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.

Imbalance and dizziness caused by unilateral vestibular schwannomas correlate with vestibulo-ocular reflex precision and bias.

Imbalance and dizziness are disabling symptoms for many patients with vestibular schwannomas (VS) but symptom severity typically does not correlate with the vestibulo-ocular reflex (VOR) amplitude-based metrics used to assess peripheral vestibular damage. In this study, we tested the hypothesis that imbalance and dizziness in patients with VS relate to VOR metrics that are not based on response amplitude. Twenty-four patients with unilateral, sporadic VS tumors were studied, and objective (balance) and subjective (dizziness) vestibular dysfunction was quantified. The VOR was tested using two yaw-axis motion stimuli, low-frequency en-bloc sinusoidal, and high-frequency head-on-body impulsive rotations. Imbalance correlated with VOR precision (the inverse of the trial-to-trial variability) and with low-frequency VOR dynamics (quantified with the time constant), and these two metrics were also strongly correlated. Dizziness correlated with the VOR bias caused by an imbalance in static central vestibular tone, but not with dynamic VOR metrics. VOR accuracy (mean response amplitude relative to the ideal response) was not correlated with the severity of imbalance or dizziness or with measures of VOR precision or time constant. Imbalance in patients with VS, therefore, scales with VOR precision and time constant, both of which appear to reflect the central vestibular signal-to-noise ratio, but not with VOR slow-phase accuracy, which is based on the magnitude of the central vestibular signals. Dizziness was related to the presence of a static central tone imbalance but not to any VOR metrics, suggesting that abnormal perception in VS may be affected by factors that are not captured by yaw-axis VOR measurements.NEW & NOTEWORTHY The severity of symptoms associated with unilateral vestibular schwannomas (VS) is poorly correlated with standard yaw-axis vestibulo-ocular reflex (VOR) metrics that are based on response amplitude. In this study, we show that the balance and perceptual dysfunction experienced by patients with VS scales with VOR metrics that capture information about the central signal-to-noise ratio (balance) and central static tone (dizziness), but are not correlated with the VOR gain, which reflects central signal amplitude.

The predictive power of geographic health care utilization for unintentional fatal fall rates.

BACKGROUND: Falls are the leading cause of fatal and nonfatal injuries among adults over 65 years old. The increase in fall mortality rates is likely multifactorial. With a lack of key drivers identified to explain rising rates of death from falls, accurate predictive modelling can be challenging, hindering evidence-based health resource and policy efforts. The objective of this work is to examine the predictive power of geographic utilization and longitudinal trends in mortality from unintentional falls amongst different demographic and geographic strata. METHODS: This is a nationwide, retrospective cohort study using the United States Centers for Disease Control (CDC) Web-based Injury Statistics Query and Reporting System (WISQARS) database. The exposure was death from an unintentional fall as determined by the CDC. Outcomes included aggregate and trend crude and age-adjusted death rates. Health care utilization, reimbursement, and cost metrics were also compared. RESULTS: Over 2001 to 2018, 465,486 total deaths due to unintentional falls were recorded with crude and age-adjusted rates of 8.42 and 7.76 per 100,000 population respectively. Comparing age-adjusted rates, males had a significantly higher age-adjusted death rate (9.89 vs. 6.17; p <  0.00001), but both male and female annual age-adjusted mortality rates are expected to rise (Male: + 0.25 rate/year, R2= 0.98; Female: + 0.22 rate/year, R2= 0.99). There were significant increases in death rates commensurate with increasing age, with the adults aged 85 years or older having the highest aggregate (201.1 per 100,000) and trending death rates (+ 8.75 deaths per 100,000/year, R2= 0.99). Machine learning algorithms using health care utilization data were accurate in predicting geographic age-adjusted death rates. CONCLUSIONS: Machine learning models have high accuracy in predicting geographic age-adjusted mortality rates from health care utilization data. In the United States from 2001 through 2018, adults aged 85+ years carried the highest death rate from unintentional falls and this rate is forecasted to accelerate.

Bayesian optimal adaptation explains age-related human sensorimotor changes.

