Vestibular Control Of Balance Research Articles

Vestibular control of standing balance following 24h of sleep deprivation.

Sleep deprivation alters cognitive and sensorimotor function, but its effects on the control of standing balance are inconclusive. The vestibular system is critical for standing balance, and is modified by sleep deprivation; however, how sleep deprivation affects vestibular-evoked balance responses is unknown. Thus, this study aimed to examine the effect of 24h of sleep deprivation on the vestibular control of standing balance. During both a well-rested (i.e., control) and sleep deprivation condition, nine females completed two 90-s trials of bilateral, binaural stochastic electrical vestibular stimulation (EVS) and two 120-s trials of quiet stance on a force plate. Quiet stance performance was assessed by center of pressure displacement parameters. Mediolateral ground reaction force (ML force) and surface electromyography (EMG) of the right medial gastrocnemius (MG) were sampled simultaneously with the EVS signal to quantify vestibular control of balance within the frequency (gain and coherence) and time (cumulant density) domains. Twenty-four hours of sleep deprivation did not affect quiet stance performance. Sleep deprivation also had limited effect on EVS-MG EMG and EVS-ML Force coherence (less than control at 8-10.5Hz, greater at ~ 16Hz); however, gain of EVS-MG EMG (< 8, 11-24Hz) and EVS-ML force (0.5-9Hz) was greater for sleep deprivation than control. Sleep deprivation did not alter peak-to-peak amplitude of EVS-MG EMG (p = 0.51) or EVS-ML force (p = 0.06) cumulant density function responses. Despite no effect on quiet stance parameters, the observed increase in vestibular-evoked balance response gain suggests 24-h sleep deprivation may lead to greater sensitivity of the central nervous system when transforming vestibular-driven signals for standing balance control.

The effects of four hours of normobaric hypoxia on the vestibular control of balance.

Whole-body vestibular-evoked balance responses decrease following ~ 55min of normobaric hypoxia. It is unclear how longer durations of hypoxia affect the vestibular control of balance at the muscle and whole-body levels. This study examined how four hours of normobaric hypoxia influenced the vestibular control of balance. Fifteen participants (4 females; 11 males) stood on a force plate with vision occluded and head rotated rightward while subjected to three blocks of binaural, bipolar stochastic electrical vestibular stimulation (EVS; 0-25Hz, root mean square amplitude = 1.1mA) consisting of two, 90-s trials. The relationship between EVS and anteroposterior (AP) forces or medial gastrocnemius (MG) electromyography (EMG) was estimated in the time and frequency domains at baseline (BL; 0.21 fraction of inspired oxygen-FIO2) and following two (H2) and four (H4) hours of normobaric hypoxia (0.11 FIO2). The EVS-MG EMG short-latency peak and peak-to-peak amplitudes were smaller than BL at H2 and H4, but the medium-latency peak amplitude was only lower at H4. The EVS-AP force medium-latency peak amplitude was lower than BL at H4, but the short-latency peak and peak-to-amplitudes were unchanged. The EVS-MG EMG coherence and gain were reduced compared to BL at H2 and H4 across multiple frequencies ≥ 7Hz, whereas EVS-AP force coherence was blunted at H4 (≤ 4Hz), but gain was unaffected. Overall, the central nervous system's response to vestibular-driven signals during quiet standing wasdecreased forup to four hours of normobaric hypoxia, and vestibular-evoked responses recorded within postural muscles may be more sensitive than the whole-body response.

Bed rest impairs the vestibular control of balance.

