Effect of adaptation to telescopic spectacles on the initial human horizontal vestibuloocular reflex

Stereoscopic 3D Optic Flow Distortions Caused by Mismatches between Image Acquisition and Display Parameters

We analyze the impact of common stereoscopic 3D (S3D) depth distortion on S3D optic flow in virtual reality (VR) environments. The depth distortion is introduced by mismatches between the image acquisition and display parameter. The results show that such S3D distortions induce large S3D optic flow distortions and may even induce partial/full optic flow reversal within a certain depth range, depending on the viewer's moving speed and the magnitude of S3D distortion introduced. We hypothesize that the S3D optic flow distortion may be a source of intra-sensory conflict that may be a source of visually induced motion sickness (VIMS) in S3D.

The Video Head Impulse Test and the Influence of Daily Use of Spectacles to Correct a Refractive Error

Objective

To determine the influence of daily use of spectacles to correct a refractive error, on the vestibulo-ocular reflex (VOR) gain measured with the video head impulse test (vHIT).

Study design

This prospective study enrolled subjects between 18 and 80 years old with and without a refractive error. Subjects were classified into three groups: (1) contact lenses, (2) spectacles, and (3) control group without visual impairment. Exclusion criteria comprised ophthalmic pathology, history of vestibular disorders, and alternated use of spectacles and contact lenses in daily life. Corrective spectacles were removed seconds before testing. One examiner performed all vHIT’s under standardized circumstances using the EyeSeeCam system. This system calculated the horizontal VOR gain for rightward and leftward head rotations separately.

Results

No statistically significant difference was found in VOR gain between the control group (n = 16), spectacles group (n = 48), and contact lenses group (n = 15) (p = 0.111). Both the spectacles group and contact lenses group showed no statistically significant correlation between VOR gain and amount of refractive error, for rightwards (p = 0.071) and leftwards (p = 0.716) head rotations. There was no statistical significant difference in VOR gain between testing monocularly or binocularly (p = 0.132) and between testing with or without wearing contact lenses (p = 0.800).

Conclusion

In this study, VOR gain was not influenced by wearing corrective spectacles or contact lenses on a daily basis. Based on this study, no corrective measures are necessary when performing the vHIT on subjects with a refractive error, regardless of the way of correction.

Epidemiology of vestibulo-ocular reflex function: data from the Baltimore Longitudinal Study of Aging

OBJECTIVE

To determine age-related changes in vestibulo-ocular reflex (VOR) function in community-dwelling adults, and evaluate these for associations with demographic characteristics and cardiovascular risk factors.

STUDY DESIGN

Cross-sectional analysis within the Baltimore Longitudinal Study of Aging (BLSA), a longitudinal prospective cohort study.

SETTING

Vestibular testing laboratory within an acute care teaching hospital.

PATIENTS

Community-dwelling adults enrolled in the BLSA.

INTERVENTION(S)

Horizontal VOR gain measurement using video head-impulse testing and visual acuity testing.

MAIN OUTCOME MEASURE(S)

VOR gain was calculated as the ratio of eye velocity to head velocity. Demographic and cardiovascular risk factor data were collected through study questionnaires.

RESULTS

One hundred nine subjects were analyzed with mean age (SD) 69.9 years (14.2), with a range from 26 to 92 years. VOR gain remained stable from age 26 to 79 after which it significantly declined at a rate of 0.012/year (p = 0.033) in adjusted analyses. Individuals aged 80 years or older had a nearly 8-fold increased odds of VOR gain less than 0.80 relative to those aged less than 80 years in multivariate models (prevalence of 13.2% vs. 2.8%; OR 7.79, 95% CI: 1.04-58.38). Otherwise, VOR gain did not differ significantly across demographic or cardiovascular risk groups.

CONCLUSION

We report age-related decline in VOR function in individuals aged 80 years and older. Further analyses are in progress to establish the significance of these VOR abnormalities to functional and mobility outcomes in older individuals.

Evaluation of quantitative head impulse testing using search coils versus video-oculography in older individuals

OBJECTIVE

To evaluate the validity of 2D video-oculography (VOG) compared with scleral search coils for horizontal AVOR gain estimation in older individuals.

STUDY DESIGN

Cross-sectional validation study.

SETTING

Tertiary care academic medical center.

PATIENTS

Six individuals age 70 and older.

