Vergence Movements Research Articles

The frequency of horizontal saccades in near and far symmetrical disparity vergence

In a natural environment, saccade and vergence eye movements shift gaze in different directions and distances. In a laboratory setting, targets can be positioned precisely to elicit symmetrical vergence movements; however, saccades occur during the vergence movement even though the stimulus should not stimulate a saccadic response. These saccades may facilitate the response when the kinematics of the vergence component are modest as indicated by reduced velocities. Hence, the purpose of this study is to assess whether the frequency of saccades within vergence responses are correlated with vergence peak velocity. Ten subjects with normal binocular vision participated in this study. Eye movements were quantified using a limbus tracking system. Stimuli included 4° symmetrical convergence and divergence steps with an initial vergence angle at far (2° and 6°, respectively) and near (12° and 16°, respectively) which are known to evoke different vergence peak velocities. A saccade detecting algorithm was utilized to compute the percentage of saccades present within all vergence responses. A repeated measures ANOVA confirmed with a post hoc Bonferroni test demonstrated that convergence steps at near were slower than convergence steps at far, whereas divergence steps at far were slower than divergence steps at near in all subjects (p<0.02). When the vergence peak velocity was slow, a greater number of saccades was observed. The average vergence peak velocities were inversely correlated to the number of saccades observed within the transient portion defined as after the latency to 400ms of the movement (r=−0.41; p=0.008), between 400ms and 1s of the response (r=−0.35; p=0.03) and within the steady-state period occurring between 1s and 3s of the response (r=−0.44; p=0.005). Peak velocity of vergence is dependent on the stimulus initial vergence angle. An increased prevalence of saccades was observed in vergence responses with reduced peak velocity, compared to responses with greater peak velocity. Prior research supports that saccades increase the peak velocity of vergence during combined vergence and saccadic tasks. This may in part explain the increased presence of saccades within vergence responses with reduced peak velocities. The recruitment of saccades may be utilized because of the longer period of diplopia resulting from slower vergence movements.

Visual Discomfort with Stereo 3D Displays when the Head is Not Upright.

Properly constructed stereoscopic images are aligned vertically on the display screen, so on-screen binocular disparities are strictly horizontal. If the viewer's inter-ocular axis is also horizontal, he/she makes horizontal vergence eye movements to fuse the stereoscopic image. However, if the viewer's head is rolled to the side, the on-screen disparities now have horizontal and vertical components at the eyes. Thus, the viewer must make horizontal and vertical vergence movements to binocularly fuse the two images. Vertical vergence movements occur naturally, but they are usually quite small. Much larger movements are required when viewing stereoscopic images with the head rotated to the side. We asked whether the vertical vergence eye movements required to fuse stereoscopic images when the head is rolled cause visual discomfort. We also asked whether the ability to see stereoscopic depth is compromised with head roll. To answer these questions, we conducted behavioral experiments in which we simulated head roll by rotating the stereo display clockwise or counter-clockwise while the viewer's head remained upright relative to gravity. While viewing the stimulus, subjects performed a psychophysical task. Visual discomfort increased significantly with the amount of stimulus roll and with the magnitude of on-screen horizontal disparity. The ability to perceive stereoscopic depth also declined with increasing roll and on-screen disparity. The magnitude of both effects was proportional to the magnitude of the induced vertical disparity. We conclude that head roll is a significant cause of viewer discomfort and that it also adversely affects the perception of depth from stereoscopic displays.

A Novel Method to Eliminate Blind Areas for Moving Double Cameras System Based on Biomimetic Eyes

Based on the characteristics of binocular vergence eye movements, A new type of method to reduce blind areas caused by double cameras in motion platform was put forward, and a model for control binocular vergence eye movements was established according to neural pathways of human binocular oculomotor system. With the model, it is guaranteed that target areas can remain in the public vision field of the double cameras all the time during the double cameras platform moving. This feature is very important for the moving double cameras system, since 3D reconstruction needs the double cameras to have an overlapping field of vision. To evaluate the effectiveness of the proposed method, some experiments on the model were conducted . The simulation and experimental results show that the model can realize adaptive binocular vergence movements. Compared with the traditional methods, this new one can eliminate blind areas for moving double cameras system.

