Ocular versus extraocular control of posture and equilibrium

Effects of aerobic exercise on balance and mobility in individuals with visual impairment: a systematic review

BACKGROUND

This meta-analysis aimed to assess the impact of aerobic exercise on the balance and mobility of visually impaired individuals and identify potential factors affecting these outcomes.

METHODS

A systematic review was conducted following PRISMA guidelines. The eligibility criteria for the study included visual impairment in the participant population, type of visual impairment, aerobic exercise intervention, controlled trials, age, design, and outcomes. Data were extracted based on these seven questions, and a narrative synthesis approach was employed for analysis. A comprehensive search of the database yielded 1987 articles, and ultimately, 14 randomized controlled trials involving 685 participants were selected for inclusion.

RESULTS

The study findings indicate that aerobic exercise can greatly enhance the static balance ability [SMD = 1.23, 95%CI 0.80-1.66, p < 0.00001] and dynamic balance ability [SMD = 1.04, 95% CI 0.09-1.99, p = 0.03] of individuals with visual impairment. However, it appeared that mobility did not exhibit significant improvement [SMD = -0.49, 95% CI -1.06 to 0.09, p = 0.10]. Through subgroup analysis, it was found that exercise frequency, age, and degree of visual impairment significantly affect static balance.

CONCLUSIONS

Engaging in aerobic exercise 3-6 times per week, with each session lasting between 30 and 60 min, and continuing for a cycle of 8-16 weeks has been shown to enhance both dynamic and static balance in individuals with visual impairments.

Diabetic older women without peripheral neuropathy amplify body sway but are capable of improving postural stability during a saccadic gaze task

BACKGROUND

Diabetic older people tend to present deteriorated performance in balance and locomotion activities, even those without peripheral neuropathy. There is evidence that saccadic eye movements are used to reduce body sway in young and older healthy adults, but it has not been shown that diabetic older people preserve this visuomotor adaptation capacity.

RESEARCH QUESTION

Are diabetic older women without peripheral neuropathy capable of improving postural stability during a saccadic gaze task?

METHODS

Seventeen type 2 diabetic older women (68.2 ± 10.7 years old) and seventeen healthy women, age-matched controls (66.0 ± 8.4 years old) voluntarily participated in the study. All participants were instructed to stand upright, barefoot, as stable as possible, for 30 s. Participants maintained their feet parallel to each other, at standard and narrow bases of support, while either fixating on a stationary target (fixation condition) or performing horizontal saccadic eye movements to follow a target (eccentricity of 11° of visual angle), which continuously disappeared and reappeared immediately on the opposite side (saccade 0.5 Hz and saccade 1.1 Hz conditions).

RESULTS

Results indicated that the diabetic group clearly had deteriorated postural control, as shown by increased values of mean sway amplitude and mean sway velocity. However, diabetic and control groups were similarly capable of using saccadic eye movements to improve their postural stability, reducing their sway velocity compared to a gaze fixation condition.

SIGNIFICANCE

Diabetes per se (without peripheral neuropathy) amplifies postural sway of older women as compared to their healthy age-matched controls. However, diabetic older women without peripheral neuropathy are capable of improving postural stability during a saccadic gaze task.

Center of mass-based posturography for free living environment applications

BACKGROUND

Postural assessment is crucial as risk of falling is a major health problem for the elderly. The most widely used devices are force and balance plates, while center of pressure is the most studied parameter as measure of neuromuscular imbalances of the body sway. In out-of-laboratory conditions, where the use of plates is unattainable, the center of mass can serve as an alternative. This work proposes a center of mass-based posturographic measurement for free living applications.

METHODS

Ten healthy and ten Parkinson's disease individuals (age = 26.1 ± 1.5, 70.4 ± 6.2 years, body mass index = 21.7 ± 2.2, 27.6 ± 2.8 kg/m², respectively) participated in the study. A stereophotogrammetric system and a force plate were used to acquire the center of pressure and the 5th lumbar vertebra displacements during the Romberg test. The center of mass was estimated using anthropometric measures. Posturographic parameters were extracted from center of pressure, center of mass and 5th lumbar vertebra trajectories. Normalized root mean squared difference was used as metric to compare the trajectories; Spearman's correlation coefficient was computed among the posturographic parameters.

FINDINGS

Low values of the metric indicated a good agreement between 5th lumbar vertebra trajectory and both center of pressure and center of mass trajectories. Statistically significant correlations were found among the postural variables.

INTERPRETATION

A method to perform posturography tracking the movement of the 5th lumbar vertebra as an approximation of center of mass has been presented and validated. The method requires the solely kinematic tracking of one anatomical landmark with no need of plates for free living applications.

Keeping balance during head-free smooth pursuit: The role of aging

Standing balance is often more unstable when visually pursuing a moving target than when fixating on a stationary one. These effects are common in both young and older adults when the head is restrained during visual task performance. The present study focused on the role of head motion on standing balance during smooth pursuit as a function of age. Three predictions were tested: a) standing balance is compromised to a greater extent in older than young adults by gaze target pursuit compared to fixation, b) older adults pursue a moving target with greater and more variable head rotation than young adults, and c) greater and more variable head rotation during the smooth pursuit task is associated with greater Center of Pressure (CoP) sway. Twenty-two (22) older (age: 71.7 ± 8.1, 12 M / 10 F) and twenty-three (23) young adults (age: 23.6 ± 2.5, 12 M / 11 F) stood on a force plate while either fixating a stationary or smoothly pursuing a horizontally moving target (31.9° peak-to-peak visual angle). CoP (Bertec Balance Plate), head kinematics (Vicon Motion Analysis) and head-unconstrained gaze (Pupil Labs Invisible) were synchronously recorded. The root means square (RMS) of CoP velocity increased during smooth pursuit compared to fixation regardless of age (p < .05), while the interquartile CoP range increased only in older and not in young participants (p < .05). We also calculated the head rotation range (peak to peak cycle amplitude) of motion and variability (SD of range of motion) across the cycles of the smooth pursuit task. Older adults pursued the moving target employing more variable (p = .022) head yaw rotation than young participants although the mean range of head rotation was similar between groups (p =. 077). The amplitude and variability of head yaw rotation did not correlate with CoP sway measures. Results suggest that head-free pursuing of a moving target decreased balance to a greater extent in old than young individuals when compared to fixation. Nevertheless, postural sway during head-free smooth pursuit was not associated with the extent or variability of head rotation.

Is There a Correlation between Dental Occlusion, Postural Stability and Selected Gait Parameters in Adults?

Background: There is still an ongoing debate about the role of the craniomandibular system, including occlusal conditions, on postural stability. This study aims to assess the role of antero-posterior malocclusion on postural control and plantar pressure distribution during standing and walking. Methods: 90 healthy volunteers (aged 19 to 35) were qualified for the study. The subjects were assigned to three groups, depending on the occlusion type. Each group (Angle Class I, II and III) consisted of 30 people. The research procedure included a clinical occlusal assessment performed by a dentist. Postural control measurements were carried out using a force platform by measuring plantar pressure distribution during standing (six trials with and without visual control) and walking test conditions. Results: The tendency to shift the CoP forward is demonstrated by Angle Class II subjects and backwards by Class I and III subjects (p < 0.001). Individuals with a malocclusion demonstrated significantly higher selected stabilographic parameters while standing on both feet (with eyes open and closed) and during the single-leg test with eyes open (p < 0.05). The analysis of the dynamic test results showed no significant correlations between Angle Classes and the selected gait parameters. Conclusions: Analyses conducted among individuals with malocclusions showed the impact of occlusion on static postural stability. In order to diagnose and effectively treat malocclusion, a multidisciplinary approach with the participation of dentists and physiotherapy specialists is necessary, with the use of stabilometric and kinematic posture assessments.

