Gravity estimation and verticality perception

Rod and frame test parameters for neuropsychology studies

BACKGROUND

The rod and frame test (RFT), a measure of field dependence-independence, recently has reemerged as a measure of research interest and potential diagnostic value in neuropsychology. In the standard RFT, the subject experiences offsetting visual cues from a frame surrounding an embedded rod, while the subject's postural/vestibular cues provide the sense of verticality as the subject attempts to set the rod to vertical. The paper shows that RFTs not adhering to RFT parameters can reduce the test's visual framework impact experienced by the subject. Comparisons of neuropsychological studies will highlight that correct adherence to RFT testing conditions can strengthen RFT effects.

METHOD

This review presents the parameters that have been studied which impact on subject performance on the RFT. It identifies how computer administered RFTs have been applied to enhance the study of the RFT parameters and make the RFT more accessible to the study of different diagnostic groups. The article also critiques studies by identifying how the RFT's parameters, study's design and statistical analysis may have diminished identifying the full effects of the RFT experience.

RESULTS

Parameters impacting judgments of verticality of the rod can include: perceived size of rod and frame, the gap between the ends of the rod and surrounding frame, presentation of the rod within an encompassing 3D visual framework that visually blocks out the surrounding environment, a dark room, instructions stressing egocentric vs allocentric strategies, double frame surrounding the rod to assess global perception effects, etc. Details are presented how gap size likely affected results in neuropsychology studies. Potentially, these and other experiments may be studied using computer administered RFTs.

CONCLUSIONS

Based on the descriptions of computer administered RFTs, this article suggested that incorporating these technologies can provide better understanding underlying the RFT, and in turn, understanding neuropsychology processes.

Change in sensory integration and regularity of postural sway with the suspensory strategy during static standing balance

Background and aim

The suspensory strategy, a method for controlling postural balance in the vertical direction of the center of mass (COM), is considered by the elderly as a means of balance control. The vertical COM control might alter the sensory integration and regularity of postural sway, which in turn impacts balance. However, to date, this was not confirmed. Thus, this study aimed at investigating the influence of the suspensory strategy achieved through knee flexion on the static standing balance.

Methods

Nineteen participants were monitored at knee flexion angles of 0°, 15°, and 65°. Time-frequency analysis and sample entropy were employed to analyze the COM data. Time-frequency analysis was utilized to assess the energy content across various frequency bands and corresponding percentage of energy within each frequency band. The outcomes of time-frequency are hypothesized to reflect the balance-related sensory input and sensory weights. Sample entropy was applied to evaluate the regularity of the COM displacement patterns.

Results

Knee flexion led to a decreased COM height. The highest energy content was observed at 65° knee flexion, in contrast with the lowest energy observed at 0° in both the anterior-posterior (AP) and medial-lateral (ML) directions. Additionally, the ultra-low-frequency band was more pronounced at 65° than that at 0° or 15° in the ML direction. Furthermore, the COM amplitudes were notably higher at 65° than those at 0° and 15° in the AP and ML directions, respectively. The sample entropy values were lower at 65° and 15° than those at 0° in the ML direction, with the lowest value observed at 65° in the vertical direction.

Conclusion

The suspensory strategy could enhance the sensory input and cause sensory reweighting, culminating in a more regular balance control. Such suspensory strategy-induced postural control modifications may potentially provide balance benefits for people with declining balance-related sensory, central processing, and musculoskeletal system functions.

Neuronal Representations Supporting Three-Dimensional Vision in Nonhuman Primates

The visual system must reconstruct the dynamic, three-dimensional (3D) world from ambiguous two-dimensional (2D) retinal images. In this review, we synthesize current literature on how the visual system of nonhuman primates performs this transformation through multiple channels within the classically defined dorsal (where) and ventral (what) pathways. Each of these channels is specialized for processing different 3D features (e.g., the shape, orientation, or motion of objects, or the larger scene structure). Despite the common goal of 3D reconstruction, neurocomputational differences between the channels impose distinct information-limiting constraints on perception. Convergent evidence further points to the little-studied area V3A as a potential branchpoint from which multiple 3D-fugal processing channels diverge. We speculate that the expansion of V3A in humans may have supported the emergence of advanced 3D spatial reasoning skills. Lastly, we discuss future directions for exploring 3D information transmission across brain areas and experimental approaches that can further advance the understanding of 3D vision.