A dual visual-local feedback model of the vergence eye movement system

Pure vergence movements are the eye movements that we make when we change our binocular fixation between targets differing in distance but not in direction relative to the head. Pure vergence is slow and controlled by visual feedback. Saccades are the rapid eye movements that we make between targets differing in direction. Saccades are extremely fast and controlled by a local, non-visual feedback loop. Usually, we change our fixation between targets that differ in both distance and direction. Then, vergence eye movements are combined with saccades. A number of models have been proposed to explain the dynamics of saccade-related vergence movements. The models have in common that visual input is ignored for the duration of the responses. This type of control is realistic for saccades but not for vergence. Here, I present computations performed to investigate if a model using dual visual and local feedback can replace the current models. Simulations and stability analysis lead to a model that computes an estimate of target vergence instead of retinal disparity and uses this signal as the main drive. Further analysis shows that the model describes the dynamics of pure vergence responses over the full physiological range, saccade-related vergence movements, and vergence adaptation. The structure of the model leads to new hypotheses about the control of vergence.

Do brainstem omnipause neurons terminate saccades?

Saccade-generating burst neurons (BN) are inhibited by omnipause neurons (OPN), except during saccades. OPN activity pauses before saccade onset and resumes at the saccade end. Microstimulation of OPN stops saccades in mid-flight, which shows that OPN can end saccades. However, OPN pause duration does not correlate well with saccade duration, and saccades are normometric after OPN lesions. We tested whether OPN were responsible for stopping saccades both in late-onset Tay-Sachs, which causes premature saccadic termination, and in individuals with cerebellar hypermetria. We studied gaze shifts between two targets at different distances aligned on one eye, which consist of a disjunctive saccade followed by vergence. High-frequency conjugate oscillations during the vergence movements that followed saccades were present in all subjects studied, indicating OPN silence. Thus, mechanisms other than OPN discharge (e.g., cerebellar caudal fastigial nucleus-promoting inhibitory BN discharge) must contribute to saccade termination.

Saccades and Vergence Performance in a Population of Children with Vertigo and Clinically Assessed Abnormal Vergence Capabilities

PurposeEarly studies reported some abnormalities in saccade and vergence eye movements in children with vertigo and vergence deficiencies. The purpose of this study was to further examine saccade and vergence performance in a population of 44 children (mean age: 12.3±1.6 years) with vertigo symptoms and with different levels of vergence abnormalities, as assessed by static orthoptic examination (near point of convergence, prism bar and cover-uncover test).MethodsThree groups were identified on the basis of the orthoptic tests: group 1 (n = 13) with vergence spasms and mildly perturbed orthoptic scores, group 2 (n = 14) with moderately perturbed orthoptic scores, and group 3 (n = 17) with severely perturbed orthoptic scores. Data were compared to those recorded from 28 healthy children of similar ages. Latency, accuracy and peak velocity of saccades and vergence movements were measured in two different conditions: gap (fixation offset 200 ms prior to target onset) and simultaneous paradigms. Binocular horizontal movements were recorded by a photoelectric device.ResultsGroup 2 of children with vergence abnormalities showed significantly longer latency than normal children in several types of eye movements recorded. For all three groups of children with vergence abnormalities, the gain was poor, particularly for vergence movement. The peak velocity values did not differ between the different groups of children examined.InterpretationEye movement measures together with static orthoptic evaluation allowed us to better identify children with vergence abnormalities based on their slow initiation of eye movements. Overall, these findings support the hypothesis of a central deficit in the programming and triggering of saccades and vergence in these children.

A continuum of myofibers in adult rabbit extraocular muscle: force, shortening velocity, and patterns of myosin heavy chain colocalization