Prolonged bed rest impairs standing balance but the underlying mechanisms are uncertain. Previous research suggests strength loss is not the cause, leaving impaired sensorimotor control as an alternative. Here we examine vestibular control of posture in 18male volunteers before and after 60days of bed rest. Stochastic vestibular stimulation (SVS) was used to evoke sway responses before, 1 and 6days after bed rest under different head yaw orientations. The directional accuracy and precision of these responses were calculated from ground reaction force vectors. Bed rest caused up to 63% increases in spontaneous standing sway and 31% reductions in leg strength, changes which were uncorrelated. The increase in sway was exacerbated when the eyes were closed. Mean directions of SVS-evoked sway responses were unaffected, being directed towards the anodal ear and rotating in line with head orientation in the same way before and after bed rest. However, individual trial analysis revealed 25%-30% increases in directional variability, which were significantly correlated with the increase in spontaneous sway (r=0.48-0.71; P≤0.044) and were still elevated on day 6 post-bed rest. This reveals that individual sway responses may be inappropriately oriented, a finding masked by the averaging process. Our results confirm that impaired balance following prolonged bedrest is not related to loss of strength. Rather, they demonstrate that the sensorimotor transformation process which converts vestibular feedback into appropriately directed balance responses is impaired. KEY POINTS: Prolonged inactivity impairs balance but previous research suggests this is not caused by loss of strength. Here we investigated vestibular control of balance before and after 60days of bed rest using electrical vestibular stimulation (EVS) to evoke sway responses. Spontaneous sway significantly increased and muscle strength reduced following bed rest, but, in keeping with previous research, these two effects were not correlated. While the overall accuracy of EVS-evoked sway responses was unaffected, their directional variability significantly increased following bed rest, and this was correlated with the increases in spontaneous sway. We have shown that the ability to transform head-centred vestibular feedback into an appropriately directed body sway response is negatively affected by prolonged inactivity; this may contribute to the impaired balance commonly observed following bed rest.

Real-world characterization of vestibular contributions during locomotion.

The vestibular system, which encodes our head movement in space, plays an important role in maintaining our balance as we navigate the environment. While in-laboratory research demonstrates that the vestibular system exerts a context-dependent influence on the control of balance during locomotion, differences in whole-body and head kinematics between indoor treadmill and real-world locomotion challenge the generalizability of these findings. Thus, the goal of this study was to characterize vestibular-evoked balance responses in the real world using a fully portable system. While experiencing stochastic electrical vestibular stimulation (0-20 Hz, amplitude peak ± 4.5 mA, root mean square 1.25 mA) and wearing inertial measurement units (IMUs) on the head, low back, and ankles, 10 participants walked outside at 52 steps/minute (∼0.4 m/s) and 78 steps/minute (∼0.8 m/s). We calculated time-dependent coherence (a measure of correlation in the frequency domain) between the applied stimulus and the mediolateral back, right ankle, and left ankle linear accelerations to infer the vestibular control of balance during locomotion. In all participants, we observed vestibular-evoked balance responses. These responses exhibited phasic modulation across the stride cycle, peaking during the middle of the single-leg stance in the back and during the stance phase for the ankles. Coherence decreased with increasing locomotor cadence and speed, as observed in both bootstrapped coherence differences (p < 0.01) and peak coherence (low back: 0.23 ± 0.07 vs. 0.16 ± 0.14, p = 0.021; right ankle: 0.38 ± 0.12 vs. 0.25 ± 0.10, p < 0.001; left ankle: 0.33 ± 0.09 vs. 0.21 ± 0.09, p < 0.001). These results replicate previous in-laboratory studies, thus providing further insight into the vestibular control of balance during naturalistic movements and validating the use of this portable system as a method to characterize real-world vestibular responses. This study will help support future work that seeks to better understand how the vestibular system contributes to balance in variable real-world environments.

The effects of acute normobaric hypoxia on vestibular-evoked balance responses in humans.