Dynamic changes of the direction and angle of radiographic ocular lateral deviation in patients with lateropulsion after stroke onset

OBJECTIVE

To examine if radiographic ocular lateral deviation (rOLD) could be provoked in stroke patients with mild-to-moderate lateropulsion according to vertical perception.

METHODS

In this single-center, retrospective study, acute stroke patients with mild-to-moderate lateropulsion assessed by the Scale for Contraversive Pushing were enrolled. Computed tomography or magnetic resonance imaging was performed on all patients on admission and then according to their conditions. The direction and angle of rOLD were compared among three groups according to the responsible lesion: lateral medullary (LM), pontine (P), and hemispheric (H).

RESULTS

Sixty-six patients (male, 47; average age, 67 years) were enrolled and divided into the LM (n = 37), P (n = 8), and H (n = 21) groups. All patients had body tilt. Patients in the LM group showed body tilt to the ipsilesional side during hospitalization, while those in the P and H groups tilted to the contralesional side. All patients had rOLD at the final assessment at an average of 13 days after onset; patients in the P and H groups showed contralateral rOLD, while those in the LM group showed ipsilateral rOLD if they did not have cerebellar or pontine lesions. Significant decreases in the angle and changes in direction of rOLD according to lesion site were observed during hospitalization.

CONCLUSION

Serial changes in rOLD findings after stroke onset are different according to the responsible lesion. The direction of rOLD in most patients is in accordance with vertical perception after the acute stage of stroke.

Association Between Body Tilt and Egocentric Estimates Near Upright

The mechanisms underlying geocentric (orientations of an object or the body relative to 'gravity') and egocentric estimates (object orientation relative to the 'body') have each been examined; however, little is known regarding the association between these estimates, especially when the body is nearly upright. To address this, we conducted two psychophysical experiments. In Experiment 1, participants estimated the direction of a visual line (subjective visual vertical; SVV) and their own body relative to gravity (subjective body tilt; SBT) and the direction of a visual line relative to the body longitudinal axis (subjective visual body axis; SVBA) during a small-range whole-body roll tilt. We evaluated the correlations between performance on each of these tasks as covariates of actual body tilt angles. Our results showed a significant correlation of performance (estimation errors) on the SVBA task with performance on the SBT task but not performance on the SVV task at the group level after adjusting for the actual body tilt angles, suggesting a link between the estimates for SVBA and SBT tasks. To confirm this relationship, in Experiment 2, we further assessed whether manipulating the subjective direction of the body axis by providing visual feedback in the SVBA task subsequently affected SBT performance. We found that feedback in the SVBA task significantly shifted the SBT angles even when the actual body angles were identical. The observed association between SVBA and SBT performance supports at least a partially shared mechanism underlying body tilt and egocentric estimates when the body is nearly upright.

Off-Vertical Body Orientation Delays the Perceived Onset of Visual Motion

The integration of vestibular, visual and body cues is a fundamental process in the perception of self-motion and is commonly experienced in an upright posture. However, when the body is tilted in an off-vertical orientation these signals are no longer aligned relative to the influence of gravity. In this study, the perceived timing of visual motion was examined in the presence of sensory conflict introduced by manipulating the orientation of the body, generating a mismatch between body and vestibular cues due to gravity and creating an ambiguous vestibular signal of either head tilt or translation. In a series of temporal-order judgment tasks, participants reported the perceived onset of a visual scene simulating rotation around the yaw axis presented in virtual reality with a paired auditory tone while in an upright, supine and side-recumbent body position. The results revealed that the perceived onset of visual motion was further delayed from zero (i.e., true simultaneity between visual onset and a reference auditory tone) by approximately an additional 30 ms when viewed in a supine or side-recumbent orientation compared to an upright posture. There were also no significant differences in the timing estimates of the visual motion between all the non-upright orientations. This indicates that the perceived timing of visual motion is negatively impacted by the presence of conflict in the vestibular and body signals due to the direction of gravity and body orientation, even when the mismatch is not in the direct plane of the axis of rotation.