INTERVENTIONS

Simultaneous eye movement recording with scleral search coil (over right eye) and EyeSeeCam VOG camera (over left eye) during horizontal head impulses.

MAIN OUTCOME MEASURES

Best estimate search coil and VOG horizontal AVOR gain, presence of compensatory saccades using both eye movement recording techniques.

RESULTS

We observed a significant correlation between search coil and VOG best estimate horizontal AVOR gain (r = 0.86, p = 0.0002). We evaluated individual head impulses and found that the shapes of the head movement and eye movement traces from the coil and VOG systems were similar. Specific features of eye movements seen in older individuals, including overt and covert corrective saccades and anticompensatory eye movements, were captured by both the search coil and VOG systems.

CONCLUSION

These data suggest that VOG is a reasonable proxy for search coil eye movement recording in older subjects to estimate VOR gain and the approximate timing of corrective eye movements. VOG offers advantages over the conventional search coil method; it is portable and easy to use, allowing for quantitative VOR estimation in diverse settings such as a routine office-based practice, at the bedside, and potentially in larger scale population analyses.

Effects of induced astigmatism on foot placement strategies when stepping onto a raised surface

PURPOSE

Large changes in spectacle prescription can increase falls risk in older people. We investigated the effect of induced astigmatism (a common cause of distorted or blurred vision in older people) on locomotor stepping patterns to determine whether the orientation of astigmatic changes could have differential effects on gait safety when negotiating steps and stairs.

METHODS

10 older adults (mean age 76.0±6.4 years) walked up to and stepped onto a raised block whilst wearing their spectacle prescription and when blurred with ±3.00D cylinders at axes 45°, 90°, 135° and 180°. Gait measurements included foot placement before the block, toe clearance over the block edge and foot placement on the block.

RESULTS

Induced astigmatism with axes at 90°, providing magnification in the horizontal meridian only, caused no change in stepping pattern. Induced astigmatism with axes at 180° caused foot placement changes in the anterior or posterior direction according to whether magnification was positive or negative in the vertical meridian (block perceived higher or lower respectively). Induced astigmatism with axes oblique at 45° and 135° (causing the block to be perceived as a parallelogram sloping downwards either to the right or left) caused gait changes in the anterior and posterior, vertical and lateral directions. Changes in lateral foot placement appeared to be an attempt to maintain constant foot clearance levels over the block edge by stepping over the perceived 'lower' side of the 'sloping' block.

CONCLUSIONS

Astigmatic changes with oblique axes had the greatest effect on gait. Clinicians, including optometrists, physiotherapists, occupational therapists and nurses should counsel older patients about the effects of astigmatism on gait safety. Furthermore, partial prescribing of astigmatic corrections should be considered to reduce the risk of falling.

Roll aftereffects: influence of tilt and inter-stimulus interval

A theme in sensory perception is that exposure to a stimulus causes perception of subsequent stimuli to be shifted in the opposite direction. Such phenomenon is known as aftereffect and has been extensively described in the visual system as well as recently described for the vestibular system during translation. It is known from aviation studies that after a maneuver in roll, pilots can experience a false perception of roll in the opposite direction. The magnitude and duration of this effect as well as the potential influence of the gravity vector have not previously been defined. In the current paper this roll aftereffect (RAE) is examined in response to whole-body roll about an earth-horizontal axis in eight healthy human subjects. The peak velocity of a 0.5-s-duration roll was varied based on previous responses to find the point where subjects perceived no motion. Without a preceding stimulus, the starting position (upright, 9° left, or 9° right) did not influence roll perception. The RAE was measured in a completely dark room using an adapting (first interval) stimulus consisting of 9° of roll over 1.5 s (peak velocity, 12°/s), delivered 0.5, 3, or 6 s prior to test (second interval) stimulus. A significant RAE was seen in all subjects. Half a second after the adapting stimulus, a test stimulus had to be on average 1.5 ± 0.4°/s in the opposite direction to be perceived as stationary. When the subject remained upright after the adapting stimulus, the RAE diminished with time, although it remained significantly larger at 3 and 6 s when the subject remained tilted after the adapting stimulus. These data demonstrate that roll perception can be influenced by small preceding stimuli and tilt causes a persistence of the RAE.

