Postural adaptations to repeated optic flow stimulation in older adults

Adaptation of Postural Sway in a Standing Position during Tilted Video Viewing Using Virtual Reality: A Comparison between Younger and Older Adults

This study aimed to investigate the effects of wearing virtual reality (VR) with a head-mounted display (HMD) on body sway in younger and older adults. A standing posture with eyes open without an HMD constituted the control condition. Wearing an HMD and viewing a 30°-tilt image and a 60°-tilt image in a resting standing position were the experimental conditions. Measurements were made using a force plate. All conditions were performed three times each and included the X-axis trajectory length (mm), Y-axis trajectory length (mm), total trajectory length (mm), trajectory length per unit time (mm/s), outer peripheral area (mm2), and rectangular area (mm2). The results showed a significant interaction between generation and condition in Y-axis trajectory length (mm) and total trajectory length (mm), with an increased body center-of-gravity sway during the viewing of tilted VR images in older adults than in younger adults in both sexes. The results of this study show that body sway can be induced by visual stimulation alone with VR without movement, suggesting the possibility of providing safe and simple balance training to older adults.

Visual Field Dependence Persists in Age-Related Central Visual Field Loss

Purpose

To examine whether the age-related increase in visual field dependence persists in older adults with central field loss (CFL).

Methods

Twenty individuals with CFL were grouped into participants with age-related binocular CFL (CFL, n = 9), age-related monocular CFL/relative scotomata (mCFL, n = 8), and CFL occurring at a young age (yCFL, n = 3). Seventeen controls were age-matched to the older CFL groups (OA) and three to the yCFL group (yOA). Participants judged the tilt direction of a rod presented at various orientations under conditions with and without a visual reference. Visual field dependence was determined as the difference in judgment bias between trials with and without the visual reference. Visual field dependence was examined between groups and relative to visual acuity and contrast sensitivity.

Results

All older groups performed similarly without the visual reference. The CFL group showed greater visual field dependence than the OA group (Mann-Whitney U test; U = 39, P = 0.045). However, there was no group difference when considering all three older groups (Kruskal-Wallis ANOVA; H(2, N = 34) = 4.31, P = 0.116). Poorer contrast sensitivity correlated with greater visual field dependence (P = 0.017; ρ = -0.43).

Conclusions

Visual field dependence persists in older adults with CFL and seems exacerbated in those with dense binocular scotomata. This could be attributed to the sensitivity of the spared peripheral retina to orientation and motion cues. The relationship with contrast sensitivity further suggests that a decline in visual function is associated with an increase in visual field dependence beyond the effects of normal aging. These observations can guide tailored care and rehabilitation in older adults with CFL.

Sensory conflicts through short, discrete visual input manipulations: Identification of balance responses to varied input characteristics

Human balance control relies on various sensory modalities, and conflict of sensory input may result in postural instability. Virtual reality (VR) technology allows to train balance under conflicting sensory information by decoupling visual from somatosensory and vestibular systems, creating additional demands on sensory reweighting for balance control. However, there is no metric for the design of visual input manipulations that can induce persistent sensory conflicts to perturb balance. This limits the possibilities to generate sustained sensory reweighting processes and design well-defined training approaches. This study aimed to investigate the effects that different onset characteristics, amplitudes and velocities of visual input manipulations may have on balance control and their ability to create persistent balance responses. Twenty-four young adults were recruited for the study. The VR was provided using a state-of-the-art head-mounted display and balance was challenged in two experiments by rotations of the visual scene in the frontal plane with scaled constellations of trajectories, amplitudes and velocities. Mean center of pressure speed was recorded and revealed to be greater when the visual input manipulation had an abrupt onset compared to a smooth onset. Furthermore, the balance response was greatest and most persistent when stimulus velocity was low and stimulus amplitude was large. These findings show clear dissociation in the state of the postural system for abrupt and smooth visual manipulation onsets with no indication of short-term adaption to abrupt manipulations with slow stimulus velocity. This augments our understanding of how conflicting visual information affect balance responses and could help to optimize the conceptualization of training and rehabilitation interventions.

Sinusoidal Optic Flow Perturbations Reduce Transient but Not Continuous Postural Stability: A Virtual Reality-Based Study

Optic flow perturbations induced by virtual reality (VR) are increasingly used in the rehabilitation of postural control and gait. Here, VR offers the possibility to decouple the visual from the somatosensory and vestibular system. By this means, it enables training under conflicting sensorimotor stimulation that creates additional demands on sensory reweighting and balance control. Even though current VR-interventions still lack a well-defined standardized metric to generate optic flow perturbations that can challenge balance in a repeatable manner, continuous oscillations of the VR are typically used as a rehabilitation tool. We therefore investigated if continuous sensory conflicts induced by optic flow perturbations can challenge the postural system sustainably. Eighteen young adults (m = 8, f = 10, age = 24.1 ± 2.0 yrs) were recruited for the study. The VR was provided using a state-of-the-art head-mounted display including the virtual replica of the real environment. After familiarization in quiet stance without and with VR, bipedal balance was perturbed by sinusoidal rotations of the visual scenery in the sagittal plane with an amplitude of 8° and a frequency of 0.2 Hz. Postural stability was quantified by mean center of mass speed derived from 3D-kinematics. A rmANOVA found increased postural instability only during the first perturbation cycle, i.e., the first 5 s. Succeeding the first perturbation cycle, visual afferents were downregulated to reduce the destabilizing influence of the sensory conflicts. In essence, only the transient beginning of sinusoidal oscillation alters balance compared to quiet standing. Therefore, continuous sinusoidal optic flow perturbations appear to be not suitable for balance training as they cannot trigger persisting sensory conflicts and hence challenge the postural system sustainably. Our study provides rationale for using unexpected and discrete optic flow perturbation paradigms to induce sustainable sensory conflicts.

