the grasp of a cane, the sensory role of this assistive tool was often limited to a mediation interface. Its role was restricted to transmit the interaction forces between its tip and the ground to the hand. While most studies involve participants standing in an unstable way, such as the tandem stance, in this paper we study LG from a different perspective. We attached a handle of a cane firmly to a stationary support. Thus, we can focus on the role of the hand receptors in the LG mechanism. LG condition was ensured through the tactile information gathered by FSR sensors placed on the handle surface. Moreover, participants involved in our study stood in a usual way. The study involved twelve participants in an experiment composed of two conditions: standing relaxed while lightly gripping an equipped handle attached to the ground, and standing in the same way without gripping the handle. Spatial and frequency analyses confirmed the results reported in the literature with other approaches.

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.

Immediate Effects of Vibrotactile Biofeedback Instructions on Human Postural Control

Vibrotactile biofeedback can improve balance and consequently be helpful in fall prevention. However, it remains unclear how different types of stimulus presentations affect not only trunk tilt, but also Center of Pressure (CoP) displacements, and whether an instruction on how to move contributes to a better understanding of vibrotactile feedback.Based on lower back tilt angles (L5), we applied individualized multi-directional vibrotactile feedback to the upper torso by a haptic vest in 30 healthy young adults. Subjects were equally distributed to three instruction groups (attractive - move in the direction of feedback, repulsive - move in the opposite direction of feedback & no instruction - with attractive stimuli). We conducted four conditions with eyes closed (feedback on/off, Narrow Stance with head extended, Semi-Tandem stance), with seven trials of 45s each. For CoP and L5, we computed Root Mean Square (RMS) of position/angle and standard deviation (SD) of velocity, and for L5 additionally, the percentage in time above threshold. The analysis consisted of mixed model ANOVAs and t-tests (α-level: 0.05).In the attractive and repulsive groups feedback significantly decreased the percentage above threshold (p<0.05). Feedback decreased RMS of L5, whereas RMS of CoP and SD of velocity in L5 and COP increased (p<0.05). Finally, an instruction on how to move contributed to a better understanding of the vibrotactile biofeedback.

The Importance of Being in Touch

This paper describes a series of studies resulting from the finding that when free floating in weightless conditions with eyes closed, all sense of one's spatial orientation with respect to the aircraft can be lost. But, a touch of the hand to the enclosure restores the sense of spatial anchoring within the environment. This observation led to the exploration of how light touch of the hand can stabilize postural control on Earth even in individuals lacking vestibular function, and can override the effect of otherwise destabilizing tonic vibration reflexes in leg muscles. Such haptic stabilization appears to represent a long loop cortical reflex with contact cues at the hand phase leading EMG activity in leg muscles, which change the center of pressure at the feet to counteract body sway. Experiments on dynamic control of balance in a device programmed to exhibit inverted pendulum behavior about different axes and planes of rotation revealed that the direction of gravity not the direction of balance influences the perceived upright. Active control does not improve the accuracy of indicating the upright vs. passive exposure. In the absence of position dependent gravity shear forces on the otolith organs and body surface, drifting and loss of control soon result and subjects are unaware of their ongoing spatial position. There is a failure of dynamic path integration of the semicircular canal signals, such as occurs in weightless conditions.

The effects of light touch on gait and dynamic balance during normal and tandem walking in individuals with an incomplete spinal cord injury

STUDY DESIGN

Prospective cross-sectional study OBJECTIVES: To investigate the effect of adding haptic input during walking in individuals with incomplete spinal cord injury (iSCI).

SETTING

Research laboratory.

METHODS

Participants with iSCI and age- and sex-matched able-bodied (AB) individuals walked normally (SCI n = 18, AB n = 17) and in tandem (SCI n = 12, AB n = 17). Haptic input was added through light touch on a railing. Step parameters, and mediolateral and anterior-posterior margins of stability (means and standard deviations) were calculated. Surface electromyography data were collected bilaterally from the tibialis anterior (TA), soleus (SOL), and gluteus medius (GMED) and integrated over a stride. Repeated measures ANOVAs examined within- and between-group differences (α = 0.05). Cutaneous and proprioceptive sensation of individuals with iSCI were correlated to changes in outcome measures that were affected by haptic input.

RESULTS

When walking normally, adding haptic input decreased stride velocity, step width, stride length, MOS_ML, MOS_ML_SD, MOS_AP, and MOS_AP_SD, and increased GMED activity on the limb opposite the railing. During tandem walking, haptic input had no effect; however, individuals with iSCI had a larger step width SD and MOS_ML_SD compared with the AB group. Sensory abilities of individuals with iSCI were not correlated to any of the outcome measures that significantly changed with added haptic input.

CONCLUSIONS

Added haptic input improved balance control during normal but not in tandem walking. Sensory abilities did not impact the use of added haptic input during walking.

Comparing the effect of haptic modalities on walking balance control: Is using one or two arms better?

BACKGROUND

Adding haptic input by lightly touching a railing or using haptic anchors may improve walking balance control. Typical use of the railing(s) and haptic anchors requires the use of one and two arms in an extended position, respectively. It is unclear whether it is arm configuration and/or the number of arms used or the addition of sensory input that affects walking balance control.

RESEARCH QUESTION

This study examined whether using one arm or two arms to add haptic input through light touch on a railing or using the haptic anchors affects walking balance control.

METHODS

In this study, young adults (n = 24) walked while using (actual use) or pretending to use (pretend use) the railing(s) and haptic anchors with one or two arms. Inertial-based sensors (Mobility Lab, APDM) were used to measure stride velocity, relative time spent in double support (%DS), and peak normalized medio-lateral trunk velocity (pnMLTV).

RESULTS

Using two arms lead to a decrease in pnMLTV compared to using one arm and pnMLTV was lower in the actual use trials compared to the pretend use trials for the anchors only. Stride velocity and %DS did not change between trials when one or two arms were used or when participants actually or pretended to use the haptic tools. Participants walked slower when using the railing compared to the anchors.

