Coding of Velocity Storage in the Vestibular Nuclei

Evaluation of the Effects of Optokinetic Stimuli and Dual-Task Performance on Vestibulo-Ocular Reflex Function in Children With Attention Deficit and Hyperactivity Disorder

OBJECTIVES

This study investigated the effects of optokinetic stimuli and dual-task performance on vestibulo-ocular reflex (VOR) function. The study primarily focused on understanding the effects of attention deficit and hyperactivity disorder (ADHD) and its subtypes.

STUDY DESIGN

A case-control study.

SETTING

Tertiary medical centre.

METHODS

Thirty-eight children diagnosed with ADHD and 40 typically developing (TD) children aged 8-18 were included in the study. According to their diagnostic reports, children diagnosed with ADHD were also examined in three subtypes: predominantly inattentive (ADHD-PI), predominantly hyperactive-impulsive (ADHD-HI) and the combined type. Functional head impulse test (fHIT) was applied to all participants in three conditions-with no additional stimulus, optokinetic stimulation and dual-task. Correct responses (CR) were determined for each group, and the conditions were compared. For the dual-task test performance, children were given a counting task.

RESULTS

The findings of the study are significant. The CR values obtained from the fHIT tests applied under three different conditions were lower in the ADHD group compared to the control group. CR values for all fHIT conditions and all semicircular canals were lower in the ADHD-PI subtype and higher in the ADHD-HI subtype compared to other subgroups.

CONCLUSION

This study has significant practical implications underscoring its relevance. fHIT, when applied with different protocols, can provide valuable information about the vestibular and cognitive states of children with ADHD. These results are particularly significant as the diagnosis of ADHD often relies on subjective interpretations, and fHIT offers a more objective and reliable method of evaluation.

A system of feed-forward cerebellar circuits that extend and diversify sensory signaling

Sensory signals are processed by the cerebellum to coordinate movements. Numerous cerebellar functions are thought to require the maintenance of a sensory representation that extends beyond the input signal. Granule cells receive sensory input, but they do not prolong the signal and are thus unlikely to maintain a sensory representation for much longer than the inputs themselves. Unipolar brush cells (UBCs) are excitatory interneurons that project to granule cells and transform sensory input into prolonged increases or decreases in firing, depending on their ON or OFF UBC subtype. Further extension and diversification of the input signal could be produced by UBCs that project to one another, but whether this circuitry exists is unclear. Here we test whether UBCs innervate one another and explore how these small networks of UBCs could transform spiking patterns. We characterized two transgenic mouse lines electrophysiologically and immunohistochemically to confirm that they label ON and OFF UBC subtypes and crossed them together, revealing that ON and OFF UBCs innervate one another. A Brainbow reporter was used to label UBCs of the same ON or OFF subtype with different fluorescent proteins, which showed that UBCs innervate their own subtypes as well. Computational models predict that these feed-forward networks of UBCs extend the length of bursts or pauses and introduce delays-transformations that may be necessary for cerebellar functions from modulation of eye movements to adaptive learning across time scales.

Cognition in vestibular disorders: state of the field, challenges, and priorities for the future

Vestibular disorders are prevalent and debilitating conditions of the inner ear and brain which affect balance, coordination, and the integration of multisensory inputs. A growing body of research has linked vestibular disorders to cognitive problems, most notably attention, visuospatial perception, spatial memory, and executive function. However, the mechanistic bases of these cognitive sequelae remain poorly defined, and there is a gap between our theoretical understanding of vestibular cognitive dysfunction, and how best to identify and manage this within clinical practice. This article takes stock of these shortcomings and provides recommendations and priorities for healthcare professionals who assess and treat vestibular disorders, and for researchers developing cognitive models and rehabilitation interventions. We highlight the importance of multidisciplinary collaboration for developing and evaluating clinically relevant theoretical models of vestibular cognition, to advance research and treatment.

A system of feed-forward cerebellar circuits that extend and diversify sensory signaling

Sensory signals are processed by the cerebellum to coordinate movements. Numerous cerebellar functions are thought to require the maintenance of a sensory representation that extends beyond the input signal. Granule cells receive sensory input, but they do not prolong the signal and are thus unlikely to maintain a sensory representation for much longer than the inputs themselves. Unipolar brush cells (UBCs) are excitatory interneurons that project to granule cells and transform sensory input into prolonged increases or decreases in firing, depending on their ON or OFF UBC subtype. Further extension and diversification of the input signal could be produced by UBCs that project to one another, but whether this circuitry exists is unclear. Here we test whether UBCs innervate one another and explore how these small networks of UBCs could transform spiking patterns. We characterized two transgenic mouse lines electrophysiologically and immunohistochemically to confirm that they label ON and OFF UBC subtypes and crossed them together, revealing that ON and OFF UBCs innervate one another. A Brainbow reporter was used to label UBCs of the same ON or OFF subtype with different fluorescent proteins, which showed that UBCs innervate their own subtypes as well. Computational models predict that these feed-forward networks of UBCs extend the length of bursts or pauses and introduce delays-transformations that may be necessary for cerebellar functions from modulation of eye movements to adaptive learning across time scales.

The Vestibular Time Constant and Clinical Response to Antimotion Sickness Medication

OBJECTIVE

The therapeutic effects of antimotion sickness medications involve suppression of several components along the vestibular system. Scopolamine-based medications have proved to be the most effective anti-seasickness agents. However, there is high variability in individual responses. The vestibular nuclei, in which the vestibular time constant is modulated, contain acetylcholine receptors which are affected by scopolamine. The hypothesis of the study was that successful seasickness prevention by scopolamine requires vestibular suppression to be reflected by the shortening of the vestibular time constant.

DESIGN

Subjects were 30 naval crew members suffering from severe seasickness and were treated with oral scopolamine. The study participants were defined as responsive or non-responsive to the anti-seasickness medication according to the clinical outcome: successful response to scopolamine was defined as a reduction of seasickness severity from the highest score of 7 according to the Wiker scale to 4 or less. Scopolamine and placebo were assigned to each subject in a crossover, double-blind design. The horizontal semicircular canal time constant was evaluated by a computerized rotatory chair before, 1 and 2 hours after drug or placebo administration.

RESULTS

The vestibular time constant was significantly shortened from 16.01 ± 3.43 seconds to 12.55 ± 2.40 seconds ( p < 0.001) in the scopolamine-responsive group but not in the nonresponsive group. In contrast, vestibular time constant values were 13.73 ± 4.08 and 12.89 ± 4.48 for baseline and 2 hours measurements, respectively. This change was not statistically significant.

CONCLUSIONS

Reduction in the vestibular time constant after scopolamine administration can be used to predict whether motion sickness alleviation will occur. This will enable the administration of appropriate pharmaceutical treatment without the need for prior exposure to sea conditions.