Assessment of the perception of vertical subjectivity in children born preterm

Children born preterm have increased rates of paediatric mortality and morbidity. Prematurity has been associated with impaired visual perception and visuo-motor integration. The alteration of the perception of verticality translates into alterations of the vestibular system at central and/or peripheral level, which may manifest itself in symptoms such as imbalance, dizziness or even vertigo. The aim of this study was to compare subjective visual vertical (SVV) test scores in children born preterm with those of children born at term at ages between 7 and 10. One hundred ten children with no neurodevelopmental disorder of 7 to 10 years of age were studied using a mobile application on a smartphone attached to a wall by means of a rotating plate. The SVV test was compared between two groups: a group of 55 preterm children (53 very preterm children born under 32 weeks of gestational age and 2 preterm with very low birth weight) and another group of 55 children born at term (after 37 weeks of gestational age). The SVV results were analysed for comparison with respect to prematurity, sex and age. We found no significant differences in the SVV study in the comparison between preterm and term children. In addition, no significant differences were observed regarding sex or age between 7 and 10 years.  Conclusion: We found no alterations in the perception of vertical subjectivity in children between 7 and 10 years of age, with antecedents of very preterm birth and/or very low birth weight. What is Known: • The different studies published so far suggest the existence of balance disorders in premature children, although in most of these studies the children are examined at an age when the vestibular system is not mature and with non-specific tests for the study of the vestibular system. What is New: • We compared the results of the subjective visual vertical (SVV) test in a group of 55 preterm children (53 very preterm children born under 32 weeks of gestational age and 2 preterm with very low weight at birth) and in a group of 55 children born at term (after 37 weeks of gestational age), at the ages of 7 to 10 years and observed no differences. • We conclude that, if there had been any vestibular alterations due to very premature birth, these must have been compensated by the age of 7.

Health Implications of Virtual Architecture: An Interdisciplinary Exploration of the Transferability of Findings from Neuroarchitecture

Virtual architecture has been increasingly relied on to evaluate the health impacts of physical architecture. In this health research, exposure to virtual architecture has been used as a proxy for exposure to physical architecture. Despite the growing body of research on the health implications of physical architecture, there is a paucity of research examining the long-term health impacts of prolonged exposure to virtual architecture. In response, this paper considers: what can proxy studies, which use virtual architecture to assess the physiological response to physical architecture, tell us about the impact of extended exposure to virtual architecture on human health? The paper goes on to suggest that the applicability of these findings to virtual architecture may be limited by certain confounding variables when virtual architecture is experienced for a prolonged period of time. This paper explores the potential impact of two of these confounding variables: multisensory integration and gravitational perception. This paper advises that these confounding variables are unique to extended virtual architecture exposure and may not be captured by proxy studies that aim to capture the impact of physical architecture on human health through acute exposure to virtual architecture. While proxy studies may be suitable for measuring some aspects of the impact of both physical and virtual architecture on human health, this paper argues that they may be insufficient to fully capture the unintended consequences of extended exposure to virtual architecture on human health. Therefore, in the face of the increasing use of virtual architectural environments, the author calls for the establishment of a subfield of neuroarchitectural health research that empirically examines the physiological impacts of extended exposure to virtual architecture in its own right.

Comparing the impact of the method of adjustment and forced-choice methodologies on subjective visual vertical bias and variability

BACKGROUND

Previous research suggested that the method of adjustment and forced choice variants of the subjective visual vertical (SVV) produce comparable estimates of both bias and variability. However, variants of the SVV that utilize a method of adjustment procedure are known to be heavily influenced by task parameters, including the stimulus rotation speed, which was not accounted for in previous SVV research comparing the method of adjustment to forced-choice.

OBJECTIVE

The aim of the present study was to determine if (1) the SVV with a forced-choice procedure produces both bias and variability estimates that are comparable to those obtained using a method of adjustment procedure, (2) to see if rotation speed impacts the comparability of estimates and (3) quantify correlations between the estimates produced by different procedures.

METHODS

Participants completed a variant of the SVV which utilized a forced-choice procedure as well as two variants of the SVV using a method of adjustment procedure with two different rotation speeds (6°/s and 12°/s).

RESULTS

We found that the bias estimates were similar across all three conditions tested and that the variability estimates were greater in the SVV variants that utilized a method of adjustment procedure. This difference was more pronounced when the rotation speed was slower (6°/s).

