Erik Viirre

The human horizontal vestibulo-ocular reflex during combined linear and angular acceleration.

Abstractu2002We employed binocular magnetic search coils to study the vestibulo-ocular reflex (VOR) and visually enhanced vestibulo-ocular reflex (VVOR) of 15 human subjects undergoing passive, whole-body rotations about a vertical (yaw) axis delivered as a series of pseudorandom transients and sinusoidal oscillations at frequencies from 0.8 to 2.0 Hz. Rotations were about a series of five axes ranging from 20 cm posterior to the eyes to 10 cm anterior to the eyes. Subjects were asked to regard visible or remembered targets 10 cm, 25 cm, and 600 cm distant from the right eye. During sinusoidal rotations, the gain and phase of the VOR and VVOR were found to be highly dependent on target distance and eccentricity of the rotational axis. For axes midway between or anterior to the eyes, sinusoidal gain decreased progressively with increasing target proximity, while, for axes posterior to the otolith organs, gain increased progressively with target proximity. These effects were large and highly significant. When targets were remote, rotational axis eccentricity nevertheless had a small but significant effect on sinusoidal gain. For sinusoidal rotational axes midway between or anterior to the eyes, a phase lead was present that increased with rotational frequency, while for axes posterior to the otolith organs phase lag increased with rotational frequency. Transient trials were analyzed during the first 25 ms and from 25 to 80 ms after the onset of the head rotation. During the initial 25 ms of transient head rotations, VOR and VVOR gains were not significantly influenced by rotational eccentricity or target distance. Later in the transient responses, 25–80 ms from movement onset, both target distance and eccentricity significantly influenced gain in a manner similar to the behavior during sinusoidal rotation. Vergence angle generally remained near the theoretically ideal value during illuminated test conditions (VVOR), while in darkness vergence often varied modestly from the ideal value. Regression analysis of instantaneous VOR gain as a function of vergence demonstrated only a weak correlation, indicating that instantaneous gain is not likely to be directly dependent on vergence. A model was proposed in which linear acceleration as sensed by the otoliths is scaled by target distance and summed with angular acceleration as sensed by the semicircular canals to control eye movements. The model was fit to the sinusoidal VOR data collected in darkness and was found to describe the major trends observed in the data. The results of the model suggest that a linear interaction exists between the canal and otolithic inputs to the VOR.

Migraine as a Cause of Sudden Hearing Loss

Sudden heating loss is common, but unexplained in many cases. Although usually attributed to a viral infection of the inner ear in mort patients, the abrupt onset of the hearing loss in many patients argues against a viral etiology. We present 13 cases of unexplained sudden healing loss who meet the diagnostic criteria for migraine. All had the sudden onset of hearing loss and other neurologic phenomena that could be attributed to vasospasm, including vertigo, amaurosis fugax, hemiplegia, facial pain, chest pain, and visual aura. We suggest that vasospasm of the cochlear vasculature was the cause of the sudden hearing loss in these patients. A personal and family history of migraine should be sought in patient; with sudden heating loss and when found, a trial of antispas modic agents should be considered.

Visual-vestibular interaction during standing, walking, and running.

In artificial laboratory situations where subjects undergo repetitive self-generated or externally imposed head rotations, visual-vestibular interaction during the wearing of telescopic spectacles can markedly augment gain of the vestibulo-ocular reflex (VOR). The present study was conducted to determine whether the wearing of these aids for the visually impaired is associated with similar visual-vestibular interaction during more natural activities. Angular eye and head movements of unrestrained normal volunteers were measured using magnetic search coils. In some subjects, head translations and rotations were also monitored by a flux gate magnetometer array. Measurements were performed of the VOR in darkness, and of the visually enhanced VOR (VVOR) in lit conditions, during three natural activities: 1) standing quietly; 2) walking in place; 3) running in place. These data were compared with similar measurements during repetitive voluntary head oscillations at 0.8 Hz in pitch or yaw. During VVOR, subjects viewed a target placed 6 to 10 m away and remembered this target during VOR trials in darkness. To assess the effects of altering visual-vestibular interactions, VVOR testing during normal vision was augmented by wearing of binocular telescopic spectacles of 2X, 4X, and 6X powers. Dorsoventral and mediolateral head translations were consistently phase-locked with pitch and yaw head rotations, respectively, such that head translation at least partially compensated for rotational disturbances of gaze. Angular velocity of the head was greater during walking than during standing, and was greater still during running, with a greater increase in each case for pitch as compared with yaw. Eye movements were phase compensatory for head movements. VOR gain (eye velocity divided by head velocity) was near 1.0 in both pitch and yaw during standing and during actively generated head rotation. During walking and running there was a significant decrease in angular VOR gain in pitch to approximately 0.75 (P < 0.0005). During ambulatory activities, normal and magnified vision were associated with VVOR gain enhancement in pitch and yaw that was statistically significant, but substantially less than was telescope magnification and markedly lower than was the corresponding VVOR gain measured during active head rotation. Measurements of unmagnified VOR and VVOR during walking and running showed that gain was lower than the ideal value of 1.0. However, since translational head perturbations during these activities partially offset the visual effects of angular disturbances, lower gains may nevertheless be associated with retinal image stability at typical indoor target distances. In contrast with performance during repetitive, uniplanar motion, vision has very limited influence on VOR gain during natural activities.