Extraocular muscle (EOM) myofibers do not fit the traditional fiber typing classifications normally used in noncranial skeletal muscle, in part, due to the complexity of their individual myofibers. With single skinned myofibers isolated from rectus muscles of normal adult rabbits, force and shortening velocity were determined for 220 fibers. Each fiber was examined for myosin heavy chain (MyHC) isoform composition by densitometric analysis of electrophoresis gels. Rectus muscle serial sections were examined for coexpression of eight MyHC isoforms. A continuum was seen in single myofiber shortening velocities as well as force generation, both in absolute force (g) and specific tension (kN/m(2)). Shortening velocity correlated with MyHCIIB, IIA, and I content, the more abundant MyHC isoforms expressed within individual myofibers. Importantly, single fibers with similar or identical shortening velocities expressed significantly different ratios of MyHC isoforms. The vast majority of myofibers in both the orbital and global layers expressed more than one MyHC isoform, with up to six isoforms in single fiber segments. MyHC expression varied significantly and unpredictably along the length of single myofibers. Thus EOM myofibers represent a continuum in their histological and physiological characteristics. This continuum would facilitate fine motor control of eye position, speed, and direction of movement in all positions of gaze and with all types of eye movements-from slow vergence movements to fast saccades. To fully understand how the brain controls eye position and movements, it is critical that this significant EOM myofiber heterogeneity be integrated into hypotheses of oculomotor control.

Changes in vergence dynamics due to repetition

Vergence insufficiency is frequent in many populations including children with vertigo in the absence of measurable vestibular dysfunction. Orthoptic exercises are typically used to improve vergence and the clinical practice suggests that simple repetition of vergence movements improves it. Objective eye movement recordings were used to asses the dynamics and spatial–temporal properties of convergence (8.7°) and divergence (2.7°) along the midline while these movements were repeated 80 times. Eight children, aged on average 13 years and showing vertigo symptoms accompanied with vergence insufficiency, participated. For both, convergence and divergence the velocity increased and the overall duration decreased; the amplitude of the mean transient component of the response changed significantly. These findings are compatible with models of double mode control of vergence eye movements (transient – open-loop vs. sustained – closed loop). Due to simple repetitions a real improvement in the dynamics of vergence along the midline occurred.

The neural control of fast vs. slow vergence eye movements

When looking between targets located in three-dimensional space, information about relative depth is sent from the visual cortex to the motor control centers in the brainstem, which are responsible for generating appropriate motor commands to move the eyes. Surprisingly, how the neurons in the brainstem use the depth information supplied by the visual cortex to precisely aim each eye on a visual target remains highly controversial. This review will consider the results of recent studies that have focused on determining how individual neurons contribute to realigning gaze when we look between objects located at different depths. In particular, the results of new experiments provide compelling evidence that the majority of saccadic neurons dynamically encode the movement of an individual eye, and show that the time-varying discharge of the saccadic neuron population encodes the drive required to account for vergence facilitation during disconjugate saccades. Notably, these results suggest that an additional input (i.e. from a separate vergence subsystem) is not required to shape the activity of motoneurons during disconjugate saccades. Furthermore, whereas motoneurons drive both fast and slow vergence movements, saccadic neurons discharge only during fast vergence movements, emphasizing the existence of distinct premotor pathways for controlling fast vs. slow vergence. Taken together, these recent findings contradict the traditional view that the brain is circuited with independent pathways for conjugate and vergence control, and thus provide an important new insight into how the brain controls three-dimensional gaze shifts.

Motor nucleus activity fails to predict extraocular muscle forces in ocular convergence

For a given eye position, firing rates of abducens neurons (ABNs) generally (Mays et al. 1984), and lateral rectus (LR) motoneurons (MNs) in particular (Gamlin et al. 1989a), are higher in converged gaze than when convergence is relaxed, whereas LR and medial rectus (MR) muscle forces are slightly lower (Miller et al. 2002). Here, we confirm this finding for ABNs, report a similarly paradoxical finding for neurons in the MR subdivision of the oculomotor nucleus (MRNs), and, for the first time, simultaneously confirm the opposing sides of these paradoxes by recording physiological LR and MR forces. Four trained rhesus monkeys with binocular eye coils and custom muscle force transducers on the horizontal recti of one eye fixated near and far targets, making conjugate saccades and symmetric and asymmetric vergence movements of 16-27°. Consistent with earlier findings, we found in 44 ABNs that the slope of the rate-position relationship for symmetric vergence (k(V)) was lower than that for conjugate movement (k(C)) at distance, i.e., mean k(V)/k(C) = 0.50, which implies stronger LR innervation in convergence. We also found in 39 MRNs that mean k(V)/k(C) = 1.53, implying stronger MR innervation in convergence as well. Despite there being stronger innervation in convergence at a given eye position, we found both LR and MR muscle forces to be slightly lower in convergence, -0.40 and -0.20 g, respectively. We conclude that the relationship of ensemble MN activity to total oculorotary muscle force is different in converged gaze than when convergence is relaxed. We conjecture that LRMNs with k(V) < k(C) and MRMNs with k(V) > k(C) innervate muscle fibers that are weak, have mechanical coupling that attenuates their effective oculorotary force, or serve some nonoculorotary, regulatory function.