Fast Saccadic Eye Movements Contribute to the Worsened Postural Sway in Older Adults Who Have Experienced Falls

The purpose of this study was to examine changes and between-group differences in postural sway during saccadic eye movement in older adults (n = 152). The participants were stratified into older adults who have experienced a fall (n = 58) (faller group) and those who have not (n = 94) (non-faller group). We measured postural sway during saccadic eye movement. Saccadic eye movement speed was such that the target was displayed at 0.5 Hz, 2 Hz, and 3 Hz. Postural sway was measured based on path length, velocity, and length between the maximal and minimal position of center of pressure in mediolateral and anteroposterior direction. In the faller group, path length, velocity, and mediolateral displacement of the center of pressure increased significantly during 3 Hz saccadic eye movement stimulation. However, in the non-faller group, there was no significant change in the center of pressure parameters during saccadic eye movement stimulation. Mediolateral displacement of the center of pressure increased significantly in both groups during saccadic eye movement, especially at 3 Hz. Therefore, rapid saccadic eye movement stimulation can contribute to the worsened postural sway in older adults who have experienced falls, and rapid external environmental stimuli may contribute to the deterioration of the upright standing stability in older adults.

Decreased Saccadic Eye Movement Speed Correlates with Dynamic Balance in Older Adults

This study aimed to determine the change in saccadic eye movement (SEM) speed according to age (young older; 65–72 years, middle older; 73–80 years, old older: over 81 years) in the elderly and identify the correlation between SEM speed and balance ability. We recruited 128 elderly individuals and measured their SEM speed and balance. The SEM speed was measured to allow the target to appear once every 2 s (0.5 Hz), twice per second (2 Hz), or thrice per second (3 Hz). The SEM performance time was 1 min with a washout period of 1 min. Balance ability was measured using the functional reach test (FRT), timed up-and-go test (TUG), and walking speed (WS). As age increased, FRT, TUG, and WS decreased and SEM speed was significantly decreased in old older than in young older adults at 3 HZ. In all participants, the 3 Hz SEM speed was significantly correlated with TUG and WS. Therefore, SEM speed may be inadequate or decreased in response to rapid external environmental stimuli and may be a factor that deteriorates the ability to balance in older adults.

The Differentiation of Self-Motion From External Motion Is a Prerequisite for Postural Control: A Narrative Review of Visual-Vestibular Interaction

The visual system is a source of sensory information that perceives environmental stimuli and interacts with other sensory systems to generate visual and postural responses to maintain postural stability. Although the three sensory systems; the visual, vestibular, and somatosensory systems work concurrently to maintain postural control, the visual and vestibular system interaction is vital to differentiate self-motion from external motion to maintain postural stability. The visual system influences postural control playing a key role in perceiving information required for this differentiation. The visual system’s main afferent information consists of optic flow and retinal slip that lead to the generation of visual and postural responses. Visual fixations generated by the visual system interact with the afferent information and the vestibular system to maintain visual and postural stability. This review synthesizes the roles of the visual system and their interaction with the vestibular system, to maintain postural stability.

Idiosyncratic multisensory reweighting as the common cause for motion sickness susceptibility and adaptation to postural perturbation

Numerous empirical and modeling studies have been done to find a relationship between postural stability and the susceptibility to motion sickness (MS). However, while the demonstration of a causal relationship between postural stability and the susceptibility to MS is still lacking, recent studies suggest that motion sick individuals have genuine deficits in selecting and reweighting multimodal sensory information. Here we investigate how the adaptation to changing postural situations develops and how the dynamics in multisensory integration is modulated on an individual basis along with MS susceptibility. We used a postural task in which participants stood on a posturographic platform with either eyes open (EO) or eyes closed (EC) during three minutes. The platform was static during the first minute (baseline phase), oscillated harmonically during the second minute (perturbation phase) and returned to its steady state for the third minute (return phase). Principal component (PC) analysis was applied to the sequence of short-term power density spectra of the antero-posterior position of the center of pressure. Results showed that the less motion-sick a participant is, the more similar is his balance between high and low frequencies for EO and EC conditions (as calculated from the eigenvector of the first PC). By fitting exponential decay models to the first PC score in the return phase, we estimated, for each participant in each condition, the sluggishness to return to the baseline spectrum. We showed that the de-adaptation following platform oscillation depends on the susceptibility to MS. These results suggest that non motion-sick participants finely adjust their spectrum in the perturbation phase (i.e. reweighting) and therefore take longer to return to their initial postural control particularly with eyes closed. Thus, people have idiosyncratic ways of doing sensory reweighting for postural control, these processes being tied to MS susceptibility.

Trunk and head displacements stabilized to perform both horizontal and vertical saccadic eye movements

Vision is crucial for humans to interact with their surrounding environment, and postural sway is reduced to allow short eye movements. However, the extent of subtle changes in postural control for horizontal and vertical eye movements remains unclear. The goal of this study was to investigate the effects of vertical and horizontal eye movements on head and trunk control in young adults. Fifteen healthy adults (23.4 ± 4.7 years) stood upright in three conditions for 60 s: fixation, horizontal, and vertical guided eye movements. In fixation, participants had to fixate on a stationary target. In both the horizontal and vertical eye movements, the target was presented with a frequency of 0.5 Hz and a visual angle of 11°. Eye displacement was monitored using a SMI eye tracker (ETG2.0) and trunk and head sway were monitored using infrared markers (Optotrak 3020, NDI). The mean sway amplitude was lower in both directions for eye movements and lowest in the vertical direction compared to the fixation condition. The sway area was also lower in vertical eye movement than in the fixation condition. We also found that the sway reduction was greater at head than at trunk level. The median frequency sway in the anterior-posterior direction was higher in both eye movements than in fixation. Based upon these results, we suggest that to perform short eye movements, postural sway is more strongly controlled at the head level than at the trunk and in vertical eye movements than in horizontal movements.

Visuomotor control dynamics of quiet standing under single and dual task conditions in younger and older adults

Visual input facilitates stable postural control; however, ageing alters visual gaze strategies and visual input processing times. Understanding the complex interaction between visual gaze behaviour and the effects of age may inform future interventions to improve postural control in older adults. The purpose of this study was to determine effects of age and dual task on gaze and postural sway dynamics, and the sway-gaze complexity coupling to explore the coupling between sensory input and motor output. Ten older and 10 younger adults performed single and dual task quiet standing while gaze behaviour and centre of mass motion were recorded. The complexity and stability of postural sway, saccade characteristics, visual input duration and complexity of gaze were calculated in addition to sway-gaze coupling quantified by cross-sample entropy. Dual tasking increased complexity and decreased stability of sway with increased gaze complexity and visual input duration, suggesting greater automaticity of sway with greater exploration of the visual field but with longer visual inputs to maintain postural stability in dual task conditions. In addition, older adults had lower complexity and stability of sway than younger adults indicating less automated and stable postural control. Older adults also demonstrated lower gaze complexity, longer visual input durations and greater sway-gaze coupling. These findings suggest older adults adopted a strategy to increase the capacity for visual information input, whilst exploring less of the visual field than younger adults.