SIGNIFICANCE

The importance of considering the number of arms is highlighted in the improved balance control when using two arms with either tool. The augmented sensory input adds to the stabilizing effect of arm configuration for the anchors but not the railings. These results have implications for future research and rehabilitation efforts emphasizing sensorimotor integration to improve walking balance control.

From One to Two: Can Visual Feedback Improve the Light Touch Effects on Postural Sway?

The postural control is improved by implicit somatosensory information from lightly touching a rigid bar or explicit visual information about the postural sway. Whether these two additional sources provided at the same time further reduce the postural sway is still unknown. Participants stood on a force plate as quiet as possible lightly touching the bar while received or not visual feedback of the center of pressure position on a monitor screen. Postural sway reduced similarly with the light touch regardless of the additional visual feedback. The findings suggested that providing explicit visual feedback of the center of pressure does not increase the light touch effects on the postural sway. The importance of the implicit somatosensory information on postural control is discussed.

Balance in Blind Subjects: Cane and Fingertip Touch Induce Similar Extent and Promptness of Stance Stabilization

Subjects with low vision often use a cane when standing and walking autonomously in everyday life. One aim of this study was to assess differences in the body stabilizing effect produced by the contact of the cane with the ground or by the fingertip touch of a firm surface. Another aim was to estimate the promptness of balance stabilization (or destabilization) on adding (or withdrawing) the haptic input from cane or fingertip. Twelve blind subjects and two subjects with severe visual impairment participated in two experimental protocols while maintaining the tandem Romberg posture on a force platform. In one protocol, subjects lowered the cane to a second platform on the ground and lifted it in sequence at their own pace. In the other protocol, they touched an instrumented pad with the index finger and withdrew the finger from the pad in sequence. In both protocols, subjects were asked to exert a force not granting mechanical stabilization. Under steady-state condition, the finger touch or the contact of the cane with the ground significantly reduced (to ∼78% and ∼86%, respectively) the amplitude of medio-lateral oscillation of the centre of foot pressure (CoP). Oscillation then increased when haptic information was removed. The delay to the change in body oscillation after the haptic shift was longer for addition than withdrawal of the haptic information (∼1.4 s and ∼0.7 s, respectively; p < 0.001), but was not different between the two haptic conditions (finger and cane). Similar stabilizing effects of input from cane on the ground and from fingertip touch, and similar latencies to integrate haptic cue from both sources, suggest that the process of integration of the input for balance control is initiated by the haptic stimulus at the interface cane-hand. Use of a tool is as helpful as the fingertip input, and does not produce different stabilization. Further, the latencies to haptic cue integration (from fingertip or cane) are similar to those previously found in a group of sighted subjects, suggesting that integration delays for automatic balance stabilization are not modified by visual impairment. Haptic input from a tool is easily exploited by the neural circuits subserving automatic balance stabilization in blind people, and its use should be enforced by sensory-enhancing devices and appropriate training.

Effect of force magnitude of touch on the components of postural sway

BACKGROUND

Lightly touching the tip of the index finger on an external surface reduces the postural sway during upright standing due to the additional somatosensory information provided by the touch to the postural control system. But when the individuals apply more force, it provides more mechanical support. However, because most of the studies investigated only two levels of force, whether the control mechanisms of postural sway are affected by the different force levels was unknown.

RESEARCH QUESTION

To examine the influence of the magnitude of force (up to 1, 2, 4, 6 or 8 N) applied to the touch bar on the mechanisms used to control the postural sway during quiet standing with eyes open or closed.

METHODS

Ten young right-handed adults stood for 35 s on a force platform, with feet apart, while touched a rigid bar with different force levels with eyes open or closed. The amplitude and velocity of the center of pressure and its components, Rambling and Trembling trajectories, respectively, related to more supraspinal and spinal control mechanisms, were assessed.

RESULTS

The touch reduced all trajectories, mainly of the Rambling component and with closed eyes. There was a floor effect of the touch force as amplitudes and velocities were minimal at 4 N of force.

SIGNIFICANCE

The component of postural sway under the supraspinal neural control is more affected by different force magnitudes applied to the touch bar.

Not just a light fingertip touch: A facilitation of functional integration between body sway and visual search in older adults

BACKGROUND

Prior studies demonstrated that, compared to no fingertip touch (NT), a reduction in body sway resulting from the effects of light fingertip touch (LT) facilitates the performance of visual search, buttressing the concept of functional integration. However, previous findings may be confounded by different arm postures required between the NT and LT conditions. Furthermore, in older adults, how LT influences the interactions between body sway and visual search has not been established.

RESEARCH QUESTIONS

(1) Are LT effects valid after excluding the influences of different upper limb configurations? (2) Is functional integration is feasible for older adults?

METHODS

Twenty-two young (age = 21.3 ± 2.0) and 22 older adults (age = 71.8 ± 4.1) were recruited. Participants performed visual inspection and visual searches under NT and LT conditions.

RESULTS

The older group significantly reduced AP sway (p < 0.05) in LT compared to NT conditions, of which the LT effects on postural adaptation were more remarkable in older than young adults (p < 0.05). In addition, the older group significantly improved search accuracy (p < 0.05) from the LT to the NT condition, and these effects were equivalent between groups.

SIGNIFICANCE

After controlling for postural configurations, the results demonstrate that light fingertip touch reduces body sway and concurrently enhances visual search performance in older adults. These findings confirmed the effects of LT on postural adaptation as well as supported functional integration in older adults.

Muscle co-contraction in elderly people change due to postural stability during single-leg standing

BACKGROUND

Muscle co-contraction is the simultaneous contraction of agonist and antagonist muscles crossing a joint, and it increases with age. This study primarily aimed to clarify the difference in the effect of a light fingertip contact to stationary surface on postural sway and muscle co-contraction during single-leg standing (SLS) between young and elderly groups; the secondary aim was to reveal the quantitative difference in the muscle co-contraction of the ankle joint among the three different support structure conditions in the elderly group.