Vision Therapy in Adults with Convergence Insufficiency: Clinical and Functional Magnetic Resonance Imaging Measures

This research quantified clinical measurements and functional neural changes associated with vision therapy in subjects with convergence insufficiency (CI). Convergence and divergence 4° step responses were compared between 13 control adult subjects with normal binocular vision and four CI adult subjects. All CI subjects participated in 18 h of vision therapy. Clinical parameters quantified throughout the therapy included: nearpoint of convergence, recovery point of convergence, positive fusional vergence at near, near dissociated phoria, and eye movements that were quantified using peak velocity. Neural correlates of the CI subjects were quantified with functional magnetic resonance imaging scans comparing random vs. predictable vergence movements using a block design before and after vision therapy. Images were quantified by measuring the spatial extent of activation and the average correlation within five regions of interests (ROI). The ROIs were the dorsolateral prefrontal cortex, a portion of the frontal lobe, part of the parietal lobe, the cerebellum, and the brain stem. All measurements were repeated 4 months to 1 year post-therapy in three of the CI subjects. Convergence average peak velocities to step stimuli were significantly slower (p = 0.016) in CI subjects compared with controls; however, significant differences in average peak velocities were not observed for divergence step responses (p = 0.30). The investigation of CI subjects participating in vision therapy showed that the nearpoint of convergence, recovery point of convergence, and near dissociated phoria significantly decreased. Furthermore, the positive fusional vergence, average peak velocity from 4° convergence steps, and the amount of functional activity within the frontal areas, cerebellum, and brain stem significantly increased. Several clinical and cortical parameters were significantly correlated. Convergence peak velocity was significantly slower in CI subjects compared with controls, which may result in asthenopic complaints reported by the CI subjects. Vision therapy was associated with and may have evoked clinical and cortical activity changes.

Models of Horizontal Eye Movements, Part II: A 3rd Order Linear Saccade Model

There are five different types of eye movements: saccades, smooth pursuit, vestibular ocular eye movements, optokinetic eye movements, and vergence eye movements. The purpose of this book is focused primarily on mathematical models of the horizontal saccadic eye movement system and the smooth pursuit system, rather than on how visual information is processed. A saccade is a fast eye movement used to acquire a target by placing the image of the target on the fovea. Smooth pursuit is a slow eye movement used to track a target as it moves by keeping the target on the fovea. The vestibular ocular movement is used to keep the eyes on a target during brief head movements. The optokinetic eye movement is a combination of saccadic and slow eye movements that keeps a full-field image stable on the retina during sustained head rotation. Each of these movements is a conjugate eye movement, that is, movements of both eyes together driven by a common neural source. A vergence movement is a non-conjugate eye movement allowing the eyes to track targets as they come closer or farther away. In this book, a 2009 version of a state-of-the-art model is presented for horizontal saccades that is 3rd-order and linear, and controlled by a physiologically based time-optimal neural network. The oculomotor plant and saccade generator are the basic elements of the saccadic system. The control of saccades is initiated by the superior colliculus and terminated by the cerebellar fastigial nucleus, and involves a complex neural circuit in the mid brain. This book is the second part of a book series on models of horizontal eye movements.