The association between visual attention and body movement-controlled video games, balance and mobility in older adults

Background

Body movement-controlled video games involving physical motion and visual attention may have the potential to train both abilities simultaneously. Our purpose was to determine the associations between performance in these games and visual attention, balance and mobility in a group of older adults. The long-term goal is to identify the optimal type of interactive games with regards to training potential.

Methods

Fifty healthy adults aged 65+ years participated in this cross-sectional study. Visual attention was measured with static and dynamic versions of a useful field of view (UFV) and a multiple object tracking (MOT) test. Balance was measured with a force plate in bi-pedal quiet stance test (QST) and one-legged stance (OLST). Gait variability and walking speed were assessed with the Five Meter Walk Test (5MWT). Four Microsoft™ Xbox® 360 Kinect™ interactive video games were chosen based on the apparent levels of visual attention demand.

Results

Visual attention (UFV and MOT) was significantly associated with performance in Xbox® Kinect™ games that appeared to have a high visual attention demand (p < 0.05), while there was minimal or no significant association with games with apparent low visual attention demand. Balance and mobility show correlations with visual attention, and with Xbox games.

Conclusion

The results suggest that there are relationships between visual attention, balance, mobility and Xbox® Kinect™ game performance. Since different Xbox® games were associated with different balance, mobility and visual attention scores, a variety of such games, rather than a single game, may be most effective for training for falls prevention.

Vestibulo-perceptual influences upon the vestibulo-spinal reflex

The vestibular system facilitates gaze and postural stability via the vestibulo-ocular (VOR) and vestibulo-spinal reflexes, respectively. Cortical and perceptual mechanisms can modulate long-duration VOR responses, but little is known about whether high-order neural phenomena can modulate short-latency vestibulo-spinal responses. Here, we investigate this by assessing click-evoked cervical vestibular myogenic-evoked potentials (VEMPS) during visual roll motion that elicited an illusionary sensation of self-motion (i.e. vection). We observed that during vection, the amplitude of the VEMPs was enhanced when compared to baseline measures. This modulation in VEMP amplitude was positively correlated with the subjective reports of vection strength. That is, those subjects reporting greater subjective vection scores exhibited a greater increase in VEMP amplitude. Control experiments showed that simple arousal (cold-induced discomfort) also increased VEMP amplitude but that, unlike vection, it did not modulate VEMP amplitude linearly. In agreement, small-field visual roll motion that did not induce vection failed to increase VEMP amplitude. Taken together, our results demonstrate that vection can modify the response of vestibulo-collic reflexes. Even short-latency brainstem vestibulo-spinal reflexes are influenced by high-order mechanisms, illustrating the functional importance of perceptual mechanisms in human postural control. As VEMPs are inhibitory responses, we argue that the findings may represent a mechanism whereby high-order CNS mechanisms reduce activity levels in vestibulo-collic reflexes, necessary for instance when voluntary head movements need to be performed.

Perceptual-motor styles

Even for a stereotyped task, sensorimotor behavior is generally variable due to noise, redundancy, adaptability, learning or plasticity. The sources and significance of different kinds of behavioral variability have attracted considerable attention in recent years. However, the idea that part of this variability depends on unique individual strategies has been explored to a lesser extent. In particular, the notion of style recurs infrequently in the literature on sensorimotor behavior. In general use, style refers to a distinctive manner or custom of behaving oneself or of doing something, especially one that is typical of a person, group of people, place, context, or period. The application of the term to the domain of perceptual and motor phenomenology opens new perspectives on the nature of behavioral variability, perspectives that are complementary to those typically considered in the studies of sensorimotor variability. In particular, the concept of style may help toward the development of personalised physiology and medicine by providing markers of individual behaviour and response to different stimuli or treatments. Here, we cover some potential applications of the concept of perceptual-motor style to different areas of neuroscience, both in the healthy and the diseased. We prefer to be as general as possible in the types of applications we consider, even at the expense of running the risk of encompassing loosely related studies, given the relative novelty of the introduction of the term perceptual-motor style in neurosciences.

The effects of stroboscopic balance training on cortical activities in athletes with chronic ankle instability

OBJECTIVES

To investigate the effect of a 6-week stroboscopic balance training program on cortical activities in athletes with chronic ankle instability.

DESIGN

Randomized controlled trial.

SETTING

Single-center.

PARTICIPANTS

Thirty-nine participants were assigned to the strobe group (SG, n = 13), non-strobe group (NSG, n = 13), and control group (CG, n = 13).

MAIN OUTCOME MEASURES

Cortical activity and balance velocity were evaluated while the athletes were on the HUBER balance device. Electroencephalographic measurements of cortical activity were made at the transition from bipedal stance to single-leg stance.

RESULTS

The SG showed significant increases in Cz theta and alpha values and COP-v (center of pressure velocity) between pretest and posttest (p < 0.001, p = 0.003, p < 0.001). Posttest Cz theta was significantly higher in the SG compared to the CG (p = 0.009) and posttest Cz alpha was significantly higher in the SG compared to the NSG (p = 0.039) and CG (p = 0.001). Posttest COP-v was significantly higher in the SG than in the CG (p = 0.031) and NSG (p = 0.03).

CONCLUSIONS

Stroboscopic training may be clinically beneficial to improve balance parameters in athletes with CAI, and may have utility in sport-specific activity phases of rehabilitation to reduce visual input and increase motor control.

Gazing down increases standing and walking postural steadiness

When walking on an uneven surface or complex terrain, humans tend to gaze downward. This behaviour is usually interpreted as an attempt to acquire useful information to guide locomotion. Visual information, however, is not used exclusively for guiding locomotion; it is also useful for postural control. Both locomotive and postural control have been shown to be sensitive to the visual flow arising from the respective motion of the individual and the three-dimensional environment. This flow changes when a person gazes downward and may present information that is more appropriate for postural control. To investigate whether downward gazing can be used for postural control, rather than exclusively for guiding locomotion, we quantified the dynamics of standing and walking posture in healthy adults, under several visual conditions. Through these experiments we were able to demonstrate that gazing downward, just a few steps ahead, resulted in a steadier standing and walking posture. These experiments indicate that gazing downward may serve more than one purpose and provide sufficient evidence of the possible interplay between the visual information used for guiding locomotion and that used for postural control. These findings contribute to our understanding of the control mechanism/s underlying gait and posture and have possible clinical implications.