CONCLUSIONS

The results of this study suggest that forced-choice and method of adjustment methodologies yield similar bias estimates and different variability estimates. Given these results, we recommend utilizing forced-choice procedures unless (a) forced-choice is not feasible or (b) response variability is unimportant. We also recommend that clinicians consider the SVV methods when interpreting a patient's test results, especially for variability metrics.

Backward Walking Styles and Impact on Spatiotemporal Gait Characteristics

Forward walking (FW) is a common balance assessment tool. However, its sensitivity is limited by the ceiling effect. Reverse gait, such as backward walking (BW), has been reported to have more advantages than FW for balance assessment. Three factors related to postural instability (i.e., increased speeds, restricted arm swing, and reduced visual feedback) during BW were investigated to determine BW conditions that have the potential to predict falls. Three-dimensional analyses were used to analyze seven walking conditions. FW and BW at self-selected and fast speeds were analyzed to identify the effects of speed. Walking with normal arm swings, crossed arms, and abducted arms during BW was tested to determine the effects of arm position. BW with closed and open eyes was compared to investigate the effects of visual feedback. BW had a significantly shorter step length than FW at high speeds. When the arms were abducted, the stance phase (%) was significantly lower compared to when arms were crossed during BW. Moreover, BW with closed eyes revealed significantly higher mediolateral center of mass (COM) displacements than with open eyes. We observed that BW with fast speeds, a crossed arm position, and closed eyes has the potential to help assess fall risk because it requires higher balance ability through spatiotemporal and COM adjustment.

Parallel processing, hierarchical transformations, and sensorimotor associations along the 'where' pathway

Visually guided behaviors require the brain to transform ambiguous retinal images into object-level spatial representations and implement sensorimotor transformations. These processes are supported by the dorsal ‘where’ pathway. However, the specific functional contributions of areas along this pathway remain elusive due in part to methodological differences across studies. We previously showed that macaque caudal intraparietal (CIP) area neurons possess robust 3D visual representations, carry choice- and saccade-related activity, and exhibit experience-dependent sensorimotor associations (Chang et al., 2020b). Here, we used a common experimental design to reveal parallel processing, hierarchical transformations, and the formation of sensorimotor associations along the ‘where’ pathway by extending the investigation to V3A, a major feedforward input to CIP. Higher-level 3D representations and choice-related activity were more prevalent in CIP than V3A. Both areas contained saccade-related activity that predicted the direction/timing of eye movements. Intriguingly, the time course of saccade-related activity in CIP aligned with the temporally integrated V3A output. Sensorimotor associations between 3D orientation and saccade direction preferences were stronger in CIP than V3A, and moderated by choice signals in both areas. Together, the results explicate parallel representations, hierarchical transformations, and functional associations of visual and saccade-related signals at a key juncture in the ‘where’ pathway.

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

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

Effect of subjective vertical perception on lateral flexion posture of patients with Parkinson's disease

In a retrospective study we tested our hypothesis that the subjective postural vertical ratio (SPV ratio), i.e., the subjective postural vertical measured in relation to the lateral flexion axis, is predictive of lateral trunk flexion in patients with Parkinson's disease (PD). Twenty-five patients were included. The SPV angle, i.e., the subjective perception of a vertical position with reference to the vertical axis, and the SPV ratio, i.e., the SPV angle with reference to the axis of lateral flexion, were calculated. The SPV ratio (r = 0.698, P = 0.001) and LTF angle (r =  − 0.601, P = 0.001) correlated with change in the LTF angle at 1 year. The SPV ratio was significantly smaller in patients for whom lateral trunk flexion improved (n = 12) than in those for whom it did not improve (n = 13) (0.99 ± 0.78 vs 1.66 ± 0.71, P = 0.011). The AUC under the ROC curve of the SPV ratio for discrimination of improvement was 0.795 (95% confidence interval: 0.61–0.98). We found that the SPV ratio is associated with change in the LTF and that it can conceivably be used to predict the likelihood of improvement in PD-associated lateral trunk flexion.