In Vivo Localization of the Human Velocity Storage Mechanism and Its Core Cerebellar Networks by Means of Galvanic-Vestibular Afternystagmus and fMRI

Humans are able to estimate head movements accurately despite the short half-life of information coming from our inner ear motion sensors. The observation that the central angular velocity estimate outlives the decaying signal of the semicircular canal afferents led to the concept of a velocity storage mechanism (VSM). The VSM can be activated via visual and vestibular modalities and becomes manifest in ocular motor responses after sustained stimulation like whole-body rotations, optokinetic or galvanic vestibular stimulation (GVS). The VSM has been the focus of many computational modelling approaches; little attention though has been paid to discover its actual structural correlates. Animal studies localized the VSM in the medial and superior vestibular nuclei. A significant modulation by cerebellar circuitries including the uvula and nodulus has been proposed. Nevertheless, the corresponding neuroanatomical structures in humans have not been identified so far. The aim of the present study was to delineate the neural substrates of the VSM using high-resolution infratentorial fMRI with a fast T2* sequence optimized for infratentorial neuroimaging and via video-oculography (VOG). The neuroimaging experiment (n=20) gave first in vivo evidence for an involvement of the vestibular nuclei in the VSM and substantiate a crucial role for cerebellar circuitries. Our results emphasize the importance of cerebellar feedback loops in VSM most likely represented by signal increases in vestibulo-cerebellar hubs like the uvula and nodulus and lobule VIIIA. The delineated activation maps give new insights regarding the function and embedment of Crus I, Crus II, and lobule VII and VIII in the human vestibular system.

Vestibulo-spatial navigation: pathways and sense of direction

Aims of the present article are: 1) assessing vestibular contribution to spatial navigation, 2) exploring how age, global positioning systems (GPS) use, and vestibular navigation contribute to subjective sense of direction (SOD), 3) evaluating vestibular navigation in patients with lesions of the vestibular-cerebellum (patients with downbeat nystagmus, DBN) that could inform on the signals carried by vestibulo-cerebellar-cortical pathways. We applied two navigation tasks on a rotating chair in the dark: return-to-start (RTS), where subjects drive the chair back to the origin after discrete angular displacement stimuli (path reversal), and complete-the-circle (CTC) where subjects drive the chair on, all the way round to origin (path completion). We examined 24 normal controls (20-83 yr), five patients with DBN (62-77 yr) and, as proof of principle, two patients with early dementia (84 and 76 yr). We found a relationship between SOD, assessed by Santa Barbara Sense of Direction Scale, and subject's age (positive), GPS use (negative), and CTC-vestibular-navigation-task (positive). Age-related decline in vestibular navigation was observed with the RTS task but not with the complex CTC task. Vestibular navigation was normal in patients with vestibulo-cerebellar dysfunction but abnormal, particularly CTC, in the demented patients. We conclude that vestibular navigation skills contribute to the build-up of our SOD. Unexpectedly, perceived SOD in the elderly is not inferior, possibly explained by increased GPS use by the young. Preserved vestibular navigation in cerebellar patients suggests that ascending vestibular-cerebellar projections carry velocity (not position) signals. The abnormalities in the cognitively impaired patients suggest that their vestibulo-spatial navigation is disrupted.NEW & NOTEWORTHY Our subjective sense-of-direction is influenced by how good we are at spatial navigation using vestibular cues. Global positioning systems (GPS) may inhibit sense of direction. Increased use of GPS by the young may explain why the elderly's sense of direction is not worse than the young's. Patients with vestibulo-cerebellar dysfunction (downbeat nystagmus syndrome) display normal vestibular navigation, suggesting that ascending vestibulo-cerebellar-cortical pathways carry velocity rather than position signals. Pilot data indicate that dementia disrupts vestibular navigation.

Emerging order of anomalous eye movements with progressive drowsiness

It has been widely recognized that human alertness is reflected in the eyes (e.g., when drowsiness, miosis, slow saccades, divergence, less compensatory vestibulo-ocular reflex, and less-accurate optokinetic response and smooth pursuit emerge). Previous studies that discovered these pupil/oculomotor anomalous behaviors along with lowering alertness evaluated only one or a few of them simultaneously, thus their emergence order is yet unknown. Presently, we focused on the following five pupil/oculomotor behaviors that can be evaluated under a natural stationary environment without giving external sensory stimulations: saccades, slow-phase eye movements, vergence, pupil diameter, and blinks. We demonstrate that their anomalous behaviors emerge in the following order: first: frequent saccades; second: slow saccades; third: divergence & miosis, then slow eye movement, while elongated eyelid closure duration emerges randomly in this sequence. These results provide a basis for the oculo-pupillometry-enabling objective monitoring of progressive drowsiness.

The Neural Correlates of Spatial Disorientation in Head Direction Cells

While the brain has evolved robust mechanisms to counter spatial disorientation, their neural underpinnings remain unknown. To explore these underpinnings, we monitored the activity of anterodorsal thalamic head direction (HD) cells in rats while they underwent unidirectional or bidirectional rotation at different speeds and under different conditions (light vs dark, freely-moving vs head-fixed). Under conditions that promoted disorientation, HD cells did not become quiescent but continued to fire, although their firing was no longer direction specific. Peak firing rates, burst frequency, and directionality all decreased linearly with rotation speed, consistent with previous experiments where rats were inverted or climbed walls/ceilings in zero gravity. However, access to visual landmarks spared the stability of preferred firing directions (PFDs), indicating that visual landmarks provide a stabilizing signal to the HD system while vestibular input likely maintains direction-specific firing. In addition, we found evidence that the HD system underestimated angular velocity at the beginning of head-fixed rotations, consistent with the finding that humans often underestimate rotations. When head-fixed rotations in the dark were terminated HD cells fired in bursts that matched the frequency of rotation. This postrotational bursting shared several striking similarities with postrotational "nystagmus" in the vestibulo-ocular system, consistent with the interpretation that the HD system receives input from a vestibular velocity storage mechanism that works to reduce spatial disorientation following rotation. Thus, the brain overcomes spatial disorientation through multisensory integration of different motor-sensory inputs.

The otolith vermis: A systems neuroscience theory of the Nodulus and Uvula

The Nodulus and Uvula (NU) (lobules X and IX of the cerebellar vermis) form a prominent center of vestibular information processing. Over decades, fundamental and clinical research on the NU has uncovered many aspects of its function. Those include the resolution of a sensory ambiguity inherent to inertial sensors in the inner ear, the otolith organs; the use of gravity signals to sense head rotations; and the differential processing of self-generated and externally imposed head motion. Here, I review these works in the context of a theoretical framework of information processing called the internal model hypothesis. I propose that the NU implements a forward internal model to predict the activation of the otoliths, and outputs sensory predictions errors to correct internal estimates of self-motion or to drive learning. I show that a Kalman filter based on this framework accounts for various functions of the NU, neurophysiological findings, as well as the clinical consequences of NU lesions. This highlights the role of the NU in processing information from the otoliths and supports its denomination as the "otolith" vermis.