Saccadic Eye Movements Attenuate Postural Sway but Less in Sleep-Deprived Young Adults

Sleep deprivation affects the performance of postural control and several other aspects related to attentional mechanisms that may alter sensory cue acquisition strategies. This study aimed to examine the possible effects of horizontal saccades and ocular fixation on a target in the performance of postural control in young adults with sleep deprivation. Twenty-six adults formed two groups, tested in two evaluations. In the first evaluation, participants slept normally on the night before. In the second evaluation, 13 participants were sleep deprived (SD) and 13 slept normally (control group [CG]) on the night before. In both evaluations, each participant stood upright as still as possible, in two experimental conditions: fixating the eye on a target and performing saccadic movement toward a target presented in two different locations (0.5 Hz). Each participant performed 3 trials in each condition, lasting 62 s each. Body oscillation was obtained in both anterior-posterior and medial-lateral directions. Results showed that SD participants swayed with a larger magnitude and higher velocity after sleep deprivation in the fixation condition. In the saccadic condition, body sway magnitude and velocity were reduced but were still larger/higher in the SD participants. Sleep deprivation deteriorates the performance of postural control. Saccadic eye movements improve postural control performance even in sleep-deprived participants but are still not sufficient to avoid postural control deterioration due to sleep deprivation.

Comparison of Postural Sway, Plantar Cutaneous Sensation According to Saccadic Eye Movement Frequency in Young Adults

The crossover trial study aimed to identify the saccadic eye movement (SEM) frequency to improve postural sway (PS) and plantar cutaneous sensation (PUS) in young adults. The 17 participants randomly performed 0.5-, 2-, and 3-Hz SEM. The SEM frequency was determined to allow the target to appear once per 2 s (0.5 Hz), twice per second (2 Hz), or thrice per second (3 Hz). SEM performance time was 3 min with a washout period of 5 min. PS and PUS were measured at baseline and during 0.5-Hz, 2-Hz, and 3-Hz SEMs using a Zebris FDM 1.5 force plate. PS was determined by measuring the sway area, path length, and speed of center of pressure (COP) displacement, and PUS was determined via the plantar surface area (PSA). In PS parameters, there was a significant difference among the SEM frequencies in the COP_{sway area} PSA_{left foot} and PSA_{right foot}. Compared to that at baseline, COP_{sway area} decreased at 0.5 Hz and 2 Hz, while PSA_{left foot} and PSA_{right foot} increased at 2 Hz. These results suggest that 2 Hz SEM may improve PS and PSA.

Visual Fixations and Motion Sensitivity: Protocol for an Exploratory Study

Background

Motion sensitivity after vestibular disorders is associated with symptoms of nausea, dizziness, and imbalance in busy environments. Dizziness and imbalance are reported in places such as supermarkets and shopping malls which have unstable visual backgrounds; however, the mechanism of motion sensitivity is poorly understood.

Objective

The main aim of this exploratory observational study is to investigate visual fixations and postural sway in response to increasingly complex visual environments in healthy adults and adults with motion sensitivity.

Methods

A total of 20 healthy adults and 20 adults with motion sensitivity will be recruited for this study. Visual fixations, postural sway, and body kinematics will be measured with a mobile eye tracker device, force plate, and 3D motion capture system, respectively. Participants will be exposed to experimental tasks requiring visual fixation on letters, projected on a range of backgrounds on a large screen during quiet stance. Descriptive statistics (mean and standard deviation) will be calculated for each of the variables. One-way independent-measures analyses of variance will be performed to investigate the differences between groups for all variables.

Results

Data collection was started in May 2019 and was completed by February 2020. It was approved by Health and Disability Ethics Committees, Ministry of Health, New Zealand on November 2, 2018 (Ethics ref: 18/CEN/193). We are currently processing the data and will begin data analysis in July 2020. We expect the results to be available for publication by the end of 2020. The trial was funded by the Neurology Special Interest Group, Physiotherapy New Zealand, and the Eisdell Moore Centre in November 2018.

Conclusions

This study will provide a detailed investigation of visual fixations in response to increasingly complex visual environments. Investigating characteristics of visual fixations in healthy adults and those with motion sensitivity will provide insight into this disabling condition and may inform the development of new intervention strategies which explicitly cater to the needs of this population.

Trial Registration

Australian New Zealand Clinical Trials Registry, ACTRN12619000254190; https://tinyurl.com/yxbn7nks

International Registered Report Identifier (IRRID)

PRR1-10.2196/16805

Visually guided eye movements reduce postural sway in dyslexic children

Dyslexic children present poorer postural control performance than their peers, perhaps due to different patterns of eye movements. It has been shown that guided eye movements decrease magnitude of body sway in young and older adults, but there is no evidence whether the search for visual information that occurs during eye movements affects postural control in dyslexic children. The aim of this study was to examine the use of guided eye movements and its pattern in the performance of postural control of dyslexic children during upright quiet stance. Twelve children with dyslexia (10.8 ± 1.1 years old) and 12 non-dyslexic children (10.4 ± 1.5 years old) participated in this study. All children were instructed to maintain an upright quiet stance for 60 s either fixating on a target (fixation condition) displayed 1 m ahead in the center of a monitor at eye level, or performing eye movements to follow a target displayed on one side of a monitor, then disappearing and reappearing immediately on the opposite side with a frequency of 0.5 Hz (guided condition). Three trials for each condition were registered. Body sway was measured with an IRED (OPTOTRAK) placed on the children's back. Eye movements were tracked using eye-tracking glasses (ETG 2.0 - SMI). Dyslexic children swayed with larger amplitude under both fixation and guided conditions than non-dyslexic children. Both dyslexic and non-dyslexic children reduced postural sway magnitude under the guided compared to the fixation condition. All children were able to modulate eye movement according to the conditions (fixation and guided) and no difference in eye movements was observed between dyslexic and non-dyslexic children. Eye movements are modulated similarly based upon the visual conditions in dyslexic and non-dyslexic children, and dyslexic children are capable of using available visual information during eye movements to improve postural control, though they do not equal the performance of non-dyslexic children. Eye movement patterns seem not to be related to poor performance of postural control in dyslexic children.

Wearing a head-mounted eye tracker may reduce body sway

This study investigated the effects of wearing a head-mounted eye tracker on upright balance during different visual tasks. Twenty five young adults stood upright on a force plate while performing the visual tasks of fixation, horizontal saccades, and eyes closed, during eighteen trials wearing or not a head-mounted eye tracker. While wearing the eye tracker, participants showed a reduction in mean sway amplitude and velocity of the CoP in the AP and ML directions and more regular CoP fluctuations, in the ML axis in all conditions. Higher mean sway amplitude and velocity of CoP were observed during eyes closed than fixation and saccades. Moreover, horizontal saccades reduced mean sway velocity of CoP compared to fixation. Therefore, wearing the eye tracker minimized the body sway of young adults; however, visual task-related effects on postural stability remained unchanged.

Misperception of the subjective visual vertical in neurological patients with or without stroke: A meta-analysis

BACKGROUND

The interpretation of the verticality of the environment is crucial for a proper body balance. The subjective visual vertical test (SVV) is a widely used method to determine the visual perception of the verticality, whose alteration has been related with poor functional status.

OBJECTIVE

To analyze the visual perception of the verticality in neurological patients in comparison with healthy controls.

METHODS

We searched PubMed, Scopus, and Scielo from the start of the databases until October 2017 and manually searched the reference lists of studies comparing SVV values between neurological patients and controls. Standardized mean difference (SMD) and subgroup analysis were used to analyze differences between neurological patients and healthy subjects and between stroke and non-stroke patients, respectively.