Age-related changes in the neuromuscular control of forward and backward locomotion

Previous studies found significant modification in spatiotemporal parameters of backward walking in healthy older adults, but the age-related changes in the neuromuscular control have been considered to a lesser extent. The present study compared the intersegmental coordination, muscle activity and corresponding modifications of spinal montoneuronal output during both forward and backward walking in young and older adults. Ten older and ten young adults walked forward and backward on a treadmill at different speeds. Gait kinematics and EMG activity of 14 unilateral lower-limb muscles were recorded. As compared to young adults, the older ones used shorter steps, a more in-phase shank and foot motion, and the activity profiles of muscles innervated from the sacral segments were significantly wider in each walking condition. These findings highlight age-related changes in the neuromuscular control of both forward and backward walking. A striking feature of backward walking was the differential organization of the spinal output as compared to forward gait. In addition, the resulting spatiotemporal map patterns also characterized age-related changes of gait. Finally, modifications of the intersegmental coordination with aging were greater during backward walking. On the whole, the assessment of backward walk in addition to routine forward walk may help identifying or unmasking neuromuscular adjustments of gait to aging.

Effects of Combined Balance and Strength Training on Measures of Balance and Muscle Strength in Older Women With a History of Falls

OBJECTIVE

We investigated the effects of combined balance and strength training on measures of balance and muscle strength in older women with a history of falls.

METHODS

Twenty-seven older women aged 70.4 ± 4.1 years (age range: 65 to 75 years) were randomly allocated to either an intervention (IG, n = 12) or an active control (CG, n = 15) group. The IG completed 8 weeks combined balance and strength training program with three sessions per week including visual biofeedback using force plates. The CG received physical therapy and gait training at a rehabilitation center. Training volumes were similar between the groups. Pre and post training, tests were applied for the assessment of muscle strength (weight-bearing squat [WBS] by measuring the percentage of body mass borne by each leg at different knee flexions [0°, 30°, 60°, and 90°], sit-to-stand test [STS]), and balance. Balance tests used the modified clinical test of sensory interaction (mCTSIB) with eyes closed (EC) and opened (EO), on stable (firm) and unstable (foam) surfaces as well as spatial parameters of gait such as step width and length (cm) and walking speed (cm/s).

RESULTS

Significant group × time interactions were found for different degrees of knee flexion during WBS (0.0001 < p < 0.013, 0.441 < d < 0.762). Post hoc tests revealed significant pre-to-post improvements for both legs and for all degrees of flexion (0.0001 < p < 0.002, 0.697 < d < 1.875) for IG compared to CG. Significant group × time interactions were found for firm EO, foam EO, firm EC, and foam EC (0.006 < p < 0.029; 0.302 < d < 0.518). Post hoc tests showed significant pre-to-post improvements for both legs and for all degrees of oscillations (0.0001 < p < 0.004, 0.753 < d < 2.097) for IG compared to CG. This study indicates that combined balance and strength training improved percentage distribution of body weight between legs at different conditions of knee flexion (0°, 30°, 60°, and 90°) and also decreased the sway oscillation on a firm surface with eyes closed, and on foam surface (with eyes opened or closed) in the IG.

CONCLUSION

The higher positive effects of training seen in standing balance tests, compared with dynamic tests, suggests that balance training exercises including lateral, forward, and backward exercises improved static balance to a greater extent in older women.

Aging-related changes in cortical mechanisms supporting postural control during base of support and optic flow manipulations

Behavioral findings suggest that aging alters the involvement of cortical sensorimotor mechanisms in postural control. However, corresponding accounts of the underlying neural mechanisms remain sparse, especially the extent to which these mechanisms are affected during more demanding tasks. Here, we set out to elucidate cortical correlates of altered postural stability in younger and older adults. 3D body motion tracking and high-density electroencephalography (EEG) were measured while 14 young adults (mean age = 24 years, 43% women) and 14 older adults (mean age = 77 years, 50% women) performed a continuous balance task under four different conditions. Manipulations were applied to the base of support (either regular or tandem (heel-to-toe) stance) and visual input (either static visual field or dynamic optic flow). Standing in tandem, the more challenging position, resulted in increased sway for both age groups, but for the older adults, only this effect was exacerbated when combined with optic flow compared to the static visual display. These changes in stability were accompanied by neuro-oscillatory modulations localized to midfrontal and parietal regions. A cluster of electro-cortical sources localized to the supplementary motor area showed a large increase in theta spectral power (4-7 Hz) during tandem stance, and this modulation was much more pronounced for the younger group. Additionally, the older group displayed widespread mu (8-12 Hz) and beta (13-30 Hz) suppression as balance tasks placed more demands on postural control, especially during tandem stance. These findings may have substantial utility in identifying early cortical correlates of balance impairments in otherwise healthy older adults.

Inter-trial phase coherence of visually evoked postural responses in virtual reality

Vision plays a central role in maintaining balance. When humans perceive their body as moving, they trigger counter movements. This results in body sway, which has typically been investigated by measuring the body’s center of pressure (COP). Here, we aimed to induce visually evoked postural responses (VEPR) by simulating self-motion in virtual reality (VR) using a sinusoidally oscillating “moving room” paradigm. Ten healthy subjects participated in the experiment. Stimulation consisted of a 3D-cloud of random dots, presented through a VR headset, which oscillated sinusoidally in the anterior–posterior direction at different frequencies. We used a force platform to measure subjects’ COP over time and quantified the resulting trajectory by wavelet analyses including inter-trial phase coherence (ITPC). Subjects exhibited significant coupling of their COP to the respective stimulus. Even when spectral analysis of postural sway showed only small responses in the expected frequency bands (power), ITPC revealed an almost constant strength of coupling to the stimulus within but also across subjects and presented frequencies. Remarkably, ITPC even revealed a strong phase coupling to stimulation at 1.5 Hz, which exceeds the frequency range that has generally been attributed to the coupling of human postural sway to an oscillatory visual scenery. These findings suggest phase-locking to be an essential feature of visuomotor control.

Optic flow and attention alter locomotion differently in the young and old

BACKGROUND

Optic flow is used in the control of walking speed and aids in navigation through space. However, the influence of attention on optic flow processing during locomotion is not known.