Perspective Cues Make Eye-specific Contributions to 3-D Motion Perception

Robust 3-D visual perception is achieved by integrating stereoscopic and perspective cues. The canonical model describing the integration of these cues assumes that perspective signals sensed by the left and right eyes are indiscriminately pooled into a single representation that contributes to perception. Here, we show that this model fails to account for 3-D motion perception. We measured the sensitivity of male macaque monkeys to 3-D motion signaled by left-eye perspective cues, right-eye perspective cues, stereoscopic cues, and all three cues combined. The monkeys exhibited idiosyncratic differences in their biases and sensitivities for each cue, including left- and right-eye perspective cues, suggesting that the signals undergo at least partially separate neural processing. Importantly, sensitivity to combined cue stimuli was greater than predicted by the canonical model, which previous studies found to account for the perception of 3-D orientation in both humans and monkeys. Instead, 3-D motion sensitivity was best explained by a model in which stereoscopic cues were integrated with left- and right-eye perspective cues whose representations were at least partially independent. These results indicate that the integration of perspective and stereoscopic cues is a shared computational strategy across 3-D processing domains. However, they also reveal a fundamental difference in how left- and right-eye perspective signals are represented for 3-D orientation versus motion perception. This difference results in more effective use of available sensory information in the processing of 3-D motion than orientation and may reflect the temporal urgency of avoiding and intercepting moving objects.

Altered visual and haptic verticality perception in posterior cortical atrophy and Alzheimer's disease

There is increasing theoretical and empirical support for the brain combining multisensory information to determine the direction of gravity and hence uprightness. A fundamental part of the process is the spatial transformation of sensory signals between reference frames: eye-centred, head-centred, body-centred, etc. The question 'Am I the right way up?' posed by a patient with posterior cortical atrophy (PCA) suggests disturbances in upright perception, subsequently investigated in PCA and typical Alzheimer's disease (tAD) based on what looks or feels upright. Participants repeatedly aligned to vertical a rod presented either visually (visual-vertical) or haptically (haptic-vertical). Visual-vertical involved orienting a projected rod presented without or with a visual orientation cue (circle, tilted square (±18°)). Haptic-vertical involved orientating a grasped rod with eyes closed using a combination of side (left, right) and hand (unimanual, bimanual) configurations. Intraindividual uncertainty and bias defined verticality perception. Uncertainty was consistently greater in both patient groups than in control groups, and greater in PCA than tAD. Bias in the frontal plane was strongly directionally affected by visual cue tilt (visual-vertical) and grip side (haptic-vertical). A model was developed that assumed verticality information from multiple sources is combined in a statistically optimal way to produce observed uncertainties and biases. Model results suggest the mechanism that spatially transforms graviceptive information between body parts is disturbed in both patient groups. Despite visual dysfunction being typically considered the primary feature of PCA, disturbances were greater in PCA than tAD particularly for haptic-vertical, and are considered in light of posterior parietal vulnerability. KEY POINTS: The perception of upright requires accurate and precise estimates of orientation based on multiple noisy sensory signals. The question 'Am I the right way up?' posed by a patient with posterior cortical atrophy (PCA; purported 'visual variant Alzheimer's') suggests disturbances in the perception of upright. What looks or feels upright in PCA and typical Alzheimer's disease (tAD) was investigated by asking participants to repeatedly align to vertical a rod presented visually (visual-vertical) or haptically (haptic-vertical). PCA and tAD groups exhibited not only greater perceptual uncertainty than controls, but also exaggerated bias induced by tilted visual orientation cues (visual-vertical) and grip side (haptic-vertical). When modelled, these abnormalities, which were particularly evident in PCA haptic-vertical performance, were compatible with disruption of a mechanism that spatially transforms verticality information between body parts. The findings suggest an important role of posterior parietal cortex in verticality perception, and have implications for understanding spatial disorientation in dementia.