Fore-aft translation aftereffects

A general theme in sensory perception is that exposure to a stimulus makes it seem more neutral such that perception of subsequent stimuli is shifted in the opposite direction. The visual motion aftereffect (MAE) is an extensively studied example of this. Although similar effects have been described in other sensory systems, it has not previously been described in the vestibular system. Velocity storage has been extensively studied in the vestibular system and suggests a persistence of perception in the direction of the initial movement. The current study sought to determine how motion perception is influenced by prior movement in darkness. Thirteen human subjects (mean age 41, range 21-68) underwent whole-body fore-aft translation. The threshold of vestibular motion discrimination perception was measured using a single interval (1I) of motion lasting 0.5 s in which subjects identified their direction of motion as forward or backward using an adaptive staircase. The translation aftereffect (TAE) was measured in 2-interval (2I) experiments: The adapting stimulus moved 15 cm in 1.5 s (peak velocity 20 cm/s, peak acceleration 42 cm/s(2)). After a fixed inter-stimulus interval (ISI) of 0.5, 1.0, 1.5, or 3 s, a second stimulus lasting 0.5 s was delivered and the subject identified the perceived direction of the second test stimulus. The test stimulus was determined using an adaptive staircase. The ISI was constant within the block, but adapting stimuli directions were randomly interleaved. During the 1I condition, the response bias was near zero in all subjects. With a 2I stimulus, 8 of 13 subjects demonstrated a significant bias. At an ISI of 0.5 s, a minority of subjects demonstrated a bias in the same direction as the adapter. When the ISI was 1, 1.5, or 3 s, all subjects who demonstrated a significant TAE had one in the opposite direction of the adapter, similar to that seen for MAE. When averaged across subjects, the TAE was significant with ISIs of 1.0 s and above. These findings demonstrate that perception of vestibular stimuli depends on prior motion. This has important implications for understanding and quantifying vestibular perception.

Spatial compression produced by a stationary low-vision telescope

PURPOSE

Geometrical analysis of the monocular information for visual space perception predicts that the magnification produced by a low-vision telescope will compress the depth dimension of space. To test this prediction we measured the compression in depth of perceived shape while looking through a stationary telescope. To control for the other aspects of telescopic viewing, apart from magnification, we also measured perception while looking through a plain tube having the same field of view.

METHODS

A 2.75x Keplarian telescope was mounted 40 cm above a tabletop patterned with receding stripes. The 11.6 degrees field of view was centered on a series of rectangular stimulus cards lying flat on the table at a distance of 100 cm. Participants monocularly viewed each card through the telescope, or through a tube having the same field of view, and verbally judged the card's perceived length (in depth) relative to its width (in the frontal plane).

RESULTS

Perceptual compression of shape was calculated by dividing the perceived proportion (length/width) by the actual proportion. The telescope and the tube both produced significant perceptual compression, but perception was significantly more compressed through the telescope (0.43) than through the tube (0.52).

CONCLUSIONS

The magnification produced by a stationary low-vision telescope can result in a compression of perceived depth. In addition, other aspects of telescopic viewing, such as monocular vision, restricted head movements, and a restricted field of view, can together contribute substantially to such compression. Further research is needed to assess the clinical implications of these results.

Independent effects of simultaneous inputs from the saccule and lateral semicircular canal. Evaluation using VEMPs

OBJECTIVE

To determine the effects of stimulation of bilateral lateral semicircular canals (LSCCs) by accelerated rotation and caloric stimulation of unilateral LSCC on vestibular evoked myogenic potentials (VEMPs) in healthy volunteers.

METHODS

In experiment 1, VEMPs were recorded while subjects (n = 11) were seated in a rotational chair and angular acceleration around the earth-vertical axis was provided. Amplitudes of p13-n23 were corrected using background muscle activities. In experiment 2, subjects (n = 8) in the semilateral position kept the LSCC in the vertical position and activated the sternocleidomastoid muscle by twisting the neck. After irrigating the external auditory canal with ice water, VEMPs were recorded on the irrigated side. In experiment 3, the same setting as experiment 2 was applied (n = 6), and hot water of 44 degrees C was used for irrigation.

RESULTS

There were no significant differences in latencies of p13 or n23, and in corrected amplitudes by either rotatory or caloric stimulation.

CONCLUSIONS

Simultaneous stimulation of LSCCs has little effect on VEMPs.