METHODS

This study included eight young adults (age 23.4 ± 2.6 years) and nine community dwelling older adults (age 74.7 ± 3.4 years). The task was SLS under the following conditions: (1) no supporting structure, FR; (2) light index fingertip contact to a stationary supporting structure (to touch in force < 1 N), LT; and (3) dependence on a supporting structure for stabilization as desired, DO. Center of pressure (COP) variables [root-mean-square distance (RDIST), total excursion (TOTEX), mean velocity (MVELO), and standard deviation area (AREA-SD)] and the co-contraction index (CI) between the tibialis anterior and soleus were measured using surface electromyography.

RESULTS

With regard to the effect of the light fingertip contact to stationary surface, in the young group, TOTEX, MVELO, AREA-SD, and CI during SLS were smaller under the LT condition than under the FR condition. However, in the elderly group, only AREA-SD and CI were smaller under the LT condition than under the FR condition. No significant difference was observed in COP variables and CI under the DO condition between the young and elderly groups.

CONCLUSION

Both young and elderly groups could decrease muscle co-contraction using the light fingertip contact. On the other hand, in the elderly group, COP variables showed a limited effect from the light fingertip contact; only the "sway" domain measure (AREA-SD). Both young and elderly groups showed the smallest CI under the DO condition. Therefore, the elderly group could decrease muscle co-contraction of the ankle joint depending on postural stability.

Human bipedal instability in tree canopy environments is reduced by "light touch" fingertip support

Whether tree canopy habitats played a sustained role in the ecology of ancestral bipedal hominins is unresolved. Some argue that arboreal bipedalism was prohibitively risky for hominins whose increasingly modern anatomy prevented them from gripping branches with their feet. Balancing on two legs is indeed challenging for humans under optimal conditions let alone in forest canopy, which is physically and visually highly dynamic. Here we quantify the impact of forest canopy characteristics on postural stability in humans. Viewing a movie of swaying branches while standing on a branch-like bouncy springboard destabilised the participants as much as wearing a blindfold. However “light touch”, a sensorimotor strategy based on light fingertip support, significantly enhanced their balance and lowered their thigh muscle activity by up to 30%. This demonstrates how a light touch strategy could have been central to our ancestor’s ability to avoid falls and reduce the mechanical and metabolic cost of arboreal feeding and movement. Our results may also indicate that some adaptations in the hand that facilitated continued access to forest canopy may have complemented, rather than opposed, adaptations that facilitated precise manipulation and tool use.

Suprapostural effects of light digital touch on the modulation of postural sway can be modified by fingertip sensitivity

We investigated whether the suprapostural effects of light digital touch on the modulation of postural sway can be modifiedby fingertip sensitivity. To achieve this, we recruitedthree types of athleteswith various fingertip sensitivity levels, including swimmers (high sensitivity), basketball players (intermediate sensitivity), and rowers (low sensitivity). We recorded the center of pressure (COP) excursions in 21 swimmers (20.57±0.42years), 22 basketball players (20.79±0.75years), and 22 rowers (20.32±0.49years) during light-touch (LT) and precision light-touch (PLT) conditions. In the LT conditions, participants touched a force plate while standing with their feet shoulder-width apart. In the PLT condition, participants were instructed to precisely touch a fixed point on the plate. Compared to the LT condition, the execution of the PLT condition significantly reduced the magnitude of COP excursion in the AP axis for all groups. This effect was most pronounced in swimmers, followed by basketball players, then rowers. These findings suggest that the suprapostural effects of precision light-touch on postural control can be modified as a function of fingertip sensitivity.

Effects of visual feedback with a mirror on balance ability in patients with stroke

[Purpose] This study aimed to examine the effects of a visual feedback obtained from a mirror on balance ability during quiet standing in patients with stroke. [Subjects] Fifteen patients with stroke (9 males, 6 females) enrolled in the study. [Methods] Experimental trials (duration, 20s) included three visual conditions (eyes closed, eyes open, and mirror feedback) and two support surface conditions (stable, and unstable). Center of pressure (COP) displacements in the mediolateral and anteroposterior directions were recorded using a force platform. [Results] No effect of condition was observed along all directions on the stable surface. An effect of condition was observed on the unstable surface, with a smaller mediolateral COP distance in the mirror feedback as compared to the other two conditions. Similar results were observed for the COP speed. [Conclusion] Visual feedback from a mirror is beneficial for improving balance ability during quiet standing on an unstable surface in patients with stroke.

Bioceramic fabrics improve quiet standing posture and handstand stability in expert gymnasts

Bioceramic fabrics have been claimed to improve blood circulation, thermoregulation and muscle relaxation, thereby also improving muscular activity. Here we tested whether bioceramic fabrics have an effect on postural control and contribute to improve postural stability. In Experiment 1, we tested whether bioceramic fabrics contribute to reduce body-sway when maintaining standard standing posture. In Experiment 2, we measured the effect of bioceramic fabrics on body-sway when maintaining a more instable posture, namely a handstand hold. For both experiments, postural oscillations were measured using a force platform with four strain gauges that recorded the displacements of the center of pressure (CoP) in the horizontal plane. In half of the trials, the participants wore a full-body second skin suit containing a bioceramic layer. In the other half of the trials, they wore a 'placebo' second skin suit that had the same cut, appearance and elasticity as the bioceramic suit but did not contain the bioceramic layer. In both experiments, the surface of displacement of the CoP was significantly smaller when participants were wearing the bioceramic suit than when they were wearing the placebo suit. The results suggest that bioceramic fabrics do have an effect on postural control and improve postural stability.