RESEARCH QUESTION

Does attentional focus influence the processing of optic flow during locomotion in young and older adults?

METHODS

Auditory and visual concurrent reaction time tasks were conducted while walking on a treadmill within an anterior-posterior modulating optic flow field in young and older adults. Optic flow was generated with full field back-projected scenes shown while walking on a treadmill under three conditions: a) optic flow consistent with the treadmill speed, b) slow sinusoidal modulation of the speed (SINE), and c) reversals of optic flow velocity from congruent with walking to the opposite direction (REV). Movement in response to the scenes along with reaction times were measured.

RESULTS

The optic flow perturbations altered movement on the treadmill. Older adults responded more than young adults during the slowly changing sinusoidal perturbations, but not to the rapid reversing scenes. Our main hypothesis that sensory modality of a concurrent cognitive task influences the processing of optic flow was confirmed for the reversing optic flow condition but not for the sinusoidal optic flow. The impact of optic flow conditions on reaction times was only found during the REV condition, with impact on visual RTs being greater than auditory RTs.

SIGNIFICANCE

Taken together, the results suggest attentional focus on sensory modality of concurrent tasks while walking can impact optic flow processing for navigation and control; however, the characteristics of the optic flow (e.g. perturbation speed) play an important role.

Assessment of visual sensation, psychiatric profile and quality of life following vestibular schwannoma surgery in patients prehabituated by chemical vestibular ablation

AIMS

Preoperative chemical vestibular ablation can reduce vestibular symptoms in patients who have gone through vestibular schwannoma resection. The goal of this study was to determine whether chemical vestibular prehabituation influences the patients' post-operative perception of visual stimulation, mental status and quality of life. We also tried to find out whether increases of optokinetic nystagmus, measured by routine electronystagmography, correlate with subjective symptoms.

METHODS

We preoperatively administered (2 months prior to surgery) 0.5 - 1.0 mL of 40 mg/mL nonbuffered gentamicin in three intratympanic instillations in 11 patients. Head impulse and caloric tests confirmed reduction of vestibular function in all patients. The control group consisted of 21 patients. Quality of life in both groups was evaluated using the Glasgow Benefit Inventory, the Glasgow Health Status Inventory and the Dizziness Handicap Inventory questionnaires. Visual symptoms and optokinetic sensation were evaluated using a specific questionnaire developed by our team and by measuring gains preoperatively and postoperatively in both groups using routine electronystagmography. The psychological profile was evaluated using the Zung Self-Rating Depression Scale and the Generalised Anxiety Disorder Assessment questionnaires.

RESULTS

There were no statistically significant differences between both groups with regards to the results of the questionnaires. Patients who received preoperative gentamicin were less sensitive to visual stimulation (P<0.10) and many of them had a significantly higher gain in the optokinetic nystagmus than the control group in the preoperative stage.

CONCLUSION

Pre-treatment with gentamicin helps to lower anxiety levels in patients and improves their general postoperative status. Pre-treated patients are also less sensitive to optokinetic stimulation.

TRIAL REGISTRATION

ClinicalTrials.gov, NCT03638310.

Aging Affects the Ability to Process the Optic Flow Stimulations: A Functional Near-Infrared Spectrometry Study

Optic flow (OF) has been utilized to investigate the sensory integration of visual stimuli during postural control. It is little known how the OF speed affects the aging brain during the sensory integration process of postural control. This study was to examine the effect of OF speeds on the brain activation using functional near-infrared spectroscopy (fNIRS) and postural sway between younger and older adults. Eleven healthy younger adults (5M/6F, age 22 ± 1-year-old) and ten healthy older adults (4M/6F, age 71 ± 5-year-old) participated in this study. A virtual reality headset was used to provide the OF stimulus at different speeds. A forceplate was used to record the center-of-pressure to compute the amplitude of postural sway (peak-to-peak). Compared with younger adults, older adults showed significantly increased activation in the OF speed of 10 m/s and decreased activation in the OF speed of 20 m/s in the left dorsolateral prefrontal cortex. Older adults also showed decreased activation in the left temporoparietal region (VEST) in the OF speed of 20 m/s. A significant difference in peak-to-peak was found between groups. Our results indicated that age might be associated with the ability to process fast OF stimulation.

Postural control adaptation to optic flow in children and adults with autism spectrum disorder

BACKGROUND

Sensory reweighting is important for humans to flexibly up-weigh and down-weigh sensory information in dynamic environments. There is an element of time involved in the sensory reweighting process. A longer time spent on sensory reweighting may increase the destabilizing effect of postural control. Individuals with autism spectrum disorder (ASD) are reported to have poor postural control. It is uncertain if a different sensory reweighting process underlies the postural control deficit in children and adults with ASD.

RESEARCH QUESTION

To explore the sensory reweighting capability in ASD, the present study examined whether the temporal domains of postural control differed in children and adults, with and without ASD under various optic flow conditions.

METHODS

Thirty-three children (8-12 years old) and 33 adults (18-50 years old) with and without ASD underwent quiet standing in six radial optic flow conditions. Each condition lasted for 60 s and was shown twice to all participants. For each optic flow condition, changes in postural response within-trial and between-trials were measured.

RESULTS

Under various optic flow illusions, both children with and without ASD took a longer time to restore their posture compared with adults with and without ASD. Nonetheless, all groups demonstrated comparable abilities to adjust their posture to one that is close to the baseline position after one exposure to the optic flow stimulation.

SIGNIFICANCE

The present study showed that the temporal domains of postural control under different optic flow conditions were similar between individuals with and without ASD from the same age group. The ability to down-weigh visual information efficiently comes with the developmental progression of the sensory reweighting system. These findings suggest that the sensory reweighting process does not elucidate the postural control deficits in individuals with ASD and thus alternative explanations to determine the underlying mechanism for postural instability are needed.