Hypoxia influences standing balance and vestibular function. The purpose here was to investigate the effect of hypoxia on the vestibular control of balance. Twenty participants (10 males; 10 females) were tested over two days (normobaric hypoxia and normoxia). Participants stood on a force plate (head rotated leftward) and experienced random, continuous electrical vestibular stimulation (EVS) during trials of eyes open (EO) and closed (EC) at baseline (BL), after 5 (H1), 30 (H2) and 55-min (H3) of hypoxia, and 10-min into normoxic recovery (NR). Vestibular-evoked balance responses were quantified using cumulant density, coherence, and gain functions between EVS and anteroposterior forces. Oxyhemoglobin saturation, end-tidal oxygen and carbon dioxide decreased for H1-3 compared to BL; however, end-tidal carbon dioxide remained reduced at NR with EC (p≤0.003). EVS-AP force peak-to-peak amplitude was lower at H3 and NR than at BL (p≤0.01). At multiple frequencies, EVS-AP force coherence and gain estimates were lower at H3 and NR than BL for females; however, this was only observed for coherence for males. Overall, vestibular-evoked balance responses are blunted following normobaric hypoxia >30 min, which persists into NR and may contribute to the reported increases in postural sway.

The influence of reduced foot dorsum cutaneous sensitivity on the vestibular control of balance.

Foot sole cooling increases vestibular-evoked balance responses, but less is known about foot dorsum temperature alterations. The purpose was to determine whether decreasing cutaneous receptor sensitivity via foot dorsum cooling modulates the vestibular control of balance. Eighteen participants (9 males; 9 females) stood quietly on a force plate with feet together, eyes closed, and head rotated leftward during 4, 90-s trials (2 control; 2 cooled) of continuous electrical vestibular stimulation (EVS). Icepacks placed on the dorsum of both feet for 15min induced cooling and remained throughout the EVS trials. Monofilament testing was performed at multiple locations before and after cooling to determine tactile detection thresholds. T-type thermocouples monitored skin temperature over the tibialis anterior, soleus, foot dorsum and arch of the right leg. Vestibular-evoked balance responses were characterized using time (cumulant density) and frequency (coherence and gain) domain analyses to determine the relationship between the EVS input and motor output (anteroposterior force-AP force; right medial gastrocnemius electromyography-MG EMG). Skin temperature of the foot dorsum and arch decreased ~ 70 and 15%, respectively during cooling (p < 0.05), but was unaltered at other locations (p ≥ 0.10). Detection thresholds for the foot dorsum increased following cooling (p < 0.05). Surprisingly, cooling reduced EVS-AP force and EVS-MG EMG coherence and gain at multiple frequencies, and peak-to-peak amplitude compared to control (p < 0.05). Our results indicate that vestibular-driven balance responses are reduced following foot dorsum cooling, likely owing to alterations in cutaneous mechanoreceptor sensitivity and subsequent alterations in the transformation of vestibular cues for balance control.

Age-related reduction in peak power and increased postural displacement variability are related to enhanced vestibular-evoked balance responses in females.

Adult aging is associated with reductions in muscle function and standing balance control. However, whether sensorimotor function adapts to maintain upright posture in the presence of age-related muscle weakness is unclear. The purpose was to determine whether vestibular control of balance is altered in older compared to younger females and whether vestibular-evoked balance responses are related to muscle power. Eight young (22.6±1.8years) and eight older (69.7±6.7years) females stood quietly on a force plate, while subjected to random, continuous electrical vestibular stimulation (EVS; 0-20Hz, root mean square amplitude: 1.13mA). Medial gastrocnemius (MG) and tibialis anterior (TA) surface electromyography (EMG) and force plate anterior-posterior (AP) forces were sampled and associated with the EVS signal in the frequency and time domains. Knee extensor function was evaluated using a Biodex multi-joint dynamometer. The weaker, less powerful older females exhibited a 99 and 42% greater medium-latency peak amplitude for the TA and AP force (p<0.05), respectively, but no other differences were detected for short- and medium-latency peak amplitudes. The TA (<10Hz) and MG (<4Hz) EVS-EMG coherence and EVS-AP force coherence (<2Hz) was greater in older females than young. A strong correlation was detected for AP force medium-latency peak amplitude with center of pressure displacement variability (r=0.75; p<0.05) and TA medium-latency peak amplitude (r=0.86; p<0.05). Power was negatively correlated with AP force medium-latency peak amplitude (r=-0.47; p<0.05). Taken together, an increased vestibular control of balance may compensate for an age-related reduction in power and accompanies greater postural instability in older females than young.