Postural stabilization effects of light touch do not come from axis-specific cues of postural sway: a pilot study

A light finger touch can stabilize posture despite it not providing enough force to create mechanical support. The underlying mechanism may be due to the finger touch providing information in the axis with the greatest instability. The most appropriate way to test this hypothesis is a dual-axis paradigm, i.e., to remove sway-related information from touch in either anterior-posterior (AP) or medio-lateral (ML) axis and then measure postural sway in both axes when the standing posture is equally unstable in AP and ML axes. In this study, 16 participants stood in a feet-together stance, and center of pressure in both axes was measured. Apart from No touch and Stable conditions, the touch surface was manipulated to move in close synchrony with postural sway so as to remove information regarding postural sway in one axis (AP Sway-referenced) or two axes (AP-ML Sway-referenced). The results showed that AP Sway-referenced condition stabilized posture in both axes, whereas AP-ML Sway-referenced condition did not stabilize posture in either axis. The pilot results indicated that touch effects do not come from axis-specific sensory cues of postural sway.

Disruption of contralateral inferior parietal cortex by 1 Hz repetitive TMS modulates body sway following unpredictable removal of sway-related fingertip feedback

Contact with an earth-fixed reference augments sway-related feedback and leads to sway reduction during upright standing. We investigated the effect of repetitive transcranial magnetic stimulation (rTMS) over the left hemisphere inferior parietal gyrus (IPG) as well as middle frontal gyrus (MFG) on the progression of sway following right-hand finger tip contact onset and removal. In two experimental sessions, 12 adults received 20 min of 1 Hz rTMS stimulation at 110% passive motor threshold over the left MFG and left IPG, respectively. Before and after each stimulation interval, participants' body sway was assessed in terms of antero-posterior Center-of-Pressure (CoP) velocity. Passive touch onset and removal were timed at random intervals by controlling the vertical position of a contact plate. Progression of sway was evaluated across 6s before to 6s after each contact event. Following both contact onset and removal, a temporary increase in sway above baseline without contact was observed. After removal overshoot was especially prominent. While steady-state sway was not altered by stimulation, rTMS over the left IPG reduced overshoot compared to pre-stimulation; thus, improving sway progression on haptic deprivation. We discuss our finding in the light of altered transient postural disorientation due to intermodal sensory conflict, illusion of backwards falling and tactile attention capture.

Time-interval for integration of stabilizing haptic and visual information in subjects balancing under static and dynamic conditions

Maintaining equilibrium is basically a sensorimotor integration task. The central nervous system (CNS) continually and selectively weights and rapidly integrates sensory inputs from multiple sources, and coordinates multiple outputs. The weighting process is based on the availability and accuracy of afferent signals at a given instant, on the time-period required to process each input, and possibly on the plasticity of the relevant pathways. The likelihood that sensory inflow changes while balancing under static or dynamic conditions is high, because subjects can pass from a dark to a well-lit environment or from a tactile-guided stabilization to loss of haptic inflow. This review article presents recent data on the temporal events accompanying sensory transition, on which basic information is fragmentary. The processing time from sensory shift to reaching a new steady state includes the time to (a) subtract or integrate sensory inputs; (b) move from allocentric to egocentric reference or vice versa; and (c) adjust the calibration of motor activity in time and amplitude to the new sensory set. We present examples of processes of integration of posture-stabilizing information, and of the respective sensorimotor time-intervals while allowing or occluding vision or adding or subtracting tactile information. These intervals are short, in the order of 1–2 s for different postural conditions, modalities and deliberate or passive shift. They are just longer for haptic than visual shift, just shorter on withdrawal than on addition of stabilizing input, and on deliberate than unexpected mode. The delays are the shortest (for haptic shift) in blind subjects. Since automatic balance stabilization may be vulnerable to sensory-integration delays and to interference from concurrent cognitive tasks in patients with sensorimotor problems, insight into the processing time for balance control represents a critical step in the design of new balance- and locomotion training devices.

Effects and after-effects of voluntary intermittent light finger touch on body sway

Effects of light touch on body sway have usually been investigated with some form of constant contact. Only two studies investigated transient sway dynamics following the addition or withdrawal of light touch. This study adopted a paradigm of intermittent touch and assessed body sway during as well as following short periods of touch of varying durations to investigate whether effects and after-effects of touch differ as a function of touch duration. In a modified heel-to-toe posture, 15 blindfolded participants alternated their index finger position between no-touching and touching on a strain gauge in response to low- and high-pitched auditory cues. Five trials of 46 s duration were segmented into 11 sections: a 6-s no-touching period was followed by five pseudo-randomly ordered touching periods of 0.5-, 1-, 1.5-, 2-, and 5-s duration, each of which was followed by another 6-s no-touching interval. Consistent with previous research, compared to no-touching intervals sway was reduced during touch periods with touch durations greater than 2 s. Progressive reductions in sway were evident after touch onset. After touch withdrawal in the 2-s touch condition, postural sway increased and returned to baseline level nearly immediately. Interestingly, in the 5-s touch condition, reductions in sway persisted even after touch withdrawal in the medio-lateral and antero-posterior plane for around 2.5 s and 5.5 s, respectively. Our intermittent touch paradigm resulted in duration-dependent touch effects and after-effects; the latter is a novel finding and may result from a more persistent postural set involved in proactive sway control.

Violation of the craniocentricity principle for vestibularly evoked balance responses under conditions of anisotropic stability

The balance response direction to electrically evoked vestibular perturbation is closely tied to head orientation. Such craniocentric response organization is expected of a simple error correction process. Here we ask whether this is maintained when the body is made more stable, but with the stability being greater in one direction than another. Since it is known that vestibularly evoked balance responses become smaller as body stability increases, the following two outcomes are possible: (1) response magnitude is attenuated, but with craniocentricity maintained; and (2) anisotropy of stability is considered such that components of the response are differentially attenuated, which would violate a craniocentric organizing principle. We tested these alternatives by measuring the direction of balance responses to electrical vestibular stimulation across a range of head orientations and stance widths in healthy humans. With feet together, the response was highly craniocentric. However, when stance width was increased so that the body was more stable in the frontal plane, response direction became biased toward the sagittal direction. This resulted in a nonlinear relationship between head orientation and response direction. While stance width changes the mechanical state of the body, the effect was also present when lateral light touch was used to produce anisotropy in stability, demonstrating that a significantly altered mechanical state was not crucial. We conclude that the balance system does not simply act according to the direction of vestibular input. Instead, it appears to assign greater relevance to components of vestibular input acting in the plane of lesser body stability than the plane of greater body stability, and acts accordingly.