Pupil responses associated with the perception of gravitational vertical under directional optic flows

This study assessed the pupil responses in the sensory integration of various directional optic flows during the perception of gravitational vertical. A total of 30 healthy participants were enrolled with normal responses to conventional subjective visual vertical (SVV) which was determined by measuring the difference (error angles) between the luminous line adjusted by the participants and the true vertical. SVV was performed under various types of rotational (5°/s, 10°/s, and 50°/s) and straight (5°/s and 10°/s) optic flows presented via a head-mounted display. Error angles (°) of the SVV and changes in pupil diameters (mm) were measured to evaluate the changes in the visually assessed subjective verticality and related cognitive demands. Significantly larger error angles were measured under rotational optic flows than under straight flows (p < 0.001). The error angles also significantly increased as the velocity of the rotational optic flow increased. The pupil diameter increased after starting the test, demonstrating the largest diameter during the final fine-tuning around the vertical. Significantly larger pupil changes were identified under rotational flows than in straight flows. Pupil changes were significantly correlated with error angles and the visual analog scale representing subjective difficulties during each test. These results suggest increased pupil changes for integrating more challenging visual sensory inputs in the process of gravity perception.

An approach to drastically reduce the required legs DOFs for bipedal robots and lower-limb exoskeletons

We present a method to drastically reduce the required number of degrees-of-freedom (DOFs) needed for walking for each leg of bipedal robots and lower-limb exoskeletons. This approach releases more legs DOFs in the null space to do other tasks, instead of unnecessarily constraining them. It uses relative reference frames to control relative motion between the two feet, instead of the usual method of controlling foot movement with respect to fixed reference frames. In its basic form, it controls the bipedal walking holistically using two controllers: (1) world space control using relative feet motion and (2) null-space control of the legs posture.

Visual predictions, neural oscillations and naïve physics

Prediction is a core function of the human visual system. Contemporary research suggests the brain builds predictive internal models of the world to facilitate interactions with our dynamic environment. Here, we wanted to examine the behavioural and neurological consequences of disrupting a core property of peoples’ internal models, using naturalistic stimuli. We had people view videos of basketball and asked them to track the moving ball and predict jump shot outcomes, all while we recorded eye movements and brain activity. To disrupt people’s predictive internal models, we inverted footage on half the trials, so dynamics were inconsistent with how movements should be shaped by gravity. When viewing upright videos people were better at predicting shot outcomes, at tracking the ball position, and they had enhanced alpha-band oscillatory activity in occipital brain regions. The advantage for predicting upright shot outcomes scaled with improvements in ball tracking and occipital alpha-band activity. Occipital alpha-band activity has been linked to selective attention and spatially-mapped inhibitions of visual brain activity. We propose that when people have a more accurate predictive model of the environment, they can more easily parse what is relevant, allowing them to better target irrelevant positions for suppression—resulting in both better predictive performance and in neural markers of inhibited information processing.

An Implanted Vestibular Prosthesis Improves Spatial Orientation in Animals with Severe Vestibular Damage

Gravity is a pervasive environmental stimulus, and accurate graviception is required for optimal spatial orientation and postural stability. The primary graviceptors are the vestibular organs, which include angular velocity (semicircular canals) and linear acceleration (otolith organs) sensors. Graviception is degraded in patients with vestibular damage, resulting in spatial misperception and imbalance. Since minimal therapy is available for these patients, substantial effort has focused on developing a vestibular prosthesis or vestibular implant (VI) that reproduces information normally provided by the canals (since reproducing otolith function is very challenging technically). Prior studies demonstrated that angular eye velocity responses could be driven by canal VI-mediated angular head velocity information, but it remains unknown whether a canal VI could improve spatial perception and posture since these behaviors require accurate estimates of angular head position in space relative to gravity. Here, we tested the hypothesis that a canal VI that transduces angular head velocity and provides this information to the brain via motion-modulated electrical stimulation of canal afferent nerves could improve the perception of angular head position relative to gravity in monkeys with severe vestibular damage. Using a subjective visual vertical task, we found that normal female monkeys accurately sensed the orientation of the head relative to gravity during dynamic tilts, that this ability was degraded following bilateral vestibular damage, and improved when the canal VI was used. These results demonstrate that a canal VI can improve graviception in vestibulopathic animals, suggesting that it could reduce the disabling perceptual and postural deficits experienced by patients with severe vestibular damage.SIGNIFICANCE STATEMENT Patients with vestibular damage experience impaired vision, spatial perception, and balance, symptoms that could potentially respond to a vestibular implant (VI). Anatomic features facilitate semicircular canal (angular velocity) prosthetics but inhibit approaches with the otolith (linear acceleration) organs, and canal VIs that sense angular head velocity can generate compensatory eye velocity responses in vestibulopathic subjects. Can the brain use canal VI head velocity information to improve estimates of head orientation (e.g., head position relative to gravity), which is a prerequisite for accurate spatial perception and posture? Here we show that a canal VI can improve the perception of head orientation in vestibulopathic monkeys, results that are highly significant because they suggest that VIs mimicking canal function can improve spatial orientation and balance in vestibulopathic patients.