Treatment of Mal de Debarquement Syndrome in an Audiology-Vestibular Clinic

BACKGROUND

 Mal de Debarquement Syndrome (MdDS) has a stereotypical presentation of symptoms including continuous rocking/swaying sensations described as feeling like one is "still on the boat," following travel, especially on water vessels. MdDS is even more notorious for the duration of symptoms that can persist months or years, and historically this condition has escaped effective treatments.

PURPOSE

 This case study presents a case of classic MdDS that was effectively treated in an Audiology-Vestibular clinic. Treatment consisted of three, relatively short-lived vestibular rehabilitation sessions using the "Roll Readaptation" technique that has previously been reported in Neurology journals.

STUDY SAMPLE

 The study sample includes a 48-year-old female with a history of MdDS following two separate ocean cruises. She underwent vestibular evaluation and was treated with a treatment paradigm aimed to readapt the central vestibular system and vestibular-ocular reflex.

RESULTS

 This report focuses on a brief review of current symptomology and diagnostic criteria of MdDS, underlying pathophysiology and application of a relatively new treatment technique in an audiology clinic. This patient was shown full-field, omni-directional optokinetic (OPK) stimulus while rolling her head rhythmically for up to 4 minutes at a time. After three treatment sessions, the patient had a significant reduction in subjective symptoms and returned to full-time work. She had previously been off work for nearly 3 months.

CONCLUSION

 Individuals with MdDS suffer large daily and work life disruptions due to the persistent nature of symptoms, and their physical manifestations. In addition, they have historically had minimal treatment options. This case demonstrates that audiologists with proper equipment may have the potential to readily offer treatment for a previously "untreatable" condition.

Treatment of Gravitational Pulling Sensation in Patients With Mal de Debarquement Syndrome (MdDS): A Model-Based Approach

Perception of the spatial vertical is important for maintaining and stabilizing vertical posture during body motion. The velocity storage pathway of vestibulo-ocular reflex (VOR), which integrates vestibular, optokinetic, and proprioception in the vestibular nuclei vestibular-only (VO) neurons, has spatio-temporal properties that are defined by eigenvalues and eigenvectors of its system matrix. The yaw, pitch and roll eigenvectors are normally aligned with the spatial vertical and corresponding head axes. Misalignment of the roll eigenvector with the head axes was hypothesized to be an important contributor to the oscillating vertigo during MdDS. Based on this, a treatment protocol was developed using simultaneous horizontal opto-kinetic stimulation and head roll (OKS-VOR). This protocol was not effective in alleviating the MdDS pulling sensations. A model was developed, which shows how maladaptation of the yaw eigenvector relative to the head yaw, either forward, back, or side down, could be responsible for the pulling sensation that subjects experience. The model predicted the sometimes counter-intuitive OKS directions that would be most effective in re-adapting the yaw eigenvector to alleviate the pulling sensation in MdDS. Model predictions were consistent with the treatment of 50 patients with a gravitational pulling sensation as the dominant feature. Overall, pulling symptoms in 72% of patients were immediately alleviated after the treatment and lasted for 3 years after the treatment in 58% of patients. The treatment also alleviated the pulling sensation in patients where pulling was not the dominant feature. Thus, the OKS method has a long-lasting effect comparable to that of OKS-VOR readaptation. The study elucidates how the spatio-temporal organization of velocity storage stabilizes upright posture and how maladaptation of the yaw eigenvector generates MdDS pulling sensations. Thus, this study introduces a new way to treat gravitational pull which could be used alone or in combination with previously proposed VOR readaptation techniques.

Reduced Gain and Shortened Time Constant of Vestibular Velocity Storage as a Source of Balance and Movement Sensitivities in Gravitational Insecurity

Gravitational insecurity (GrI) involves lifetime movement and balance concerns whose pathophysiological origins are unclear. We tested whether balance symptoms in mild GrI might involve anomalies in vestibular velocity storage (VVS), a brainstem/cerebellar circuit that amplifies gain and prolongs the persistence of weak vestibular signals from small/slow head movements. A Provisional Gravitational Insecurity Index (PGrI) was developed, evaluated for psychometrics/demographics, and used to identify otherwise healthy adults with life-long balance challenges as well as sex, age, and ethnicity-matched comparison adults without such challenges. Balance confidence, sensory hypersensitivities, spatial orientation, anxiety, and hearing loss were self-reported. Standing balance under visual/proprioceptive restrictions and perrotary vestibulo-ocular nystagmus were evaluated. The PGrI showed approximated test-retest reliability and convergent and discriminant validity. When only vestibular input was available, mild GrI participants on a tilting platform used effortful hip strategies for balance significantly more than did comparison participants. Rotation testing revealed that mild GrI participants had significantly less low frequency gain and shortened VVS persistence. Combined, these two parameters correlated significantly with PGrI. The PGrI also correlated with problematic spatial orientation, but surprisingly, not to anxiety. Balance/movement issues in GrI are likely due to VVS deficiencies. Additional mechanisms may account for other GrI symptoms. Better understanding of GrI’s pathophysiological basis will be useful in informing the larger health-provider community about this condition.

A New Vestibular Stimulation Mode for Motion Sickness With Emphatic Analysis of Pica

Motion sickness (MS) was frequently introduced for rodents in research work through passive motion that disturbed vestibular signals in the presence of visual and aleatory, proprioceptive inputs. Inducement of MS in this way causes conflicting signals that activate intermixed neural circuits representing multimodal stimulation. From reductionism, a lab setup to elicit rat MS via vestibular stimulation was configured in the present study for MS study in connection with dissection of the central vestibular component causally underlying MS. The individual animal was blinded to light with a custom-made restrainer, and positioned at an inclination of 30° for otolith organs to receive unusual actions by gravitoinertial vector. Following a 2-h double-axis (earth-vertical) rotation involving angular acceleration/deceleration, a suit of behaviors characterizing the MS was observed to be significantly changed including pica (eating non-nutritive substance like kaolin), conditioned taste avoidance and locomotion (p < 0.05). Notably, for the statistical hypothesis testing, the utility of net increased amount of kaolin consumption as independent variables in data processing was expounded. In addition, Fos-immunostained neurons in vestibular nucleus complex were significantly increased in number, suggesting the rotation-induced MS was closely related to the vestibular activation. In conclusion, our work indicated that the present setup could effectively elicit the MS by disturbing vestibular signals in rat in the context of well-controlled proprioceptive inputs and lack of visual afference.

Comparing Vestibular Responses to Linear and Angular Whole-Body Accelerations in Real and Immersive Environments

The vestibular end organs differ in terms of anatomical and physiological characteristics. Sensory modalities' stimuli including visual stimuli and vestibular sensation can influence these organs differently. This paper explores differences between vestibular responses to axial tilts in physical and virtual environments. Four passive whole-body movements (linear: up-down, and angular: yaw, pitch, and roll) were applied to twenty-seven healthy participants once using a hydraulic chair (physical) and once visually using a head-mounted display (virtual). Electrovestibulography (EVestG) was used as the outcome measure to investigate the magnitude of vestibular-response-change in both ears for physical and virtual stimuli. Three features including average action potential (AP) area, AP amplitude, and mean detected firing rate change were used as indices of response. The results show that for both physical and virtual stimuli (1) generally the pitch and roll tilts produce the largest EVestG changes compared to other tilts (2) roll and pitch tilt responses are not significantly different from each other and (3) right side and left side roll tilts' responses are not significantly different. The findings indicate although visually- and physically-induced vestibular responses are different in terms of afferent activity, visual stimuli can still result in distinct responses when exposed to different axial tilts.