Interactions of touch feedback with muscle vibration and galvanic vestibular stimulation in the control of trunk posture

This study investigated the effect of touch on trunk sway in a seated position. Two touch conditions were included: touching an object with the index finger of the right hand (hand-touch) and maintaining contact with an object at the level of the spine of T10 on the mid back (back-touch). In both touch conditions, the exerted force stayed below 2N. Furthermore, the interaction of touch with paraspinal muscle vibration and galvanic vestibular stimulation (GVS) was studied. Thirteen healthy subjects with no history of low-back pain participated in this study. Subjects sat on a stool and trunk sway was measured with a motion capture system tracking a cluster marker on the trunk. Subjects performed a total of 12 trials of 60-s duration in a randomized order, combining the experimental conditions of no-touch, hand-touch or back-touch with no sensory perturbation, paraspinal muscle vibration or GVS. The results showed that touch through hand or back decreased trunk sway and decreased the effects of muscle vibration and GVS. GVS led to a large increase in sway whereas the effect of muscle vibration was only observed as an increase of drift and not of sway. In the current experimental set-up, the stabilizing effect of touch was strong enough to mask any effects of perturbations of vestibular and paraspinal muscle spindle afference. In conclusion, tactile information, whenever available, seems to play a dominant role in seated postural sway and therefore has important implications for studying trunk control.

Rapid processing of haptic cues for postural control in blind subjects

OBJECTIVES

Vision and touch rapidly lead to postural stabilization in sighted subjects. Is touch-induced stabilization more rapid in blind than in sighted subjects, owing to cross-modal reorganization of function in the blind?

METHODS

We estimated the time-period elapsing from onset of availability of haptic support to onset of lateral stabilization in a group of early- and late-onset blinds. Eleven blind (age 39.4 years±11.7SD) and eleven sighted subjects (age 30.0 years±10.0SD), standing eyes closed with feet in tandem position, touched a pad with their index finger and withdrew the finger from the pad in sequence. EMG of postural muscles and displacement of centre of foot pressure were recorded. The task was repeated fifty times, to allow statistical evaluation of the latency of EMG and sway changes following the haptic shift.

RESULTS

Steady-state sway (with or without contact with pad, no haptic shift) did not differ between blind and sighted. On adding the haptic stimulus, EMG and sway diminished in both groups, but at an earlier latency (by about 0.5 s) in the blinds (p <0.01). Latencies were still shorter in the early-than late-blinds. When the haptic stimulus was withdrawn, both groups increased EMG and sway at equally short delays.

CONCLUSIONS

Blinds are rapid in implementing adaptive postural modifications when granted an external haptic reference. Fast processing of the stabilizing haptic spatial-orientation cues may be favoured by cortical plasticity in blinds.

SIGNIFICANCE

These findings add new information to the field of sensory-guided dynamic control of equilibrium in man.

Merging machines with microsurgery: clinical experience with neuroArm

Object

It has been over a decade since the introduction of the da Vinci Surgical System into surgery. Since then, technology has been advancing at an exponential rate, and newer surgical robots are becoming increasingly sophisticated, which could greatly impact the performance of surgery. NeuroArm is one such robotic system.

Methods

Clinical integration of neuroArm, an MR-compatible image-guided robot, into surgical procedure has been developed over a prospective series of 35 cases with varying pathology.

Results

Only 1 adverse event was encountered in the first 35 neuroArm cases, with no patient injury. The adverse event was uncontrolled motion of the left neuroArm manipulator, which was corrected through a rigorous safety review procedure. Surgeons used a graded approach to introducing neuroArm into surgery, with routine dissection of the tumor-brain interface occurring over the last 15 cases. The use of neuroArm for routine dissection shows that robotic technology can be successfully integrated into microsurgery. Karnofsky performance status scores were significantly improved postoperatively and at 12-week follow-up.

Conclusions

Surgical robots have the potential to improve surgical precision and accuracy through motion scaling and tremor filters, although human surgeons currently possess superior speed and dexterity. Additionally, neuroArm's workstation has positive implications for technology management and surgical education. NeuroArm is a step toward a future in which a variety of machines are merged with medicine.

Interpreting the need for initial support to perform tandem stance tests of balance

BACKGROUND

Geriatric rehabilitation reimbursement increasingly requires documented deficits on standardized measures. Tandem stance performance can characterize balance, but protocols are not standardized. Objective The purpose of this study was to explore the impact of: (1) initial support to stabilize in position and (2) maximum hold time on tandem stance tests of balance in older adults. Design A cross-sectional secondary analysis of observational cohort data was conducted.

METHODS

One hundred seventeen community-dwelling older adults (71% female, 12% black) were assigned to 1 of 3 groups based on the need for initial support to perform tandem stance: (1) unable even with support, (2) able only with support, and (3) able without support. The able without support group was further stratified on hold time in seconds: (1) <10 (low), (2) 10 to 29, (medium), and (3) 30 (high). Groups were compared on primary outcomes (gait speed, Timed "Up & Go" Test performance, and balance confidence) using analysis of variance.

RESULTS

Twelve participants were unable to perform tandem stance, 14 performed tandem stance only with support, and 91 performed tandem stance without support. Compared with the able without support group, the able with support group had statistically or clinically worse performance and balance confidence. No significant differences were found between the able with support group and the unable even with support group on these same measures. Extending the hold time to 30 seconds in a protocol without initial support eliminated ceiling effects for 16% of the study sample. Limitations Small comparison groups, use of a secondary analysis, and lack of generalizability of results were limitations of the study.