Age-related changes in leg proprioception: implications for postural control

In addition to being a prerequisite for many activities of daily living, the ability to maintain steady upright standing is a relevant model to study sensorimotor integrative function. Upright standing requires managing multimodal sensory inputs to produce finely tuned motor output that can be adjusted to accommodate changes in standing conditions and environment. The sensory information used for postural control mainly arises from the vestibular system of the inner ear, vision, and proprioception. Proprioception (sense of body position and movement) encompasses signals from mechanoreceptors (proprioceptors) located in muscles, tendons, and joint capsules. There is general agreement that proprioception signals from leg muscles provide the primary source of information for postural control. This is because of their exquisite sensitivity to detect body sway during unperturbed upright standing that mainly results from variations in leg muscle length induced by rotations around the ankle joint. However, aging is associated with alterations of muscle spindles and their neural pathways, which induce a decrease in the sensitivity, acuity, and integration of the proprioceptive signal. These alterations promote changes in postural control that reduce its efficiency and thereby may have deleterious consequences for the functional independence of an individual. This narrative review provides an overview of how aging alters the proprioceptive signal from the legs and presents compelling evidence that these changes modify the neural control of upright standing.

Balance and gait in the elderly: A contemporary review

Background

The prevalence of balance and gait deficits increases with age and is associated with the increased incidence of falls seen in the elderly population; these falls are associated with significant morbidity and mortality.

Objectives

To review changes in gait and balance associated with aging and the effect of visual perturbations on gait and balance in the elderly to provide a basis for future research.

Methods

PubMed and Cochrane Library were searched for articles from 1980 to present pertaining to gait and balance in older adults (>60) and younger adults (<60). Search terms included balance, posture, gait, locomotion, gait variability, gait disorders, gait disturbance, elderly, aging, falls, vision, visual, vestibular, and virtual reality. The references section of queried articles was also used to find relevant studies. Studies were excluded if subjects had a diagnosed gait or balance disorder.

Results

Elderly adults show age-related decline in sensory systems and reduced ability to adapt to changes in their environment to maintain balance. Elderly adults are particularly dependent on vision to maintain postural stability. Distinct changes in spatiotemporal gait parameters are associated with aging, such as slower gait and increased gait variability, which are amplified with exposure to visual perturbations. Increased gait variability, specifically with mediolateral perturbations, poses a particular challenge for elderly adults and is linked to increased falls risk. Virtual reality training has shown promising effects on balance and gait.

Conclusion

Elderly adults show age-related decline in balance and gait with increased gait variability and an associated increased risk of falls.

Level of Evidence

5.

Slowed sensory reweighting and postural illusions in older adults: the moving platform illusion

We investigated whether postural aftereffects witnessed during transitions from a moving to a stable support are accompanied by a delayed perception of platform stabilization in older adults, in two experiments. In experiment 1, postural sway and muscle cocontraction were assessed in 11 healthy young, 11 healthy older, and 11 fall-prone older adults during blindfolded stance on a fixed platform, followed by a sway-referenced platform and then by a fixed platform again. The sway-referenced platform was more compliant for young adults, to induce similar levels of postural sway in both age groups. Participants were asked to press a button whenever they perceived that the platform had stopped moving. Both older groups showed significantly larger and longer postural sway aftereffects during platform stabilization compared with young adults, which were pronounced in fall-prone older adults. In both older groups elevated muscle cocontraction aftereffect was also witnessed. Importantly, these aftereffects were accompanied by an illusory perception of prolonged platform movement. After this, experiment 2 examined whether this illusory perception was a robust age effect or an experimental confound due to greater surface compliance in young adults, which could create a larger perceptual discrepancy between moving and stable conditions. Despite exposure to the same surface compliance levels during sway-reference, the perceptual illusion was maintained in experiment 2 in a new group of 14 healthy older adults compared with 11 young adults. In both studies, older adults took five times longer than young adults to perceive platform stabilization. This supports that sensory reweighting is inefficient in older adults. NEW & NOTEWORTHY This is the first paper to show that postural sway aftereffects witnessed in older adults after platform stabilization may be due to a perceptual illusion of platform movement. Surprisingly, in both experiments presented it took older adults five times longer than young adults to perceive platform stabilization. This supports a hypothesis of less efficient sensory reintegration in this age group, which may delay the formation of an accurate postural percept.

The motor repertoire of older adult fallers may constrain their response to balance perturbations

Older adults are at a high risk of falls, and most falls occur during locomotor activities like walking. This study aimed to improve our understanding of changes in neuromuscular control associated with increased risk of falls in older adults in the presence of dynamic balance challenges during walking. Motor module (also known as muscle synergy) analyses identified changes in the neuromuscular recruitment of leg muscles during walking with and without perturbations designed to elicit the visual perception of lateral instability. During normal walking we found that a history of falls (but not age) was associated with reduced motor module complexity and that age (but not a history of falls) was associated with increased step-to-step variability of module recruitment timing. Furthermore, motor module complexity was unaltered in the presence of optical flow perturbations. The specific effects of a history of falls on leg muscle recruitment included an absence and/or inability to independently recruit motor modules normally recruited to perform biomechanical functions important for walking balance control. These results suggest that fallers do not recruit the appropriate motor modules necessary for well-coordinated walking balance control even in the presence of perturbations. The identified changes in the modular control of walking balance in older fallers may either represent a neural deficit that leads to poor balance control or a prior history of falls that results in a compensatory motor adaptation. In either case, our study provides initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking. NEW & NOTEWORTHY This is the first study to demonstrate a reduced motor repertoire during walking in older adults with a history of falls but without any overt neurological deficits. Furthermore, using virtual reality during walking to elicit the visual perception of lateral instability, we provide initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking.

