Can a plantar pressure–based tongue-placed electrotactile biofeedback improve postural control under altered vestibular and neck proprioceptive conditions?

Improving postural stability among people with lower-limb amputations by tactile sensory substitution

Background

For people with lower-limb amputations, wearing a prosthetic limb helps restore their motor abilities for daily activities. However, the prosthesis's potential benefits are hindered by limited somatosensory feedback from the affected limb and its prosthesis. Previous studies have examined various sensory substitution systems to alleviate this problem; the prominent approach is to convert foot–ground interaction to tactile stimulations. However, positive outcomes for improving their postural stability are still rare. We hypothesized that the sensory substiution system based on surrogated tactile stimulus is capable of improving the standing stability among people with lower-limb amputations.

Methods

We designed a wearable device consisting of four pressure sensors and two vibrators and tested it among people with unilateral transtibial amputations (n = 7) and non-disabled participants (n = 8). The real-time measurements of foot pressure were fused into a single representation of foot–ground interaction force, which was encoded by varying vibration intensity of the two vibrators attached to the participants’ forearm. The vibration intensity followed a logarithmic function of the force representation, in keeping with principles of tactile psychophysics. The participants were tested with a classical postural stability task in which visual disturbances perturbed their quiet standing.

Results

With a brief familiarization of the system, the participants exhibited better postural stability against visual disturbances when switching on sensory substitution than without. The body sway was substantially reduced, as shown in head movements and excursions of the center of pressure. The improvement was present for both groups of participants and was particularly pronounced in more challenging conditions with larger visual disturbances.

Conclusions

Substituting otherwise missing foot pressure feedback with vibrotactile signals can improve postural stability for people with lower-limb amputations. The design of the mapping between the foot–ground interaction force and the tactile signals is essential for the user to utilize the surrogated tactile signals for postural control, especially for situations that their postural control is challenged.

Age-Related Changes in Standing Balance in Preschoolers Using Traditional and Nonlinear Methods

Considerable disagreement exists on the linearity of the development of standing balance in children. This study aimed to use different traditional and nonlinear methods to investigate age-related changes in standing balance in preschoolers. A sample of 118 preschoolers took part in this study. A force platform was used to record the center of pressure during standing balance over 15 s in three conditions: eyes open, eyes closed, and/or head extended backward. Detrended fluctuation analysis (DFA), recurrence quantification analysis (RQA), and traditional measures were used to evaluate standing balance. The main results are as follows: (1) Higher range and SD in the anterior-posterior (AP) direction were observed for 5-year-old than for 4-year-old children, while higher DFA coefficient (at shorter time scales) and higher determinism and laminarity in the AP direction were found for 5-year-old children compared to 3- and 4-year-old children; and (2) as sensory conditions became more challenging, all traditional measures increased and DFA coefficients (at shorter and longer time scales) decreased in the AP and mediolateral directions, while determinism and laminarity significantly declined in the AP direction. In conclusion, although increased postural sway, 5-year-old preschool children's balance performance improved, and their control strategy changed significantly compared with the younger preschoolers. Sensory perturbation (eye closure and/or head extension) changed preschoolers' balance performance and control strategy. Moreover, both traditional and nonlinear methods provided complementary information on the control of standing balance in preschoolers.

The Effects of Textured Insoles on Cortical Activity and Quiet Bipedal Standing With and Without Vision: An EEG Study

Wearing textured insoles (TIs) can reduce static postural sway, but the neurophysiological mechanisms by which these changes occur are not well understood. To address this issue, cortical activity was investigated in this study using electroencephalography (EEG) recordings from 19 scalp locations, in 15 healthy young adults (5 females; mean age = 27 ± 4.09 years) during quiet bipedal standing, under different insole conditions (textured versus smooth), with and without vision. Compared to smooth insoles (SIs), TIs significantly reduced postural sway in two measures; anterior-posterior range and standard deviation. In the EEG data, whole-head analyses showed cortical activity in the upper alpha power band was significantly reduced for textured compared to SIs. Exploratory analyses revealed this effect was significant both with and without vision, and was more pronounced over the parietal, compared to central regions, and over central compared to frontal regions. This trend was observed in low alpha and theta bands, but the effect of insole type was not significant. Textured insoles thus appear to affect not only balance outcomes but also cortical activity. The cortical activity adaptation may represent greater information becoming readily available at the cortical level, enhancing the representation of the body in space.

The effects of textured insoles on quiet standing balance in four stance types with and without vision

Background

Wearing a textured shoe insole can decrease postural sway during static balance. Previous studies assessed bipedal and/or unipedal standing. In contrast, we aimed to investigate if textured insoles modulated postural sway during four stance types (bipedal, standard Romberg, tandem Romberg, and unipedal), with and without vision.

Methods

The repeated measures design involved 28 healthy young adults (13 females; mean age = 26.86 ± 6.6 yrs) performing quiet standing in the four stance types on a force platform, under two different insole conditions (textured insole; TI vs. smooth insole; SI), with eyes open and eyes closed. Postural sway was assessed via the range and standard deviation of the COP excursions in the anterior-posterior and medial-lateral sway, and overall mean velocity.