Balance Impairment in Fahr's Disease: Mixed Signs of Parkinsonism and Cerebellar Disorder. A Case Study.

Fahr’s disease is a rare idiopathic degenerative disease characterized by calcifications in the brain, and has also been associated with balance impairment. However, a detailed analysis of balance in these patients has not been performed. A 69-year-old woman with Fahr’s disease presented with a long-lasting subjective imbalance. Balance was analyzed using both clinical (EquiScale, Timed Up and Go test, and Dizziness Handicap Inventory-short form) and instrumented tests (the sway of the body center of mass during quiet, perturbed, and self-perturbed stance, and the peak curvature of the center of mass during single stance while walking on a force-treadmill). The patient’s balance was normal during clinical tests and walking. However, during standing, a striking impairment in vestibular control of balance emerged. The balance behavior displayed mixed parkinsonian (e.g., slowness and reduced amplitude of movement) and cerebellar (e.g., increased sway during standing in all conditions and decomposition of movement) features, with a discrepancy between the high severity of the static and the low severity of the dynamic balance impairment. The balance impairment characteristics outlined in this study could help neurologists and physiatrists detect, stage, and treat this rare condition.

Age-Related Vestibular Dysfunction; Vestibulo-Ocular Reflex, Dynamic Visual Acuity, Dizziness and Falls: A Review

Background: Vestibular and vision functions are important contributors to posture control and fall avoidance. This review examines how the vestibulo-ocular reflex can be rehabilitated to help restore postural control. Methods: PubMed searches (7th April 2021) using the terms ‘vestibulo-ocular reflex’, ‘imbalance and vestibular dysfunction’, ‘vestibular dysfunction and dizziness’, ‘dynamic visual acuity’, ‘vestibular dysfunction rehabilitation’, and ‘gaze stabilization exercises’ yielded 4986, 495, 3576,1830, 3312, and 137 potentially useful publications respectively. Selections of those which were found to be the most relevant and representative of a balanced and current account of these topics, as well as selections from the most relevant reports referenced in those publications, were included in this review. Results: Just as there are age-related losses of static visual acuity even when there are no specific visual pathologies diagnosed, patients may also present with age-related loss of vestibular functions in the absence of specific vestibular pathologies. For example, cases of dizziness which are diagnosed as idiopathic might be usefully classified as age-related as the basis for the initiation of rehabilitation exercises. Conclusions: Apart from age-related loss of vestibular functions, cases diagnosed as having a particular form of vestibular pathology may have that condition exacerbated by age-related losses of vestibular functions. The effects of vestibular rehabilitation gaze stability exercises in patients with vestibular dysfunction are well established and include both improved dynamic acuity and postural stability. Improvements in posture control following rehabilitation of the vestibulo-ocular reflex are apparently due to improved peripheral and/or central vestibular balance control which has occurred in conjunction with enhanced gaze stability. The complex nature of increased fall risk suggests that an interdisciplinary approach to rehabilitation that includes vestibulo-ocular reflex rehabilitation appears likely to be associated with optimum outcomes for both pathological and age-related cases.

P85 Head motion predictability explains phase- and speed-dependent suppression of vestibular balance control during walking

P85 Head motion predictability explains phase- and speed-dependent suppression of vestibular balance control during walking

Head motion predictability explains activity-dependent suppression of vestibular balance control

Vestibular balance control is dynamically weighted during locomotion. This might result from a selective suppression of vestibular inputs in favor of a feed-forward balance regulation based on locomotor efference copies. The feasibility of such a feed-forward mechanism should however critically depend on the predictability of head movements (HMP) during locomotion. To test this, we studied in 10 healthy subjects the differential impact of a stochastic vestibular stimulation (SVS) on body sway (center-of-pressure, COP) during standing and walking at different speeds and compared it to activity-dependent changes in HMP. SVS-COP coupling was determined by correlation analysis in frequency and time domains. HMP was quantified as the proportion of head motion variance that can be explained by the average head trajectory across the locomotor cycle. SVS-COP coupling decreased from standing to walking and further dropped with faster locomotion. Correspondingly, HMP increased with faster locomotion. Furthermore, SVS-COP coupling depended on the gait-cycle-phase with peaks corresponding to periods of least HMP. These findings support the assumption that during stereotyped human self-motion, locomotor efference copies selectively replace vestibular cues, similar to what was previously observed in animal models.