Changes in Regional Brain Grey-Matter Volume Following Successful Completion of a Sensori-Motor Intervention Targeted at Healthy and Fall-Prone Older Adults

Previous studies have suggested that discrete cross-sensory events could be incorrectly combined in the brain of older adults with a history of falls, possibly undermining motor and balance control. Based on previous findings that multisensory integration is modifiable with practice, even in an ageing population, we designed a serious game, named CityQuest, to train typical, everyday multisensory processes including sensori-motor control, spatial navigation, obstacle avoidance and balance control. Played over several sessions, this game was shown to improve these functions in older adults with and without a history of falls, depending on the specific condition of the game on which they were trained. Here, using voxel-based morphometry analysis of anatomical magnetic resonance imaging (MRI) data, we investigated structural changes in the brain of a smaller group of older adults from those who successfully completed this five-week intervention. A grey-matter (GM) volume increase in the precentral gyrus, and GM volume reduction in the inferior temporal and orbitofrontal gyri, was found for all participants. Changes in GM volume within regions of the cerebellum were differentially associated with fall-prone and healthy older adults. Furthermore, a greater GM volume increase in the precentral gyrus was observed in participants who performed the full CityQuest intervention relative to those required to avoid obstacles only. Our results support previous evidence that multisensory training can affect structural changes in the older brain and have implications for programmes designed for the successful rehabilitation of perceptual and cognitive functions.

The Rubber Hand Illusion in Healthy Younger and Older Adults

Percepts about our body's position in space and about body ownership are informed by multisensory feedback from visual, proprioceptive, and tactile inputs. The Rubber Hand Illusion (RHI) is a multisensory illusion that is induced when an observer sees a rubber hand being stroked while they feel their own, spatially displaced, and obstructed hand being stroked. When temporally synchronous, the visual-tactile interactions can create the illusion that the rubber hand belongs to the observer and that the observer's real hand is shifted in position towards the rubber hand. Importantly, little is understood about whether these multisensory perceptions of the body change with older age. Thus, in this study we implemented a classic RHI protocol (synchronous versus asynchronous stroking) with healthy younger (18-35) and older (65+) adults and measured the magnitude of proprioceptive drift and the subjective experience of body ownership. As an adjunctive objective measure, skin temperature was recorded to evaluate whether decreases in skin temperature were associated with illusory percepts, as has been shown previously. The RHI was observed for both age groups with respect to increased drift and higher ratings of ownership following synchronous compared to asynchronous stroking. Importantly, no effects of age and no interactions between age and condition were observed for either of these outcome measures. No effects were observed for skin temperature. Overall, these results contribute to an emerging field of research investigating the conditions under which age-related differences in multisensory integration are observed by providing insights into the role of visual, proprioceptive, and tactile inputs on bodily percepts.

Neuroimaging of an attention demanding dual-task during dynamic postural control

Cognitive tasks impact postural control when performed concurrently as dual-tasks. This is presumed to result from capacity limitations in relevant brain regions. We used functional near-infrared spectroscopy (fNIRS) to measure brain activation of the left motor, temporal, and dorsal-lateral prefrontal brain regions of younger (n=6) and older (n=10) adults. Brain activation was measured during an auditory choice reaction task (CRT) and standing on a dynamic posturography platform, both as single-tasks and concurrently as dual-task. Body sway was assessed by median absolute deviation (MAD) of anterior-posterior translation of the center of mass (COM). Brain activation was measured as changes in oxy-hemoglobin by fNIRS. During both single- and dual-task conditions, we found that older adults had greater brain activation relative to younger adults. During dual task performance, the total activation was less than expected from the sum of individual conditions for both age groups, indicating a dual-task interference (reduction in younger adults=53% [p=0.02]; in older adults=53%; [p=0.008]). This reduction was greater for the activation attributable to the postural task (reduction younger adults=75% [p=0.03]; older adults=59% [p=0.005]) compared to the CRT task (reduction younger adults=10%, [p=0.6]; older adults=7.3%, [p=0.5]) in both age groups. Activation reduction was not accompanied by any significant changes in body sway in either group (older adults: single-task MAD=0.94cm, dual-task MAD=1.10cm, p=0.20; younger adults: single-task RMS=0.95cm, dual-task MAD=1.08cm, p=0.14). Our results indicate that neural resources devoted to postural control are reduced under dual-task conditions that engage attention.

Functional near-infrared spectroscopy (fNIRS) brain imaging of multi-sensory integration during computerized dynamic posturography in middle-aged and older adults

Studies suggest that aging affects the sensory re-weighting process, but the neuroimaging evidence is minimal. Functional Near-Infrared Spectroscopy (fNIRS) is a novel neuroimaging tool that can detect brain activities during dynamic movement condition. In this study, fNIRS was used to investigate the hemodynamic changes in the frontal-lateral, temporal-parietal, and occipital regions of interest (ROIs) during four sensory integration conditions that manipulated visual and somatosensory feedback in 15 middle-aged and 15 older adults. The results showed that the temporal-parietal ROI was activated more when somatosensory and visual information were absent in both groups, which indicated the sole use of vestibular input for maintaining balance. While both older adults and middle-aged adults had greater activity in most brain ROIs during changes in the sensory conditions, the older adults had greater increases in the occipital ROI and frontal-lateral ROIs. These findings suggest a cortical component to sensory re-weighting that is more distributed and requires greater attention in older adults.

Vertigo Perception and Quality of Life in Patients after Surgical Treatment of Vestibular Schwannoma with Pretreatment Prehabituation by Chemical Vestibular Ablation

Surgical removal of vestibular schwannoma causes acute vestibular symptoms, including postoperative vertigo and oscillopsia due to nystagmus. In general, the dominant symptom postoperatively is vertigo. Preoperative chemical vestibular ablation can reduce vestibular symptoms postoperatively. We used 1.0 mL of 40 mg/mL nonbuffered gentamicin in three intratympanic installations over 2 days, 2 months preoperatively in 10 patients. Reduction of vestibular function was measured by the head impulse test and the caloric test. Reduction of vestibular function was found in all gentamicin patient groups. After gentamicin vestibular ablation, patients underwent home vestibular exercising for two months. The control group consisted of 10 patients who underwent only home vestibular training two months preoperatively. Postoperative rates of recovery and vertigo in both groups were evaluated with the Glasgow Benefit Inventory (GBI), the Glasgow Health Status Inventory (GHSI), and the Dizziness Handicap Inventory questionnaires, as well as survey of visual symptoms by specific questionnaire developed by us. There were no statistically significant differences between both groups with regard to the results of questionnaires. Patients who received preoperative gentamicin were more resilient to optokinetic and optic flow stimulation (p < 0.05). This trial is registered with clinical study registration number NCT02963896.