Results

The main effect of insole type was statistically significant at the alpha p = 0.05 level (p = 0.045). Compared to smooth insoles, textured insoles reduced the standard deviation of anterior-posterior excursions (APSD). While simple main effect analyses revealed this was most pronounced during eyes closed bipedal standing, insole type did not provide a statistically significant interaction with either stance or vision in this measure, or any other. Postural sway showed statistically significant increases across both stance type (bipedal < standard Romberg < tandem Romberg < unipedal), and vision (eyes closed < eyes open), in almost all measures. Stance and vision did have a statistically significant interaction in each measure, reflecting greater postural disturbances with eyes closed when stance stability decreased.

Conclusions

Overall, these results support textured insole use in healthy young adults to reduce postural sway measures. This is because APSD is an index of spatial variability, where a decrease is associated with improved balance and possibly translates to reduced falls risk. Placing a novel texture in the shoe presumably modulated somatosensory inputs. It is important to understand the underlying mechanisms by which textured insoles influence postural sway. As such, utilising a healthy adult group allows us to investigate possible mechanisms of textured insoles. Future research could investigate the potential underlying mechanisms of textured insole effects at a neuromuscular and cortical level, in healthy young adults.

Effects of whole-body vibration exercise on functions required for bowling performance of male bowling player

The focus of this research is to examine the impact of 8 weeks of whole body vibration (WBV) exercise on physical factors by conducting an experiment on 17 male bowling players registered in Korea Bowling Association. Strength of truncus and lower-limb and balance were mea-sured as the physical factors and the impact of WBV exercise on each variable was examined. There was significant increase between groups in flexor-weight ratio of truncus at 60°/sec according to performance after applying WBV exercise. There was significant difference in groups before bowling event in pre and posttest as well as after bowling event in pre and posttest. There was significant difference of extensor-weight ratio in groups after bowling event in pre and posttest. There was significant increase at 60°/sec in flexor-ratio of both left and right lower-limb strength according to performance after applying WBV exercise. There was significant difference between groups before bowling event in pre and posttest as well as after bowling event in pre and posttest. Extensor-weight ratio showed significant difference between groups only before bowling event in pre and posttest. There was significant difference between groups in both left and right balance according to performance after applying WBV exercise. There was significant difference in before bowling event in pre and posttest as well as after bowling event in pre and posttest.

Critical Role of Somatosensation in Postural Control Following Spaceflight: Vestibularly Deficient Astronauts Are Not Able to Maintain Upright Stance During Compromised Somatosensation

The free-fall of orbital spaceflight effectively removes the gravitational vector used as a primary spatial orientation reference on Earth. Sustained absence of this reference drives adaptive changes in the internal perception-action models of the central nervous system (CNS), most notably in the processing of the vestibular otolith inputs. Upon landing, the return of the gravitational signal triggers a re-adaptation that restores terrestrial performance; however, during this period, the individual suffers from a functional vestibular deficiency. Here we provide evidence of a transient increase of the weighting of somatosensory inputs in postural control while the CNS resolves these vestibular deficiencies. Postural control performance was measured before and after spaceflight in 11 Shuttle astronauts and 11 matched controls and nine elderly who did not experience spaceflight. A quiet-stance paradigm was used that eliminated vision, modulated the lower extremity somatosensory cues by subtly modulating the orientation of the support surface beneath feet of subjects in all groups. Additionally, in astronauts and matched controls, we challenged the vestibular system with dynamic head tilts. Postural stability on the landing day (R+0) was substantially decreased for trials with absent visual and altered somatosensory cues, especially those also requiring dynamic head tilts ( ± 5° @ 0.33 Hz) during which 20/22 trials ended prematurely with a fall. In contrast, none of the astronauts fell during eyes-closed, dynamic head tilt trials with unaltered somatosensory cues, and only 3/22 trials resulted in falls with eyes-closed and altered somatosensory cues, but static upright head orientation. Furthermore, postural control performance of astronauts was either statistically not different or worse than that of healthy elderly subjects during the most challenging vestibular conditions on R+0. Overall, our results demonstrate a transient reweighting of sensory cues associated with microgravity-induced vestibular deficiencies, with a significant increase in reliance on somatosensory cues, which can provide an effective reference even without vision and with dynamic vestibular challenges. The translation of these results to aging population suggests that elderly individuals with visual and vestibular deficits may benefit from therapeutic interventions enhancing sensorimotor-integration to improve balance and reduce the risk of falling.