The acute, systemic effects of aerobic exercise in recently concussed adolescent student-athletes: preliminary findings

OBJECTIVES

Examine the acute effects (pre-, during, post-intervention) of two different intensities of aerobic exercise or rest on autonomic, oculomotor, and vestibular function and symptom burden in patients with a recent sport-related concussion (SRC) and compare their responses to sex-matched, age-stratified, non-concussed (HEALTHY) student-athletes.

METHODS

Student-athletes between the ages of 13 and 18 that presented to the sports medicine clinic within Day 3-7 post-SRC and from local schools were recruited for a randomized controlled trial (RCT). The participants were administered the Vestibular/Ocular Motor Screening (VOMS), King-Devick (K-D), and Post-Concussion Symptom Scale (PCSS) before and after the intervention. Heart rate variability (HRV) and mean arterial pressure (MAP) were collected before, during, and after the intervention. The intervention was either a single, 20-min session of treadmill walking at 40% (40HR) or 60% of age-predicted max heart rate (60HR), or seated, rest (NOEX).

RESULTS

30 participants completed the intervention with the SRC group treated 4.5 ± 1.3 days post-injury. Pre-exercise HRV and MAP were significantly different (p's < 0.001) during treatment but returned to pre-exercise values within 5 min of recovery in both the SRC and HEALTHY groups. Both the SRC and HEALTHY groups exhibited similar reductions pre- to post-intervention for symptom severity and count (p's < 0.05), three VOMS items (p's < 0.05) but not K-D time.

CONCLUSIONS

To date, this is the first adolescent RCT to report the acute, systemic effects of aerobic exercise on recently concussed adolescent athletes. The interventions appeared safe in SRC participants, were well-tolerated, and provided brief therapeutic benefit.

TRIAL REGISTRATION

Clinicaltrials.gov Identifier NCT03575455.

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.

Central Ocular Motor Disorders: Clinical and Topographic Anatomical Diagnosis, Syndromes and Underlying Diseases

The key to the diagnosis of ocular motor disorders is a systematic clinical examination of the different types of eye movements, including eye position, spontaneous nystagmus, range of eye movements, smooth pursuit, saccades, gaze-holding function, vergence, optokinetic nystagmus, as well as testing of the function of the vestibulo-ocular reflex (VOR) and visual fixation suppression of the VOR. This is like a window which allows you to look into the brain stem and cerebellum even if imaging is normal. Relevant anatomical structures are the midbrain, pons, medulla, cerebellum and rarely the cortex. There is a simple clinical rule: vertical and torsional eye movements are generated in the midbrain, horizontal eye movements in the pons. For example, isolated dysfunction of vertical eye movements is due to a midbrain lesion affecting the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF), with impaired vertical saccades only or vertical gaze-evoked nystagmus due to dysfunction of the Interstitial nucleus of Cajal (INC). Lesions of the lateral medulla oblongata (Wallenberg syndrome) lead to typical findings: ocular tilt reaction, central fixation nystagmus and dysmetric saccades. The cerebellum is relevant for almost all types of eye movements; typical pathological findings are saccadic smooth pursuit, gaze-evoked nystagmus or dysmetric saccades. The time course of the development of symptoms and signs is important for the diagnosis of underlying diseases: acute: most likely stroke; subacute: inflammatory diseases, metabolic diseases like thiamine deficiencies; chronic progressive: inherited diseases like Niemann-Pick type C with typically initially vertical and then horizontal saccade palsy or degenerative diseases like progressive supranuclear palsy. Treatment depends on the underlying disease. In this article, we deal with central ocular motor disorders. In a second article, we focus on clinically relevant types of nystagmus such as downbeat, upbeat, fixation pendular, gaze-evoked, infantile or periodic alternating nystagmus. Therefore, these types of nystagmus will not be described here in detail.

The Importance of Being in Touch

This paper describes a series of studies resulting from the finding that when free floating in weightless conditions with eyes closed, all sense of one's spatial orientation with respect to the aircraft can be lost. But, a touch of the hand to the enclosure restores the sense of spatial anchoring within the environment. This observation led to the exploration of how light touch of the hand can stabilize postural control on Earth even in individuals lacking vestibular function, and can override the effect of otherwise destabilizing tonic vibration reflexes in leg muscles. Such haptic stabilization appears to represent a long loop cortical reflex with contact cues at the hand phase leading EMG activity in leg muscles, which change the center of pressure at the feet to counteract body sway. Experiments on dynamic control of balance in a device programmed to exhibit inverted pendulum behavior about different axes and planes of rotation revealed that the direction of gravity not the direction of balance influences the perceived upright. Active control does not improve the accuracy of indicating the upright vs. passive exposure. In the absence of position dependent gravity shear forces on the otolith organs and body surface, drifting and loss of control soon result and subjects are unaware of their ongoing spatial position. There is a failure of dynamic path integration of the semicircular canal signals, such as occurs in weightless conditions.

Effect of the Stimulus Duration on the Adaptation of the Optokinetic Afternystagmus

Observing a rotating visual pattern covering a large portion of the visual field induces optokinetic nystagmus (OKN). If the lights are suddenly switched off, optokinetic afternystagmus (OKAN) occurs. OKAN is hypothesized to originate in the velocity storage mechanism (VSM), a central processing network involved in multi-sensory integration. During a sustained visual rotation, the VSM builds up a velocity signal. After the lights are turned off, the VSM discharges slowly, with OKAN as the neurophysiological correlate. It has been reported that the initial afternystagmus in the direction of the preceding stimulus (OKAN-I) can be followed by a reversed one (OKAN-II), which increases with stimulus duration up to 15 min. In 11 healthy adults, we investigated OKAN following optokinetic stimulus lasting 30 s, 3-, 5-, and 10-min. Analysis of slow-phase cumulative eye position and velocity found OKAN-II in only 5/11 participants. Those participants presented it in over 70% of their trials with longer durations, but only in 10% of their 30 s trials. While this confirms that OKAN-II manifests predominantly after sustained stimuli, it suggests that its occurrence is subject-specific. We also did not observe further increases with stimulus duration. Conversely, OKAN-II onset occurred later as stimulus duration increased (p = 0.02), while OKAN-II occurrence and peak velocity did not differ between the three longest stimuli. Previous studies on OKAN-I, used negative saturation models to account for OKAN-II. As these approaches have no foundation in the OKAN-II literature, we evaluated if a simplified version of a rigorous model of OKAN adaptation could be used in humans. Slow-phase velocity following the trials with 3-, 5-, and 10-min stimuli was fitted with a sum of two decreasing exponential functions with opposite signs (one for OKAN-I and one for OKAN-II). The model assumes separate mechanisms for OKAN-I, representing VSM discharge, and OKAN-II, described as a slower adaptation phenomenon. Although the fit was qualitatively imperfect, this is not surprising given the limited reliability of OKAN in humans. The estimated adaptation time constant seems comparable to the one describing the reversal of the vestibulo-ocular reflex during sustained rotation, suggesting a possible shared adaptive mechanism.