The human vertical vestibulo-ocular reflex during combined linear and angular acceleration with near-target fixation

The purpose of this study was to examine the effect of fixation target distance on the human vestibuloocular reflex (VOR) during eccentric rotation in pitch. Such rotation induces both angular and linear acceleration. Eight normal subjects viewed earth-fixed targets that were either remote or near to the eyes during wholebody rotation about an earth-horizontal axis that was either oculocentric or 15 cm posterior (eccentric) to the eyes. Eye and head movements were recorded using magnetic search coils. Using a servomotor-driven chair, passive whole-body rotations were delivered as trains of single-frequency sinusoids at frequencies from 0.8 to 2.0 Hz and as pseudorandom impulses of acceleration. In the light, the visually enhanced VOR (VVOR) was recorded while subjects were asked to fixate targets at one of several distances. In darkness, subjects were asked to remember targets that had been viewed immediately prior to the rotation. In order to eliminate slip of the retinal image of a near target when the axis of rotation of the head is posterior to the eyes, the ideal gain (compensatory eye velocity divided by head velocity) of the VVOR and VOR must exceed 1.0. Both the VOR and VVOR were found to have significantly enhanced gains during sinusoidal and pseudorandom impulses of rotation (P<0.05). Enhancement of VVOR gain was greatest at low frequencies of head rotation and decreased with increasing frequency. However, enhanced VOR gain only slightly exceeded 1.0, and VVOR gain enhancement was significantly lower than the expected ideal values for the stimulus conditions employed (P<0.05). During oculocentric rotations with near targets, both the VOR and VVOR tended to exhibit small phase leads that increased with rotational frequency. In contrast, during eccentric rotations with near targets, there were small phase lags that increased with frequency. Visual tracking contributes during ocular compensatory responses to sustained head rotation, although the latency of visual tracking reflexes exceeds 100 ms. In order to study initial vestibular responses prior to modification by visual tracking, we presented impulses of head acceleration in pseudorandom sequence of initial positions and directions, and evaluated the ocular response in the epoch from 25 to 80 ms after movement onset. As with sinusoidal rotations, pseudorandom eccentric head rotation in the presence of a near, earth-fixed target was associated with enhancement of VVOR and VOR gains in the interval from 25 to 80 ms from movement onset. Despite the inability of visual tracking to contribute to these responses, VVOR gain significantly exceeded VOR gain for pseudorandom accelerations. This gain enhancement indicates that target distance and linear motion of the head are considered by the human ocular motor system in adjustment of performance of the early VOR, prior to a contribution by visual following reflexes. Vergence was appropriate to target distance during all VVOR rotations, but varied during VOR rotations with remembered targets. For the 3-m target distance, vergence during the VOR was stable over each entire trial but slightly exceeded the ideal value. For the 0.1-m near target, instantaneous vergence during the VOR typically declined gradually in a manner not corresponding to the time course of instantaneous VOR gain change; mean vergence over entire trials ranged from 60 to 90% of ideal, corresponding to target distances for which ideal gain would be much higher than actually observed. These findings suggest a dissociation between vergence and VOR gain during eccentric rotation with near targets in the frequency range from 0.8 to 2.0 Hz.

The Virtual Retinal Display: A new Display Technology using Scanned Laser Light

The Virtual Retinal Display (VRD) is a new display technology that scans modulated low energy laser light directly onto the viewers retina to create a perception of a virtual image. This approach provides an unprecedented way to stream photons to the receptors of the eye, affording higher resolution, increased luminance, and potentially a wider field-of-view than previously possible in head coupled displays. The VRD uses video signals from a graphics board or a video camera to modulate low power coherent light from red, green and blue photon sources such as gas lasers, laser diodes and/or light emitting diodes. The modulated light is then combined and piped through a single mode optical fiber. A mechanical resonant scanner and galvanometer mirror then scan the photon stream from the fiber in two dimensions through reflective elements and semitransparent combiner such that a raster of light is imaged on the retina. The pixels produced on the retina have no persistence, yet they create the perception of a brilliant full color, and flicker-free virtual image. Developmental models of the VRD have been shown to produce VGA and SVGA image quality. This paper describes the VRD technology, the advantages that it provides, and areas of human factors research ensuing from scanning light directly onto the retina. Future applications of the VRD are discussed along with new research findings regarding the use of the VRD for people with low vision