The effect of age on postural and cognitive task performance while using vibrotactile feedback

Vibrotactile feedback (VTF) has been shown to improve balance performance in healthy people and people with vestibular disorders in a single-task experimental condition. It is unclear how age-related changes in balance affect the ability to use VTF and if there are different attentional requirements for old and young adults when using VTF. Twenty younger and 20 older subjects participated in this two-visit study to examine the effect of age, VTF, sensory condition, cognitive task, duration of time, and visit on postural and cognitive performance. Postural performance outcome measures included root mean square of center of pressure (COP) and trunk tilt, and cognitive performance was assessed using the reaction time (RT) from an auditory choice RT task. The results showed that compared with younger adults, older adults had an increase in COP in fixed platform conditions when using VTF, although they were able to reduce COP during sway-referenced platform conditions. Older adults also did not benefit fully from using VTF in their first session. The RTs for the secondary cognitive tasks increased significantly while using the VTF in both younger and older adults. Older adults had a larger increase compared with younger adults, suggesting that greater attentional demands were required in older adults when using VTF information. Future training protocols for VTF should take into consideration the effect of aging.

Visual reliance for balance control in older adults persists when visual information is disrupted by artificial feedback delays

Sensory information from our eyes, skin and muscles helps guide and correct balance. Less appreciated, however, is that delays in the transmission of sensory information between our eyes, limbs and central nervous system can exceed several 10s of milliseconds. Investigating how these time-delayed sensory signals influence balance control is central to understanding the postural system. Here, we investigate how delayed visual feedback and cognitive performance influence postural control in healthy young and older adults. The task required that participants position their center of pressure (COP) in a fixed target as accurately as possible without visual feedback about their COP location (eyes-open balance), or with artificial time delays imposed on visual COP feedback. On selected trials, the participants also performed a silent arithmetic task (cognitive dual task). We separated COP time series into distinct frequency components using low and high-pass filtering routines. Visual feedback delays affected low frequency postural corrections in young and older adults, with larger increases in postural sway noted for the group of older adults. In comparison, cognitive performance reduced the variability of rapid center of pressure displacements in young adults, but did not alter postural sway in the group of older adults. Our results demonstrate that older adults prioritize vision to control posture. This visual reliance persists even when feedback about the task is delayed by several hundreds of milliseconds.

Posture and cognition in the elderly: interaction and contribution to the rehabilitation strategies

In this paper we review the effects of aging on sensory systems and their impact on posture, balance and gait. We also address cognitive aging and attempt to specify which altered cognitive functions negatively impact balance and walking. The role of cognition in postural control is tested with dual-task experiments. This situation results in deleterious effects due to an attentional overload. Given the human cognitive system has limited capacities, we propose that simultaneously performing two tasks depends on the capacity of each individual to perform these tasks on a continuum between automatic execution to highly controlled performance. A level of maximum control exceeds the subject's attentional capacity, which makes it impossible to perform both tasks simultaneously. The subject therefore prioritizes one of the tasks. We use representative dual-task studies from the literature to illustrate the relationship between the different cognitive components and their impact on the control of posture and gait in elderly subjects with altered cognitive capacities and with elderly subjects who are fallers or who have altered sensory-motor capacities. Recently this postural-cognitive relationship was addressed with a new approach. We report how cognitive training can improve dual-task management and we attempt to define the cognitive mechanisms that may be responsible for better postural balance.

Effects of repeated optic flow stimulation on gait termination in humans

CONCLUSIONS

Because the basic strategies to stop walking are stored as motor programs, visual stimulation may have little influence on body deviation during gait termination and its time course. Walking velocity, however, demonstrated dynamic flexible changes, which may subserve the stable process of gait termination under variable circumstantial changes such as optic flow.

OBJECTIVE

The aim of this study was to examine the effect of repeated optic flow on body deviation and walking velocity during gait termination, which may be more complicated than continuous standing or walking.

METHODS

Twenty-three healthy subjects were instructed to start walking upon an acoustic cue and to stop walking when the scenery changed in a virtual reality environment. Subjects underwent eight control trials without optic flow and three sets of optic flow conditions including four trials each of optic horizontal and rotational movement randomly.

RESULTS

Repeated optic flow caused no significant change of body deviation or the time course of the gait termination process in comparison with that in the control. The walking velocity at the start of the termination process showed short-term flexibility that denoted a gradual increase over the trial for within-set and long-term flexibility that denoted a gradual decrease for between-set.

Exposição crônica em ambiente de conflito sensorial e sua influência no controle postural

O objetivo do presente estudo foi investigar o equilíbrio de indivíduos expostos cronicamente a um ambiente com conflito sensorial, antes e imediatamente após a exposição a este ambiente. Quinze pescadores participaram voluntariamente e tiveram seu deslocamento do centro de pressão (CP) mensurado por uma plataforma de força, antes e após a jornada no mar. Seis diferentes condições, nas quais as informações sensoriais dos sistemas visual e somatossensorial foram manipuladas, foram testadas. A trajetória total do centro de pressão indicou a habilidade dos participantes em utilizar cada sistema sensorial separadamente, em cada condição para o controle postural. Não houve alterações na oscilação corporal ou na dependência sensorial imediatamente após a exposição ao ambiente com conflito sensorial. Uma adaptação crônica parece evitar com que eles sejam influenciados pelos efeitos agudos da exposição a um ambiente com conflito sensorial.

