The coordination of binocular eye movements: vertical and torsional alignment

Concussed patients with visually induced dizziness exhibit increased ocular torsion and vertical vergence during optokinetic gaze-stabilization

Visually Induced Dizziness (VID) is a common post-concussion sequalae that remains poorly understood and difficult to quantify. The present study aims to identify biomarkers for VID in the form of gaze-stabilizing eye movements. Nine patients with post-commotio VID and nine age-matched healthy controls were recruited by physiotherapists at a local neurorehabilitation centre. Torsional and vergence eye movements were recorded while participants viewed a series of optokinetic rotations where the central- and peripheral regions moved coherently, incoherently, or semi-randomly. Results showed that vergence and torsional velocities were increased in VID patients, reflecting increased oculomotor gain to visual motion, and that responses correlated with symptom severity. Coherent stimulation produced fastest torsional slow-phases across all participants; when faced with confliction directional information, eye movements tended to follow the direction of the central visual field, albeit at slower velocities than during coherent motion, meaning that while torsion was sensitive to visual content of the entire visual field it expressed directional preference to the central stimulation. In conclusion, post-commotio VID was associated with faster slow-phases during optokinetic gaze-stabilization, with both vergence and torsion being correlated to symptom intensity. As torsional tracking remains inaccessible using commercial eye-trackers, vertical vergence may prove particularly accessible for clinical utility.

A Geometric Theory Integrating Human Binocular Vision With Eye Movement

A theory of the binocular system with asymmetric eyes (AEs) is developed in the framework of bicentric perspective projections. The AE accounts for the eyeball's global asymmetry produced by the foveal displacement from the posterior pole, the main source of the eye's optical aberrations, and the crystalline lens' tilt countering some of these aberrations. In this theory, the horopter curves, which specify retinal correspondence of binocular single vision, are conic sections resembling empirical horopters. This advances the classic model of empirical horopters as conic sections introduced in an ad hoc way by Ogle in 1932. In contrast to Ogle's theory, here, anatomically supported horopteric conics vary with the AEs' position in the visual plane of bifoveal fixations and their transformations are visualized in a computer simulation. Integrating horopteric conics with eye movements can help design algorithms for maintaining a stable perceptual world from visual information captured by a mobile robot's camera head. Further, this paper proposes a neurophysiologically meaningful definition for the eyes' primary position, a concept which has remained elusive despite its theoretical importance to oculomotor research. Finally, because the horopteric conic's shape is dependent on the AE's parameters, this theory allows for changes in retinal correspondence, which is usually considered preformed and stable.

Optokinetic stimulation induces vertical vergence, possibly through a non-visual pathway

Vertical vergence is generally associated with one of three mechanisms: vestibular activation during a head tilt, induced by vertical visual disparity, or as a by-product of ocular torsion. However, vertical vergence can also be induced by seemingly unrelated visual conditions, such as optokinetic rotations. This study aims to investigate the effect of vision on this latter form of vertical vergence. Eight subjects (4m/4f) viewed a visual scene in head erect position in two different viewing conditions (monocular and binocular). The scene, containing white lines angled at 45° against a black background, was projected at an eye-screen distance of 2 m, and rotated 28° at an acceleration of 56°/s2. Eye movements were recorded using a Chronos Eye-Tracker, and eye occlusions were carried out by placing an infrared-translucent cover in front of the left eye during monocular viewing. Results revealed vergence amplitudes during binocular viewing to be significantly lower than those seen for monocular conditions (p = 0.003), while torsion remained unaffected. This indicates that vertical vergence to optokinetic stimulation, though visually induced, is visually suppressed during binocular viewing. Considering that vertical vergence is generally viewed as a vestibular signal, the findings may reflect a visually induced activation of a vestibular pathway.

The Non-surgical Management of Ophthalmoplegia

Ophthalmoplegia can result from damage or dysfunction of the supranuclear eye movement pathways, the brainstem internuclear pathways, or the ocular motor nerves. Diplopia and impaired eye movements are commonly associated symptoms. The goal of non-surgical treatment is usually management of symptoms, and the primary tool used is prism. Results of recent studies in the neurorehabilitation literature have suggested that eye muscle exercise may do more than compensate for symptoms of ophthalmoplegia. They may accelerate recovery for those conditions in which recuperation is possible, and restore ocular mobility in chronic and degenerative conditions. A large, randomized, blind study is needed to investigate the efficacy of eye exercises in ophthalmoplegia.