Blast-related ear injuries among U.S. military personnel

Blast-related ear injuries are a concern during deployment because they can compromise a servicemembers situational awareness and adversely affect operational readiness. The objectives of this study were to describe blast-related ear injuries during Operation Iraqi Freedom, identify the effect of hearing protection worn at the point of injury, and explore hearing loss and tinnitus outcomes within one year after injury. The Expeditionary Medical Encounter Database was used to identify military personnel who survived blast-related injury, and it was linked with outpatient medical databases to obtain diagnoses of hearing loss and tinnitus. The prevalence of ear injuries was 30.7% (1,223 of 3,981). The most common ear injury diagnoses were inner or middle ear injury involving tinnitus and tympanic membrane (TM) rupture. Hearing protection reduced the odds of ear injury involving tinnitus. Personnel with TM rupture had higher odds of hearing loss (odds ratio [OR] = 6.65, 95% confidence interval [CI] = 5.04-8.78) and tinnitus outcomes (OR = 4.34, 95% CI = 3.12-6.04) than those without TM rupture. Ear injuries and hearing impairment are frequent consequences of blast exposure during combat deployment. Hearing protection is warranted for all servicemembers at risk of blast exposure.

Laser safety analysis of a retinal scanning display system

The Virtual Retinal Display (VRD) is a visual display that scans modulated laser light on to the retina of the viewers eye to create an image. Maximum permissible exposures (MPE) have been calculated for the VRD in both normal viewing and possible failure modes. The MPE power levels are compared to the measured power that enters the eye while viewing images with the VRD. The power levels indicate that the VRD is safe in both normal operating mode and in failure modes.

Adaptation of the VOR in patients with low VOR gains.

Six subjects with histories of vertigo and with vestibulo-ocular reflex (VOR) gains less than 0.5 were tested in an adaptation protocol. After initial VOR testing in the dark, the subjects had a computer-driven visual display system placed on their heads. The system had the capability for variation of visual image magnification. The magnification was set to be 5% greater than the subjects average VOR gain. Subjects then performed active head movements as they carried out a visual searching task looking for objects in a panoramic scene. After 6 minutes with each image, the magnification was increased by 3 to 5%. The process was repeated for a total of 5 images, for a total increase in magnification of approximately 20% over 30 minutes. The VOR gain was measured again. In 17 of 18 conditions tested, the VOR gain increased. The average increase was 16%. Significant increases in VOR gain occurred at 0.32 and 0.64 Hz. The VOR gain increase in these patients occurred in a visual environment that lowered VOR gain in normal subjects. These results suggest that the VOR has an adaptation mechanism tuned to correct for small changes in required gain. Further research is necessary to determine if this method can result in persistent VOR gain improvements and reduction in symptoms and disability in patients with vestibular disorders.

Vision with a scanning laser display: comparison of flicker sensitivity to a CRT

Abstract Laser scanning displays allow for large depth-of-focus, reduced optical aberrations, and high luminance. However, scanned laser light images have no phosphor persistence. Psychophysical tests with a 3-color laser display showed that flicker thresholds were nearly equal with a laser display relative to the CRT when both were matched in luminance, resolution, frame rate, and similar chromaticity. The threshold ratio was 1.14 (range 0.7–1.6). Differences in flicker sensitivity between displays were significantly related to the absolute energy at the cornea irrespective of the subjective luminance match. Flicker sensitivity was not enhanced by a laser scanning display without image persistence.

Demonstration of the Virtual Retinal Display: A New Display Technology Using Scanned Laser Light

The Virtual Retinal Display (VRD) is a new display technology that scans modulated low energy laser light directly onto the viewers retina to create a perception of a virtual image. This approach provides an unprecedented way to stream photons to the receptors of the eye, affording higher resolution, increased luminance, and potentially a wider field-of-view than previously possible in head coupled displays. The VRD uses video signals from a graphics board or a video camera to modulate low power coherent light from a red laser diode. A mechanical resonant scanner and galvanometer mirror then scan the photon stream from the laser diode in two dimensions through reflective elements and semitransparent combiner such that a raster of light is imaged on the retina. The pixels produced on the retina have no persistence, yet they create the perception of a brilliant full color, and flicker-free virtual image. Developmental models of the VRD have been shown to produce VGA and SVGA image quality. This demonstration exhibits the portable monochrome VRD