Postural Control in Children with Dyslexia: Effects of Emotional Stimuli in a Dual-Task Environment

The aim of this study was to compare the visual exploration strategies used during a postural control task across participants with and without dyslexia. We simultaneously recorded eye movements and postural control while children were viewing different types of emotional faces. Twenty-two children with dyslexia and twenty-two aged-matched children without dyslexia participated in the study. We analysed the surface area, the length and the mean velocity of the centre of pressure for balance in parallel with visual saccadic latency, the number of saccades and the time spent in regions of interest. Our results showed that postural stability in children with dyslexia was weaker and the surface area of their centre of pressure increased significantly when they viewed an unpleasant face. Moreover, children with dyslexia had different strategies to those used by children without dyslexia during visual exploration, and in particular when they viewed unpleasant emotional faces. We suggest that lower performance in emotional face processing in children with dyslexia could be due to a difference in their visual strategies, linked to their identification of unpleasant emotional faces. Copyright © 2017 John Wiley & Sons, Ltd.

Effects of Age-Related Macular Degeneration on Postural Sway

Purpose: To compare the impact of unilateral vs. bilateral age-related macular degeneration (AMD) on postural sway, and the influence of different visual conditions. The hypothesis of our study was that the impact of AMD will be different between unilateral and bilateral AMD subjects compared to age-matched healthy elderly.

Methods: Postural stability was measured with a platform (TechnoConcept®) in 10 elderly unilateral AMD subjects (mean age: 71.1 ± 4.6 years), 10 elderly bilateral AMD subjects (mean age: 70.8 ± 6.1 years), and 10 healthy age-matched control subjects (mean age: 69.8 ± 6.3 years). Four visual conditions were tested: both eyes viewing condition (BEV), dominant eye viewing (DEV), non-dominant eye viewing (NDEV), and eyes closed (EC). We analyzed the surface area, the length, the mean speed, the anteroposterior (AP), and mediolateral (ML) displacement of the center of pressure (CoP).

Results: Bilateral AMD subjects had a surface area (p < 0.05) and AP displacement of the CoP (p < 0.01) higher than healthy elderly. Unilateral AMD subjects had more AP displacement of the CoP (p < 0.05) than healthy elderly.

Conclusions: We suggest that ADM subjects could have poor postural adaptive mechanisms leading to increase their postural instability. Further studies will aim to improve knowledge on such issue and to develop reeducation techniques in these patients.

Importance of Proprioceptive Information for Postural Control in Children with Strabismus before and after Strabismus Surgery

The objective of this study is to examine the role of proprioception in postural balance in children with strabismus before and after realignment of their visual axes by eye surgery. Postural recordings were made with the TechnoConcept® force platform in 23 children. Several conditions were studied, whether the subjects had both eyes open, or either the dominant or the non-dominant eye open, without and with foam pads of 4 mm underfoot. Recordings were performed before and after strabismus surgery. The surface area, the length and the mean speed of the center of pressure (CoP) were analyzed. Before strabismus surgery, all children showed better stability with both eyes open with respect to the condition with the non-dominant eye open; furthermore postural stability improved in the presence of foam pads. After surgery, the surface area of CoP decreased significantly, especially in the non-dominant eye viewing condition. We suggest that strabismic children use mainly proprioceptive information in order to control their posture, but also visual inputs, which are important for obtaining a good postural stability. The alignment of the visual axes after surgery provides enhanced postural stability, suggesting, again the major role of visual inputs in the control of posture. Proprioceptive plasticity after strabismus surgery may allow better visual rehabilitation.

A Portable Sensory Augmentation Device for Balance Rehabilitation Using Fingertip Skin Stretch Feedback

Neurological disorders are the leading causes of poor balance. Previous studies have shown that biofeedback can compensate for weak or missing sensory information in people with sensory deficits. These biofeedback inputs can be easily recognized and converted into proper information by the central nervous system (CNS), which integrates the appropriate sensorimotor information and stabilizes the human posture. In this study, we proposed a form of cutaneous feedback which stretches the fingertip pad with a rotational contactor, so-called skin stretch. Skin stretch at a fingertip pad can be simply perceived and its small contact area makes it favored for small wearable devices. Taking advantage of skin stretch feedback, we developed a portable sensory augmentation device (SAD) for rehabilitation of balance. SAD was designed to provide postural sway information through additional skin stretch feedback. To demonstrate the feasibility of the SAD, quiet standing on a force plate was evaluated while sensory deficits were simulated. Fifteen healthy young adults were asked to stand quietly under six sensory conditions: three levels of sensory deficits (normal, visual deficit, and visual + vestibular deficits) combined with and without augmented sensation provided by SAD. The results showed that augmented sensation via skin stretch feedback helped subjects correct their posture and balance, especially as the deficit level of sensory feedback increased. These findings demonstrate the potential use of skin stretch feedback in balance rehabilitation.

Importance of visual inputs quality for postural stability in strabismic children

OBJECTIVE

The first goal of the present study is to analyze the postural control in strabismic children in four different visual conditions (eyes open, eyes closed and monocular viewing with squint eye and non-squint eye). Secondly, we wish to explore the effect of strabismus surgery in postural control.

METHOD

Postural stability was recorded with a platform (TechnoConcept) in 23 strabismic children aged from 4.4 to 14.8 years old (mean age: 8.4 years); 12 of these children were also examined at least two months after strabismus surgery. We analyzed the surface, the length, and the mean speed of the Center of Pressure displacement (CdP), and we calculated the Romberg's Quotient (that is for each postural parameter the ratio between eyes closed relative to eyes open).