A dataset of stereoscopic images and ground-truth disparity mimicking human fixations in peripersonal space

Binocular stereopsis is the ability of a visual system, belonging to a live being or a machine, to interpret the different visual information deriving from two eyes/cameras for depth perception. From this perspective, the ground-truth information about three-dimensional visual space, which is hardly available, is an ideal tool both for evaluating human performance and for benchmarking machine vision algorithms. In the present work, we implemented a rendering methodology in which the camera pose mimics realistic eye pose for a fixating observer, thus including convergent eye geometry and cyclotorsion. The virtual environment we developed relies on highly accurate 3D virtual models, and its full controllability allows us to obtain the stereoscopic pairs together with the ground-truth depth and camera pose information. We thus created a stereoscopic dataset: GENUA PESTO-GENoa hUman Active fixation database: PEripersonal space STereoscopic images and grOund truth disparity. The dataset aims to provide a unified framework useful for a number of problems relevant to human and computer vision, from scene exploration and eye movement studies to 3D scene reconstruction.

A rapid quantification of binocular misalignment without recording eye movements: Vertical and torsional alignment nulling

BACKGROUND

Small, innate asymmetries between the left and right otolith organs can cause ocular misalignment with symptoms that include double vision and motion sickness. Additionally, ocular misalignment affects nearly 5% of the US population. We have developed a portable, non-invasive technology that uses subjective perception of binocular visual signals to estimate relative binocular alignment.

NEW METHOD AND RESULTS

The Vertical Alignment Nulling (VAN) and Torsional Alignment Nulling (TAN) tests ask subjects to view one red and one blue line on a tablet computer while looking through color-matched red and blue filters so that each eye sees only one of the lines. Subjects align the red and blue lines, which are initially vertically offset from one another during VAN or rotated relative to one another during TAN, until they perceive a single continuous line. Ocular misalignments are inferred from actual offsets in the final line positions. During testing, all binocular visual cues are eliminated by employing active-matrix organic light-emitting diode (AMOLED) technology and testing in darkness. VAN and TAN can accurately account for visual offsets induced by prisms, and test-retest reliability is excellent, with resolution better than many current standard clinical tests.

COMPARISON WITH EXISTING METHOD(S)

VAN and TAN tests are similar to the clinical Lancaster red-green test. However, VAN and TAN employ inexpensive, hand-held hardware that can be self-administered with results that are quickly quantifiable.

CONCLUSIONS

VAN and TAN provide simple, sensitive, and quantitative measures of binocular positioning alignment that may be useful for detecting subtle abnormalities in ocular positioning.

Tilted frames of reference have similar effects on the perception of gravitational vertical and the planning of vertical saccadic eye movements

We investigated the effects of a tilted reference frame (i.e., allocentric visual context) on the perception of the gravitational vertical and saccadic eye movements along a planned egocentric vertical path. Participants (n = 5) in a darkened room fixated a point in the center of a circle on an LCD display and decided which of two sequentially presented dots was closer to the unmarked ‘6 o’clock’ position on that circle (i.e., straight down toward their feet). The slope of their perceptual psychometric functions showed that participants were able to locate which dot was nearer the vertical with a precision of 1°–2°. For three of the participants, a square frame centered at fixation and tilted (in the roll direction) 5.6° from the vertical caused a strong perceptual bias, manifest as a shift in the psychometric function, in the direction of the traditional ‘rod-and-frame’ effect, without affecting precision. The other two participants showed negligible or no equivalent biases. The same subjects participated in the saccade version of the task, in which they were instructed to shift their gaze to the 6 o’clock position as soon as the central fixation point disappeared. The participants who showed perceptual biases showed biases of similar magnitude in their saccadic endpoints, with a strong correlation between perceptual and saccadic biases across all subjects. Tilting of the head 5.6° reduced both perceptual and saccadic biases in all but one observer, who developed a strong saccadic bias. Otherwise, the overall pattern and significant correlations between results remained the same. We conclude that our observers’ saccades-to-vertical were dominated by perceptual input, which outweighed any gravitational or head-centered input.