The Scientific Contributions of Bernard Cohen (1929-2019)

Throughout Bernard Cohen's active career at Mount Sinai that lasted over a half century, he was involved in research on vestibular control of the oculomotor, body postural, and autonomic systems in animals and humans, contributing to our understanding of such maladies as motion sickness, mal de débarquement syndrome, and orthostatic syncope. This review is an attempt to trace and connect Cohen's varied research interests and his approaches to them. His influence was vast. His scientific contributions will continue to drive research directions for many years to come.

Mal de Debarquement Syndrome: A Matter of Loops?

Introduction: Mal de Debarquement Syndrome (MdDS) is a poorly understood neurological disorder affecting mostly perimenopausal women. MdDS has been hypothesized to be a maladaptation of the vestibulo-ocular reflex, a neuroplasticity disorder, and a consequence of neurochemical imbalances and hormonal changes. Our hypothesis considers elements from these theories, but presents a novel approach based on the analysis of functional loops, according to Systems and Control Theory.

Hypothesis: MdDS is characterized by a persistent sensation of self-motion, usually occurring after sea travels. We assume the existence of a neuronal mechanism acting as an oscillator, i.e., an adaptive internal model, that may be able to cancel a sinusoidal disturbance of posture experienced aboard, due to wave motion. Thereafter, we identify this mechanism as a multi-loop neural network that spans between vestibular nuclei and the flocculonodular lobe of the cerebellum. We demonstrate that this loop system has a tendency to oscillate, which increases with increasing strength of neuronal connections. Therefore, we hypothesize that synaptic plasticity, specifically long-term potentiation, may play a role in making these oscillations poorly damped. Finally, we assume that the neuromodulator Calcitonin Gene-Related Peptide, which is modulated in perimenopausal women, exacerbates this process thus rendering the transition irreversible and consequently leading to MdDS.

Conclusion and Validation: The concept of an oscillator that becomes noxiously permanent can be used as a model for MdDS, given a high correlation between patients with MdDS and sea travels involving undulating passive motion, and an alleviation of symptoms when patients are re-exposed to similar passive motion. The mechanism could be further investigated utilizing posturography tests to evaluate if subjective perception of motion matches with objective postural instability. Neurochemical imbalances that would render individuals more susceptible to developing MdDS could be investigated through hormonal profile screening. Alterations in the connections between vestibular nuclei and cerebellum, notably GABAergic fibers, could be explored by neuroimaging techniques as well as transcranial magnetic stimulation. If our hypothesis were tested and verified, optimal targets for MdDS treatment could be found within both the neural networks and biochemical factors that are deemed to play a fundamental role in loop functioning and synaptic plasticity.

Long term visuo-vestibular mismatch in freely behaving mice differentially affects gaze stabilizing reflexes

The vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) work synergistically to stabilize gaze in response to head movements. We previously demonstrated that a 14-day visuo-vestibular mismatch (VVM) protocol applied in freely behaving mice decreased the VOR gain. Here, we show for the first time that the OKR gain is also reduced and report on the recovery dynamics of both VOR and OKR after the end of the VVM protocol. Using sinusoidally-modulated stimulations, the decreases in VOR and OKR were found to be frequency-selective with larger reductions for frequencies < 0.5 Hz. Constant-velocity OKR stimulation tests demonstrated that the persistent components of the OKR were not modified while the transient, initial responses were. To identify the signals driving VOR and OKR reductions, we compared the responses of mice exposed to a high-contrast and no-contrast VVM. Despite being more robust in the high-contrast conditions, reductions were largely comparable and recovered with a similar time course. An analysis that directly compared VOR and OKR responses revealed that, alterations in the VOR were of significantly larger amplitude with significantly slower dynamics of recovery. Our findings are evidence for a frequency-selective influence of visual signals in the tuning of gaze stabilizing reflexes in normal mice.

Readaptation Treatment of Mal de Debarquement Syndrome With a Virtual Reality App: A Pilot Study

Mal de Debarquement syndrome (MdDS) is composed of constant phantom sensations of motion, which are frequently accompanied by increased sensitivity to light, inability to walk on a patterned floor, the sensation of ear fullness, head pressure, anxiety, and depression. This disabling condition generally occurs in premenopausal women within 2 days after prolonged passive motion (e.g., travel on a cruise ship, plane, or in a car). It has been previously hypothesized that MdDS is the result of maladaptive changes in the polysynaptic vestibulo-ocular reflex (VOR) pathway called velocity storage. Past research indicates that full-field optokinetic stimulation is an optimal way to activate velocity storage. Unfortunately, such devices are typically bulky and not commonly available. We questioned whether virtual reality (VR) goggles with a restricted visual field could effectively simulate a laboratory environment for MdDS treatment. A stripes program for optokinetic stimulation was implemented using Google Daydream Viewer. Five female patients (42 ± 10 years; range 26-50), whose average MdDS symptom duration was 2 months, participated in this study. Four patients had symptoms triggered by prolonged passive motion, and in one, symptoms spontaneously occurred. Symptom severity was self-scored by patients on a scale of 0-10, where 0 is no symptoms at all and 10 is the strongest symptoms that the patient could imagine. Static posturography was obtained to determine objective changes in body motion. The treatment was considered effective if the patient's subjective score improved by at least 50%. All five patients reported immediate improvement. On 2-month follow-ups, symptoms returned only in one patient. These data provide proof of concept for the limited-visual-field goggles potentially having clinical utility as a substitute for full-field optokinetic stimulation in treating patients with MdDS in clinics or via telemedicine.

Effects of exercise on symptoms, vestibular/ocular motor screening and postural stability in a college-aged sample

Aim:

To examine the effects of maximal exercise on symptoms, vestibular/ocular motor screening (VOMS) and postural stability.

Methodology:

A total of 17 college-aged individuals completed a symptom scale, VOMS and the modified Clinical Test for Sensory Interaction and Balance (m-CTSIB), followed by a graded maximal exercise treadmill test. Assessments were repeated post exercise, 20 and 40 min post-exercise.

Results:

Significant increases in total symptoms, symptom severity scores and m-CTSIB scores from baseline to immediate post exercise were reported. Following 20-min recovery, improvements were noted on symptoms, visual motion sensitivity on VOMS and m-CTSIB.

Conclusion:

Symptoms and postural stability are influenced by exercise and following 20 min of rest, returned to baseline, indicating that a period of 20 min following a suspected concussion may be needed to negate exercise effects.