RESULTS

Strabismic children are significantly more stable when they can use their visual information to control their posture. Also, postural stability was better when the non-squint eye was viewing. For the first time, we observed the important role of vision (by calculating Romberg's Quotient) in strabismic children with binocular vision in comparison of strabismic children without binocular vision. Finally, we found that eye surgery improves postural stability.

CONCLUSION

Strabismic children use visual inputs to control their posture. Moreover, binocular vision plays an important role in postural control. Strabismus surgery improves postural stability.

SIGNIFICANCE

Visual inputs from the non-squint eye and binocular vision are important to control stability.

The effect of face exploration on postural control in healthy children

The objective was to explore how face exploration affects postural control in healthy children. The novelty here is that eye movements and posture were simultaneously recorded. Three groups of children participated in the study: 12 children of 7.8±0.5 years old, 13 children of 10.4±0.5 years old and 12 children of 15.7±0.9 years old. Eye movements were recorded by video-oculography and postural stability was recorded by a platform. Children were invited to explore five emotional faces (neutral, happy, sad fear and angry). Analysis of eye movements was done on saccadic latency, percentage of exploration time spent and number of saccades for each specific region of interest (ROI): eyes, nose and mouth. Analysis of posture was made on surface area, sway length and mean velocity of the center of pressures (CoP). Results showed that visual strategies, exploration and postural control develop during childhood and adolescence. Indeed, after nine years-old, children started to look the eyes ROI firstly, then the nose ROI and finally the mouth ROI. The number of saccades decreased with the age of children. The percentage of exploration time spent in eyes ROI was longer than the others ROIs and greater for unpleasant faces (sad, fear and angry) with respect to pleasant emotional face (happy). We found that in front of sad and happy faces the surface area of the CoP was significantly larger compared to other faces (neutral and angry). These results suggest that visual strategies and postural control change during children's development and can be influenced by the emotional face.

Footwear interventions: a review of their sensorimotor and mechanical effects on balance performance and gait in older adults

Footwear interventions, including shoe insoles and foot orthoses, have the capacity to enhance balance control and gait in older people. This review assessed the evidence for the effect of footwear interventions on static and dynamic balance performance and gait in older populations and explored proposed theories for underlying sensorimotor and mechanical mechanisms. We searched the Medline, EMBASE, CINAHL (the Cumulative Index to Nursing and Allied Health Literature), and AMED databases and conducted hand searches. Of 115 relevant articles screened, 14 met the predefined inclusion criteria. Articles were grouped into one of three categories based on balance task (static balance performance during quiet standing, dynamic balance performance during walking, and dynamic balance performance during perturbed standing or functional tasks) and were scored for methodological quality using the Downs and Black Quality Index tool. Footwear interventions seem to alter underlying strategies controlling static and dynamic movement patterns through a combination of sensorimotor and mechanical mechanisms in older people, including those with chronic sensory and musculoskeletal conditions. Evidence shows a consistent trend toward footwear interventions markedly improving lateral stability measures, which are predictors of falls in the elderly. In-depth investigation of neurophysiologic responses to footwear interventions is necessary to help confirm any sensorimotor adaptations. The long-term effects of footwear interventions on balance, gait, and the prevention of falls in older people require further investigation.

Force-controlled biting alters postural control in bipedal and unipedal stance

Human posture is characterised by inherent body sway which forces the sensory and motor systems to counter the destabilising oscillations. Although the potential of biting to increase postural stability has recently been reported, the mechanisms by which the craniomandibular system (CMS) and the motor systems for human postural control are functionally coupled are not yet fully understood. The purpose of our study was, therefore, to investigate the effect of submaximum biting on postural stability and on the kinematics of the trunk and head. Twelve healthy young adults performed force-controlled biting (FB) and non-biting (NB) during bipedal narrow stance and single-leg stance. Postural stability was quantified on the basis of centre of pressure (COP) displacements, detected by use of a force platform. Trunk and head kinematics were investigated by biomechanical motion analysis, and bite forces were measured using a hydrostatic system. The results revealed that FB significantly improved postural control in terms of reduced COP displacements, providing additional evidence for the functional coupling of the CMS and human posture. Our study also showed, for the first time, that reductions in the sway of the COP were accompanied by reduced trunk and head oscillations, which might be attributable to enhanced trunk stiffness during FB. This physiological response to isometric activation of the masticatory muscles raises questions about the potential of oral motor activity as a strategy to reduce the risk of falls among the elderly or among patients with compromised postural control.