SIGNIFICANCE

No functional interaction between the saccule and LSCC was detected in VEMPs, although convergence of semicircular canal and otolith nerve inputs onto single vestibular nucleus neurons has been demonstrated electrophysiologically in animal experiments.

Time course of vestibuloocular reflex suppression during gaze shifts

Although numerous investigations have probed the status of the vestibuloocular (VOR) during gaze shifts, its exact status remains strangely elusive. The goal of the present study was to precisely evaluate the dynamics of VOR suppression immediately before, throughout, and just after gaze shifts. A torque motor was used to apply rapid (100 degrees/s), short-duration (20-30 ms) horizontal head perturbations in three Rhesus monkeys. The status of the VOR elicited by this transient head perturbation was first compared during 15, 40, and 60 degrees gaze shifts. The level of VOR suppression just after gaze-shift onset (40 ms) increased with gaze-shift amplitude in two monkeys, approaching values of 80 and 35%. In contrast, in the third monkey, the VOR was not significantly attenuated for all gaze-shift amplitudes. The time course of VOR attenuation was then studied in greater detail for all three monkeys by imposing the same short-duration head perturbations 40, 100, and 150 ms after the onset of 60 degrees gaze shifts. Overall we found a consistent trend, in which VOR suppression was maximal early in the gaze shift and progressively recovered to reach normal values near gaze-shift end. However, the high variability across subjects prevented establishing a unifying description of the absolute level and time course of VOR suppression during gaze shifts. We propose that differences in behavioral strategies may account, at least in part, for these differences between subjects.

Impaired modulation of the vestibulo-ocular reflex in Huntington's disease

The vestibulo-ocular reflex (VOR) stabilizes gaze during movement, in conjunction with other afferent information: visual, proprioceptive, and somaesthetic. The reflex can either be augmented or suppressed, depending on visual requirements, and undergoes long-term adaptation to compensate for physical changes in the subject. Importantly, over relatively short periods of time, the VOR should function consistently under the same circumstances. This study examines VOR function in patients with Huntington's disease (HD), with a view to investigating cortical influences on the reflex. Horizontal eye movements were recorded in 9 patients with HD and 7 normal subjects, using the scleral search coil technique, in response to high frequency, unpredictable head rotations imposed manually. To establish base VOR function, recordings were made in darkness, without instruction, before and after wearing x2 magnifying lenses for a period of 2 hours to adapt the reflex. Recordings were also made before adaptation, while fixating a stationary visual target (VOR augmentation), and while fixating a target moving with the head (VOR suppression). Although results suggest that the VOR is preserved in HD, with relatively normal gain values and appropriate augmentation and suppression of the reflex with visual input, patients were unable to adapt the VOR to altered visual conditions. This represents a novel finding in HD and suggests that cortical structures compromised in HD exert influences on the long-term adaptation of the VOR.

Vergence-dependent adaptation of the vestibulo-ocular reflex

The gain of the vestibulo-ocular reflex (VOR) normally depends on the distance between the subject and the visual target, but it remains uncertain whether vergence angle can be linked to changes in VOR gain through a process of context-dependent adaptation. In this study, we examined this question with an adaptation paradigm that modified the normal relationship between vergence angle and retinal image motion. Subjects were rotated sinusoidally while they viewed an optokinetic (OKN) stimulus through either diverging or converging prisms. In three subjects the diverging prisms were worn while the OKN stimulus moved out of phase with the head, and the converging prisms were worn when the OKN stimulus moved in-phase with the head. The relationship between the vergence angle and OKN stimulus was reversed in the fourth subject. After 2 h of training, the VOR gain at the two vergence angles changed significantly in all of the subjects, evidenced by the two different VOR gains that could be immediately accessed by switching between the diverged and converged conditions. The results demonstrate that subjects can learn to use vergence angle as the contextual cue that retrieves adaptive changes in the angular VOR.