RESULTS

A total of 1,916 subjects from 31 studies were included. Neurological patients misestimate the true vertical in comparison with controls (SMD = 1.05; 95% CI: 0.81, 1.28). The misperception of the verticality was higher in stroke patients (SMD = 1.35; 95% CI: 1.02, 1.68) than in patients with other neurological conditions (SMD = 0.48; 95% CI: 0.29, 0.68).

CONCLUSIONS

Neurological patients showed a misperception of the verticality, estimated using the SVV. The neurological pathology that most alters the SVV is stroke.

Effects of Ankle Muscle Fatigue and Visual Behavior on Postural Sway in Young Adults

Ankle muscle fatigue has been shown to increase body sway. In addition, body sway in quiet upright standing is reduced when saccadic eye movements are performed. The purpose of this study was to investigate the effects of visual information manipulation on postural control during ankle muscle fatigue in young adults. Twenty young adults performed: (1) two 60-s trials in quiet bipedal standing with eyes open, eyes closed, and while performing saccadic eye movements; (2) maximum voluntary isometric contractions in a leg press device, custom-made to test ankle plantar flexion force; (3) a calf raise exercise on top of a step to induce ankle muscle fatigue; and (4) a repetition of items 1 and 2. Postural sway parameters were compared with two-way ANOVAs (vision condition × fatigue; p < 0.05). Ankle muscle fatigue increased anterior-posterior and medial-lateral displacement and RMS of sway, as well as sway area. Saccadic eye movements reduced anterior-posterior displacement and RMS of sway and area of sway compared to eyes open and eyes closed conditions. Both saccadic eye movements and eyes closed increased the frequency of AP sway compared to the eyes open condition. Finally, anterior-posterior displacement, anterior-posterior RMS, and both anterior-posterior and medial-lateral sway frequency were affected by an interaction of fatigue and vision condition. Without muscle fatigue, closing the eyes increased anterior-posterior displacement and RMS of sway, compared to eyes open, while during muscle fatigue closing the eyes closed reduced anterior-posterior displacement and had no significant effect on anterior-posterior RMS. In conclusion, body sway was increased after induction of ankle muscle fatigue. Saccadic eye movements consistently reduced postural sway in fatigued and unfatigued conditions. Surprisingly, closing the eyes increased sway in the unfatigued condition but reduced sway in the fatigued condition.

Are changes in the stomatognatic system able to modify the eye balance in dyslexia?

Objectives

To clarify the link between eye muscle function and oral information by comparing 21 dyslexic readers (DR) and 14 normal readers (NR).

Methods

Changes in vertical heterophoria (VH) were measured using the Maddox Rod Test performed according to oral modifications and postural conditions. The Spearman correlation was used to assess whether reading delay was correlated with the lability index.

Results

Overall, 50% of NR children and 81% of DR experienced at least one variation in visual perception (p = 0.053). Among DR, the less reading delay they had, the higher their index of lability (p = 0.026), whereas there was no significant correlation among NR. Changes in the Maddox Test were more frequent in DR than in NR after the addition of sensory and postural stimuli, except for one specific posture. For sensory stimuli, the mean lability index was 1.35 in NR and 4.19 in DR, (p = 0.001). For postural stimuli, it was 0.71 and 2.61, (p = 0.003).

Conclusions

It is possible to modify visual perception by changing sensory or mechanical stimuli. Changes are more frequent in DR than in NR. Postural control can be improved with guided oral stimulations.

Significance

These results reinforce the importance of professional cooperation in the care of dyslexic readers.

Effect of Eye-Object Distance on Body Sway during Galvanic Vestibular Stimulation

Gazing at objects at a near distance (small eye-object distance) can reduce body sway. However, whether body sway is regulated by movement in the mediolateral or anteroposterior direction remains unclear. Galvanic vestibular stimulation (GVS) can induce body tilting in the mediolateral or anteroposterior direction. This study examined the directionality of the eye-object distance effect, using body-tilting GVS manipulations. Ten healthy subjects (aged 21.1 ± 0.3 years) stood on a force plate covered with a piece of foamed rubber and either closed their eyes or gazed at a marker located 0.5 m, 1.0 m, or 1.5 m in front of them. The GVS polarities were set to evoke rightward, forward, and backward body tilts. To compare the effects of eye-object distance in the mediolateral and anteroposterior directions, the root mean square (RMS) of the center of pressure (COP) without GVS was subtracted from the COP RMS during GVS. For swaying in the mediolateral direction, significant visual condition-related differences were found during rightward and forward GVS (p < 0.05). Thus, reductions in mediolateral body sway are more evident for smaller eye-object distances during rightward GVS. It would be appropriate to use body-tilting GVS to detect the directionality of the eye-object distance effect.

Do postural constraints affect eye, head, and arm coordination?

If a whole body reaching task is produced when standing or adopting challenging postures, it is unclear whether changes in attentional demands or the sensorimotor integration necessary for balance control influence the interaction between visuomotor and postural components of the movement. Is gaze control prioritized by the central nervous system (CNS) to produce coordinated eye movements with the head and whole body regardless of movement context? Considering the coupled nature of visuomotor and whole body postural control during action, this study aimed to understand how changing equilibrium constraints (in the form of different postural configurations) influenced the initiation of eye, head, and arm movements. We quantified the eye-head metrics and segmental kinematics as participants executed either isolated gaze shifts or whole body reaching movements to visual targets. In total, four postural configurations were compared: seated, natural stance, with the feet together (narrow stance), or while balancing on a wooden beam. Contrary to our initial predictions, the lack of distinct changes in eye-head metrics; timing of eye, head, and arm movement initiation; and gaze accuracy, in spite of kinematic differences, suggests that the CNS integrates postural constraints into the control necessary to initiate gaze shifts. This may be achieved by adopting a whole body gaze strategy that allows for the successful completion of both gaze and reaching goals.

NEW & NOTEWORTHY Differences in sequence of movement among the eye, head, and arm have been shown across various paradigms during reaching. Here we show that distinct changes in eye characteristics and movement sequence, coupled with stereotyped profiles of head and gaze movement, are not observed when adopting postures requiring changes to balance constraints. This suggests that a whole body gaze strategy is prioritized by the central nervous system with postural control subservient to gaze stability requirements.

How Eye Movements Stabilize Posture in Patients With Bilateral Vestibular Hypofunction

Chronic patients with bilateral vestibular hypofunction (BVH) complain of oscillopsia and great instability particularly when vision is excluded and on irregular surfaces. The real nature of the visual input substituting to the missing vestibular afferents and improving posture control remains however under debate. Is retinal slip involved? Do eye movements play a substantial role? The present study tends to answer this question in BVH patients by investigating their posture stability during quiet standing in four different visual conditions: total darkness, fixation of a stable space-fixed target, and pursuit of a visual target under goggles delivering visual input rate at flicker frequency inducing either slow eye movements (4.5 Hz) or saccades (1.2 Hz). Twenty one chronic BVH patients attested by both the caloric and head impulse test were examined by means of static posturography, and compared to a control group made of 21 sex-and age-matched healthy participants. The posturography data were analyzed using non-linear computation of the center of foot pressure (CoP) by means of the wavelet transform (Power Spectral Density in the visual frequency part, Postural Instability Index) and the fractional Brownian-motion analysis (stabilogram-diffusion analysis, Hausdorff fractal dimension). Results showed that posture stability was significantly deteriorated in darkness in the BVH patients compared to the healthy controls. Strong improvement of BVH patients' posture stability was observed during fixation of a visual target, pursuit with slow eye movements, and saccades, whereas the postural performance of the control group was less affected by the different visual conditions. It is concluded that BVH patients improve their posture stability by (1) using extraocular signals from eye movements (efference copy, muscle re-afferences) much more than the healthy participants, and (2) shifting more systematically than the controls to a more automatic mode of posture control when they are in dual-task conditions associating the postural task and a concomitant visuo- motor task.