Down regulation of vestibular balance stabilizing mechanisms to enable transition between motor states

The neural control of transition between posture and movement encompasses the regulation of reflex-stabilizing mechanisms to enable motion. Optimal feedback theory suggests that such transitions require the disengagement of one motor control policy before the implementation of another. To test this possibility, we investigated the continuity of the vestibular control of balance during transitions between quiet standing and locomotion and between two standing postures. Healthy subjects initiated and terminated locomotion or shifted the distribution of their weight between their feet, while exposed to electrical vestibular stimuli (EVS). The relationship between EVS and ground reaction forces was quantified using time-frequency analyses. Discontinuities corresponding to null coherence periods were observed preceding the onset of movement initiation and during the step preceding locomotion termination. These results show humans interrupt the vestibular balance stabilizing mechanisms to transition between motor states, suggesting a discrete change between motor control policies, as predicted by optimal feedback theory.

Susceptibility to Fear of Heights in Bilateral Vestibulopathy and Other Disorders of Vertigo and Balance.

Aims: To determine the susceptibility to visual height intolerance (vHI) in patients with acquired bilateral vestibulopathy (BVP). The question was whether postural instability in BVP, which is partially compensated for by visual substitution of the impaired vestibular control of balance, leads to an increased susceptibility. This is of particular importance since fear of heights is dependent on body posture, and visual control of balance at heights can no longer substitute vestibular input. For comparison susceptibility to vHI was determined in patients with other vestibular or functional disorders.Methods: A total of 150 patients aged 18 or above who had been referred to the German Center for Vertigo and Balance Disorders and diagnosed to have BVP were surveyed with a standardized questionnaire by specifically trained neurological professionals. Further, 481 patients with other vestibular or functional disorders were included.Results: Susceptibility to vHI was reported by 29% (32 % in females, 25% in males) of the patients with BVP. Patients with vHI were slightly younger (67 vs. 71 years). Seventy percent of those with vHI reported avoidance of climbing, hiking, stairs, darkness, cycling or swimming (84% of those without vHI). Mean age for onset of vHI was 40 years. Susceptibility to vHI was higher in patients with other vertigo disorders than in those with BVP: 64% in those with phobic postural vertigo, 61% in vestibular migraine, 56% in vestibular paroxysmia, 54% in benign paroxysmal positional vertigo, 49% in unilateral vestibulopathy and 48% in Menière's disease.Conclusions: The susceptibility to vHI in BVP was not higher than that of the general population (28%).This allows two explanations that need not be alternatives but contribute to each other: (1) Patients with a bilateral peripheral vestibular deficit largely avoid exposure to heights because of their postural instability. (2) The irrational anxiety to fall from heights triggers increased susceptibility to vHI, not the objective postural instability. However, patients with BVP do not exhibit increased comorbid anxiety disorders. This view is supported by the significantly increased susceptibility to vHI in other vestibular syndromes, which are characterized by an increased comorbidity of anxiety disorders.

Differential effects of vision upon the accuracy and precision of vestibular-evoked balance responses.