Electrical tongue stimulation normalizes activity within the motion-sensitive brain network in balance-impaired subjects as revealed by group independent component analysis

Multivariate analysis of functional magnetic resonance imaging (fMRI) data allows investigations into network behavior beyond simple activations of individual regions. We apply group independent component analysis to fMRI data collected in a previous study looking at the sustained neuromodulatory effects of electrical tongue stimulation in balance-impaired individuals. Twelve subjects with balance disorders viewed optic flow in an fMRI scanner before and after 5 days of electrical tongue stimulation. Nine healthy controls also viewed the visual stimuli but did not receive any stimulation. Multiple regression of the 47 estimated components found two that were modulated by the visual stimuli. Component 7, comprised primarily of the primary visual cortex (V1), responded to all visual stimuli and showed no difference in task-related activity between the healthy controls and the balance-impaired subjects before or after stimulation. Component 11 responded only to motion in the visual field and contained multiple cortical and subcortical regions involved in processing information pertinent to balance. Two-sample t-tests of the calculated signal change revealed that the task-related activity of this network is greater in balance-impaired subjects compared with controls before stimulation (p=0.02), but that this network hypersensitivity decreases after electrical tongue stimulation (p=0.001).

High-resolution fMRI detects neuromodulation of individual brainstem nuclei by electrical tongue stimulation in balance-impaired individuals

High-resolution functional magnetic resonance imaging (fMRI) can be used to precisely identify blood oxygen level dependent (BOLD) activation of small structures within the brainstem not accessible with standard fMRI. A previous study identified a region within the pons exhibiting sustained neuromodulation due to electrical tongue stimulation, but was unable to precisely identify the neuronal structure involved. For this study, high-resolution images of neural activity induced by optic flow were acquired in nine healthy controls and nine individuals with balance dysfunction before and after information-free tongue stimulation. Subjects viewed optic flow videos to activate the structures of interest. Sub-millimeter in-plane voxels of structures within the posterior fossa were acquired using a restricted field of view. Whole-brain functional imaging verified that global activation patterns due to optic flow were consistent with previous studies. Optic flow activated the visual association cortices, the vestibular nuclei, and the superior colliculus, as well as multiple regions within the cerebellum. The anterior cingulate cortex showed decreased activity after stimulation, while a region within the pons had increased post-stimulation activity. These observations suggest the pontine region is the trigeminal nucleus and that tongue stimulation interfaces with the balance-processing network within the pons. This high-resolution imaging allows detection of activity within individual brainstem nuclei not possible using standard resolution imaging.

Sustained cortical and subcortical neuromodulation induced by electrical tongue stimulation

This pilot study aimed to show that information-free stimulation of the tongue can improve behavioral measures and induce sustained neuromodulation of the balance-processing network in individuals with balance dysfunction. Twelve balance-impaired subjects received one week of cranial nerve non-invasive neuromodulation (CN-NINM). Before and after the week of stimulation, postural sway and fMRI activation were measured to monitor susceptibility to optic flow. Nine normal controls also underwent the postural sway and fMRI tests but did not receive CN-NINM. Results showed that before CN-NINM balance-impaired subjects swayed more than normal controls as expected (p ≤ 0.05), and that overall sway and susceptibility to optic flow decreased after CN-NINM (p ≤ 0.005 & p ≤ 0.05). fMRI showed upregulation of visual sensitivity to optic flow in balance-impaired subjects that decreased after CN-NINM. A region of interest analysis indicated that CN-NINM may induce neuromodulation by increasing activity within the dorsal pons (p ≤ 0.01).

The dynamics of visual reweighting in healthy and fall-prone older adults

Multisensory reweighting (MSR) is an adaptive process that prioritizes the visual, vestibular, and somatosensory inputs to provide the most reliable information for postural stability when environmental conditions change. This process is thought to degrade with increasing age and to be particularly deficient in fall-prone versus healthy older adults. In the present study, the authors investigate the dynamics of sensory reweighting, which is not well-understood at any age. Postural sway of young, healthy, and fall-prone older adults was measured in response to large changes in the visual motion stimulus amplitude within a trial. Absolute levels of gain, and the rate of adaptive gain change were examined when visual stimulus amplitude changed from high to low and from low to high. Compared with young adults, gains in both older adult groups were higher when the stimulus amplitude was high. Gains in the fall-prone elderly were higher than both other groups when the stimulus amplitude was low. Both older groups demonstrated slowed sensory reweighting over prolonged time periods when the stimulus amplitude was high. The combination of higher vision gains and slower down weighting in older adults suggest deficits that may contribute to postural instability.

A influência dos sistemas sensoriais na plataforma de força: estudo do equilíbrio corporal em idosas com e sem queixa de tontura

OBJETIVO: analisar as respostas de dois grupos de idosas (com e sem queixa de tontura) submetidas a diferentes condições sensoriais em uma plataforma de força. MÉTODOS: para a realização desse estudo utilizou-se uma plataforma de força AMTI OR6-5 com uma frequência de aquisição de 100 Hz. Foram realizadas coletas em quatro condições distintas: olhos abertos (OA), olhos fechados (OF) sobre apenas a plataforma de força, olhos abertos (OAA) e olhos fechados (OFA) sobre uma almofada de 20 cm de espessura. As variáveis analisadas foram o centro de força ântero-posterior (COPap), centro de força médio-lateral (COPml), 95% da elipse da área, velocidade de oscilação e comprimento da oscilação.Comparações entre os grupos e entre as condições dentro dos grupos, foram realizadas por meio do teste de Duncan. RESULTADOS: os resultados mostraram não haver diferenças entre os grupos em nenhuma variável, porém a condição OFA causou maiores perturbações no equilíbrio corporal das idosas. CONCLUSÃO: conclui-se que o aumento da dificuldade imposta pelas quatro condições de testes, alterou a oscilação corporal das idosas avaliadas.

Attenuation of the evoked responses with repeated exposure to proprioceptive disturbances is muscle specific

In response to repetitive proprioceptive disturbances (vibration) applied to postural muscles, the evoked response has been shown to decrease in amplitude within the first few trials. The present experiment investigated whether this attenuation of the response to vibration stimulation (90Hz, 5s) was muscle specific or would be transferred to the antagonist muscles. Sixteen participants stood upright with eyes closed. One half of the participants practiced 15 tibialis vibrations followed by 15 calf vibrations (TIB-CALF order), while the other half practiced the opposite order (CALF-TIB order). Antero-posterior trunk displacements were measured at the level of C7 and centre of foot pressure (COP). EMG activity of the tibialis anterior (TA) and gastrocnemius lateralis (GL) was also measured. Results showed that evoked postural responses as well as EMG activity decreased with practice when vibration was applied to either calf or tibialis muscles. However, such attenuation of the response appeared muscle specific since it did not generalise when the same vibration stimulus was later applied onto the antagonist muscles.