CONCLUSIONS

Requiring initial support to stabilize in tandem stance appears to reflect meaningful deficits in balance-related mobility measures, so failing to consider support may inflate balance estimates and confound hold time comparisons. Additionally, 10-second maximum hold times limit discrimination of balance in adults with a higher level of function. For community-dwelling older adults, we recommend timing for at least 30 seconds and documenting initial support for consideration when interpreting performance.

Effect of haptic supplementation provided by a fixed or mobile stick on postural stabilization in elderly people

BACKGROUND

Haptic supplementation by a light touch with the index finger on a stable surface has been widely shown to increase postural stability. With a view to a potential application in the domain of mobility aids, it should, however, be demonstrated that haptic supplementation is effective even if provided by a mobile support.

OBJECTIVE

The present experiment aimed at determining whether haptic supplementation was effective in elderly people when provided by a light grip on an unstable stick support.

METHODS

Ten young and 11 older adults were tested in an upright position in 6 experimental conditions, in which the mobility of the stick support and its resistance to body sway were manipulated. Classical center-of-pressure (COP) variables (i.e. root mean square variability, range and area) were computed together with power spectral analysis and stabilogram diffusion analysis (SDA) variables of COP.

RESULTS

The results suggest that the stabilizing effect of haptic supplementation is independent of age and the nature of the support (fixed or mobile) when transient sway-related contact forces at the fingertip and proprioceptive cues are of sufficient magnitude. The results also indicate that haptic supplementation attenuates the age-related increase in energy consumption during the postural task even in the mobile support condition on a low-resistance surface. The results of SDA suggest that the availability of sway-related haptic cues reduces reliance on increased muscle activity around the ankle over short time intervals of postural control. After some time, haptic supplementation eventually leads to well-coordinated postural corrections.

CONCLUSIONS

In summary, haptic supplementation improves postural control mechanisms independent of age due to enhanced perception of self-motion through sensory interaction with the environment.

Unperceivable noise to active light touch effects on fast postural sway

Human postural sway during quiet standing is reduced when a fingertip lightly touches a stable surface. The tactile feedback information from the fingertip has been considered responsible for this effect of light touch. Studies have shown that a noise-like minute stimulation to the sensory system can improve the system's weak signal detection. In the present study, we investigated whether a noise-like unperceivable vibration on the fingertip enhances its tactile sensation and facilitates the effect of light touch during quiet standing. Thirteen volunteers maintained quiet standing while lightly touching a touch surface with the index fingertip. Based on each subject's vibrotactile threshold (VT), a noise-like vibration was applied to the touch surface at amplitudes under (0.5VT) or at VT (1.0VT), in addition to the normal light touch condition (no vibration, 0VT). The results showed that the mean velocities of the foot center of pressure (CoP) in both the anteroposterior (AP) and mediolateral (ML) directions were significantly reduced at 0.5VT compared to 0VT and 1.0VT (P<0.05), while there was no significant difference between 1.0VT and 0VT (P>0.05). Frequency analysis of CoP revealed that the power of high-frequency fluctuation (1-10Hz) was significantly reduced at 0.5VT (P<0.05), whereas no significant change was observed in that of low-frequency sway (below 1Hz) (P>0.05). These results indicate that an unperceivable noise-like vibration can facilitate the effect of light touch on postural stability, by further reducing fast postural sway.

Contrasting effects of finger and shoulder interpersonal light touch on standing balance

Sway is reduced by light nonsupporting touch between parts of the body and a fixed surface. This effect is assumed to reflect augmentation of sensory cues for sway by point-of-contact reaction forces. It has been shown that movement of the contact surface can increase sway relative to an earth-fixed contact. Light touch contact with another person, for example, holding hands, affords a moving contact due to partner sway. We asked whether interpersonal light touch (IPLT) would increase sway relative to standing alone. We expected effects on sway to vary as a function of the site of contact and the postural stability of each partner. Eight pairs of participants, standing in either normal bipedal or tandem Romberg stance with eyes closed and using IPLT (finger to finger or shoulder to shoulder) or no contact, provided 4 trials of 30-s duration in each of 12 posture-touch combinations. Sway (SD of the rate of change of upper trunk position at C7) was reliably less with IPLT compared with no contact, with two exceptions: in normal stance, shoulder contact with a partner in tandem stance, and in tandem Romberg stance, finger contact with a partner in the same stance, increased sway. Otherwise, the reduction in sway was greater with shoulder than with finger contact. Measures of interpersonal synchronization based on cross-correlations and coherence analysis between the partners' C7 movements suggest different control factors operate to reduce sway in IPLT with the hand or shoulder contact.

Children with developmental coordination disorder benefit from using vision in combination with touch information for quiet standing

In two experiments, the ability to use multisensory information (haptic information, provided by lightly touching a stationary surface, and vision) for quiet standing was examined in typically developing (TD) children, adults, and in seven-year-old children with Developmental Coordination Disorder (DCD). Four sensory conditions (no touch/no vision, with touch/no vision, no touch/with vision, and with touch/with vision) were employed. In experiment 1, we tested four-, six- and eight-year-old TD children and adults to provide a developmental landscape for performance on this task. In experiment 2, we tested a group of seven-year-old children with DCD and their age-matched TD peers. For all groups, touch robustly attenuated standing sway suggesting that children as young as four years old use touch information similarly to adults. Touch was less effective in children with DCD compared to their TD peers, especially in attenuating their sway velocity. Children with DCD, unlike their TD peers, also benefited from using vision to reduce sway. The present results suggest that children with DCD benefit from using vision in combination with touch information for standing control possibly due to their less well developed internal models of body orientation and self-motion. Internal model deficits, combined with other known deficits such as postural muscles activation timing deficits, may exacerbate the balance impairment in children with DCD.