Beyond Rehabilitation of Acuity, Ocular Alignment, and Binocularity in Infantile Strabismus

Infantile strabismus impairs the perception of all attributes of the visual scene. High spatial frequency components are no longer visible, leading to amblyopia. Binocularity is altered, leading to the loss of stereopsis. Spatial perception is impaired as well as detection of vertical orientation, the fastest movements, directions of movement, the highest contrasts and colors. Infantile strabismus also affects other vision-dependent processes such as control of postural stability. But presently, rehabilitative therapies for infantile strabismus by ophthalmologists, orthoptists and optometrists are restricted to preventing or curing amblyopia of the deviated eye, aligning the eyes and, whenever possible, preserving or restoring binocular vision during the critical period of development, i.e., before ~10 years of age. All the other impairments are thus ignored; whether they may recover after strabismus treatment even remains unknown. We argue here that medical and paramedical professionals may extend their present treatments of the perceptual losses associated with infantile strabismus. This hypothesis is based on findings from fundamental research on visual system organization of higher mammals in particular at the cortical level. In strabismic subjects (as in normal-seeing ones), information about all of the visual attributes converge, interact and are thus inter-dependent at multiple levels of encoding ranging from the single neuron to neuronal assemblies in visual cortex. Thus if the perception of one attribute is restored this may help to rehabilitate the perception of other attributes. Concomitantly, vision-dependent processes may also improve. This could occur spontaneously, but still should be assessed and validated. If not, medical and paramedical staff, in collaboration with neuroscientists, will have to break new ground in the field of therapies to help reorganize brain circuitry and promote more comprehensive functional recovery. Findings from fundamental research studies in both young and adult patients already support our hypothesis and are reviewed here. For example, presenting different contrasts to each eye of a strabismic patient during training sessions facilitates recovery of acuity in the amblyopic eye as well as of 3D perception. Recent data also demonstrate that visual recoveries in strabismic subjects improve postural stability. These findings form the basis for a roadmap for future research and clinical development to extend presently applied rehabilitative therapies for infantile strabismus.

Analysis of sensory system aspects of postural stability during quiet standing in adolescent idiopathic scoliosis patients

Background

The aim of this study was to quantitatively analyze quite standing postural stability of adolescent idiopathic scoliosis (AIS) patients in respect to three sensory systems (visual, vestibular, and somatosensory).

Method

In this study, we analyzed the anterior-posterior center of pressure (CoP) signal using discrete wavelet transform (DWT) between AIS patients (n = 32) and normal controls (n = 25) during quiet standing.

Result

The energy rate (∆E_EYE%) of the CoP signal was significantly higher in the AIS group than that in the control group at levels corresponding to vestibular and somatosensory systems (p < 0.01).

Conclusions

This implies that AIS patients use strategies to compensate for possible head position changes and spinal asymmetry caused by morphological deformations of the spine through vestibular and somatosensory systems. This could be interpreted that such compensation could help them maintain postural stability during quiet standing. The interpretation of CoP signal during quiet standing in AIS patients will improve our understanding of changes in physical exercise ability due to morphological deformity of the spine. This result is useful for evaluating postural stability before and after treatments (spinal fusion, bracing, rehabilitation, and so on).

Visually fixating or tracking another person decreases balance control in young and older females walking in a real-world scenario

Balance control during overground walking was assessed in 10 young (23.6 ± 3.4) and 10 older (71.0 ± 5.5 years) healthy females during free gaze, and when fixating or tracking another person in an everyday use waiting room. Balance control was characterised by medial/lateral sacrum acceleration dispersion, and gaze fixations were simultaneously assessed with eye tracking equipment. The results showed decreased balance control when fixating a stationary (p = 0.003, g_av = 0.19) and tracking a walking (p = 0.027, g_av = 0.16) person compared to free gaze. The older adults exhibited reduced baseline stability throughout, but the decrease caused by the visual tasks was not more profound than the younger adults. The decreased balance control when fixating on or tracking the observed person was likely due to more challenging conditions for interpreting retinal flow, which facilitated less reliable estimates of self-motion through vision. The older adults either processed retinal flow during the tasks as effectively as the young adults, or they adopted a more rigid posture to facilitate visual stability, which masked any ageing effect of the visual tasks. The decrease in balance control, the first to be shown in this context, may warrant further investigation in those with ocular or vestibular dysfunction.

Gaze position interferes in body sway in young adults

Postural control is influenced by eye movements. Gaze fixation, which comprises a component of ocular vergence, is important in the acquisition of highly specific task information, but its relation to postural control is little investigated. The aim of the study was to investigate the effects of gaze fixation position (central and lateral fixations) on postural sway in young adults. Forty young adults with ages ranging from 20 to 35 years were invited to participate in the study. Postural sway was measured in quiet stance in bipedal support in three 60-s trials under the following conditions: gaze fixation on a target positioned in front of participant, gaze fixation on a target positioned on right side of participant, and gaze fixation on a target positioned on left side of participant. The following center of pressure parameters (COP) in the anteroposterior (AP) and mediolateral directions (ML) were analyzed for each of the trials: body sway displacement, mean velocity of sway, root mean square (RMS) of sway, and median frequency. In addition, detrended fluctuation analysis (DFA) exponent, in anteroposterior and medio-lateral directions, was calculated. The COP presented greater AP and ML displacement (p<0.03, effect size=1.37; and p<0.03, effect size=1.64, respectively) and RMS AP and ML (p<0.04, effect size=1.66; and p<0.02, effect size=2.50, respectively) for lateral gaze fixation compared to central gaze fixation. These results suggest that gaze fixation on a laterally positioned target increases body sway in anteroposterior and mediolateral directions.

Smooth pursuits decrease balance control during locomotion in young and older healthy females

Dynamic balance control-characterised as movement of the trunk and lower limbs-was assessed during fixation of a fixed target, smooth pursuits and saccadic eye movements in ten young (22.9 ± 1.5 years) and ten older (72.1 ± 8.2 years) healthy females walking overground. Participants were presented with visual stimuli to initiate eye movements, and posture and gaze were assessed with motion analysis and eye tracking equipment. The results showed an increase in medial/lateral (ML) trunk movement (C7: p = 0.012; sacrum: p = 0.009) and step-width variability (p = 0.052) during smooth pursuits compared to a fixed target, with no changes for saccades compared to a fixed target. The elders demonstrated greater ML trunk movement (sacrum: p = 0.037) and step-width variability (p = 0.037) than the younger adults throughout, although this did not interact with the eye movements. The findings showed that smooth pursuits decreased balance control in young and older adults similarly, which was likely a consequence of more complicated retinal flow. Since healthy elders are typically already at a postural disadvantage, further decreases in balance caused by smooth pursuits are undesirable.