The brain uses information from different sensory systems to guide motor behavior, and aging is associated with simultaneous decline in the quality of sensory information provided to the brain and deterioration in motor control. Correlations between age-dependent decline in sensory anatomical structures and behavior have been demonstrated in many sensorimotor systems, and it has recently been suggested that a Bayesian framework could explain these relationships. Here we show that age-dependent changes in a human sensorimotor reflex, the vestibuloocular reflex, are explained by a Bayesian optimal adaptation in the brain occurring in response to death of motion-sensing hair cells. Specifically, we found that the temporal dynamics of the reflex as a function of age emerge from ( r = 0.93, P < 0.001) a Kalman filter model that determines the optimal behavioral output when the sensory signal-to-noise characteristics are degraded by death of the transducers. These findings demonstrate that the aging brain is capable of generating the ideal and statistically optimal behavioral response when provided with deteriorating sensory information. While the Bayesian framework has been shown to be a general neural principle for multimodal sensory integration and dynamic sensory estimation, these findings provide evidence of longitudinal Bayesian processing over the human life span. These results illuminate how the aging brain strives to optimize motor behavior when faced with deterioration in the peripheral and central nervous systems and have implications in the field of vestibular and balance disorders, as they will likely provide guidance for physical therapy and for prosthetic aids that aim to reduce falls in the elderly. NEW & NOTEWORTHY We showed that age-dependent changes in the vestibuloocular reflex are explained by a Bayesian optimal adaptation in the brain that occurs in response to age-dependent sensory anatomical changes. This demonstrates that the brain can longitudinally respond to age-related sensory loss in an ideal and statistically optimal way. This has implications for understanding and treating vestibular disorders caused by aging and provides insight into the structure-function relationship during aging.

Relationship between vestibular sensitivity and multisensory temporal integration.

A single event can generate asynchronous sensory cues due to variable encoding, transmission, and processing delays. To be interpreted as being associated in time, these cues must occur within a limited time window, referred to as a "temporal binding window" (TBW). We investigated the hypothesis that vestibular deficits could disrupt temporal visual-vestibular integration by determining the relationships between vestibular threshold and TBW in participants with normal vestibular function and with vestibular hypofunction. Vestibular perceptual thresholds to yaw rotation were characterized and compared with the TBWs obtained from participants who judged whether a suprathreshold rotation occurred before or after a brief visual stimulus. Vestibular thresholds ranged from 0.7 to 16.5 deg/s and TBWs ranged from 13.8 to 395 ms. Among all participants, TBW and vestibular thresholds were well correlated ( R2 = 0.674, P < 0.001), with vestibular-deficient patients having higher thresholds and wider TBWs. Participants reported that the rotation onset needed to lead the light flash by an average of 80 ms for the visual and vestibular cues to be perceived as occurring simultaneously. The wide TBWs in vestibular-deficient participants compared with normal functioning participants indicate that peripheral sensory loss can lead to abnormal multisensory integration. A reduced ability to temporally combine sensory cues appropriately may provide a novel explanation for some symptoms reported by patients with vestibular deficits. Even among normal functioning participants, a high correlation between TBW and vestibular thresholds was observed, suggesting that these perceptual measurements are sensitive to small differences in vestibular function. NEW & NOTEWORTHY While spatial visual-vestibular integration has been well characterized, the temporal integration of these cues is not well understood. The relationship between sensitivity to whole body rotation and duration of the temporal window of visual-vestibular integration was examined using psychophysical techniques. These parameters were highly correlated for those with normal vestibular function and for patients with vestibular hypofunction. Reduced temporal integration performance in patients with vestibular hypofunction may explain some symptoms associated with vestibular loss.

An Engineering Model to Test for Sensory Reweighting: Nonhuman Primates Serve as a Model for Human Postural Control and Vestibular Dysfunction.