Haptic touch reduces sway by increasing axial tone

It is unclear how haptic touch with a stable surface reduces postural sway. We hypothesized that haptic input enhances postural stability due to alterations in axial postural tone. We measured the influence of heavy and light touch (LT) of the hands on a stable bar on axial postural tone and postural sway during stance in 14 healthy adults. A unique "Twister" device measured hip torque by fixing the upper body in space while oscillating the surface in yaw ±10 at 1 deg/s. Subjects were tested while: (1) standing quietly with their arms at their sides, (2) lightly touching a rigid bar in front of them and (3) firmly gripping the bar. Horizontal and vertical sway was not restricted by the device's yaw fixation, therefore, the subjects remained in a state of active postural control during the three touch conditions. Haptic touch significantly increased hip postural tone by 44% during light touch, from 2.5±0.9 to 3.6±1.0 Nm (P=0.005), and by 40% during firm grip to 3.5±0.8 Nm (P=0.005). Increases in hip postural tone were associated with a reduction in postural sway (r=-0.55, P=0.001). This is the first study showing that axial postural tone can be modified by remote somatosensory input and provides a potential explanation for how light touch improves postural stability. Changes in subjects' perception from trunk to surface rotation when changing from no touch (NT) to haptic touch, suggests that the CNS changes from using a global, to a local, trunk reference frame for control of posture during touch. The increase of hip postural tone during touching and gripping can be explained as a suppression of hip muscle shortening reactions that normally assist axial rotation.

Stance stability with unilateral and bilateral light touch of an external stationary object

Unilateral light fingertip touch of a stationary object has a significant stabilizing effect on postural sway during stance. The purpose of this study was to find out if this effect is enhanced by bilateral light touch of parallel stationary objects. The postural sway of 54 healthy subjects was tested in four stance conditions: no touch; unilateral left light touch of the left handle of a walker; unilateral right light touch of the right handle of the same walker; and bilateral light touch of the two handles. During testing, subjects stood blindfolded on two foam pads placed on the left and right force plates of the Tetrax balance system. Testing in each condition lasted 45 s and was executed twice in a random order. As expected, postural sway was significantly reduced by unilateral left or right light fingertip touch. It was significantly further decreased by bilateral light touch. In addition, light touch conditions were associated with a reduction in pressure fluctuations between the heel and forefoot of the same foot as well as those of the contralateral foot, with a concomitant increase in weight shift fluctuations between the two feet. The decrease in postural sway with bilateral light touch suggests cortical modulation of the bilateral touch inputs, with enhancement of the stabilizing response.

Modulation of soleus H reflex due to stance pattern and haptic stabilization of posture

The quiet stance is a complicated motor act requiring sophisticated sensorimotor integration to balance an artificial inverted pendulum with the ankle musculature. The objective of this study was to characterize the effects of stance pattern (bilateral stance vs. unilateral stance) and directional influence of light finger touch (medial-lateral vs. anterior-posterior) in unilateral stance upon responsiveness of the soleus H reflex. Sixteen healthy volunteers (mean age, 24.25+/-1.77 years) participated in four postural tasks with the eyes open, including the bilateral stance (BS), the unilateral stance without finger touch (USNT), and with finger touch in the medial-lateral direction (USML) and anterior-posterior direction (USAP). Meanwhile, the soleus H reflex, the pre-stimulus background activity of ankle antagonist pairs, and center of pressure (CoP) sway were measured. In reference to the BS, the USNT resulted in a significant stance effect on suppression of the soleus H reflex (H/M(max)) associated with enhancement of CoP sway. Among the conditions of unilateral stance, there was a marked directional effect of finger touch on modulation of the H/M(max). A greater disinhibition of the H/M(max) in consequence to light touch in the ML direction than in the AP direction was noted (H/M(max): USML>USAP>USNT). This directional modulation of the soleus H reflex concurred with haptic stabilization of posture in unilateral stance, showing a more pronounced reduction in CoP sway in the USML condition than in the USAP condition. However, alteration in postural sway and modulation of the soleus H reflex were not mutually correlated when stance pattern or touch vector varied. In conclusion, gating of the soleus H reflex indicated adaptation of an ankle strategy to stance pattern and haptic stabilization of posture. Relative to bilateral stance, postural maintenance in unilateral stance relied less on reflexive correction of the soleus. When finger touch was provided in line with prevailing postural threat in the lateral direction, postural stability in unilateral stance was better secured than finger touch in anterior-posterior direction, resulting in more pronounced disinhibition of the monosynaptic reflex pathway.

Prediction of object contact during grasping

The maximum grip aperture (MGA) during prehension is linearly related to the size of objects to be grasped and is adapted to the haptically sensed object size when there is a discrepancy between visual and haptic information. We have investigated what information is used to drive this adaptation process and how the onset of fingertip forces on the object is triggered. Subjects performed a reach-to-grasp task, where the object seen and the object grasped physically never were the same. We measured the movements of the index finger and the thumb and the contact forces between each fingertip and the object. The subjects' adaptation of the MGA was unrelated both to different fingertip velocities at the moment of object contact, or the fingertip forces. Instead, the 'timing' of contact between the fingers and the object was most consistently influenced by introducing a size discrepancy. Specifically, if the object was larger than expected, the moment of contact occurred earlier, and if the object was decreased in size, then the contact occurred later. During adaptation, these timing differences were markedly reduced. Also, the motor command for applying forces on the object seemed to be released in anticipation of the predicted moment of contact. We therefore conclude that the CNS dynamically predicts when contact between the fingertips and objects occur and that aperture adaptation is primarily driven by timing prediction errors.

Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.