Key points Effective balance control requires the transformation of vestibular signals from head‐ to foot‐centred coordinates in order to move the body in an appropriate direction.This transformation process has previously been studied by analysing the directional accuracy of the averaged sway response to multiple electrical vestibular stimuli (EVS).Here we studied trial‐by‐trial variability of EVS responses to measure any changes in directional precision which may be masked by the averaging process.We found that vision increased directional variability without influencing the mean sway direction, demonstrating that response accuracy and precision are dissociable.These results emphasise the importance of single trial analysis in determining the efficacy of vestibular control of balance. Vestibular information must be transformed from head‐ to‐foot‐centred coordinates for balance control. This transformation process has previously been investigated using electrical vestibular stimulation (EVS), which evokes a sway response fixed in head coordinates. The craniocentric nature of the response has been demonstrated by analysing average responses to multiple stimuli. This approach misses any trial‐by‐trial variability which would reflect poor balance control. Here we performed single‐trial analysis to measure this directional variability (precision), and compared this to mean performance (accuracy). We determined the effect of vision upon both parameters. Standing volunteers adopted various head orientations (0, ±30 and ±60 deg yaw) while EVS‐evoked response direction was determined from ground reaction force vectors. As previously reported, mean force direction was orientated towards the anodal ear, and rotated in line with head yaw. Although vision caused a ∼50% reduction in response magnitude, it had no influence on the direction of the mean sway response, indicating that accuracy was unaffected. However, individual trial analysis revealed up to 30% increases in directional variability with the eyes open. This increase was inversely correlated with the size of the force response. The paradoxical observation that vision reduces the precision of the balance response may be explained by a multi‐sensory integration process. As additional veridical sensory information becomes available, this lessens the relative contribution of vestibular input, causing a simultaneous reduction in both the magnitude and the precision of the response to EVS. Our novel approach demonstrates the importance of single‐trial analysis in revealing the efficacy of vestibular reflexes.

Vestibular-Evoked Responses Indicate a Functional Role for Intrinsic Foot Muscles During Standing Balance

Maintaining standing balance involves multisensory processing and integration to produce dynamic motor responses. Electrical vestibular stimulation (EVS) delivered over the mastoid processes can be used to explore the vestibular control of balance. The purpose of this study was to determine whether intrinsic foot muscles exhibit vestibular-evoked balance responses and to characterize the traits associated with these responses. Electromyography (EMG) of the abductor hallucis (AH), abductor digiti minimi (ADM) and medial gastrocnemius (MG) and anterior–posterior (AP) forces were sampled while quietly standing participants were subjected to a random continuous EVS signal (peak-to-peak amplitude = ±3 mA). The relationship between EVS input and motor output was characterized in both the frequency (coherence) and time (cumulant density) domains. When head orientation was rotated in yaw from left to right, the biphasic cumulant density function was inverted for all muscle (EVS–EMG) and whole-body (EVS–AP forces) balance responses. When vision was occluded, the EVS–EMG and EVS–AP forces coherence function amplitude increased at low frequencies (<2 Hz) and was accompanied by a heightened medium-latency peak amplitude for all muscles as well as the whole-body balance response (AP forces) compared to when static visual cues were present. The enhanced coherence amplitudes at lower frequencies may highlight a mechanism for the increase in postural sway from vision to occluded vision. The current findings indicate that the vestibular control of standing balance can be represented by the intrinsic foot muscles and implicate a postural role for these muscles in modulating quiet standing.

Co-ordination of the upper and lower limbs for vestibular control of balance.