Quantitative animal models are critically needed to provide proof of concept for the investigation of rehabilitative balance therapies (e.g., invasive vestibular prostheses) and treatment response prior to, or in conjunction with, human clinical trials. This paper describes a novel approach to modeling the nonhuman primate postural control system. Our observation that rhesus macaques and humans have even remotely similar postural control motivates the further application of the rhesus macaque as a model for studying the effects of vestibular dysfunction, as well as vestibular prosthesis-assisted states, on human postural control. Previously, system identification methodologies and models were only used to describe human posture. However, here we utilized pseudorandom, roll-tilt balance platform stimuli to perturb the posture of a rhesus monkey in normal and mild vestibular (equilibrium) loss states. The relationship between rhesus monkey trunk sway and platform roll-tilt was determined via stimulus-response curves and transfer function results. A feedback controller model was then used to explore sensory reweighting (i.e., changes in sensory reliance), which prevented the animal from falling off of the tilting platform. Conclusions involving sensory reweighting in the nonhuman primate for a normal sensory state and a state of mild vestibular loss led to meaningful insights. This first-phase effort to model the balance control system in nonhuman primates is essential for future investigations toward the effects of invasive rehabilitative (balance) technologies on postural control in primates, and ultimately, humans.

Advances in the diagnosis and treatment of vestibular disorders: psychophysics and prosthetics.

Although vestibular disorders are common and often disabling, they remain difficult to diagnose and treat. For these reasons, considerable interest has been focused on developing new ways to identify peripheral and central vestibular abnormalities and on new therapeutic options that could benefit the numerous patients who remain symptomatic despite optimal therapy. In this review, I focus on the potential utility of psychophysical vestibular testing and vestibular prosthetics. The former offers a new diagnostic approach that may prove to be superior to the current tests in some circumstances; the latter may be a way to provide the brain with information about head motion that restores some elements of the information normally provided by the vestibular labyrinth.

Vestibular implants studied in animal models: clinical and scientific implications.

Damage to the peripheral vestibular system can result in debilitating postural, perceptual, and visual symptoms. A potential new treatment for this clinical problem is to replace some aspects of peripheral vestibular function with an implant that senses head motion and provides this information to the brain by stimulating branches of the vestibular nerve. In this review I consider animal studies performed at our institution over the past 15 years, which have helped elucidate how the brain processes information provided by a vestibular (semicircular canal) implant and how this information could be used to improve the problems experienced by patients with peripheral vestibular damage.

Contribution of intravestibular sensory conflict to motion sickness and dizziness in migraine disorders.

Migraine is associated with enhanced motion sickness susceptibility and can cause episodic vertigo [vestibular migraine (VM)], but the mechanisms relating migraine to these vestibular symptoms remain uncertain. We tested the hypothesis that the central integration of rotational cues (from the semicircular canals) and gravitational cues (from the otolith organs) is abnormal in migraine patients. A postrotational tilt paradigm generated a conflict between canal cues (which indicate the head is rotating) and otolith cues (which indicate the head is tilted and stationary), and eye movements were measured to quantify two behaviors that are thought to minimize this conflict: suppression and reorientation of the central angular velocity signal, evidenced by attenuation ("dumping") of the vestibuloocular reflex and shifting of the rotational axis of the vestibuloocular reflex toward the earth vertical. We found that normal and migraine subjects, but not VM patients, displayed an inverse correlation between the extent of dumping and the size of the axis shift such that the net "conflict resolution" mediated through these two mechanisms approached an optimal value and that the residual sensory conflict in VM patients (but not migraine or normal subjects) correlated with motion sickness susceptibility. Our findings suggest that the brain normally controls the dynamic and spatial characteristics of central vestibular signals to minimize intravestibular sensory conflict and that this process is disrupted in VM, which may be responsible for the enhance motion intolerance and episodic vertigo that characterize this disorder.

Cerebellar contributions to self-motion perception: evidence from patients with congenital cerebellar agenesis.

The cerebellum was historically considered a brain region dedicated to motor control, but it has become clear that it also contributes to sensory processing, particularly when sensory discrimination is required. Prior work, for example, has demonstrated a cerebellar contribution to sensory discrimination in the visual and auditory systems. The cerebellum also receives extensive inputs from the motion and gravity sensors in the vestibular labyrinth, but its role in the perception of head motion and orientation has received little attention. Drawing on the lesion-deficit approach to understanding brain function, we evaluated the contributions of the cerebellum to head motion perception by measuring perceptual thresholds in two subjects with congenital agenesis of the cerebellum. We used a set of passive motion paradigms that activated the semicircular canals or otolith organs in isolation or combination, and compared results of the agenesis patients with healthy control subjects. Perceptual thresholds for head motion were elevated in the agenesis subjects for all motion protocols, most prominently for paradigms that only activated otolith inputs. These results demonstrate that the cerebellum increases the sensitivity of the brain to the motion and orientation signals provided by the labyrinth during passive head movements.