Impaired plantar sensitivity among the obese is associated with increased postural sway

Impaired foot plantar sensitivity has been hypothesized among individuals who are obese, and may contribute to their impaired balanced during quiet standing. The objective of this study was to investigate the effects of obesity on plantar sensitivity, and explore the relationship between plantar sensitivity and balance during quiet standing. Thirty-nine young adults from the university population participated in the study including 19 obese and 20 non-obese adults. Plantar sensitivity was measured as the force threshold at which an increasing force applied to the plantar surface of the foot was first perceived, and the force threshold at which a decreasing force was last perceived. Measurements were obtained while standing, and at two locations on the plantar surface of the dominant foot. Postural sway during quiet standing was then measured under three different sensory conditions. Results indicated less sensitive plantar sensitivity and increased postural sway among the obese, and statistically significant correlations between plantar sensitivity and postural sway that were characterized as weak to moderate in strength. As such, impaired plantar sensitivity among individuals who are obese may be a mechanism by which obesity degrades standing balance among these individuals.

Postural control in strabismic children: importance of proprioceptive information

OBJECTIVE

To examine the effect of proprioceptive information during postural control in strabismic children.

METHODS

Postural stability was recorded with a platform (Techno Concept®) in 12 strabismic children aged from 4.9 to 10 years and data were compared to that of 12 control age-matched children. Two postural positions were performed: Romberg and Tandem. Two postural conditions: without and with foam pad. We analyzed the surface area, the length, the mean speed of the center of pressure (CoP) and the effect of proprioceptive information.

RESULTS

Strabismic children are more instable than control age-matched children. The surface, the length and the mean speed of CoP are significantly higher in strabismic children than in control age-matched children. Both groups are more instable in Tandem position than in Romberg position. Finally, strabismic children use more proprioceptive information than control age-matched children.

CONCLUSION

For both Romberg and Tandem position, strabismic children are more instable than control age-matched children. Strabismic children use proprioceptive information more than control age-matched children to control their posture.

SIGNIFICANCE

Proprioceptive inputs are important for control posture particularly for strabismic population.

Saccades improve postural control: a developmental study in normal children

INTRODUCTION

Dual-task performance is known to affect postural stability in children. This study focused on the effect of oculomotor tasks like saccadic eye movements on postural stability, studied in a large population of children by recording simultaneously their eye movements and posture.

MATERIALS AND METHODS

Ninety-five healthy children from 5.8 to 17.6 years old were examined. All children were free of any vestibular, neurological, ophtalmologic and orthoptic abnormalities. Postural control was measured with a force platform TechnoConcept®, and eye movements with video oculography (MobilEBT®). Children performed two oculomotor tasks: fixation of a stable central target and horizontal saccades. We measured the saccade latency and the number of saccades during fixation as well as the surface, length and mean velocity of the center of pressure.

RESULTS

During postural measurement, we observed a correlation between the age on the one hand and a decrease in saccade latency as well as an improvement in the quality of fixation on the other. Postural sway decreases with age and is reduced in the dual task (saccades) in comparison with a simple task of fixation.

DISCUSSION - CONCLUSION

These results suggest a maturation of neural circuits controlling posture and eye movements during childhood. This study also shows the presence of an interaction between the oculomotor system and the postural system. Engaging in oculomotor tasks results in a reduction of postural sway.

The effect of performing a dual task on postural control in children with autism

The aim of the study was to explore the effect of eye movements (saccades and pursuits) on postural stability in children with autism versus typically developing children of comparable age. Postural stability was recorded with a platform (Techno Concept) in seven children with autism (mean age: 6 ± 0.8) while fixating a target or making saccades or pursuit eye movements. Data was compared to that of seven age-matched typically developing children. Surface area and mean speed of the center of pressure (CoP) were measured. Autistic children (AC) were more instable than typically developing children (TD), both in simple as well as dual task conditions. Performing a dual task thus affects AC and TD children in a different way. AC stability is not improved during saccades or pursuit eye movements in the dual task condition; in contrast, saccades significantly improve postural stability in TD children. The postural instability observed in AC during simple as well as dual task supports the hypothesis that such children have deficits in cerebellar functions.

Electrotactile vision substitution for 3D trajectory following

Navigation for blind persons represents a challenge for researchers in vision substitution. In this field, one of the used techniques to navigate is guidance. In this study, we develop a new approach for 3D trajectory following in which the requested task is to track a light path using computer input devices (keyboard and mouse) or a rigid body handled in front of a stereoscopic camera. The light path is visualized either on direct vision or by way of a electro-stimulation device, the Tongue Display Unit, a 12 × 12 matrix of electrodes. We improve our method by a series of experiments in which the effect of the modality of perception and that of the input device. Preliminary results indicated a close correlation between the stimulated and recorded trajectories.