Measuring Vestibular Contributions to Age-Related Balance Impairment: A Review

Aging is associated with progressive declines in both the vestibular and human balance systems. While vestibular lesions certainly contribute to imbalance, the specific contributions of age-related vestibular declines to age-related balance impairment is poorly understood. This gap in knowledge results from the absence of a standardized method for measuring age-related changes to the vestibular balance pathways. The purpose of this manuscript is to provide an overview of the existing body of literature as it pertains to the methods currently used to infer vestibular contributions to age-related imbalance.

The potential of noisy galvanic vestibular stimulation for optimizing and assisting human performance

Noisy galvanic vestibular stimulation (nGVS) is an emerging non-invasive brain stimulation technique. It involves applying alternating currents of different frequencies and amplitudes presented in a random, or noisy, manner through electrodes on the mastoid bones behind the ears. Because it directly activates vestibular hair cells and afferents and has an indirect effect on a variety of brain regions, it has the potential to impact many different functions. The objective of this review is twofold: (1) to review how nGVS affects motor, sensory, and cognitive performance in healthy adults; and (2) to discuss potential clinical applications of nGVS. First, we introduce the technique. We then describe the regions receiving and processing vestibular information. Next, we discuss the effects of nGVS on motor, sensory, and cognitive function in healthy adults. Subsequently, we outline its potential clinical applications. Finally, we highlight other electrical stimulation technologies and discuss why nGVS offers an alternative or complementary approach. Overall, nGVS appears promising for optimizing human performance and as an assistive technology, though further research is required.

The gravitational imprint on sensorimotor planning and control

Humans excel at learning complex tasks, and elite performers such as musicians or athletes develop motor skills that defy biomechanical constraints. All actions require the movement of massive bodies. Of particular interest in the process of sensorimotor learning and control is the impact of gravitational forces on the body. Indeed, efficient control and accurate internal representations of the body configuration in space depend on our ability to feel and anticipate the action of gravity. Here we review studies on perception and sensorimotor control in both normal and altered gravity. Behavioral and modeling studies together suggested that the nervous system develops efficient strategies to take advantage of gravitational forces across a wide variety of tasks. However, when the body was exposed to altered gravity, the rate and amount of adaptation exhibited substantial variation from one experiment to another and sometimes led to partial adjustment only. Overall, these results support the hypothesis that the brain uses a multimodal and flexible representation of the effect of gravity on our body and movements. Future work is necessary to better characterize the nature of this internal representation and the extent to which it can adapt to novel contexts.

Optimized but Not Maximized Cue Integration for 3D Visual Perception

Reconstructing three-dimensional (3D) scenes from two-dimensional (2D) retinal images is an ill-posed problem. Despite this, 3D perception of the world based on 2D retinal images is seemingly accurate and precise. The integration of distinct visual cues is essential for robust 3D perception in humans, but it is unclear whether this is true for non-human primates (NHPs). Here, we assessed 3D perception in macaque monkeys using a planar surface orientation discrimination task. Perception was accurate across a wide range of spatial poses (orientations and distances), but precision was highly dependent on the plane's pose. The monkeys achieved robust 3D perception by dynamically reweighting the integration of stereoscopic and perspective cues according to their pose-dependent reliabilities. Errors in performance could be explained by a prior resembling the 3D orientation statistics of natural scenes. We used neural network simulations based on 3D orientation-selective neurons recorded from the same monkeys to assess how neural computation might constrain perception. The perceptual data were consistent with a model in which the responses of two independent neuronal populations representing stereoscopic cues and perspective cues (with perspective signals from the two eyes combined using nonlinear canonical computations) were optimally integrated through linear summation. Perception of combined-cue stimuli was optimal given this architecture. However, an alternative architecture in which stereoscopic cues, left eye perspective cues, and right eye perspective cues were represented by three independent populations yielded two times greater precision than the monkeys. This result suggests that, due to canonical computations, cue integration for 3D perception is optimized but not maximized.