Vestibular ablation and a semicircular canal prosthesis affect postural stability during head turns.

In our study, we examined postural stability during head turns for two rhesus monkeys: one animal study contrasted normal and mild bilateral vestibular ablation and a second animal study contrasted severe bilateral vestibular ablation with and without prosthetic stimulation. The monkeys freely stood, unrestrained on a balance platform and made voluntary head turns between visual targets. To quantify each animals' posture, motions of the head and trunk, as well as torque about the body's center of mass, were measured. In the mildly ablated animal, we observed less foretrunk sway in comparison with the normal state. When the canal prosthesis provided electric stimulation to the severely ablated animal, it showed a decrease in trunk sway during head turns. Because the rhesus monkey with severe bilateral vestibular loss exhibited a decrease in trunk sway when receiving vestibular prosthetic stimulation, we propose that the prosthetic electrical stimulation partially restored head velocity information. Our results provide an indication that a semicircular canal prosthesis may be an effective way to improve postural stability in patients with severe peripheral vestibular dysfunction.

Vestibular Prostheses Investigated in Animal Models.

Loss of peripheral vestibular function results in debilitating postural, perceptual, and visual symptoms. A new approach to treating this clinical problem is to replace some aspects of peripheral vestibular function with a prosthesis that senses head motion and provides this information to the brain by stimulating the vestibular nerve. In this paper, I review studies done in animals over the past 15 years which lay the groundwork for transferring this approach to human patients with severe peripheral vestibular damage. The animal studies demonstrate that the visual and perceptual defects associated with peripheral vestibular damage can be improved with a vestibular implant, but the data on postural control remain less conclusive at this point in time.

Electrical stimulation of semicircular canal afferents affects the perception of head orientation.

Patients with vestibular dysfunction have visual, perceptual, and postural deficits. While there is considerable evidence that a semicircular canal prosthesis that senses angular head velocity and stimulates canal ampullary nerves can improve vision by augmenting the vestibulo-ocular reflex, no information is available regarding the potential utility of a canal prosthesis to improve perceptual deficits. In this study, we investigated the possibility that electrical stimulation of canal afferents could be used to modify percepts of head orientation. Two rhesus monkeys were trained to align a light bar parallel to gravity, and were tested in the presence and absence of electrical stimulation provided by an electrode implanted in the right posterior canal. While the monkeys aligned the light bar close to the true earth-vertical without stimulation, when the right posterior canal was stimulated their responses deviated toward their left ear, consistent with a misperception of head tilt toward the right. The deviation of the light bar from the earth-vertical exceeded the torsional deviation of the eyes, indicating that the perceptual changes were not simply visual in origin. Eye movements recorded during electrical stimulation in the dark were consistent with isolated activation of right posterior canal afferents, with no evidence of otolith stimulation. These results demonstrate that electrical stimulation of canal afferents affects the perception of head orientation, and therefore suggest that motion-modulated stimulation of canal afferents by a vestibular prosthesis could potentially improve vestibular percepts in patients lacking normal vestibular function.

Comparative utility of vestibular function tests in patients with peripheral and central vestibular dysfunction.

Background: Bithermal caloric irrigation, video head impulse test (vHIT), and rotational testing are commonly used to assess peripheral vestibular function, but the relative clinical utility of each test in differentiating patients with peripheral vestibulopathy is debated. Objectives: To determine whether (1) the combination of two or more vestibular tests enhances diagnostic utility over a single test; (2) abnormal test results on vestibular tests correlate with one another. Methods: Retrospective analysis of data collected from multidisciplinary vestibular clinics at two academic medical centers from 2016 to 2022. Results: 150 patients (54.10 ± 15.09 years, 88 females) were included. No individual test was significantly better at predicting the presence of peripheral vestibular damage (p > 0.05). vHIT test results improved significantly when combined with either the caloric test (p = 0.007) or rotary chair test (p = 0.039). Caloric and rotational testing had high sensitivity (74.65% and 76.06%, respectively) and specificity (83.54% and 78.48%, respectively). vHIT demonstrated excellent specificity (89.87%) but poor sensitivity (47.89%). Caloric, vHIT, and rotary chair tests results did not correlate with one another (p > 0.05). Conclusions: Vestibular function tests have comparable diagnostic utility, yet each offers unique advantages. Caloric and rotational testing may be best suited for screening peripheral damage and vHIT may function ideally as a confirmatory test.