Angle of gaze and optic flow direction modulate body sway

Optic flow is a crucial signal in maintaining postural stability. We sought to investigate whether the activity of postural muscles and body sway was modulated by eye position during the view of radial optic flow stimuli. We manipulated the spatial distribution of dot speed and the fixation point position to simulate specific heading directions combined with different gaze positions. The experiments were performed using stabilometry and surface electromyography (EMG) on 24 right-handed young, healthy volunteers. Center of pressure (COP) signals were analyzed considering antero-posterior and medio-lateral oscillation, COP speed, COP area, and the prevalent direction of oscillation of body sway. We found a significant main effect of body side in all COP parameters, with the right body side showing greater oscillations. The different combinations of optic flow and eye position evoked a non-uniform direction of oscillations in females. The EMG analysis showed a significant main effect for muscle and body side. The results showed that the eye position modulated body sway without changing the activity of principal leg postural muscles, suggesting that the extraretinal input regarding the eye position is a crucial signal that needs to be integrated with perceptual optic flow processing in order to control body sway.

Cortical Correlates of Human Balance Control

Balance control is a fundamental component of human every day motor activities such as standing or walking, and its impairment is associated with an increased risk of falling. However, in humans the exact neurobiological mechanisms underlying balance control are still unclear. Specifically, although previous studies have identified a number of cortical regions that become significantly activated during real or imagined balancing, the interactions within and between the relevant cortical regions remain to be investigated. The working hypothesis of this study is that cortical networks contribute to an optimization of balance control, and that this contribution can be revealed by partial directed coherence—a time-variant, frequency-selective and directed functional connectivity analysis tool. Electroencephalographic activity was recorded in 37 subjects during single-leg balancing on a stable as well as an unstable surface. Results of this study show that in the transition from balancing on a stable surface to an unstable surface, two topographically delimitable connectivity networks (weighted directed networks) are established; one associated with the alpha and one with the theta frequency band. The theta network sequence can be described as a set of subnetworks (modules) comprising the frontal, central and parietal cortex with individual temporal and spatial developments within and between those modules. In the alpha network, the occipital electrodes O1 and O2 act as a source, and the interactions propagate predominantly in the directions from occipital to parietal and to centro-parietal areas. These important findings indicate that balance control is supported by at least two functional cortical networks.

Saccadic Eye Movement Improves Plantar Sensation and Postural Balance in Elderly Women

Vision, proprioception and plantar sensation contribute to the control of postural balance (PB). Reduced plantar sensation alters postural response and is at an increased risk of fall, and eye movements reduce the postural sway. Therefore, the aim of this study was to study the improvement of plantar sensation and PB after saccadic eye movement (SEM) and pursuit eye movement (PEM) in community-dwelling elderly women. Participants (104 females; 75.11 ± 6.25 years) were randomly allocated into the SEM group (n = 52) and PEM groups (n = 52). The SEM group performed eye fixation and SEM for 5 minutes, and the PEM group performed eye fixation and PEM for 5 minutes. The plantar sensation was measured according to the plantar surface area of the feet in contact with the floor surface before and after the intervention. Before and after SEM and PEM with the eyes open and closed, PB was measured as the area (mm(2)), length (cm), and velocity (cm/s) of the fluctuation of the center of pressure (COP). The plantar sensation of both feet improved in both groups (p < 0.01). Significant decreases in the area, length, and velocity of the COP were observed in the eye open and close in both groups (p < 0.01). The length and velocity of the COP significantly decreased in the SEM group compared to the PEM group (p < 0.05). In conclusion, SEM and PEM are effective interventions for improving plantar sensation and PB in elderly women, with greater PB improvement after SEM.

Beyond deficit or compensation: new insights on postural control after long-term total visual loss

Loss of vision is well known to affect postural control in blind subjects. This effect has classically been framed in terms of deficit or compensation depending on whether body sway increases or decreases in comparison with that of sighted subjects with the eyes open. However, studies have shown that postural responses can be modulated by the context and that changes in postural sway may not necessarily mean a worsened or improved postural control. The goal of our study was to test whether balance is affected by the context in blind subjects. Additional to the quantification of center of pressure (COP) displacement, measurements of body motion (COG) and the correspondent net neuromuscular response (COP-COG) were evaluated in anterior-posterior and medial-lateral directions. Thirty-eight completely blind and thirty-two sighted subjects participated of this study. The volunteers were asked to stand barefoot on a force platform for 60 s in two different conditions: feet apart and feet together. Sighted participants performed the tests with both the eyes open and eyes closed. Results showed that the COP-COG displacements in the blind group were greater than those of the sighted group with eyes open in almost all conditions tested, but not in eyes closed condition. However, the COP and COG results confirmed that the postural responses were context dependent. Together these results suggest that total visual loss does not just lead to a balance deficit or compensation, but to a specific postural signature that might imply in enhancing COP, COG and/or COP-COG in specific postural conditions.

Eye Movements Affect Postural Control in Young and Older Females

Visual information is used for postural stabilization in humans. However, little is known about how eye movements prevalent in everyday life interact with the postural control system in older individuals. Therefore, the present study assessed the effects of stationary gaze fixations, smooth pursuits, and saccadic eye movements, with combinations of absent, fixed and oscillating large-field visual backgrounds to generate different forms of retinal flow, on postural control in healthy young and older females. Participants were presented with computer generated visual stimuli, whilst postural sway and gaze fixations were simultaneously assessed with a force platform and eye tracking equipment, respectively. The results showed that fixed backgrounds and stationary gaze fixations attenuated postural sway. In contrast, oscillating backgrounds and smooth pursuits increased postural sway. There were no differences regarding saccades. There were also no differences in postural sway or gaze errors between age groups in any visual condition. The stabilizing effect of the fixed visual stimuli show how retinal flow and extraocular factors guide postural adjustments. The destabilizing effect of oscillating visual backgrounds and smooth pursuits may be related to more challenging conditions for determining body shifts from retinal flow, and more complex extraocular signals, respectively. Because the older participants matched the young group's performance in all conditions, decreases of posture and gaze control during stance may not be a direct consequence of healthy aging. Further research examining extraocular and retinal mechanisms of balance control and the effects of eye movements, during locomotion, is needed to better inform fall prevention interventions.