Sensory Re-Weighting in Human Bipedal Postural Control: The Effects of Experimentally-Induced Plantar Pain

The present study was designed to assess the effects of experimentally-induced plantar pain on the displacement of centre of foot pressure during unperturbed upright stance in different sensory conditions of availability and/or reliability of visual input and somatosensory input from the vestibular system and neck. To achieve this goal, fourteen young healthy adults were asked to stand as still as possible in three sensory conditions: (1) No-vision, (2) Vision, and (3) No-vision - Head tilted backward, during two experimental conditions: (1) a No-pain condition, and (2) a condition when a painful stimulation was applied to the plantar surfaces of both feet (Plantar-pain condition). Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) experimentally-induced plantar pain increased CoP displacements in the absence of vision (No-vision condition), (2) this deleterious effect was more accentuated when somatosensory information from the vestibular and neck was altered (No-vision - Head tilted backward condition) and (3) this deleterious effect was suppressed when visual information was available (Vision condition). From a fundamental point of view, these results lend support to the sensory re-weighting hypothesis whereby the central nervous system dynamically and selectively adjusts the relative contributions of sensory inputs (i.e. the sensory weightings) in order to maintain balance when one or more sensory channels are altered by the task (novel or challenging), environmental or individual conditions. From a clinical point of view, the present findings further suggest that prevention and treatment of plantar pain may be relevant for the preservation or improvement of balance control, particularly in situations (or individuals) in which information provided by the visual, neck proprioceptive and vestibular systems is unavailable or disrupted.

Postural control in strabismic children versus non strabismic age-matched children

BACKGROUND

Several studies have shown that achieving a dual task modifies postural control; however, their results are conflicting. The goal of the present study is to compare the effect of a simple task (eye fixation) to the effect of a dual task (saccadic eye movements) on postural balance in strabismic and in non strabismic children.

METHODS

Postural stability was recorded with a platform (Techno Concept©) in ten strabismic children aged from 5.4 to 13.8 years (mean age: 8.8 ± 2.5 years). Data were compared to that of ten age-matched non strabismic control children. We analyzed the surface area, the length and the mean speed of the center of pressure (CoP).

RESULTS

Strabismic children are more instable than control children. The surface (340 mm(2)), the length (402 mm) and the mean speed (16 mm/s) of the CoP are significantly higher in strabismic children than in control children (160 mm(2), 280 mm and 11 mm/s, respectively). In addition, both strabismic and control children have a better postural control during saccades than during a fixation task. Surface, length and mean speed of the CoP are significantly reduced during the saccade task (214 mm(2), 306 mm and 12.5 mm/s, respectively) than during the fixation task (285 mm(2), 376 mm and 14.8 mm/s, respectively).

CONCLUSIONS

Abnormal postural control in strabismic children could be due to their visual deficits. The postural improvement observed in a dual task (saccades) vs. a simple task (fixation) might be due to the fact that postural control becomes more automatic during saccadic eye movements.

Differing effects of prosaccades and antisaccades on postural stability

The goal of the study was to examine the effect of different types of eye movements on postural stability. Ten healthy young adults (25 ± 3 years) participated in the study. Postural control was measured by the TechnoConcept© platform and recorded in Standard Romberg and Tandem Romberg conditions while participants performed five oculomotor tasks: two fixation tasks (central fixation cross, without and with distractors), two prosaccade tasks toward peripheral targets displayed 4° to the left or to the right of the fixation cross (reactive saccades induced by a gap 0 ms paradigm and voluntary saccades induced by an overlap 600 ms paradigm) and one antisaccade task (voluntary saccade made in the opposite direction of the visual target). The surface, the length, and the mean speed of the center of pressure were analyzed. We found that saccadic eye movements improved postural stability with respect to the fixation tasks. Furthermore, antisaccades were found to decrease postural stability compared to prosaccades (reactive as well as voluntary saccades). This result is in line with the U-shaped nonlinear model described by Lacour et al. (Neurophysiol Clin 38:411-421, 2008), showing that a secondary task performed during a postural task could increase (prosaccade task) or decrease (antisacade task) postural stability depending on its complexity. We suggest that the different degree of attentional resources needed for performing prosaccade or antisaccade tasks are, most likely, responsible for the different effect on postural control.

A mathematical model for incorporating biofeedback into human postural control

BACKGROUND

Biofeedback of body motion can serve as a balance aid and rehabilitation tool. To date, mathematical models considering the integration of biofeedback into postural control have represented this integration as a sensory addition and limited their application to a single degree-of-freedom representation of the body. This study has two objectives: 1) to develop a scalable method for incorporating biofeedback into postural control that is independent of the model's degrees of freedom, how it handles sensory integration, and the modeling of its postural controller; and 2) to validate this new model using multidirectional perturbation experimental results.

METHODS

Biofeedback was modeled as an additional torque to the postural controller torque. For validation, this biofeedback modeling approach was applied to a vibrotactile biofeedback device and incorporated into a two-link multibody model with full-state-feedback control that represents the dynamics of bipedal stance. Average response trajectories of body sway and center of pressure (COP) to multidirectional surface perturbations of subjects with vestibular deficits were used for model parameterization and validation in multiple perturbation directions and for multiple display resolutions. The quality of fit was quantified using average error and cross-correlation values.

RESULTS

The mean of the average errors across all tactor configurations and perturbations was 0.24° for body sway and 0.39 cm for COP. The mean of the cross-correlation value was 0.97 for both body sway and COP.