Baseline concussion testing is often completed by athletes in a rested state, prior to beginning their respective sports season. However, concussions are often sustained while the athlete has undergone physical exertion and is in a fatigued state. Therefore, it may be inaccurate to compare fatigue-state sideline assessments with resting-state baselines. The purpose of this study was to compare the effects of pre- and post-exercise performance on common concussion assessment tools. Worse performance was noted between pre- and immediately post-exercise on these tools. A duration of 20 min after exercise, performance returned to pretest levels, indicating that clinicians should wait at least 20 min before conducting a sideline examination after a sustained concussion, to cancel out the effects of exercise.

Velocity storage mechanism drives a cerebellar clock for predictive eye velocity control

Predictive motor control is ubiquitously employed in animal kingdom to achieve rapid and precise motor action. In most vertebrates large, moving visual scenes induce an optokinetic response (OKR) control of eye movements to stabilize vision. In goldfish, the OKR was found to be predictive after a prolonged exposure to temporally periodic visual motion. A recent study showed the cerebellum necessary to acquire this predictive OKR (pOKR), but it remained unclear as to whether the cerebellum alone was sufficient. Herein we examined different fish species known to share the basic architecture of cerebellar neuronal circuitry for their ability to acquire pOKR. Carps were shown to acquire pOKR like goldfish while zebrafish and medaka did not, demonstrating the cerebellum alone not to be sufficient. Interestingly, those fish that acquired pOKR were found to exhibit long-lasting optokinetic after nystagmus (OKAN) as opposed to those that didn’t. To directly manipulate OKAN vestibular-neurectomy was performed in goldfish that severely shortened OKAN, but pOKR was acquired comparable to normal animals. These results suggest that the neuronal circuitry producing OKAN, known as the velocity storage mechanism (VSM), is required to acquire pOKR irrespective of OKAN duration. Taken together, we conclude that pOKR is acquired through recurrent cerebellum-brainstem parallel loops in which the cerebellum adjusts VSM signal flow and, in turn, receives appropriately timed eye velocity information to clock visual world motion.

The Superiority of the Otolith System

BACKGROUND

The peripheral vestibular end organ is considered to consist of semi-circular canals (SCC) for detection of angular accelerations and the otoliths for detection of linear accelerations. However, otoliths being phylogenetically the oldest part of the vestibular sensory organs are involved in detection of all motions.

SUMMARY

This study elaborates on this property of the otolith organ, as this concept can be of importance for the currently designed vestibular implant devices. Key Message: The analysis of the evolution of the inner ear and examination of clinical examples shows the robustness of the otolith system and inhibition capacity of the SCC. The otolith system must be considered superior to the SCC system as illustrated by evolution, clinical evidence, and physical principles.

Velocity storage: its multiple roles

Our research described in this article was motivated by the puzzling finding of the Skylab M131 experiments: head movements made while rotating that are nauseogenic and disorienting on Earth are innocuous in a weightless, 0-g environment. We describe a series of parabolic flight experiments that directly addressed this puzzle and discovered the gravity-dependent responses to semicircular canal stimulation, consistent with the principles of velocity storage. We describe a line of research that started in a different direction, investigating dynamic balancing, but ended up pointing to the gravity dependence of angular velocity-to-position integration of semicircular canal signals. Together, these lines of research and the theoretical framework of velocity storage provide an answer to at least part of the M131 puzzle. We also describe recently discovered neural circuits by which active, dynamic vestibular, multisensory, and motor signals are interpreted as either appropriate for action and orientation or as conflicts evoking motion sickness and disorientation.

Vestibular, locomotor, and vestibulo-autonomic research: 50 years of collaboration with Bernard Cohen

My collaboration on the vestibulo-ocular reflex with Bernard Cohen began in 1972. Until 2017, this collaboration included studies of saccades, quick phases of nystagmus, the introduction of the concept of velocity storage, the relationship of velocity storage to motion sickness, primate and human locomotion, and studies of vasovagal syncope. These studies have elucidated the functioning of the vestibuloocular reflex, the locomotor system, the functioning of the vestibulo-sympathetic reflex, and how blood pressure and heart rate are controlled by the vestibular system. Although it is virtually impossible to review all the contributions in detail in a single paper, this article traces a thread of modeling that I brought to the collaboration, which, coupled with Bernie Cohen's expertise in vestibular and sensory-motor physiology and clinical insights, has broadened our understanding of the role of the vestibular system in a wide range of sensory-motor systems. Specifically, the paper traces how the concept of a relaxation oscillator was used to model the slow and rapid phases of ocular nystagmus. Velocity information that drives the slow compensatory eye movements was used to activate the saccadic system that resets the eyes, giving rise to the relaxation oscillator properties and simulated nystagmus as well as predicting the types of unit activity that generated saccades and nystagmic beats. The slow compensatory component of ocular nystagmus was studied in depth and gave rise to the idea that there was a velocity storage mechanism or integrator that not only is a focus for visual-vestibular interaction but also codes spatial orientation relative to gravity as referenced by the otoliths. Velocity storage also contributes to motion sickness when there are visual-vestibular as well as orientation mismatches in velocity storage. The relaxation oscillator concept was subsequently used to model the stance and swing phases of locomotion, how this impacted head and eye movements to maintain gaze in the direction of body motion, and how these were affected by Parkinson's disease. Finally, the relaxation oscillator was used to elucidate the functional form of the systolic and diastolic beats during blood pressure and how vasovagal syncope might be initiated by cerebellar-vestibular malfunction.

Posterior semicircular canal paroxysmal positional vertigo triggers a new type of windmill nystagmus

Background

Periodic alternating nystagmus is a rare condition characterised by spontaneous horizontal nystagmus that periodically reverses direction, indicating an alteration of the velocity storage mechanism. Windmill nystagmus is a peculiar and rare variant of periodic alternating horizontal nystagmus with a superimposed periodic alternating vertical nystagmus. It is generally observed in blind patients.

Case report

This paper presents the unique case of a normally sighted patient with a windmill nystagmus triggered by an episode of benign paroxysmal positional vertigo due to bilateral posterior canalolithiasis. Videonystagmography revealed an anticlockwise up-beating nystagmus followed by a clockwise down-beating nystagmus with a cycle lasting 2 minutes, followed by a brief burst of horizontal left-beating nystagmus.

Conclusion

This case report represents the first observation of a new type of windmill nystagmus, probably provoked by a malfunction of the velocity storage mechanism, gaze-stabilisation and short-adaptation networks, with a loss of cerebellar inhibition.