An adaptive fuzzy system for the control of the vergence angle on a robotic head

An important issue in realizing robots with stereo vision is the efficient control of the vergence angle. In an active robotic vision system the vergence angle along with the pan and tilt ones determines uniquely the fixation point in the 3D space. The vergence control involves the adjustment of the angle between the two cameras’ axes towards the fixation point and, therefore, it enables the robot to perceive depth and to compute obstacle maps. Vergence movement is directly related to the binocular fusion. Additionally, the decision for convergence or divergence is extracted either by motion affine models or by mathematical ones. In this paper, a new method for extracting the cameras’ movement direction is presented. The movement decision is performed by an adaptive fuzzy control system, the inputs of which are the zero-mean normalized cross correlation (ZNCC) and the depth estimations at each time step. The proposed system is assessed on a 4 d.o.f. robotic head, yet it can be utilized in any active binocular system, since it is computationally inexpensive and it is independent to a priori camera calibration.

Models of Horizontal Eye Movements, Part I: Early Models of Saccades and Smooth Pursuit

There are five different types of eye movements: saccades, smooth pursuit, vestibular ocular eye movements, optokinetic eye movements, and vergence eye movements. The purpose of this book is focused primarily on mathematical models of the horizontal saccadic eye movement system and the smooth pursuit system, rather than on how visual information is processed. A saccade is a fast eye movement used to acquire a target by placing the image of the target on the fovea. Smooth pursuit is a slow eye movement used to track a target as it moves by keeping the target on the fovea. The vestibular ocular movement is used to keep the eyes on a target during brief head movements. The optokinetic eye movement is a combination of saccadic and slow eye movements that keeps a full-field image stable on the retina during sustained head rotation. Each of these movements is a conjugate eye movement, that is, movements of both eyes together driven by a common neural source. A vergence movement is a non-con ugate eye movement allowing the eyes to track targets as they come closer or farther away. In this book, early models of saccades and smooth pursuit are presented. The smooth pursuit system allows tracking of a slow moving target to maintain its position on the fovea. Models of the smooth pursuit have been developed using systems control theory, all involving a negative feedback control system that includes a time delay, controller and plant in the forward loop, with unity feedback. The oculomotor plant and saccade generator are the basic elements of the saccadic system. The oculomotor plant consists of three muscle pairs and the eyeball. A number of oculomotor plant models are described here beginning with the Westheimer model published in 1954, and up through our 1995 model involving a 4$^{th}$ order oculomotor plant model. The work presented here is not an exhaustive coverage of the field, but focused on the interests of the author. In Part II, a state-of-art model of the saccade s stem is presented, including a neural network that controls the system. Table of Contents: Introduction / Smooth Pursuit Models / Early Models of the Horizontal Saccadic Eye Movement System / Velocity and Acceleration Estimation / 1995 Linear Homeomorphic Saccadic Eye Movement Model

2A2-F15 輻輳機構を有するズームカメラステレオビジョンにおけるキャリブレーションとズーム立体視の実現

We report the stereo vision for 'Vergence Zoom Stereo' which is vision system with vergence structure and two zoom cameras. In this vision system, unlike static stereo vision system, stereo parameters change according to vergence movement and zoom control. Rotation axes estimation, zoom centers estimation and changes of intrinsic parameters according to change of zoom factor are necessary for estimation of stereo parameters for stereo vision in vergence zoom stereo. We estimated stereo parameters by using these calibration data, rotation angles available from high resolution encoder and zoom factor. And We realized stereo vision for vergence zoom stereo by these estimated parameters.

Experimental platform for moving double-camera system based on binocular vergence eye movements

A control model of binocular vergence eye movements is presented. The control model can reduce blind areas caused by the double cameras in motion platform. In order to validate the model performance, an experimental platform and its control system based on TMS320LF2407 are designed. The control system has its compacted configuration and high reliability. The simulation and experimental results show that the control system can realize binocular vergence movements. Compared with the conventional moving double cameras system, this new system can considerably reduce blind areas.

A PHYSICAL SYSTEM FOR BINOCULAR VISION THROUGH SACCADE GENERATION AND VERGENCE CONTROL

The human naturally possesses a robust and effective visual system that utilizes saccade and vergence eye movements to explore the visual environment. This article presents a system that provides the functional biological equivalent which consists of a pair of cameras that provide for saccade and vergence eye movements. Included in this article is a detailed description of a simplified equivalent of the saccade generation module (typically from the superior colliculus (SC)) based on a FLANN image segmentation method and a visual cortex (VC) equivalent model based on a hierarchical disparity estimation model for vergence control. These two models cooperate to provide the systematic means for the autonomous exploration of the scene. Combining saccade and vergence movements, we are able to selectively reconstruct the 3D relative positions of objects in the scene and segment the image of the object under vergence.