Beyond the cover test: the motor half of the sensorimotor exam

The cover tests fulfill both the "sensory" and the "motor" requirements of the sensorimotor exam because they not only detect and quantify strabismus but also evaluate control. Control is achieved through various types of vergence, but vergence is not the only type of eye movement that is integral to fusion. The function of all eye movements is to position and maintain similar images on corresponding areas of the retinae in order to sustain binocular vision. Therefore, a complete motor exam should include evaluation of all eye movement systems. Such an exam can be performed without expensive and complicated equipment, without supplementary education, without significantly extending exam time, and within the confines of an average exam lane, because much of the exam is concealed within the standard sensorimotor evaluation. The term "sensorimotor exam" underscores the fact that sensory and motor fusion, and their respective cortical pathways, are inextricably linked, and function simultaneously and cooperatively to achieve normal visual experience.

Short-term adaptation of accommodation, accommodative vergence and disparity vergence facility

Previous studies have found that subjects can increase the velocity of accommodation using visual exercises such as pencil push ups, flippers, Brock strings and the like and myriad papers have shown improvement in accommodation facility (speed) and sufficiency (amplitude) using subjective tests following vision training but few have objectively measured accommodation before and after training in either normal subjects or in patients diagnosed with accommodative infacility (abnormally slow dynamics). Accommodation is driven either directly by blur or indirectly by way of neural crosslinks from the vergence system. Until now, no study has objectively measured both accommodation and accommodative-vergence before and after vision training and the role vergence might play in modifying the speed of accommodation. In the present study, accommodation and accommodative-vergence were measured with a Purkinje Eye Tracker/optometer before and after normal subjects trained in a flipper-like task in which the stimulus stepped between 0 and 2.5 diopters and back for over 200 cycles. Most subjects increased their speed of accommodation as well as their speed of accommodative vergence. Accommodative vergence led the accommodation response by approximately 77 ms before training and 100 ms after training and the vergence lead was most prominent in subjects with high accommodation and vergence velocities and the vergence leads tended to increase in conjunction with increases in accommodation velocity. We surmise that volitional vergence may help increase accommodation velocity by way of vergence-accommodation cross links.

Perception of Relative Depth Interval: Systematic Biases in Perceived Depth

Given an estimate of the binocular disparity between a pair of points and an estimate of the viewing distance, or knowledge of eye position, it should be possible to obtain an estimate of their depth separation. Here we show that, when points are arranged in different vertical geometric configurations across two intervals, many observers find this task difficult. Those who can do the task tend to perceive the depth interval in one configuration as very different from depth in the other configuration. We explore two plausible explanations for this effect. The first is the tilt of the empirical vertical horopter: Points perceived along an apparently vertical line correspond to a physical line of points tilted backwards in space. Second, the eyes can rotate in response to a particular stimulus. Without compensation for this rotation, biases in depth perception would result. We measured cyclovergence indirectly, using a standard psychophysical task, while observers viewed our depth configuration. Biases predicted from error due either to cyclovergence or to the tilted vertical horopter were not consistent with the depth configuration results. Our data suggest that, even for the simplest scenes, we do not have ready access to metric depth from binocular disparity.

Binocular vision

This essay reviews major developments - empirical and theoretical - in the field of binocular vision during the last 25years. We limit our survey primarily to work on human stereopsis, binocular rivalry and binocular contrast summation, with discussion where relevant of single-unit neurophysiology and human brain imaging. We identify several key controversies that have stimulated important work on these problems. In the case of stereopsis those controversies include position vs. phase encoding of disparity, dependence of disparity limits on spatial scale, role of occlusion in binocular depth and surface perception, and motion in 3D. In the case of binocular rivalry, controversies include eye vs. stimulus rivalry, role of "top-down" influences on rivalry dynamics, and the interaction of binocular rivalry and stereopsis. Concerning binocular contrast summation, the essay focuses on two representative models that highlight the evolving complexity in this field of study.