CONCLUSIONS

The biofeedback model developed in this study is capable of capturing experimental response trajectory shapes with low average errors and high cross-correlation values in both the anterior-posterior and medial-lateral directions for all perturbation directions and spatial resolution display configurations considered. The results validate that biofeedback can be modeled as an additional torque to the postural controller without a need for sensory reweighting. This novel approach is scalable and applicable to a wide range of movement conditions within the fields of balance and balance rehabilitation. The model confirms experimental results that increased display resolution does not necessarily lead to reduced body sway. To our knowledge, this is the first theoretical confirmation that a spatial display resolution of 180° can be as effective as a spatial resolution of 22.5°.

Effect of saccades in tongue electrotactile stimulation for vision substitution applications

The visual substitution paradigm aims to facilitate the life of blind people. Generally one uses electro-stimulating devices where electrodes are arranged into arrays to stimulate the skin or the tongue mucosa to send signals of visual type to the subjects. When an electro-stimulation signal is applied continuously (e.g. when static visual scenes are displayed for a long period of time), the receptors of the affected region can get saturated and the patient may lose the displayed information. We propose here some mechanisms that ameliorate the quality of perception of the electro-stimulation information. The electrical signal is encoded as 2D scenes projected onto the tongue via a Tongue Display Unit, i.e. an electro-tactile stimulator formed by a 12×12 matrix of electrodes. We propose to apply stochastic saccades on this signal. Our assumption is that this eye-inspired mechanism should make the visual substitution more efficient (by improving the perception) because of the reduction of the tactile receptors saturation. The influence of saccades was evaluated by a series of experiments. Results revealed a benefit on the persistence of perception due to saccades. This work helps to prevent the saturation of receptors on the tongue. Therefore increasing the quality of vision by the way of the electro-stimulation. It allows new enhancement features to retinal prosthesis devices which suffer from the same phenomenon.

Influence of sparkle and saccades on tongue electro-stimulation-based vision substitution of 2D vectors

Vision substitution by electro-stimulation has been studied since the 60s beginning with P. Bach-y-Rita. Camera pictures or movies encoded in gray levels are displayed using an electro-stimulation display device on the surface of a body part, such as the skin or the tongue. Medical-technical devices have been developed on this principle to compensate for sensory-motor disabilities such as blindness or loss of balance, or to guide specific actions, such as surgery. However, the electrical signals of stationary or moving slowly moving objects, displayed on a Tongue display unit (TDU), are quickly lost due to saturation of receptors undergoing electrostimulation. We propose to add random saccades or sparkle to the displayed visual scene to increase the quality of pattern recognition by the subjects. In the present experimental trimodal study (normal vision, TDU vision substitution, or both), we show that the presence of a moderate sparkle level enhances the perception of the direction of lines drawn on a TDU and reduces the response time.

Effect of a dual task on postural control in dyslexic children

Several studies have examined postural control in dyslexic children; however, their results were inconclusive. This study investigated the effect of a dual task on postural stability in dyslexic children. Eighteen dyslexic children (mean age 10.3±1.2 years) were compared with eighteen non-dyslexic children of similar age. Postural stability was recorded with a platform (TechnoConcept®) while the child, in separate sessions, made reflex horizontal and vertical saccades of 10° of amplitude, and read a text silently. We measured the surface and the mean speed of the center of pressure (CoP). Reading performance was assessed by counting the number of words read during postural measures. Both groups of children were more stable while performing saccades than while reading a text. Furthermore, dyslexic children were significantly more unstable than non-dyslexic children, especially during the reading task. Finally, the number of words read by dyslexic children was significantly lower than that of non-dyslexic children and, in contrast to the non-dyslexic children. In line with the U-shaped non-linear interaction model, we suggest that the attention consumed by the reading task could be responsible for the loss of postural control in both groups of children. The postural instability observed in dyslexic children supports the hypothesis that such children have a lack of integration of multiple sensorimotor inputs.

When does mechanical plantar stimulation promote sensory re-weighing: standing on a firm or compliant surface?

The purpose of this study was to investigate the effect of different types of mechanical stimulation of the sole on standing postural stability in healthy, young adults. Fifty subjects (34 women, 16 men; mean age 23 ± 2 (mean ± SE)) stood barefoot on fixed force plates both with open and closed eyes on firm surface and then on compliant surface (foam). A modified clinical test of sensory interaction on balance protocol was employed to assess the center of gravity (COG) excursions along anteroposterior (AP) and mediolateral (ML) axes on each surface and visual condition. After the baseline measurement, a stimulation was applied with an elastic spiked layer topped to the firm and then foam surface, and the COG excursions were measured during the stimulation, and then at least 30 min after the stimulation of the spiked layer, we used 10 min of manual static and glide pressure applied to the plantar surface of both feet. Immediately after manual stimulation, static balance parameters were measured again. Results showed that after manual stimulation, the sway path with closed eyes decreased significantly on the AP and ML directions on firm surface conditions. The spiked layer caused significantly decreased sway path on firm platform in both directions, but it was ineffective on compliant surface. Our results established that the activation of plantar mechanoreceptors by 10-min manual stimulation can partially compensate subjects for the absence of visual input and the lack of accurate pressure information from the supporting surface, too.