Age-Related Changes in Temporal Binding Involving Auditory and Vestibular Inputs.

Maintaining balance involves the combination of sensory signals from the visual, vestibular, proprioceptive, and auditory systems. However, physical and biological constraints ensure that these signals are perceived slightly asynchronously. The brain only recognizes them as simultaneous when they occur within a period of time called the temporal binding window (TBW). Aging can prolong the TBW, leading to temporal uncertainty during multisensory integration. This effect might contribute to imbalance in the elderly but has not been examined with respect to vestibular inputs. Here, we compared the vestibular-related TBW in 13 younger and 12 older subjects undergoing 0.5 Hz sinusoidal rotations about the earth-vertical axis. An alternating dichotic auditory stimulus was presented at the same frequency but with the phase varied to determine the temporal range over which the two stimuli were perceived as simultaneous at least 75% of the time, defined as the TBW. The mean TBW among younger subjects was 286 ms (SEM ± 56 ms) and among older subjects was 560 ms (SEM ± 52 ms). TBW was related to vestibular sensitivity among younger but not older subjects, suggesting that a prolonged TBW could be a mechanism for imbalance in the elderly person independent of changes in peripheral vestibular function.

Vestibular labyrinth contributions to human whole-body motion discrimination.

To assess the contributions of the vestibular system to whole-body motion discrimination in the dark, we measured direction recognition thresholds as a function of frequency for yaw rotation, superior-inferior translation ("z-translation"), interaural translation ("y-translation"), and roll tilt for 14 normal subjects and for 3 patients following total bilateral vestibular ablation. The patients had significantly higher average threshold measurements than normal (p < 0.01) for yaw rotation (depending upon frequency, 5.4× to 15.7× greater), z-translation (8.3× to 56.8× greater), y-translation (1.7× to 4.5× greater), and roll tilt (1.3× to 3.0× greater)--establishing the predominant contributions of the vestibular system for these motions in the dark.

The Next Challenges of Vestibular Implantation in Humans.

Patients with bilateral vestibulopathy suffer from a variety of complaints, leading to a high individual and social burden. Available treatments aim to alleviate the impact of this loss and improve compensatory strategies. Early experiments with electrical stimulation of the vestibular nerve in combination with knowledge gained by cochlear implant research, have inspired the development of a vestibular neuroprosthesis that can provide the missing vestibular input. The feasibility of this concept was first demonstrated in animals and later in humans. Currently, several research groups around the world are investigating prototype vestibular implants, in the form of vestibular implants as well as combined cochlear and vestibular implants. The aim of this review is to convey the presentations and discussions from the identically named symposium that was held during the 2021 MidWinter Meeting of the Association for Research in Otolaryngology, with researchers involved in the development of vestibular implants targeting the ampullary nerves. Substantial advancements in the development have been made. Yet, research and development processes face several challenges to improve this neuroprosthesis. These include, but are not limited to, optimization of the electrical stimulation profile, refining the surgical implantation procedure, preserving residual labyrinthine functions including hearing, as well as gaining regulatory approval and establishing a clinical care infrastructure similar to what exists for cochlear implants. It is believed by the authors that overcoming these challenges will accelerate the development and increase the impact of a clinically applicable vestibular implant.

Vestibular dysfunction in neurofibromatosis type 2-related schwannomatosis.