Functional links between sensory representations, choice activity, and sensorimotor associations in parietal cortex

Three-dimensional (3D) representations of the environment are often critical for selecting actions that achieve desired goals. The success of these goal-directed actions relies on 3D sensorimotor transformations that are experience-dependent. Here we investigated the relationships between the robustness of 3D visual representations, choice-related activity, and motor-related activity in parietal cortex. Macaque monkeys performed an eight-alternative 3D orientation discrimination task and a visually guided saccade task while we recorded from the caudal intraparietal area using laminar probes. We found that neurons with more robust 3D visual representations preferentially carried choice-related activity. Following the onset of choice-related activity, the robustness of the 3D representations further increased for those neurons. We additionally found that 3D orientation and saccade direction preferences aligned, particularly for neurons with choice-related activity, reflecting an experience-dependent sensorimotor association. These findings reveal previously unrecognized links between the fidelity of ecologically relevant object representations, choice-related activity, and motor-related activity.

Stumbling reactions in hypo and hyper gravity - muscle synergies are robust across different perturbations of human stance during parabolic flights

The control of bipedal stance and the capacity to regain postural equilibrium after its deterioration in variable gravities are crucial prerequisites for manned space missions. With an emphasize on natural orthograde posture, computational techniques synthesize muscle activation patterns of high complexity to a simple synergy organization. We used nonnegative matrix factorization to identify muscle synergies during postural recovery responses in human and to examine the functional significance of such synergies for hyper-gravity (1.75 g) and hypo-gravity (0.25 g). Electromyographic data were recorded from leg, trunk and arm muscles of five human exposed to five modes of anterior and posterior support surface translations during parabolic flights including transitional g-levels of 0.25, 1 and 1.75 g. Results showed that in 1 g four synergies accounted for 99% of the automatic postural response across all muscles and perturbation directions. Each synergy in 1 g was correlated to the corresponding one in 0.25 and 1.75 g. This study therefore emphasizes the similarity of the synergy organization of postural recovery responses in Earth, hypo- and hyper-gravity conditions, indicating that the muscle synergies and segmental strategies acquired under terrestrial habits are robust and persistent across variable and acute changes in gravity levels.

The Vestibular Drive for Balance Control Is Dependent on Multiple Sensory Cues of Gravity

Vestibular signals, which encode head movement in space as well as orientation relative to gravity, contribute to the ongoing muscle activity required to stand. The strength of this vestibular contribution changes with the presence and quality of sensory cues of balance. Here we investigate whether the vestibular drive for standing balance also depends on different sensory cues of gravity by examining vestibular-evoked muscle responses when independently varying load and gravity conditions. Standing subjects were braced by a backboard structure that limited whole-body sway to the sagittal plane while load and vestibular cues of gravity were manipulated by: (a) loading the body downward at 1.5 and 2 times body weight (i.e., load cues), and/or (b) exposing subjects to brief periods (20 s) of micro- (<0.05 g) and hyper-gravity (∼1.8 g) during parabolic flights (i.e., vestibular cues). A stochastic electrical vestibular stimulus (0–25 Hz) delivered during these tasks evoked a vestibular-error signal and corrective muscles responses that were used to assess the vestibular drive to standing balance. With additional load, the magnitude of the vestibular-evoked muscle responses progressively increased, however, when these responses were normalized by the ongoing muscle activity, they decreased and plateaued at 1.5 times body weight. This demonstrates that the increased muscle activity necessary to stand with additional load is accompanied a proportionally smaller increase in vestibular input. This reduction in the relative vestibular contribution to balance was also observed when we varied the vestibular cues of gravity, but only during an absence (<0.05 g) and not an excess (∼1.8 g) of gravity when compared to conditions with normal 1 g gravity signals and equivalent load signals. Despite these changes, vestibular-evoked responses were observed in all conditions, indicating that vestibular cues of balance contribute to upright standing even in the near absence of a vestibular signal of gravity (i.e., micro-gravity). Overall, these experiments provide evidence that both load and vestibular cues of gravity influence the vestibular signal processing for the control of standing balance.