Influences of arm proprioception and degrees of freedom on postural control with light touch feedback

Lightly touching a stable surface with one fingertip strongly stabilizes standing posture. The three main features of this phenomenon are fingertip contact forces maintained at levels too low to provide mechanical support, attenuation of postural sway relative to conditions without fingertip touch, and center of pressure (CP) lags changes in fingertip shear forces by approximately 250 ms. In the experiments presented here, we tested whether accurate arm proprioception and also whether the precision fingertip contact afforded by the arm's many degrees of freedom are necessary for postural stabilization by finger contact. In our first experiment, we perturbed arm proprioception and control with biceps brachii vibration (120-Hz, 2-mm amplitude). This degraded postural control, resulting in greater postural sway amplitudes. In a second study, we immobilized the touching arm with a splint. This prevented precision fingertip contact but had no effect on postural sway amplitude. In both experiments, the correlation and latency of fingertip contact forces to postural sway were unaffected. We conclude that postural control is executed based on information about arm orientation as well as tactile feedback from light touch, although precision fingertip contact is not essential. The consistent correlation and timing of CP movement and fingertip forces across conditions in which postural sway amplitude and fingertip contact are differentially disrupted suggests posture and the fingertip are controlled in parallel with feedback from the fingertip in this task.

Effects of Maintaining Touch Contact on Predictive and Reactive Balance

Light touch contact between the body and an environmental referent reduces fluctuations of center of pressure (CoP) in quiet standing although the contact forces are insufficient to provide significant forces to stabilize standing balance. Maintenance of upright standing posture (with light touch contact) may include both predictive and reactive components. Recently Dickstein et al. (2003) demonstrated that reaction to temporally unpredictable displacement of the support surface was affected by light touch raising the question whether light touch effects also occur with predictable disturbance to balance. We examined the effects of shoulder light touch on SD of CoP rate (dCoP) during balance perturbations associated with forward sway induced by pulling on (voluntary), or being pulled by (reactive), a hand-held horizontal load. Prior to perturbation, SD dCoP was lower with light touch, corresponding to previous findings. Immediately after perturbation, SD dCoPAP was greater with light touch in the case of voluntary pull, whereas no difference was found for reflex pull. However, in the following time course, light touch contact again resulted in a significantly lower SD dCoP and faster stabilization of SD dCoP. We conclude that shoulder light touch contact affects immediate postural responses to voluntary pull but also stabilization after voluntary and reflex perturbation. We suggest that in voluntary perturbation CoP fluctuations are differentially modulated in anterioposterior and mediolateral directions to maintain light touch, which not only provides augmented sensory feedback about body self-motion, but may act as a “constraint” to the postural control system when preparing postural adjustments.

The postural disorientation induced by neck muscle vibration subsides on lightly touching a stationary surface or aiming at it

The aim of this study was to investigate whether the standing body spatial disorientation, induced by neck muscle vibration, and the related post-effects can be suppressed by light finger touch (LFT) of a stationary surface. Continuous (60 s) vibration of dorsal neck or sternocleidomastoid muscle was administered with eyes closed. The center of foot pressure (CFP) displacement, measured by a stabilometric platform, indicated the degree of vibration-induced body tilt. We also investigated whether sensory information from LFT itself was necessary or anticipation of a more secure posture was enough for reducing vibration effects. To this aim, we administered a vibration pulse (5 s) to dorsal neck or sternocleidomastoid muscle and during reaching to the stationary surface. CFP was recorded during both vibration and post-vibration condition and during the aiming task. Neck vibration induced significant CFP displacement in the direction opposite to vibration site. Post-vibration, CFP slowly returned to control values with ample oscillations. LFT during vibration reduced body tilt. LFT was more effective when fingertip contact was in the plane of the greatest tilt. LFT applied during either vibration or post-vibration period reduced post-vibration effects. Reaching toward the stationary surface was enough for reducing vibration-induced body tilt to values close to those observed during actual LFT. The novel conclusions of this study are: 1) LFT is able to relieve the effects of vibration-induced abnormal proprioceptive input from the neck, a segment central to postural control and orientation; 2) LFT during vibration also attenuates vibration post-effects, further suggesting that its action is not merely mechanical; 3) the intention to stabilize the body generates a new postural 'set' sufficient for diminishing body tilt.

Acquiring and adapting a novel audiomotor map in human grasping

For sensorimotor transformations to be executed accurately, there must be mechanisms that can both establish and modify mappings between sensory and motor coordinates. Such mechanisms were investigated in normal subjects using a reach-to-grasp task. First, we replaced the normal input of visual information about object size with auditory information, i.e., we attempted to establish an 'audiomotor map'. The size of the object was log linearly related to the frequency of the sound, and we measured the maximum grip aperture (MGA) during the reaching phase to determine if the subjects had learned the relationship. Second, we changed the frequency-object size relationship to study adaptation in the newly acquired map. Our results demonstrate that learning of an audiomotor map consisted of three distinct phases: during the first stage (approximately 10-15 trials) subjects simply used MGAs large enough to grasp any reasonably sized object and there were no overt signs of learning. During the second stage, there was a period of fast learning where the slope of the relationship between MGA and object size became steeper until the third stage where the slope was constant. In contrast, when sensorimotor adaptation was studied in the established audiomotor map, there was rapid learning from the start of a size perturbation. We conclude that different learning strategies are employed when sensorimotor transformations are established compared to when existing transformations are modified.

Time course of haptic stabilization of posture

Contact of the index finger with a stable surface greatly attenuates postural sway in blindfolded subjects. The time course of postural stabilization was measured after subjects made finger contact with a surface. Subjects (n=12) were tested standing in a heel-to-toe stance in 25 s duration trials. The subject stood with the index finger above but not contacting a laterally placed surface, and made finger contact when cued midway through the trial. Fingertip contact forces stabilized with a time constant of less than 0.5 s and postural stabilization occurred rapidly following fingertip contact. Sway amplitude of center of pressure of the feet decreased by half with a time constant of less than 1.6 s. The stereotypical pattern of force changes at the fingertip leading correlated changes in center of pressure by approximately 300 ms was evident within the first 0.5 s of finger contact. We conclude that the fingertip can serve as a sensory-motor probe that is stabilized nearly immediately on contact with a surface and that from the moment of contact the fingertip contributes sensory signals used to control sway.