Neurofibromatosis type 2-related schwannomatosis is a genetic disorder characterized by neurologic tumours, most typically vestibular schwannomas that originate on the vestibulo-cochlear nerve(s). Although vestibular symptoms can be disabling, vestibular function has never been carefully analysed in neurofibromatosis type 2-related schwannomatosis. Furthermore, chemotherapy (e.g. bevacizumab) can reduce tumour volume and improve hearing in neurofibromatosis type 2-related schwannomatosis, but nothing is known about its vestibular effects. In this report, we studied the three primary vestibular-mediated behaviours (eye movements, motion perception and balance), clinical vestibular disability (dizziness and ataxia), and imaging and hearing in eight untreated patients with neurofibromatosis type 2-related schwannomatosis and compared their results with normal subjects and patients with sporadic, unilateral vestibular schwannoma tumours. We also examined how bevacizumab affected two patients with neurofibromatosis type 2-related schwannomatosis. Vestibular schwannomas in neurofibromatosis type 2-related schwannomatosis degraded vestibular precision (inverse of variability, reflecting a reduced central signal-to-noise ratio) but not vestibular accuracy (amplitude relative to ideal amplitude, reflecting the central signal magnitude) and caused clinical disability. Bevacizumab improved vestibular precision and clinical disability in both patients with neurofibromatosis type 2-related schwannomatosis but did not affect vestibular accuracy. These results demonstrate that vestibular schwannoma tumours in our neurofibromatosis type 2-related schwannomatosis population degrade the central vestibular signal-to-noise ratio, while bevacizumab improves the signal-to-noise ratio, changes that can be explained mechanistically by the addition (schwannoma) and suppression (bevacizumab) of afferent neural noise.

Vestibular damage affects the precision and accuracy of navigation in a virtual visual environment.

Vestibular information is available to the brain during navigation, as are the other self-generated (idiothetic) and external (allothetic) sensorimotor cues that contribute to central estimates of position and motion. Rodent studies provide strong evidence that vestibular information contributes to navigation but human studies have been less conclusive. Furthermore, sex-based differences have been described in human navigation studies performed with the head stationary, a situation where dynamic vestibular (and other idiothetic) information is absent, but sex differences in the utilization of vestibular information have not been described. Here, we studied men and women with severe bilateral vestibular damage as they navigated through a visually barren virtual reality environment and compared their performance to normal men and women. Two navigation protocols were employed, which either activated dynamic idiothetic cues ('dynamic task', navigate by turning, walking in place) or eliminated them ('static task', navigate with key presses, head stationary). For both protocols, we employed a standard 'triangle completion task' in which subjects moved to two visual targets in series and then were required to return to their perceived starting position without localizing visual information. The angular and linear 'accuracy' (derived from response error) and 'precision' (derived from response variability) were calculated. Comparing performance 'within tasks', navigation on the dynamic paradigm was worse in male vestibular-deficient patients than in normal men but vestibular-deficient and normal women were equivalent; on the static paradigm, vestibular-deficient men (but not women) performed better than normal subjects. Comparing performance 'between tasks', normal men performed better on the dynamic than the static paradigm while vestibular-deficient men and both normal and vestibular-deficient women were equivalent on both tasks. Statistical analysis demonstrated that for the angular precision metric, sex had a significant effect on the interaction between vestibular status and the test paradigm. These results provide evidence that humans use vestibular information when they navigate in a virtual visual environment and that men and women may utilize vestibular (and visual) information differently. On our navigation paradigm, men used vestibular information to improve navigation performance, and in the presence of severe vestibular damage, they utilized visual information more effectively. In contrast, we did not find evidence that women used vestibular information while navigating on our virtual task, nor did we find evidence that they improved their utilization of visual information in the presence of severe vestibular damage.

Frequency dependence of vestibuloocular reflex thresholds.

How the brain processes signals in the presence of noise impacts much of behavioral neuroscience. Thresholds provide one way to assay noise. While perceptual thresholds have been widely investigated, vestibuloocular reflex (VOR) thresholds have seldom been studied and VOR threshold dynamics have never, to our knowledge, been reported. Therefore, we assessed VOR thresholds as a function of frequency. Specifically, we measured horizontal VOR thresholds evoked by yaw rotation in rhesus monkeys, using standard signal detection approaches like those used in earlier human vestibular perceptual threshold studies. We measured VOR thresholds ranging between 0.21 and 0.76°/s; the VOR thresholds increased slightly with frequency across the measured frequency range (0.2-3 Hz). These results do not mimic the frequency response of human perceptual thresholds that have been shown to increase substantially as frequency decreases below 0.5 Hz. These reported VOR threshold findings could indicate a qualitative difference between vestibular responses of humans and nonhuman primates, but a more likely explanation is an additional dynamic neural mechanism that does not influence the VOR but, rather, influences perceptual thresholds via a decision-making process included in direction recognition tasks.