Effect of Vergence on Human Ocular Following Response (OFR)

The human ocular following response (OFR) is a preattentive, short-latency visual-field-holding mechanism, which is enhanced if the moving stimulus is applied in the wake of a saccade. Since most natural gaze shifts incorporate both saccadic and vergence components, we asked whether the OFR was also enhanced during vergence. Ten subjects viewed vertically moving sine-wave gratings on a video monitor at 45 cm that had a temporal frequency of 16.7 Hz, contrast of 32%, and spatial frequency of 0.17, 0.27, or 0.44 cycle/deg. In Fixation/OFR experiments, subjects fixed on a white central dot on the video monitor, which disappeared at the beginning of each trial, just as the sinusoidal grating started moving up or down. We measured the change in eye position in the 70- to 150-ms open-loop interval following stimulus onset. Group mean downward responses were larger (0.14 degrees) and made at shorter latency (85 ms) than upward responses (0.10 degrees and 96 ms). The direction of eye drifts during control trials, when gratings remained stationary, was unrelated to the prior response. During vergence/OFR experiments, subjects switched their fixation point between the white dot at 45 cm and a red spot at 15 cm, cued by the disappearance of one target and appearance of the other. When horizontal vergence velocity exceeded 15 degrees/s, motion of sinusoidal gratings commenced and elicited the vertical OFR. Subjects showed significantly (P<0.001) larger OFR when the moving stimulus was presented during convergence (group mean increase of 46%) or divergence (group mean increase of 36%) compared with following fixation. Since gaze shifts between near and far are common during natural activities, we postulate that the increase of OFR during vergence movements reflects enhancement of early cortical motion processing, which serves to stabilize the visual field as the eyes approach their new fixation point.

An examination of binocular reading fixations based on sentence corpus data

Binocular eye movements of normal adult readers were examined as they read single sentences. Analyses of horizontal and vertical fixation disparities indicated that the most prevalent type of disparate fixation is crossed (i.e., the left eye is located further to the right than the right eye) while the left eye frequently fixates somewhat above the right eye. The Gaussian distribution of the binocular fixation point peaked 2.6 cm in front of the plane of text, reflecting the prevalence of horizontally crossed fixations. Fixation disparity accumulates during the course of successive saccades and fixations within a line of text, but only to an extent that does not compromise single binocular vision. In reading, the version and vergence system interact in a way that is qualitatively similar to what has been observed in simple nonreading tasks. Finally, results presented here render it unlikely that vergence movements in reading aim at realigning the eyes at a given saccade target word.

Dynamic Characterization of Agonist and Antagonist Oculomotoneurons During Conjugate and Disconjugate Eye Movements

In this report, we provide the first quantitative characterization of the relationship between the spike train dynamics of medial rectus oculomotoneurons (OMNs) and eye movements during conjugate and disconjugate saccades. We show that a simple, first-order model (i.e., containing eye position and velocity terms) provided an adequate model of neural discharges during both on and off-directed conjugate saccades, while a second-order model, which included a decaying slide term, significantly improved the ability to fit neuronal responses by approximately 10% (P<0.05). To understand how the same neurons drove disconjugate eye movements, we evaluated whether sensitivities estimated during conjugate saccades could be used to predict responses during disconjugate saccades. For the majority of neurons (68%), a conjugate-based model failed, and instead neurons preferentially encoded the position and velocity of the ipsilateral eye. Similar to our previous results with abducens motoneurons, we also found that position and velocity sensitivities of OMNs decreased with increasing velocity, and the simulated population drive of OMNs during disconjugate saccades was less (approximately 10%) than during conjugate saccades. Taken together, our results provide evidence that the activation of the antagonist, as well as agonist, motoneuron pools must be considered to understand the neural control of horizontal eye movements across different oculomotor behaviors. Moreover, we propose that the undersampling of smaller motoneurons (e.g., nontwitch) was likely to account for the missing drive observed during disconjugate saccades; these cells are thought to be more specialized for vergence movements and thus could provide the additional input required to command disconjugate eye movements.