Age-related effects of increasing postural challenge on eye movement onset latencies to visual targets

When a single light cue is given in the visual field, our eyes orient towards it with an average latency of 200 ms. If a second cue is presented at or around the time of the response to the first, a secondary eye movement occurs that represents a reorientation to the new target. While studies have shown that eye movement latencies to 'single-step' targets may or may not be lengthened with age, secondary eye movements (during 'double-step' displacements) are significantly delayed with increasing age. The aim of this study was to investigate whether the postural challenge posed simply by standing (as opposed to sitting) results in significantly longer eye movement latencies in older adults compared to the young. Ten young (<35 years) and 10 older healthy adults (>65 years) participated in the study. They were required to fixate upon a central target and move their eyes in response to 2 types of stimuli: (1) a single-step perturbation of target position either 15° to the right or left and (2) a double-step target displacement incorporating an initial target jump to the right or left by 15°, followed after 200 ms, by a shift of target position to the opposite side (e.g. +15° then -15°). All target displacement conditions were executed in sit and stand positions with the participant at the same distance from the targets. Eye movements were recorded using electro-oculography. Older adults did not show significantly longer eye movement latencies than the younger adults for single-step target displacements, and postural configuration (stand compared to sit) had no effect upon latencies for either group. We categorised double-step trials into those during which the second light changed after or before the onset of the eye shift to the first light. For the former category, young participants showed faster secondary eye shifts to the second light in the standing position, while the older adults did not. For the latter category of double-step trial, young participants showed no significant difference between sit and stand secondary eye movement latencies, but older adults were significantly longer standing compared to sitting. The older adults were significantly longer than the younger adults across both postural conditions, regardless of when the second light change occurred during the eye shift to the first light. We suggest that older adults require greater time and perhaps attentional processes to execute eye movements to unexpected changes in target position when faced with the need to maintain standing balance.

Sensorimotor and cognitive factors associated with the age-related increase of visual field dependence: a cross-sectional study

Reliance on the visual frame of reference for spatial orientation (or visual field dependence) has been reported to increase with age. This has implications on old adults' daily living tasks as it affects stability, attention, and adaptation capacities. However, the nature and underlying mechanisms of this increase are not well defined. We investigated sensorimotor and cognitive factors possibly associated with increased visual field dependence in old age, by considering functions that are both known to degrade with age and important for spatial orientation and sensorimotor control: reliance on the (somatosensory-based) egocentric frame of reference, visual fixation stability, and attentional processing of complex visual scenes (useful field of view, UFOV). Twenty young, 18 middle-aged, and 20 old adults completed a visual examination, three tests of visual field dependence (RFT, RDT, and GEFT), a test of egocentric dependence (subjective vertical estimation with the body erect and tilted at 70°), a visual fixation task, and a test of visual attentional processing (UFOV®). Increased visual field dependence with age was associated with reduced egocentric dependence, visual fixation stability, and visual attentional processing. In addition, visual fixation instability and reduced UFOV were correlated. Results of middle-aged adults fell between those of the young and old, revealing the progressive nature of the age effects we evaluated. We discuss results in terms of reference frame selection with respect to ageing as well as visual and non-visual information processing. Inter-individual differences amongst old adults are highlighted and discussed with respect to the functionality of increased visual field dependence.

Analysis of lower limb movement to determine the effect of manipulating the appearance of stairs to improve safety: a linked series of laboratory-based, repeated measures studies

Background

Falls on stairs are a common and dangerous problem for older people. This series of studies evaluated whether or not selected changes to the appearance of stairs could make them safer for older people to negotiate.

Objectives

To determine the effect of (1) a step edge highlighter and its position and (2) an optimised horizontal–vertical (H–V) visual illusion placed on a step riser on gait safety during stair descent and ascent.

Design

A series of studies using a repeated measures, laboratory-based design, investigating gait control and safety in independently mobile older people.

Setting

The University of Bradford Vision and Mobility Laboratory.

Participants

Fit and healthy older people aged 60 years of age or more, independently mobile, reasonably active and with normal healthy eyes and corrected vision.

Interventions

A step edge highlighter in a variety of offsets from the stair edge and an optimised H–V visual illusion placed on the stair riser. The H–V illusion was provided on a staircase by horizontal step edge highlighters on the tread edges and vertical stripes on the step risers.

Main outcome measures

Gait parameters that are important for safe stepping in ascent and descent, particularly toe clearance during stair ascent and heel clearance during stair descent.

Results

The step edge highlighter increased the precision of heel clearance during stepping and its positioning relative to the tread edge determined the extent of heel clearance over the tread edge. Positioning the highlighter away from the tread edge, as is not uncommonly provided by friction strips, decreased heel clearance significantly and led to greater heel scuffs. Although psychophysics experiments suggested that higher spatial frequencies of the H–V illusion might provide greater toe clearance on stair ascent, gait trials showed similar increased toe clearances for all spatial frequencies. When a 12 cycle per step spatial frequency H–V illusion was used, toe clearance increases of approximately 1 cm (17.5%) occurred without any accompanying changes in other important gait parameters or stability measures.

Conclusions

High-contrast tread edge highlighters present on steps and stairs and positioned flush with the edge of the tread or as near to this as possible should improve stair descent safety in older people. A H–V illusion positioned on the riser of a raised surface/walkway (e.g. kerbs) and/or the top and/or bottom of a stairway is likely to increase foot clearance over the associated step/stair edge, and appears not to lead to any decrement in postural stability. Thus, their use is likely to reduce trip risk and hence improve stair ascent safety. The effect of the step and stair modifications should be assessed in older people with visual impairment. The only other remaining assessment that could be made would be to assess fall prevalence on steps and stairs, perhaps in public buildings, with and without these modifications.

Funding

The National Institute for Health Research Public Health Research programme.

Saccadic and smooth pursuit eye movements attenuate postural sway similarly

Saccadic eye movements reduce body sway, yet visually pursuing a moving dot seems to increase body sway. However, how these two types of eye movements affect postural control remains ambiguous, particularly for smooth pursuit eye movements. The aim of this study was to examine the effects of saccade and smooth pursuit eye movements on body sway magnitude during low and high frequencies. Ten young adults (19.5 ± 1.9 years) participants were required to stand upright, barefoot for 70s using a bipedal stance, with feet hip width apart, fixating or pursuing a target that was displayed on a monitor positioned 100 cm away from their eyes. Each participant performed three trials using both types of eye movements, in particular, slow and fast saccades, and slow and fast smooth pursuit movements. Body sway was obtained using reflective markers attached to a participant's head and trunk, which were recorded by two video cameras. The results indicated that body sway was reduced during both saccadic eye movements and smooth pursuit movements when compared to fixation, independent of visual frequencies. These results suggested similarities in the control of saccades and smooth pursuit on postural control.

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.

The Glenn A. Fry award lecture 2013: blurred vision, spectacle correction, and falls in older adults

This article reviews the literature on how blurred vision contributes to falls, gait, and postural control and discusses how these are influenced by spectacle correction. Falls are common and represent a very serious health risk for older people. They are not random events as studies have shown that falls are linked to a range of intrinsic and extrinsic risk factors. Vision provides a significant input to postural control in addition to providing information about the size and position of hazards and obstacles in the travel pathway and allows us to safely negotiate steps and stairs. Many studies have shown that reduced vision is a significant risk factor for falls. However, randomized controlled trials of optometric interventions and cataract surgery have not shown the expected reduction in falls rate, which may be due to magnification changes (and thus vestibulo-ocular reflex gain) in those participants who have large changes in refractive correction. Epidemiological studies have also shown that progressive addition lens and bifocal wearers are twice as likely to fall as non-multifocal wearers, laboratory-based studies have shown safer adaptive gait with single-vision glasses than progressive addition lenses or bifocals, and a randomized controlled trial has shown that an additional pair of distance vision single-vision glasses for outdoor use can reduce falls rate. Clinical recommendations to help optometrists prevent their frail, older patients from falling are suggested.