Key points When standing and holding an earth‐fixed object, galvanic vestibular stimulation (GVS) can evoke upper limb responses to maintain balance.In the present study, we determined how these responses are affected by grip context (no contact, light grip and firm grip), as well as how they are co‐ordinated with the lower limbs to maintain balance.When GVS was applied during firm grip, hand and ground reaction forces were generated.The directions of these force vectors were co‐ordinated such that the overall body sway response was always aligned with the inter‐aural axis (i.e. craniocentric).When GVS was applied during light grip (< 1 N), hand forces were secondary to body movement, suggesting that the arm performed a mostly passive role.These results demonstrate that a minimum level of grip is required before the upper limb becomes active in balance control and also that the upper and lower limbs co‐ordinate for an appropriate whole‐body sway response. Vestibular stimulation can evoke responses in the arm when it is used for balance. In the present study, we determined how these responses are affected by grip context, as well as how they are co‐ordinated with the rest of the body. Galvanic vestibular stimulation (GVS) was used to evoke balance responses under three conditions of manual contact with an earth‐fixed object: no contact, light grip (< 1 N) (LG) and firm grip (FG). As grip progressed along this continuum, we observed an increase in GVS‐evoked hand force, with a simultaneous reduction in ground reaction force (GRF) through the feet. During LG, hand force was secondary to the GVS‐evoked body sway response, indicating that the arm performed a mostly passive role. By contrast, during FG, the arm became actively involved in driving body sway, as revealed by an early force impulse in the opposite direction to that seen in LG. We then examined how the direction of this active hand vector was co‐ordinated with the lower limbs. Consistent with previous findings on sway anisotropy, FG skewed the direction of the GVS‐evoked GRF vector towards the axis of baseline postural instability. However, this was effectively cancelled by the hand force vector, such that the whole‐body sway response remained aligned with the inter‐aural axis, maintaining the craniocentric principle. These results show that a minimum level of grip is necessary before the upper limb plays an active role in vestibular‐evoked balance responses. Furthermore, they demonstrate that upper and lower‐limb forces are co‐ordinated to produce an appropriate whole‐body sway response.

The effect of fear of falling on vestibular feedback control of balance.

Vestibular sensation contributes to cervical‐head stabilization and fall prevention. To what extent fear of falling influences the associated vestibular feedback processes is currently undetermined. We used galanic vestibular stimulation (GVS) to induce vestibular reflexes while participants stood at ground level and on a narrow walkway at 3.85 m height to induce fear of falling. Fear was confirmed by questionnaires and elevated skin conductance. Full‐body kinematics was measured to differentiate the whole‐body centre of mass response (CoM) into component parts (cervical, axial trunk, appendicular short latency, and medium latency). We studied the effect of fear of falling on each component to discern their underlying mechanisms. Statistical parametric mapping analysis provided sensitive discrimination of early GVS and height effects. Kinematic analysis revealed responses at 1 mA stimulation previously believed marginal through EMG and force plate analysis. The GVS response comprised a rapid, anode‐directed cervical‐head acceleration, a short‐latency cathode‐directed acceleration (cathodal buckling) of lower extremities and pelvis, an anode‐directed upper thorax acceleration, and subsequently a medium‐latency anode‐directed acceleration of all body parts. At height, head and upper thorax early acceleration were unaltered, however, short‐latency lower extremity acceleration was increased. The effect of height on balance was a decreased duration and increased rate of change in the CoM acceleration pattern. These results demonstrate that fear modifies vestibular control of balance, whereas cervical‐head stabilization is governed by different mechanisms unaffected by fear of falling. The mechanical pattern of cathodal buckling and its modulation by fear of falling both support the hypothesis that short‐latency responses contribute to regulate balance.

The internal representation of head orientation differs for conscious perception and balance control.

We tested perceived head-on-feet orientation and the direction of vestibular-evoked balance responses in passively and actively held head-turned postures. The direction of vestibular-evoked balance responses was not aligned with perceived head-on-feet orientation while maintaining prolonged passively held head-turned postures. Furthermore, static visual cues of head-on-feet orientation did not update the estimate of head posture for the balance controller. A prolonged actively held head-turned posture did not elicit a rotation in the direction of the vestibular-evoked balance response despite a significant rotation in perceived angular head posture. It is proposed that conscious perception of head posture and the transformation of vestibular signals for standing balance relying on this head posture are not dependent on the same internal representation. Rather, the balance system may operate under its own sensorimotor principles, which are partly independent from perception. Vestibular signals used for balance control must be integrated with other sensorimotor cues to allow transformation of descending signals according to an internal representation of body configuration. We explored two alternative models of sensorimotor integration that propose (1) a single internal representation of head-on-feet orientation is responsible for perceived postural orientation and standing balance or (2) conscious perception and balance control are driven by separate internal representations. During three experiments, participants stood quietly while passively or actively maintaining a prolonged head-turned posture (>10min). Throughout the trials, participants intermittently reported their perceived head angular position, and subsequently electrical vestibular stimuli were delivered to elicit whole-body balance responses. Visual recalibration of head-on-feet posture was used to determine whether static visual cues are used to update the internal representation of body configuration for perceived orientation and standing balance. All three experiments involved situations in which the vestibular-evoked balance response was not orthogonal to perceived head-on-feet orientation, regardless of the visual information provided. For prolonged head-turned postures, balance responses consistent with actual head-on-feet posture occurred only during the active condition. Our results indicate that conscious perception of head-on-feet posture and vestibular control of balance do not rely on the same internal representation, but instead treat sensorimotor cues in parallel and may arrive at different conclusions regarding head-on-feet posture. The balance system appears to bypass static visual cues of postural orientation and mainly use other sensorimotor signals of head-on-feet position to transform vestibular signals of head motion, a mechanism appropriate for most daily activities.