Development of a conversion model between mechanical and electrical vestibular stimuli

The vestibular end-organs encode for linear and angular head accelerations in space contributing to our internal representation of self-motion. Activation of the vestibular system with transmastoid electrical current has recently grown in popularity; however, a direct relationship between electrically evoked and mechanically evoked vestibular responses remains elusive in humans. We have developed and tested a mechanical-to-electrical vestibular stimulus conversion model incorporating physiological activation of primary vestibular afferents identified in nonhuman primates. We compared ocular torsional responses between mechanical (chair rotation) and model-derived electrical (binaural-bipolar) stimuli in separate experiments for an angular velocity step change (±10 deg/s over 1 s, ±4-mA peak amplitude; n = 10) and multisine angular velocities (±10 deg/s, 9.7 mA peak to peak, 0.05-1 Hz; n = 5), respectively. Perception of whole body rotation (n = 18) to our step-change stimuli was also evaluated. Ocular torsional slow-phase velocity responses between stimulation types were similar (paired two one-sided tests of equivalence: multiple P < 0.002; one-sample t test: P = 0.178) and correlated (Pearson's coefficient: multiple P < 0.001). Bootstrap analysis of perceived angular velocity likewise showed similarity in perceptual decay dynamics. These data suggest that central processing between stimuli was similar, and our vestibular stimulus conversion model with a conversion factor of ∼0.4 mA per deg/s for an angular velocity step change can generate electrical stimuli that replicates dynamic vestibular activation elicited by mechanical whole body rotations. This proposed vestibular conversion model represents an initial framework for using electrical stimuli to generate mechanically equivalent activation of primary vestibular afferents for use in biomedical applications and immersive reality technologies.NEW & NOTEWORTHY With the growing popularity of electrical vestibular stimulation in biomedical and immersive reality applications, a direct conversion model between electrical and mechanical vestibular stimuli is needed. We developed a model to generate electrical stimuli mimicking the physiological activation of vestibular afferents evoked by mechanical rotations. Ocular and perceptual responses evoked by mechanical and model-derived electrical stimuli were similar, thus providing a critical first step toward generation of electrically induced vestibular responses that have a realistic mechanical equivalent.

A review on screening tests for vestibular disorders

Although many studies have reported on tests of the vestibular system a valid and reliable, evidence-based screening battery for easy clinical use remains elusive. Many screening tests attempt to assess the vestibulo-ocular reflex. Therefore, head shaking, the Dix-Hallpike maneuver, the supine roll test, and head impulse tests are discussed. Other tests address the spatial orientation functions of the vestibular system, such as the Bucket Test and the Fukuda Stepping test. Still, other tests are based on the known correlates with balance skills, both static and dynamic, such as tandem walking and the modern variation of the Romberg test, the modified Clinical Test of Sensory Interaction and Balance. This review provides a critical overview of the literature on some of these tests and their value for clinical use and in epidemiological studies.

Galvanic vestibular stimulation: from basic concepts to clinical applications

Galvanic vestibular stimulation (GVS) plays an important role in the quest to understand sensory signal processing in the vestibular system under normal and pathological conditions. It has become a highly relevant tool to probe neuronal computations and to assist in the differentiation and treatment of vestibular syndromes. Following its accidental discovery, GVS became a diagnostic tool that generates eye movements in the absence of head/body motion. With the possibility to record extracellular and intracellular spikes, GVS became an indispensable method to activate or block the discharge in vestibular nerve fibers by cathodal and anodal currents, respectively. Bernie Cohen, in his attempt to decipher vestibular signal processing, has used this method in a number of hallmark studies that have added to our present knowledge, such as the link between selective electrical stimulation of semicircular canal nerves and the generation of directionally corresponding eye movements. His achievements paved the way for other major milestones including the differential recruitment order of vestibular fibers for cathodal and anodal currents, pronounced discharge adaptation of irregularly firing afferents, potential activation of hair cells, and fiber type-specific activation of central circuits. Previous disputes about the structural substrate for GVS are resolved by integrating knowledge of ion channel-related response dynamics of afferents, fiber type-specific innervation patterns, and central convergence and integration of semicircular canal and otolith signals. On the basis of solid knowledge of the methodology, specific waveforms of GVS are currently used in clinical diagnosis and patient treatment, such as vestibular implants and noisy galvanic stimulation.

Discharge properties of morphologically identified vestibular neurons recorded during horizontal eye movements in the goldfish

Computational capability and connectivity are key elements for understanding how central vestibular neurons contribute to gaze-stabilizing eye movements during self-motion. In the well-characterized and segmentally distributed hindbrain oculomotor network of goldfish, we determined afferent and efferent connections along with discharge patterns of descending octaval nucleus (DO) neurons during different eye motions. Based on activity correlated with horizontal eye and head movements, DO neurons were categorized into two complementary groups that either increased discharge during both contraversive (type II) eye (e) and ipsiversive (type I) head (h) movements (eIIhI) or vice versa (eIhII). Matching time courses of slow-phase eye velocity and corresponding firing rates during prolonged visual and head rotation suggested direct causality in generating extraocular motor commands. The axons of the dominant eIIhI subgroup projected either ipsi- or contralaterally and terminated in the abducens nucleus, Area II, and Area I with additional recurrent collaterals of ipsilaterally projecting neurons within the parent nucleus. Distinct feedforward commissural pathways between bilateral DO neurons likely contribute to the generation of eye velocity signals in eIhII cells. The shared contribution of DO and Area II neurons to eye velocity storage likely represents an ancestral condition in goldfish that is clearly at variance with the task separation between mammalian medial vestibular and prepositus hypoglossi neurons. This difference in signal processing between fish and mammals might correlate with a larger repertoire of visuo-vestibular-driven eye movements in the latter species that potentially required a shift in sensitivity and connectivity within the hindbrain-cerebello-oculomotor network.

NEW & NOTEWORTHY We describe the structure and function of neurons within the goldfish descending octaval nucleus. Our findings indicate that eye and head velocity signals are processed by vestibular and Area II velocity storage integrator circuitries whereas the velocity-to-position Area I neural integrator generates eye position solely. This ancestral condition differs from that of mammals, in which vestibular neurons generally lack eye position signals that are processed and stored within the nucleus prepositus hypoglossi.

The neural basis of motion sickness

Although motion of the head and body has been suspected or known as the provocative cause for the production of motion sickness for centuries, it is only within the last 20 yr that the source of the signal generating motion sickness and its neural basis has been firmly established. Here, we briefly review the source of the conflicts that cause the body to generate the autonomic signs and symptoms that constitute motion sickness and provide a summary of the experimental data that have led to an understanding of how motion sickness is generated and can be controlled. Activity and structures that produce motion sickness include vestibular input through the semicircular canals, the otolith organs, and the velocity storage integrator in the vestibular nuclei. Velocity storage is produced through activity of vestibular-only (VO) neurons under control of neural structures in the nodulus of the vestibulo-cerebellum. Separate groups of nodular neurons sense orientation to gravity, roll/tilt, and translation, which provide strong inhibitory control of the VO neurons. Additionally, there are acetylcholinergic projections from the nodulus to the stomach, which along with other serotonergic inputs from the vestibular nuclei, could induce nausea and vomiting. Major inhibition is produced by the GABA_B receptors, which modulate and suppress activity in the velocity storage integrator. Ingestion of the GABA_B agonist baclofen causes suppression of motion sickness. Hopefully, a better understanding of the source of sensory conflict will lead to better ways to avoid and treat the autonomic signs and symptoms that constitute the syndrome.

Adaptation of spatio-temporal convergent properties in central vestibular neurons in monkeys

The spatio-temporal convergent (STC) response occurs in central vestibular cells when dynamic and static inputs are activated. The functional significance of STC behavior is not fully understood. Whether STC is a property of some specific central vestibular neurons, or whether it is a response that can be induced in any neuron at some frequencies is unknown. It is also unknown how the change in orientation of otolith polarization vector (orientation adaptation) affects STC behavior. A new complex model, that includes inputs with regular and irregular discharges from both canal and otolith afferents, was applied to experimental data to determine how many convergent inputs are sufficient to explain the STC behavior as a function of frequency and orientation adaptation. The canal-otolith and otolith-only neurons were recorded in the vestibular nuclei of three monkeys. About 42% (11/26 canal-otolith and 3/7 otolith-only) neurons showed typical STC responses at least at one frequency before orientation adaptation. After orientation adaptation in side-down head position for 2 h, some canal-otolith and otolith-only neurons altered their STC responses. Thus, STC is a property of weights of the regular and irregular vestibular afferent inputs to central vestibular neurons which appear and/or disappear based on stimulus frequency and orientation adaptation. This indicates that STC properties are more common for central vestibular neurons than previously assumed. While gravity-dependent adaptation is also critically dependent on stimulus frequency and orientation adaptation, we propose that STC behavior is also linked to the neural network responsible for localized contextual learning during gravity-dependent adaptation.

Vestibular Injury After Low-Intensity Blast Exposure

The increased use of close range explosives has led to a higher incidence of exposure to blast-related head trauma. Exposure to primary blast waves is a significant cause of morbidity and mortality. Active service members and civilians who have experienced blast waves report high rates of vestibular dysfunction, such as vertigo, oscillopsia, imbalance, and dizziness. Accumulating evidence suggests that exposure to blast-wave trauma produces damage to both the peripheral and central vestibular system; similar to previous findings that blast exposure results in damage to auditory receptors. In this study, mice were exposed to a 63 kPa peak blast-wave over pressure and were examined for vestibular receptor damage as well as behavioral assays to identify vestibular dysfunction. We observed perforations to the tympanic membrane in all blast animals. We also observed significant loss of stereocilia on hair cells in the cristae and macule up to 1 month after blast-wave exposure; damage that is likely permanent. Significant reductions in the ability to perform the righting reflex and balance on a rotating rod that lasted several weeks after blast exposure were prominent behavioral effects. We also observed a significant reduction in horizontal vestibuloocular reflex gain and phase lags in the eye movement responses that lasted many weeks following a single blast exposure event. OKN responses were absent immediately following blast exposure, but began to return after several weeks’ recovery. These results show that blast-wave exposure can lead to peripheral vestibular damage (possibly central deficits as well) and provides some insight into causes of vestibular dysfunction in blast-trauma victims.

Hypothesis: The Vestibular and Cerebellar Basis of the Mal de Debarquement Syndrome

The Mal de Debarquement syndrome (MdDS) generally follows sea voyages, but it can occur after turbulent flights or spontaneously. The primary features are objective or perceived continuous rocking, swaying, and/or bobbing at 0.2 Hz after sea voyages or 0.3 Hz after flights. The oscillations can continue for months or years and are immensely disturbing. Associated symptoms appear to be secondary to the incessant sensation of movement. We previously suggested that the illness can be attributed to maladaptation of the velocity storage integrator in the vestibular system, but the actual neural mechanisms driving the MdDS are unknown. Here, based on experiments in subhuman primates, we propose a series of postulates through which the MdDS is generated: (1) The MdDS is produced in the velocity storage integrator by activation of vestibular-only (VO) neurons on either side of the brainstem that are oscillating back and forth at 0.2 or 0.3 Hz. (2) The groups of VO neurons are driven by signals that originate in Purkinje cells in the cerebellar nodulus. (3) Prolonged exposure to roll, either on the sea or in the air, conditions the roll-related neurons in the nodulus. (4) The prolonged exposure causes a shift of the pitch orientation vector from its original position aligned with gravity to a position tilted in roll. (5) Successful treatment involves exposure to a full-field optokinetic stimulus rotating around the spatial vertical countering the direction of the vestibular imbalance. This is done while rolling the head at the frequency of the perceived rocking, swaying, or bobbing. We also note experiments that could be used to verify these postulates, as well as considering potential flaws in the logic. Important unanswered questions: (1) Why does the MdDS predominantly affect women? (2) What aspect of roll causes the prolongation of the tilted orientation vector, and why is it so prolonged in some individuals? (3) What produces the increase in symptoms of some patients when returning home after treatment, and how can this be avoided? We also posit that the same mechanisms underlie the less troublesome and shorter duration Mal de Debarquement.

A unified internal model theory to resolve the paradox of active versus passive self-motion sensation

Brainstem and cerebellar neurons implement an internal model to accurately estimate self-motion during externally generated (‘passive’) movements. However, these neurons show reduced responses during self-generated (‘active’) movements, indicating that predicted sensory consequences of motor commands cancel sensory signals. Remarkably, the computational processes underlying sensory prediction during active motion and their relationship to internal model computations during passive movements remain unknown. We construct a Kalman filter that incorporates motor commands into a previously established model of optimal passive self-motion estimation. The simulated sensory error and feedback signals match experimentally measured neuronal responses during active and passive head and trunk rotations and translations. We conclude that a single sensory internal model can combine motor commands with vestibular and proprioceptive signals optimally. Thus, although neurons carrying sensory prediction error or feedback signals show attenuated modulation, the sensory cues and internal model are both engaged and critically important for accurate self-motion estimation during active head movements.

When seated in a car, we can detect when the vehicle begins to move even with our eyes closed. Structures in the inner ear called the vestibular, or balance, organs enable us to sense our own movement. They do this by detecting head rotations, accelerations and gravity. They then pass this information on to specialized vestibular regions of the brain.

Experiments using rotating chairs and moving platforms have shown that passive movements – such as car journeys and rollercoaster rides – activate the brain’s vestibular regions. But recent work has revealed that voluntary movements – in which individuals start the movement themselves – activate these regions far less than passive movements. Does this mean that the brain ignores signals from the inner ear during voluntary movements? Another possibility is that the brain predicts in advance how each movement will affect the vestibular organs in the inner ear. It then compares these predictions with the signals it receives during the movement. Only mismatches between the two activate the brain’s vestibular regions.

To test this theory, Laurens and Angelaki created a mathematical model that compares predicted signals with actual signals in the way the theory proposes. The model accurately predicts the patterns of brain activity seen during both active and passive movement. This reconciles the results of previous experiments on active and passive motion. It also suggests that the brain uses similar processes to analyze vestibular signals during both types of movement.

These findings can help drive further research into how the brain uses sensory signals to refine our everyday movements. They can also help us understand how people recover from damage to the vestibular system. Most patients with vestibular injuries learn to walk again, but have difficulty walking on uneven ground. They also become disoriented by passive movement. Using the model to study how the brain adapts to loss of vestibular input could lead to new strategies to aid recovery.