The tongue display unit (TDU) for electrotactile spatiotemporal pattern presentation

The Tongue Display Unit (TDU) is a 144-channel programmable pulse generator that delivers dc-balanced voltage pulses suitable for electrotactile (electrocutaneous) stimulation of the anterior-dorsal tongue, through a matrix of surface electrodes. This article reviews the theory of operation and a design overview of the TDU, as well as selected applications. These include sensory substitution, tactile information display and neurorehabilitation via induced neuroplasticity.

Predicting losses of balance during upright stance: evaluation of a novel approach based on wearable accelerometers

The study of postural sway during quiet stance has proved to be a useful approach to investigate the function of the balance system. Recent studies have suggested that providing information on postural sway to vestibular patients through biofeedback may improve their balance awareness and therefore reduce their risk of falling. One drawback common to these approaches is related to timing: informing a patient about current balance conditions may not allow enough time to react and avoid a fall. Here we propose a new technique for predicting relevant balance related events based on the recording of inertial information on trunk and thigh movement using wearable devices. We have developed a regressive model for the prediction of quiet stance dynamics of the center of body mass (CM), based on these sensory data. Our preliminary results show that, with careful signal processing, such approach may allow to learn quiet stance dynamics based on the inverted pendulum model and use it in predicting critical balance conditions with a few hundreds of milliseconds advance. When these predictions are then used for event-detection the system provides accurate results and is thus promising for the development of a fall prevention device.

Postural control and attentional demand during adolescence

In the present study we aimed to determine the attentional cost of postural control during adolescence by studying the influence of a cognitive task on concurrent postural control. 38 teenagers aged 12 to 17years and 13 young adults (mean age=26.1) stood barefoot on a force platform in a semi-tandem position. A dual-task paradigm consisted of performing a Stroop or a COUNTING BACKWARD task while simultaneously standing quietly on a firm or foam support surface. Different centre of pressure (CoP) measures were calculated (90% confidence ellipse area, mean velocity, root mean square on the antero-posterior (AP) and medio-lateral (ML) axes). The number and percentage of correct responses in the cognitive tasks were also recorded. Our results indicate (1) higher values of surface, ML mean velocity and ML RMS in the COUNTING BACKWARD task in adolescents aged 12 to 15 than in teenagers aged 16 to 17 and in adults, regardless of the complexity of the postural task and, (2) better cognitive performances in the Stroop than in the COUNTING BACKWARD task. The difference in the dual-task performance between the different age groups and particularly the existence of a turning point around 14-15years of age might be due to 1) difficulties in properly allocating attentional resources to two simultaneous tasks and/or, 2) the inability to manage increased cognitive requests because of a limited information processing capacity in adolescents aged 14-15years.

Effectiveness of a tongue-placed electrotactile biofeedback to improve ankle force sense following plantar-flexor muscles fatigue

To assess the effectiveness of a tongue-placed electrotactile biofeedback system to improve ankle force sense following plantar-flexor muscles fatigue, 11 young healthy adults were asked to perform an isometric contra-lateral force ankle-matching task in two experimental conditions of No fatigue and Fatigue of the plantar-flexor muscles and two conditions of No biofeedback and Biofeedback. The underlying principle of the biofeedback consisted of supplying subjects with supplementary information about the force developed by the plantar-flexor muscles through electrical stimulation of the tongue. Measures of the overall accuracy and the variability of the force ankle-matching performances were determined using the absolute error and the variable error, respectively. Results showed that (1) the Fatigue condition yielded increased absolute and variable errors relative to the No fatigue condition in the No biofeedback condition, whereas (2) no significant difference between the two No fatigue and Fatigue conditions was observed in the Biofeedback condition. These results suggest that subjects were able to integrate augmented sensory information delivered through electrotactile stimulation of the tongue to suppress the adverse effect of plantar-flexor muscles fatigue on ankle force sense.

Sensory weighting of force and position feedback in human motor control tasks

In daily life humans integrate force and position feedback from mechanoreceptors, proprioception, and vision. With handling relatively soft, elastic objects, force and position are related and can be integrated to improve the accuracy of an estimate of either one. Sensory weighting between different sensory systems (e.g., vision and proprioception) has been extensively studied. This study investigated whether similar weighting can be found within the proprioceptive sensory system, more specifically between the modalities force and position. We hypothesized that sensory weighting is governed by object stiffness: position feedback is weighted heavier on soft objects (large deflections), while force feedback is weighted heavier on stiff objects (small deflections). Subjects were instructed to blindly reproduce either position or force while holding a one degree of freedom haptic manipulator that simulated a linear spring with one of four predetermined stiffnesses. In catch trials the spring was covertly replaced by a nonlinear spring. The difference in force (DeltaF) and position (DeltaX) between the regular and the catch trials revealed the sensory weighting between force and position feedback. A maximum likelihood estimation model predicted that: (1) task instruction did not affect the outcome measures, and (2) force feedback is weighted heavier with increasing object stiffness as was hypothesized. Both effects were found experimentally, and the subjects' sensory weighting closely resembled the optimal model predictions. To conclude, this study successfully demonstrated sensory weighting within the proprioceptive system.