Stroke affects locomotor steering responses to changing optic flow directions

BACKGROUND

Stroke patients manifest steering difficulties during walking, which may arise from an altered perception of visual motion.

OBJECTIVE

To examine the ability of stroke patients to control their heading direction while walking in a virtual environment (VE) describing translational optic flows (OFs) expanding from different directions.

METHODS

The authors evaluated 10 stroke patients and 11 healthy people while they were walking overground and visualizing a VE in a helmet-mounted display. Participants were instructed to walk straight in the VE and were randomly exposed to an OF having a focus of expansion (FOE) located in 5 possible locations (0 degrees, +/-20 degrees, and +/-40 degrees to the right or left). The body's center of mass (CoM) trajectory, heading direction, and horizontal body reorientation were recorded with a Vicon-512 system.

RESULTS

Healthy participants veered opposite to the FOE location in the physical world, with larger deviations occurring at the most eccentric FOE locations. Stroke patients displayed altered steering behaviors characterized either by an absence of CoM trajectory corrections, multiple errors in the heading direction, or systematic veering to the nonparetic side. Both groups displayed relatively small CoM trajectory corrections that led to large virtual heading errors.

CONCLUSIONS

The control of heading of locomotion in response to different OF directions is affected by stroke. An altered perception of heading direction and/or a poor integration of sensory and motor information are likely causes. This altered response to OF direction while walking may contribute to steering difficulties after stroke.

Aging effects on the structure underlying balance abilities tests

Balance impairment is one of the biggest risk factors for falls reducing inactivity, resulting in nursing care. Therefore, balance ability is crucial to maintain the activities of independent daily living of older adults. Many tests to assess balance ability have been developed. However, few reports reveal the structure underlying results of balance performance tests comparing young and older adults. Covariance structure analysis is a tool that is used to test statistically whether factorial structure fits data. This study examined aging effects on the factorial structure underlying balance performance tests. Participants comprised 60 healthy young women aged 22 ± 3 years (young group) and 60 community-dwelling older women aged 69 ± 5 years (older group). Six balance tests: postural sway, one-leg standing, functional reach, timed up and go (TUG), gait, and the EquiTest were employed. Exploratory factor analysis revealed that three clearly interpretable factors were extracted in the young group. The first factor had high loadings on the EquiTest, and was interpreted as 'Reactive'. The second factor had high loadings on the postural sway test, and was interpreted as 'Static'. The third factor had high loadings on TUG and gait test, and was interpreted as 'Dynamic'. Similarly, three interpretable factors were extracted in the older group. The first factor had high loadings on the postural sway test and the EquiTest and therefore was interpreted as 'Static and Reactive'. The second factor, which had high loadings on the EquiTest, was interpreted as 'Reactive'. The third factor, which had high loadings on TUG and the gait test, was interpreted as 'Dynamic'. A covariance structure model was applied to the test data: the second-order factor was balance ability, and the first-order factors were static, dynamic and reactive factors which were assumed to be measured based on the six balance tests. Goodness-of-fit index (GFI) of the models were acceptable (young group, GFI=0.931; older group, GFI=0.923). Static, dynamic and reactive factors relating to balance ability had loadings 0.21, 0.24, and 0.76 in the young group and 0.71, 0.28, and 0.43 in the older group, respectively. It is suggested that the common factorial structure of balance abilities were static, dynamic and reactive, and that for young people reactive balance ability was characterized and explained by balance ability, whereas for older people it was static balance ability.

Aging affects the ability to use optic flow in the control of heading during locomotion

Perceived self-motion from optic flow is implicated in the control of locomotion. Aging, which affects visual perception and sensorimotor integration, may result in an inability to use optic flow to guide heading while walking. The purpose of this study was to examine whether advanced age could impact on the steering of locomotion, when changing optic flow directions were presented in an immersive virtual environment (VE). Nine young adults (21.56 +/- 3.20 years) and nine older adults (66.11 +/- 3.95 years) participated in the study. Subjects were asked to walk while viewing a VE through a head-mounted display unit (Kaiser). The VE viewed by the subjects was a large room displayed as an expanding translational optic flow, with the focus of expansion (FOE) located at neutral, 20 degrees or 40 degrees to the right or left. Their task was to walk straight with respect to the VE. Kinematic data in 3D were collected, from which the body's centre of mass (CoM) position and heading direction were calculated. Young subjects were able to make proper heading adjustments in the VE, with respect to FOE shifts, but not older individuals. Young subjects altered their CoM trajectory so that it was oriented in the direction opposite to the FOE in the physical environment and resulted in small deviation in the VE. The older adults did not adjust their locomotor patterns in response to the different flows presented and maintained similar walking trajectories across all trials. Advanced age results in an altered control of steering of locomotion in response to changing directions of optic flow. This may be related to an impaired perception and/or use of the optic flow, or due to inherent problems in sensorimotor integration.

Neurobiology of a Simple Memory

Habituation is one of the simplest forms of memory, yet its neurobiological mechanisms remain largely unknown in mammalian systems. This review summarizes recent multidisciplinary analyses of the neurobiology of mammalian odor habituation including in vitro and in vivo synaptic physiology, sensory physiology, behavioral pharmacology, and computational modeling approaches. The findings show that a metabotropic glutamate receptor–mediated depression of afferent synapses to the olfactory cortex is necessary and perhaps sufficient to account for cortical sensory adaptation and short-term behavioral habituation. Furthermore, long-term habituation is an N-methyl-d-aspartate (NMDA) receptor–dependent process within the olfactory bulb. Thus there is both a pharmacological and anatomical distinction between short-term and long-term memory for habituation. The differential locus of change underlying short- and long-term memory leads to predictable differences in their behavioral characteristics, such as specificity.