Neuromuscular plasticity and rehabilitation of the ocular near response

The near response is composed of cross-coupled interactions between convergence and other distance-related oculomotor responses including accommodation, vertical vergence, and cyclovergence. The cross-coupling interactions are analogous to the body postural reflexes that maintain balance. Near-response couplings guide involuntary motor responses during voluntary shifts of distance and direction of gaze without feedback from defocus or retinal-image disparity. They optimize the disparity stimulus for stereoscopic depth perception and can be modified by optically induced sensory demands placed on binocular vision. In natural viewing conditions, the near response is determined by passive orbital mechanics and active-adaptable tonic components. For example, the normal coupling of vertical vergence with convergence in tertiary gaze is partly a byproduct of passive orbital mechanics. Both, adapted changes of vertical vergence in response to anisophoria, produced by unequal ocular magnification (aniseikonia), and adapted changes in the orientation of Listing's plane in response to torsional disparities can be achieved by a combination of passive orbital mechanics and neural adjustments for the control of the vertical vergence and cyclovergence. Adaptive adjustments are coupled with gaze direction, convergence angle, and head tilt. Several adaptation studies suggest that it is possible to achieve non-linear changes in the coupling of both vertical vergence and cyclovergence with gaze direction. This coupling can be achieved with changes in neural control signals of ocular elevator muscles that are cross-coupled with both convergence and direction of tertiary gaze. These linear and non-linear coupling interactions can be adapted to compensate for (1) anisophoria induced by spectacle corrections for anisometropia, (2) accommodative esotropia, (3) convergence excess and insufficiency, and (4) non-concomitant deviations with ocular torticollis associated with trochlear palsy. The adaptable near-response couplings form the basis of an area of orthoptics that optimizes visual performance by facilitating our natural ability to calibrate neural pathways underlying binocular postural reflexes.

Acute superior oblique palsy in the monkey: effects of viewing conditions on ocular alignment and modelling of the ocular motor plant

We investigated the immediate and long-term changes in static eye alignment with acute superior oblique palsy (SOP) in the monkey. When the paretic eye was patched immediately after the lesion for 6-9 days, vertical alignment slowly improved. When the patch was removed and binocular viewing was allowed, alignment slowly worsened. In contrast when a monkey was not patched immediately after the lesion vertical alignment did not improve. We also show that a model of the eye plant can reproduce the observed acute deficit induced by SOP, but only by abandoning Robinson's symmetric simplification of the reciprocal innervation relationship within pairs of agonist-antagonist muscles. The model also demonstrated that physiologic variability in orbital geometry can have a large impact on SOP deficits.

Variation of binocular-vertical fusion amplitude with convergence

PURPOSE

The maximum binocular vertical disparity that can be fused with disparity vergence (vertical-fusion amplitude or VFA), varies with convergence angle. VFA is larger for convergence responses to near than to far viewing distances; however, the clinical norms for changes in VFA with convergence have not been established. VFA at several convergence angles was measured to obtain a quantitative description of the changes in VFA with convergence.

METHODS

Fifty-six adults took part in the study. Horizontal and vertical disparity stimuli were presented on a computer monitor by using the red-green anaglyphic technique. Stimulus to convergence was altered either by changing horizontal disparity on the computer monitor (experiment I: nine horizontal disparities: 1.2-22.5 PD [Delta]) or by changing the binocular viewing distance (experiment II: five viewing distances: 25-300 cm). Convergence was held constant during an experimental session, while vertical disparity was incremented in steps of 0.05 Delta after a subjective report of fusion, until the subject reported diplopia. The maximum vertical disparity that could be fused was defined as the VFA.

RESULTS

VFA increased linearly over the range of convergence stimuli (y = 0.10x + 1.62) and intersubject variability of VFA increased marginally with the amount of convergence. Linear regression equations with similar slopes and y-intercepts were observed in experiments I and II.

CONCLUSIONS

The results of the experiments provide a quantitative description of a linear relationship between VFA and convergence. The linear regression equation could be used in a clinical setting to establish norms and to screen for vertical vergence abnormalities.