Rapid motor learning in the translational vestibulo-ocular reflex

Motor learning was induced in the translational vestibulo-ocular reflex (TVOR) when monkeys were repeatedly subjected to a brief (0.5 sec) head translation while they tried to maintain binocular fixation on a visual target for juice rewards. If the target was world-fixed, the initial eye speed of the TVOR gradually increased; if the target was head-fixed, the initial eye speed of the TVOR gradually decreased. The rate of learning acquisition was very rapid, with a time constant of approximately 100 trials, which was equivalent to <1 min of accumulated stimulation. These learned changes were consolidated over >or=1 d without any reinforcement, indicating induction of long-term synaptic plasticity. Although the learning generalized to targets with different viewing distances and to head translations with different accelerations, it was highly specific for the particular combination of head motion and evoked eye movement associated with the training. For example, it was specific to the modality of the stimulus (translation vs rotation) and the direction of the evoked eye movement in the training. Furthermore, when one eye was aligned with the heading direction so that it remained motionless during training, learning was not expressed in this eye, but only in the other nonaligned eye. These specificities show that the learning sites are neither in the sensory nor the motor limb of the reflex but in the sensory-motor transformation stage of the reflex. The dependence of the learning on both head motion and evoked eye movement suggests that Hebbian learning may be one of the underlying cellular mechanisms.

Gaze-, eye-, and head-movement dynamics during closed- and open-loop gaze pursuit

Horizontal step-ramp stimuli were used to examine gaze-, eye-, and head-movement dynamics during head-unrestrained pursuit in two rhesus monkeys. In a first series of experiments, we characterized and compared head-restrained (HR) and -unrestrained (HU) pursuit responses to unpredictable, nonperiodic, constant velocity (20-80 degrees/s) stimuli. When the head was free to move, both monkeys used a combination of eye and head motion to initially fixate and then pursue the target. The pursuit responses (i.e., gaze responses) were highly stereotyped and nearly identical among the HR and HU conditions for a given step-ramp stimulus. In the HU condition, initial eye and initial head acceleration tended to increase as a function of target velocity but did not vary systematically with initial target eccentricity. In a second series of experiments, step-ramp stimuli (40 degrees/s) were presented, and, approximately 125 ms after pursuit onset, a constant retinal velocity error (RVE) was imposed for a duration of 300 ms. In each monkey, HR and HU gaze velocity was similarly affected by stabilizing the target with respect to the monkey's fovea (i.e., RVE = 0 degrees/s) and by moving the target with constant retinal velocity errors (i.e., RVE = +/- 10 degrees/s). In the HU condition, changes in both eye and head velocity trajectories contributed to the observed gaze velocity responses to imposed RVEs. We conclude that eye and head movements are not independently controlled during HU pursuit but rather are controlled, at least in part, by a shared upstream controller within the pursuit pathways.

New tests of vestibular function

The vestibulo-ocular reflex (VOR) is the only drive for short-latency eye movements stabilizing the retina during externally imposed, sudden, high-head accelerations. New strategies can exploit this unique VOR feature to study it under conditions relevant to the daily lives of patients, and to exclude the contributions from confounding nonvestibular mechanisms. Testing of the yaw vestibulo-ocular reflex (VOR) during random, whole-body rotational transients at < or = 2800 degrees/s2 delivered about centered and eccentric axes enables measurement of gains and millisecond latencies of the canal and otolith VORs in humans. Repeated measurements in acute unilateral deafferentation show sequential recovery of canal and otolith VORs to contralesional rotation, but severe and permanent deficits to ipsilesional rotation. Patients with bilateral loss of caloric responses show severe bilateral loss of VORs to transient rotation, suggesting that the apparent preservation of their VORs during sinusoidal rotations at moderate frequencies may be due instead to somatosensory inputs. Since visual acuity is degraded by retinal image motion, dynamic visual acuity (DVA) measured during imposed head-on-body or whole-body transient motion can correlate closely with VOR performance only if optotypes are presented during directionally and temporally unpredictable, high-acceleration head motion. Prediction and efference copy are relentlessly employed by vestibulopathic patients to enable good DVA during predictable or low-acceleration head motion. The linear VOR to transient lateral acceleration is strongly dependent upon viewing distance. The latency of this otolith VOR is slightly longer and more variable than the canal VOR. Unlike the canal VOR, the otolith VOR does not develop a strong directional asymmetry in unilateral deafferentation. The otolith VOR is bilaterally attenuated in bilateral vestibulopathy, and loses target distance dependence in cerebellar degeneration.

Ocular motor disorders

Our detailed understanding of the physiology and anatomy of the ocular motor system allows an accurate differential diagnosis of pathological eye movement patterns. This review covers important clinical studies and studies in basic research relevant for the neurologist published during the past year.