Vestibular control of standing balance is enhanced with increased cognitive load.

When cognitive load is elevated during a motor task, cortical inhibition and reaction time are increased; yet, standing balance control is often unchanged. This disconnect is likely explained by compensatory mechanisms within the balance system such as increased sensitivity of the vestibulomotor pathway. This study aimed to determine the effects of increased cognitive load on the vestibular control of standing balance. Participants stood blindfolded on a force plate with their head facing left and arms relaxed at their sides for two trials while exposed to continuous electrical vestibular stimulation (EVS). Participants either stood quietly or executed a cognitive task (double-digit arithmetic). Surface electromyography (EMG) and anterior-posterior ground-body forces (APF) were measured in order to evaluate vestibular-evoked balance responses in the frequency (coherence and gain) and time (cumulant density) domains. Total distance traveled for anterior-posterior center of pressure (COP) was assessed as a metric of balance variability. Despite similar distances traveled for COP, EVS-medial gastrocnemius (MG) EMG and EVS-APF coherence and EVS-TA EMG and EVS-MG EMG gain were elevated for multiple frequencies when standing with increased cognitive load. For the time domain, medium-latency peak amplitudes increased by 13-54% for EVS-APF and EVS-EMG relationships with the cognitive task compared to without. Peak short-latency amplitudes were unchanged. These results indicate that reliance on vestibular control of balance is enhanced when cognitive load is elevated. This augmented neural strategy may act to supplement divided cortical processing resources within the balance system and compensate for the acute neuromuscular modifications associated with increased cognitive demand.

Neuroticism modulates brain visuo-vestibular and anxiety systems during a virtual rollercoaster task.

Different lines of research suggest that anxiety‐related personality traits may influence the visual and vestibular control of balance, although the brain mechanisms underlying this effect remain unclear. To our knowledge, this is the first functional magnetic resonance imaging (fMRI) study that investigates how individual differences in neuroticism and introversion, two key personality traits linked to anxiety, modulate brain regional responses and functional connectivity patterns during a fMRI task simulating self‐motion. Twenty‐four healthy individuals with variable levels of neuroticism and introversion underwent fMRI while performing a virtual reality rollercoaster task that included two main types of trials: (1) trials simulating downward or upward self‐motion (vertical motion), and (2) trials simulating self‐motion in horizontal planes (horizontal motion). Regional brain activity and functional connectivity patterns when comparing vertical versus horizontal motion trials were correlated with personality traits of the Five Factor Model (i.e., neuroticism, extraversion‐introversion, openness, agreeableness, and conscientiousness). When comparing vertical to horizontal motion trials, we found a positive correlation between neuroticism scores and regional activity in the left parieto‐insular vestibular cortex (PIVC). For the same contrast, increased functional connectivity between the left PIVC and right amygdala was also detected as a function of higher neuroticism scores. Together, these findings provide new evidence that individual differences in personality traits linked to anxiety are significantly associated with changes in the activity and functional connectivity patterns within visuo‐vestibular and anxiety‐related systems during simulated vertical self‐motion. Hum Brain Mapp 38:715–726, 2017. © 2016 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc.