Quantifying virtual self-motion sensations induced by galvanic vestibular stimulation.

BACKGROUND: The vestibular system provides a comprehensive estimate of self-motion in 3D space. Widely used to artificially stimulate the vestibular system, binaural-bipolar square-wave Galvanic Vestibular Stimulation (GVS) elicits a virtual sensation of roll rotation. Postural responses to GVS have been clearly delineated, however quantifying the perceived virtual rotation vector has not been fully realised. OBJECTIVE: We aimed to quantify the perceived virtual roll rotation vector elicited by GVS using a psychophysical approach on a 3D turntable. METHODS: Participants were placed supine on the 3D turntable and rotated around the naso-occipital axis while supine and received square-wave binaural-bipolar GVS or sham stimulation. GVS amplitudes and intensities were systematically manipulated. The turntable motion profile consisted of a velocity step of 20°/s2 until the trial velocity between 0-20°/s was reached, followed by a 1°/s ramp until the end of the trial. In a psychophysical adaptive staircase procedure, we systematically varied the roll velocity to identify the exact velocity that cancelled the perceived roll sensation induced by GVS. RESULTS: Participants perceived a virtual roll rotation towards the cathode of approximately 2°/s velocity for 1 mA GVS and 6°/s velocity for 2.5 mA GVS. The observed values were stable across repetitions. CONCLUSIONS: Our results quantify for the first time the perceived virtual roll rotations induced by binaural-bipolar square-wave GVS. Importantly, estimates were based on perceptual judgements, in the absence of motor or postural responses and in a head orientation where the GVS-induced roll sensation did not interact with the perceived direction of gravity. This is an important step towards applications of GVS in different settings, including sensory substitution or Virtual Reality.

Influence of panoramic cues during prolonged roll-tilt adaptation on the percept of vertical.

The percept of vertical, which mainly relies on vestibular and visual cues, is known to be affected after sustained whole-body roll tilt, mostly at roll positions adjacent to the position of adaptation. Here we ask whether the viewing of panoramic visual cues during the adaptation further influences the percept of the visual vertical. Participants were rotated in the frontal plane to a 90° clockwise tilt position, which was maintained for 4-minutes. During this period, the subject was either kept in darkness, or viewed panoramic pictures that were either veridical (aligned with gravity) or oriented along the body longitudinal axis. Errors of the subsequent subjective visual vertical (SVV), measured at various tilt angles, showed that the adaptation effect of panoramic cues is local, i.e. for a narrow range of tilts in the direction of the adaptation angle. This distortion was found irrespective of the orientation of the panoramic cues. We conclude that sustained exposure to panoramic and vestibular cues does not adapt the subsequent percept of vertical to the direction of the panoramic cue. Rather, our results suggest that sustained panoramic cues affect the SVV by an indirect effect on head orientation, with a 90° periodicity, that interacts with a vestibular cue to determine the percept of vertical.

Processing of an ambiguous time phrase in posttraumatic stress disorder: Eye movements suggest a passive, oncoming perception of the future.

Metaphorically, the future can be perceived as approaching us (time-moving metaphor) or as being approached by us (ego-moving metaphor). Also, in line with findings that our eyes look more up when thinking about the future than the past, the future's location can be conceptualized in upwards terms. Eye movements were recorded in 19 participants with PTSD and 20 healthy controls. Participants with PTSD showed downward and healthy controls upward eye movements while processing an ego/time-moving ambiguous phrase, suggesting a passive (time-moving) outlook toward the future. If replicated, our findings may have implications for the conceptualization and treatment of PTSD.

Asymmetry in Gaze-Holding Impairment in Acute Unilateral Ischemic Cerebellar Lesions Critically Depends on the Involvement of the Caudal Vermis and the Dentate Nucleus.

Stabilizing the eyes in space when looking at a target is provided by a brainstem/cerebellar gaze-holding network, including the flocculus/paraflocculus complex (non-human primate studies) and the caudal vermis, biventer, and inferior semilunar lobule (human studies). Previous research suggests that acute lateralized cerebellar lesions preferentially lead to gaze-evoked nystagmus (GEN) on ipsilesional gaze. Here, we further characterize the effect of unilateral cerebellar lesions on gaze-holding and hypothesize that the side-specific magnitude of gaze-holding impairment depends on the lesion location. Nine patients (age range = 31-62 years) with acute/subacute (≤ 10 days old) MRI-confirmed unilateral cerebellar stroke were included. Horizontal gaze holding was quantified while looking at a slowly moving (0.5°/s) flashing target (gaze angle = ±40°). Asymmetry in eye-drift velocity was calculated and compared with the different MRI patterns of cerebellar lesions. Individual peak eye-drift velocities (range = 1.7-8.8°/s) occurred at the most eccentric eye positions (gaze angle = 28-38°). We found significantly asymmetric eye-drift velocity (EDV) in eight out of nine patients. The four patients with MRI-confirmed involvement of the caudal vermis and the dentate nucleus all presented with ipsilesionally-predominant EDV, while in the five patients with lesions restricted to the cerebellar hemisphere, EDV was stronger on contralesional gaze in three out of four found with an asymmetric EDV. Involvement of the caudal vermis and the dentate nucleus is critical for determining the directional GEN asymmetry in unilateral cerebellar lesions. Thus, our findings support the occurrence of GEN without floccular/parafloccular lesions and suggest that the EDV asymmetry in relation to the side of the lesion provides information about the involvement of specific structures.

Effects of prolonged roll-tilt on the subjective visual and haptic vertical in healthy human subjects.

BACKGROUND: While verticality perception is normally accurate when upright, a systematic bias ("post-tilt bias") is seen after prolonged roll-tilt. The source of the bias could either be central (shifting "null" position) or related to changes in torsional eye-position. OBJECTIVE: To study the mechanisms of the post-tilt bias in vision-dependent and vision-independent paradigms and to characterize the impact of optokinetic stimulation. METHODS: The subjective visual-vertical (SVV) and subjective haptic-vertical (SHV) were measured after static roll-tilt (±90deg ear-down ("adaptation") position; duration = 5 min; n = 9 subjects). To assess the effect of visual stimuli, a control condition (darkness) was compared with an optokinetic stimulus (clockwise/counter-clockwise rotation, 60deg/sec) during adaptation. RESULTS: A significant post-tilt bias was more frequent for the SVV than the SHV (72% vs. 54%, p = 0.007) with shifts pointing towards or away from the adaptation position with similar frequency. Exponential-decay time-constants were comparable for both paradigms and directions of shifts. The optokinetic stimulus had no effect on the bias for either paradigm. CONCLUSIONS: Emerging in both vision-dependent and vision-independent paradigms, the results support the hypothesis that the post-tilt bias results from a shift in the internal estimate of direction of gravity, while optokinetic nystagmus seems not to be a major contributor.

Otoconial loss or lack of otoconia - An overlooked or ignored diagnosis of balance deficits.

HYPOTHESIS: Lack of otoconia or otoconial loss may be the major reason for increasing imbalance with age, posttraumatic dizziness and residual dizziness as well as other so far unexplained imbalance affecting probably millions of people. BACKGROUND: It is written in every textbook that we need sensation of gravity for stable gait and stance, especially on two legs. Lack of otoconia is known to cause lifelong balance problems in animals. Loss of otoconia is happening in aging humans, like shown by increasing incidence of benign paroxysmal positional vertigo (BPPV) and in histological sections. While hundreds of papers have been published on BPPV, increasing imbalance with age and increasing falls, none has ever described the loss of otoconia as a major reason for this imbalance. Maybe this is due to the problems to proof this hypothesis in an individual patient. We will explain why otoconial loss may cause dizziness, postural and locomotor instability in patients with no other identifiable cause or in addition to other causes. Several reasons can cause otoconial loss and lead to the described symptoms. We will describe the symptoms and the tests which could in combination support the diagnosis. CONCLUSION: Our hypothesis argues for the new diagnosis in many patients with so far undiagnosed or incorrectly or incompletely diagnosed dizziness or imbalance.

Precision of perceived direction of gravity in partial bilateral vestibulopathy correlates with residual utricular function.

OBJECTIVE: Gait imbalance in patients with bilateral-vestibular-deficiency (BVD) was linked to increased variability in perceived direction of gravity while upright. We hypothesized this to be true also when roll-tilted. Moreover, as utricular input is essential for spatial orientation, we predicted the variability of perceived vertical to correlate inversely with utricular function. METHODS: Subjective visual vertical (SVV) and haptic vertical (SHV) were measured in various roll-orientations (0°/±45°/±90°) and postural adjustments along earth-vertical/earth-horizontal were collected in patients with partial BVD (n = 10) and healthy controls (n = 11). Patients with bilaterally-absent bone-conducted ocular vestibular-evoked myogenic-potentials (oVEMPs) were compared to those with (partially) preserved oVEMPs. RESULTS: For the SVV (p < 0.001) and SHV (p = 0.004) variability was larger in patients than controls. Compared to those with (partially) preserved oVEMPs, patients with bilaterally-absent oVEMPs had higher SVV (p = 0.024) and SHV (p = 0.006) variability. Self-positioning along earth-horizontal was more variable in BVD-patients compared to controls (p < 0.001). Again, variability was higher in those with bilaterally-absent oVEMPs (p = 0.032). SVV/SHV-variability was correlated (R2 = 0.61, slope = 1.06 [95%-CI = 0.80-1.54]) in BVD-patients. CONCLUSION: With variability correlating amongst the different paradigms and with oVEMP-responses, this emphasizes the role of bilaterally intact utricular input for precise perception of gravity. SIGNIFICANCE: In BVD-patients with bilaterally-absent oVEMPs intensified vestibular rehabilitation should be considered.

Compensatory saccades in head impulse testing influence the dynamic visual acuity of patients with unilateral peripheral vestibulopathy1.

BACKGROUND: Both the dynamic visual acuity (DVA) test and the video head-impulse test (vHIT) are fast and simple ways to assess peripheral vestibulopathy. After losing peripheral vestibular function, some patients show better DVA performance than others, suggesting good compensatory mechanisms. It seems possible that compensatory covert saccades could be responsible for improved DVA. OBJECTIVE: To investigate VOR gain and compensatory saccades with vHIT and compare them to the DVA of patients with unilateral peripheral vestibulopathy. METHODS: VOR gain deficit and compensatory saccades were measured with vHIT. VOR gain was calculated for each trial as mean eye velocity divided by mean head velocity during 4 samples between 24 ms - 40 ms after peak head acceleration. DVA was then assessed. VHIT was analyzed for percentage of covert saccades and for cumulative overt saccade amplitude. Twenty-four patients with unilateral vestibular deficit were included. A control group of 113 healthy subjects provided normal data. RESULTS: On the affected side, pathologic values for DVA (mean 0.83 logMAR±0.25 SD) and VOR gain (mean 0.16±0.13) were obtained, whereas the healthy side showed normal values (0.53 logMAR±0.15 for DVA and 0.89±0.18 for VOR gain). Yet, DVA performance on the affected side was significantly better in patients with higher covert saccade percentage (p = 0.012) and lower cumulative overt saccade amplitude (p < 0.001). CONCLUSION: Compensatory covert saccades seen in vHIT correlate with improved performance of DVA-testing in patients with unilateral peripheral vestibular loss. Hence, in addition to testing peripheral vestibulopathy, our results indicate a way for assessing rehabilitatory compensation in such patients by DVA in addition to vHIT.

Does gravity influence the visual line bisection task?

The visual line bisection task (LBT) is sensitive to perceptual biases of visuospatial attention, showing slight leftward (for horizontal lines) and upward (for vertical lines) errors in healthy subjects. It may be solved in an egocentric or allocentric reference frame, and there is no obvious need for graviceptive input. However, for other visual line adjustments, such as the subjective visual vertical, otolith input is integrated. We hypothesized that graviceptive input is incorporated when performing the LBT and predicted reduced accuracy and precision when roll-tilted. Twenty healthy right-handed subjects repetitively bisected Earth-horizontal and body-horizontal lines in darkness. Recordings were obtained before, during, and after roll-tilt (±45°, ±90°) for 5 min each. Additionally, bisections of Earth-vertical and oblique lines were obtained in 17 subjects. When roll-tilted ±90° ear-down, bisections of Earth-horizontal (i.e., body-vertical) lines were shifted toward the direction of the head (P < 0.001). However, after correction for vertical line-bisection errors when upright, shifts disappeared. Bisecting body-horizontal lines while roll-tilted did not cause any shifts. The precision of Earth-horizontal line bisections decreased (P ≤ 0.006) when roll-tilted, while no such changes were observed for body-horizontal lines. Regardless of the trial condition and paradigm, the scanning direction of the bisecting cursor (leftward vs. rightward) significantly (P ≤ 0.021) affected line bisections. Our findings reject our hypothesis and suggest that gravity does not modulate the LBT. Roll-tilt-dependent shifts are instead explained by the headward bias when bisecting lines oriented along a body-vertical axis. Increased variability when roll-tilted likely reflects larger variability when bisecting body-vertical than body-horizontal lines.

Static roll-tilt over 5 minutes locally distorts the internal estimate of direction of gravity.

The subjective visual vertical (SVV) indicates perceived direction of gravity. Even in healthy human subjects, roll angle-dependent misestimations, roll overcompensation (A-effect, head-roll > 60° and <135°) and undercompensation (E-effect, head-roll < 60°), occur. Previously, we demonstrated that, after prolonged roll-tilt, SVV estimates when upright are biased toward the preceding roll position, which indicates that perceived vertical (PV) is shifted by the prior tilt (Tarnutzer AA, Bertolini G, Bockisch CJ, Straumann D, Marti S. PLoS One 8: e78079, 2013). Hypothetically, PV in any roll position could be biased toward the previous roll position. We asked whether such a "global" bias occurs or whether the bias is "local". The SVV of healthy human subjects (N = 9) was measured in nine roll positions (-120° to +120°, steps = 30°) after 5 min of roll-tilt in one of two adaptation positions (±90°) and compared with control trials without adaptation. After adapting, adjustments were shifted significantly (P < 0.05) toward the previous adaptation position for nearby roll-tilted positions (±30°, ±60°) and upright only. We computationally simulated errors based on the sum of a monotonically increasing function (producing roll undercompensation) and a mixture of Gaussian functions (representing roll overcompensation centered around PV). In combination, the pattern of A- and E-effects could be generated. By shifting the function representing local overcompensation toward the adaptation position, the experimental postadaptation data could be fitted successfully. We conclude that prolonged roll-tilt locally distorts PV rather than globally shifting it. Short-term adaptation of roll overcompensation may explain these shifts and could reflect the brain's strategy to optimize SVV estimates around recent roll positions. Thus postural stability can be improved by visually-mediated compensatory responses at any sustained body-roll orientation.

Quantitative analysis of benign paroxysmal positional vertigo fatigue under canalithiasis conditions.

In our daily life, small flows in the semicircular canals (SCCs) of the inner ear displace a sensory structure called the cupula which mediates the transduction of head angular velocities to afferent signals. We consider a dysfunction of the SCCs known as canalithiasis. Under this condition, small debris particles disturb the flow in the SCCs and can cause benign paroxysmal positional vertigo (BPPV), arguably the most common form of vertigo in humans. The diagnosis of BPPV is mainly based on the analysis of typical eye movements (positional nystagmus) following provocative head maneuvers that are known to lead to vertigo in BPPV patients. These eye movements are triggered by the vestibulo-ocular reflex, and their velocity provides an indirect measurement of the cupula displacement. An attenuation of the vertigo and the nystagmus is often observed when the provocative maneuver is repeated. This attenuation is known as BPPV fatigue. It was not quantitatively described so far, and the mechanisms causing it remain unknown. We quantify fatigue by eye velocity measurements and propose a fluid dynamic interpretation of our results based on a computational model for the fluid-particle dynamics of a SCC with canalithiasis. Our model suggests that the particles may not go back to their initial position after a first head maneuver such that a second head maneuver leads to different particle trajectories causing smaller cupula displacements.

Detection and grading of endolymphatic hydrops in Menière disease using MR imaging.

BACKGROUND AND PURPOSE: Endolymphatic hydrops has been recognized as the underlying pathophysiology of Menière disease. We used 3T MR imaging to detect and grade endolymphatic hydrops in patients with Menière disease and to correlate MR imaging findings with the clinical severity. MATERIALS AND METHODS: MR images of the inner ear acquired by a 3D inversion recovery sequence 4 hours after intravenous contrast administration were retrospectively analyzed by 2 neuroradiologists blinded to the clinical presentation. Endolymphatic hydrops was classified as none, grade I, or grade II. Interobserver agreement was analyzed, and the presence of endolymphatic hydrops was correlated with the clinical diagnosis and the clinical Menière disease score. RESULTS: Of 53 patients, we identified endolymphatic hydrops in 90% on the clinically affected and in 22% on the clinically silent side. Interobserver agreement on detection and grading of endolymphatic hydrops was 0.97 for cochlear and 0.94 for vestibular hydrops. The average MR imaging grade of endolymphatic hydrops was 1.27 ± 0.66 for 55 clinically affected and 0.65 ± 0.58 for 10 clinically normal ears. The correlation between the presence of endolymphatic hydrops and Menière disease was 0.67. Endolymphatic hydrops was detected in 73% of ears with the clinical diagnosis of possible, 100% of probable, and 95% of definite Menière disease. CONCLUSIONS: MR imaging supports endolymphatic hydrops as a pathophysiologic hallmark of Menière disease. High interobserver agreement on the detection and grading of endolymphatic hydrops and the correlation of MR imaging findings with the clinical score recommend MR imaging as a reliable in vivo technique in patients with Menière disease. The significance of MR imaging detection of endolymphatic hydrops in an additional 22% of asymptomatic ears requires further study.

Visually guided adjustments of body posture in the roll plane.

Body position relative to gravity is continuously updated to prevent falls. Therefore, the brain integrates input from the otoliths, truncal graviceptors, proprioception and vision. Without visual cues estimated direction of gravity mainly depends on otolith input and becomes more variable with increasing roll-tilt. Contrary, the discrimination threshold for object orientation shows little modulation with varying roll orientation of the visual stimulus. Providing earth-stationary visual cues, this retinal input may be sufficient to perform self-adjustment tasks successfully, with resulting variability being independent of whole-body roll orientation. We compared conditions with informative (earth-fixed) and non-informative (body-fixed) visual cues. If the brain uses exclusively retinal input (if earth-stationary) to solve the task, trial-to-trial variability will be independent from the subject's roll orientation. Alternatively, central integration of both retinal (earth-fixed) and extra-retinal inputs will lead to increasing variability when roll-tilted. Subjects, seated on a motorized chair, were instructed to (1) align themselves parallel to an earth-fixed line oriented earth-vertical or roll-tilted 75° clockwise; (2) move a body-fixed line (aligned with the body-longitudinal axis or roll-tilted 75° counter-clockwise to it) by adjusting their body position until the line was perceived earth-vertical. At 75° right-ear-down position, variability increased significantly (p < 0.05) compared to upright in both paradigms, suggesting that, despite the earth-stationary retinal cues, extra-retinal input is integrated. Self-adjustments in the roll-tilted position were significantly (p < 0.01) more precise for earth-fixed cues than for body-fixed cues, underlining the importance of earth-stable visual cues when estimates of gravity become more variable with increasing whole-body roll.

Antihysteresis of perceived longitudinal body axis during continuous quasi-static whole-body rotation in the earth-vertical roll plane.

Estimation of subjective whole-body tilt in stationary roll positions after rapid rotations shows hysteresis. We asked whether this phenomenon is also present during continuous quasi-static whole-body rotation and whether gravitational cues are a major contributing factor. Using a motorized turntable, 8 healthy subjects were rotated continuously about the earth-horizontal naso-occipital axis (earth-vertical roll plane) and the earth-vertical naso-occipital axis (earth-horizontal roll plane). In both planes, three full constant velocity rotations (2°/s) were completed in clockwise and counterclockwise directions (acceleration = 0.05°/s(2), velocity plateau reached after 40 s). Subjects adjusted a visual line along the perceived longitudinal body axis (pLBA) every 2 s. pLBA deviation from the longitudinal body axis was plotted as a function of whole-body roll position, and a sine function was fitted. At identical whole-body earth-vertical roll plane positions, pLBA differed depending on whether the position was reached by a rotation from upright or by passing through upside down. After the first 360° rotation, pLBA at upright whole-body position deviated significantly in the direction of rotation relative to pLBA prior to rotation initiation. This deviation remained unchanged after subsequent full rotations. In contrast, earth-horizontal roll plane rotations resulted in similar pLBA before and after each rotation cycle. We conclude that the deviation of pLBA in the direction of rotation during quasi-static earth-vertical roll plane rotations reflects static antihysteresis and might be a consequence of the known static hysteresis of ocular counterroll: a visual line that is perceived that earth-vertical is expected to be antihysteretic, if ocular torsion is hysteretic.

Velocity storage contribution to vestibular self-motion perception in healthy human subjects.

Self-motion perception after a sudden stop from a sustained rotation in darkness lasts approximately as long as reflexive eye movements. We hypothesized that, after an angular velocity step, self-motion perception and reflexive eye movements are driven by the same vestibular pathways. In 16 healthy subjects (25-71 years of age), perceived rotational velocity (PRV) and the vestibulo-ocular reflex (rVOR) after sudden decelerations (90°/s(2)) from constant-velocity (90°/s) earth-vertical axis rotations were simultaneously measured (PRV reported by hand-lever turning; rVOR recorded by search coils). Subjects were upright (yaw) or 90° left-ear-down (pitch). After both yaw and pitch decelerations, PRV rose rapidly and showed a plateau before decaying. In contrast, slow-phase eye velocity (SPV) decayed immediately after the initial increase. SPV and PRV were fitted with the sum of two exponentials: one time constant accounting for the semicircular canal (SCC) dynamics and one time constant accounting for a central process, known as velocity storage mechanism (VSM). Parameters were constrained by requiring equal SCC time constant and VSM time constant for SPV and PRV. The gains weighting the two exponential functions were free to change. SPV were accurately fitted (variance-accounted-for: 0.85 ± 0.10) and PRV (variance-accounted-for: 0.86 ± 0.07), showing that SPV and PRV curve differences can be explained by a greater relative weight of VSM in PRV compared with SPV (twofold for yaw, threefold for pitch). These results support our hypothesis that self-motion perception after angular velocity steps is be driven by the same central vestibular processes as reflexive eye movements and that no additional mechanisms are required to explain the perceptual dynamics.

Visual contribution to postural stability: Interaction between target fixation or tracking and static or dynamic large-field stimulus.

Stationary visual information has a stabilizing effect on posture, whereas moving visual information is destabilizing. We compared the influence of a stationary or moving fixation point to the influence of stationary or moving large-field stimulation, as well as the interaction between a fixation point and a large-field stimulus. We recorded body sway in 20 healthy subjects who were fixating a stationary or oscillating dot (vertical or horizontal motion, 1/3 Hz, +/-12 degrees amplitude, distance 96 cm). In addition, a large-field random dot pattern (extension: approximately 80 x 70 degrees) was stationary, moving or absent. Visual fixation of a stationary dot in darkness did not reduce antero-posterior (AP) sway compared to the situation in total darkness, but slightly reduced lateral sway at frequencies below 0.5 Hz. In contrast, fixating a stationary dot on a stationary large-field pattern reduced both AP and lateral body sway at all frequencies (0.1-2 Hz). Ocular tracking of the oscillating dot caused a peak in body sway at 1/3 Hz, i.e. the stimulus frequency, but there was no influence of large-field stimulus at this frequency. A stationary large-field pattern, however, reduced AP and lateral sway at frequencies between 0.1 and 2 Hz when subjects tracked a moving dot, compared to tracking in darkness. Our results demonstrate that a stationary large-field pattern has a stabilizing effect in all conditions, independent of whether the eyes are fixing on a stationary target or tracking a moving target.

Roll-dependent modulation of the subjective visual vertical: contributions of head- and trunk-based signals.

Precision and accuracy of the subjective visual vertical (SVV) modulate in the roll plane. At large roll angles, systematic SVV errors are biased toward the subject's body-longitudinal axis and SVV precision is decreased. To explain this, SVV models typically implement a bias signal, or a prior, in a head-fixed reference frame and assume the sensory input to be optimally tuned along the head-longitudinal axis. We tested the pattern of SVV adjustments both in terms of accuracy and precision in experiments in which the head and the trunk reference frames were not aligned. Twelve subjects were placed on a turntable with the head rolled about 28 degrees counterclockwise relative to the trunk by lateral tilt of the neck to dissociate the orientation of head- and trunk-fixed sensors relative to gravity. Subjects were brought to various positions (roll of head- or trunk-longitudinal axis relative to gravity: 0 degrees , +/-75 degrees ) and aligned an arrow with perceived vertical. Both accuracy and precision of the SVV were significantly (P < 0.05) better when the head-longitudinal axis was aligned with gravity. Comparing absolute SVV errors for clockwise and counterclockwise roll tilts, statistical analysis yielded no significant differences (P > 0.05) when referenced relative to head upright, but differed significantly (P < 0.001) when referenced relative to trunk upright. These findings indicate that the bias signal, which drives the SVV toward the subject's body-longitudinal axis, operates in a head-fixed reference frame. Further analysis of SVV precision supports the hypothesis that head-based graviceptive signals provide the predominant input for internal estimates of visual vertical.

Dynamic aspects of trochlear nerve palsy.

Trochlear nerve palsy leads to kinematic aberrations of both the paretic and the unaffected eye. During dynamic head roll, the rotation axis of the covered paretic or unaffected eye deviates inward, while the rotation axis of the viewing paretic or unaffected eye aligns with the line of sight; this convergence of rotation axes increases with gaze moving in the direction of the unaffected eye. During downward saccades, the trajectories of both eyes curve towards the unaffected side; these curvatures increase when the head is rolled to the affected side and gaze directed to the unaffected side. Hence, during both vestibular evoked and saccadic ocular movements, the unaffected eye shows similar kinematic aberrations as the paretic eye. While aberrations of the paretic eye can be explained by decreased force of the superior oblique (SO) muscle, aberrations of the unaffected eye may be due to increased force parallel to the paretic SO in the unaffected eye in accordance with Hering's law. This law, which forms the basis of conjugate eye movements, also seems to govern eye displacements in unilateral eye muscle palsy.

Do humans show velocity-storage in the vertical rVOR?

To investigate the contribution of the vestibular velocity-storage mechanism (VSM) to the vertical rotational vestibulo-ocular reflex (rVOR) we recorded eye movements evoked by off-vertical axis rotation (OVAR) using whole-body constant-velocity pitch rotations about an earth-horizontal, interaural axis in four healthy human subjects. Subjects were tumbled forward, and backward, at 60 deg/s for over 1 min using a 3D turntable. Slow-phase velocity (SPV) responses were similar to the horizontal responses elicited by OVAR along the body longitudinal axis, ('barbecue' rotation), with exponentially decaying amplitudes and a residual, otolith-driven sinusoidal response with a bias. The time constants of the vertical SPV ranged from 6 to 9 s. These values are closer to those that reflect the dynamic properties of vestibular afferents than the typical 20 s produced by the VSM in the horizontal plane, confirming the relatively smaller contribution of the VSM to these vertical responses. Our preliminary results also agree with the idea that the VSM velocity response aligns with the direction of gravity. The horizontal and torsional eye velocity traces were also sinusoidally modulated by the change in gravity, but showed no exponential decay.

Cyclovergence evoked by up-down acceleration along longitudinal axis in humans.

We present results of a study of torsional eye movements evoked by earth-vertical accelerations along the subject's longitudinal axis. The earth-vertical stimulus leads to a gravito-inertial acceleration vector that changes magnitude but not direction. It can therefore be viewed as a dynamic change of the gravity level. Up-down oscillations induced relatively symmetric cyclovergence (0.6-2.2 degrees peak-to-peak). Eyes intorted/extorted for higher/lower effective gravity. The phase of this modulation was small relative to chair acceleration. We contrast this behaviour to the dynamics of cycloversion in response to interaural acceleration, which shows a considerably larger phase lag. This strikingly different dynamics suggest a different processing of otolith signals during interaural and longitudinal stimulation.