Vergence Dynamics Research Articles

Gaze-shift dynamics in subjects with and without symptoms of convergence insufficiency: influence of monocular preference and the effect of training

We studied gaze-shift dynamics during several gaze-shift tasks and during reading, in five subjects with convergence insufficiency (C.I., a diminished ability to converge), and in ten subjects without C.I. Furthermore, we studied the effect of vergence training in order to verify previous claims that orthoptic exercises can improve vergence performance. We recorded binocular eye movements with the scleral coil technique. Subjects switched fixation between nearby and distant light emitting diodes (LEDs) arranged in isovergence arrays (distances 35 and 130 cm) in a dimly lit room. In both the C.I. and non-C.I. group, two classes of subjects occurred: vergence responders and saccadic responders. During pure vergence tasks, saccadic responders made saccades with no or little vergence; vergence responders made vergence movements with no or small saccadic components. In saccadic responders, fixation of nearby targets was monocular. Subjects with a preferred eye, according to our determination, used the preferred eye. The five C.I. subjects showed idiosyncratic responses with insufficient vergence during most trials. They all had a tendency to alternate fixation between the left and right eye. Vergence–version tasks always elicited larger vergence components than pure vergence tasks. During a reading task, vergence angles were more accurate than during gaze-shifts between LEDs. After the pre-training sessions, nine subjects (one of which had C.I.) practised a pure vergence task three times a day for at least 2 weeks. Vergence amplitudes of four of these subjects were larger after training. We conclude that vergence training can change oculomotor performance. Although C.I. is often associated with abnormal vergence dynamics, there are no typical C.I. vergence dynamics. Unstable monocular preferences may play a role in the aetiology of C.I.

Dynamic details of disparity convergence eye movements.

Classically, the primary tool for quantifying the dynamics of vergence and other eye movements has been the main sequence. The main sequence is a plot of peak velocity versus response amplitude and is particularly useful for comparing the dynamics of a large number of eye movements over a range of response amplitudes. However, the main sequence represents only the equivalent first-order behavior of a response and does not describe its dynamics in detail. Since the main sequence is based on only two points on the dynamic trajectory, it is sensitive to measurement artifacts and noise. A new methodology is presented which quantifies the equivalent second-order dynamics of eye movements using a larger region of the transient response. These new indexes were applied to vergence eye movements and were found to differentiate between subtle, but important differences in movement dynamics.

Dynamic model of the vergence eye movement system: simulations using matlab/ simulink

A dynamic model of the vergence eye movement system was developed and simulated using matlab/ simulink. The model was based on a dual-mode dynamic model previously written in fortran. It consisted of a fast open-loop component and a slow closed-loop component. The new model contained several important modifications. For example, in the fast component, a zero-order hold element replaced the sampler and the target trajectory estimator in the earlier model to provide more stable and accurate responses. Also, a periodicity detector was added to automatically detect periodicity in the stimulus waveform. The stored periodic stimulus, with a reduction in latency, was used to drive the fast component output. Moreover, a connection representing the efference copy signal was added from the fast component output to the disparity input to provide an accurate estimate of the stimulus waveform. Further, Robinson's model of the extraocular muscles replaced the earlier 2nd-order plant to provide more realistic muscle dynamics. The entire model, containing the fast and slow components, was simulated using a variety of stimuli such as pulses, positive and negative ramps, square-wave, and sine-wave. The responses showed dynamic characteristics similar to experimental results. Thus, this new matlab/ simulink program provides a relatively easy-to-use, versatile, and powerful simulation environment for investigating the basic as well as clinical aspects of vergence dynamics. Moreover, the simulation program has general characteristics that can be modified to represent other oculomotor systems such as the versional and accommodation systems. This provides a framework for future investigation of dynamic interactions between oculomotor systems.

The dynamics of vertical vergence.

We measured the gain and phase of vertical vergence in response to disjunctive vertical oscillations of dichoptic textured displays. The texture elements were m-scaled to equate visibility over the area of the display and were aperiodic and varied in shape so as to avoid spurious binocular matches. The display subtended 65 degrees and oscillated through peak-to-peak amplitudes from 18 arc min to 4 degrees at frequencies from 0.05 to 2 Hz - larger ranges than used in previous investigations. The gain of vergence was near 1 when the stimulus oscillated at 18 arc min at a frequency of 0.1 Hz or less. As the amplitude of stimulus oscillation increased from 18 arc min to 4 degrees, vergence gain decreased at all frequencies, which is evidence of a nonlinearity. Gain declined with increasing stimulus frequency but was still about 0.5 at 2 Hz for an amplitude of 18 arc min. Phase lag increased from less than 10 degrees at a stimulus frequency of 0.05 Hz to between 100 degrees and 145 degrees at 2 Hz. Overall, the dynamics of vertical vergence resemble the dynamics of horizontal vergence and cyclovergence.

Voluntary binocular gaze-shifts in the plane of regard: Dynamics of version and vergence

We studied the dynamics of voluntary, horizontal, binocular gaze-shifts between pairs of continuously visible, real three-dimensional targets. Subjects were stabilized on a biteboard to allow full control of target angles, which were made to differ only in distance (pure vergence), only in direction (pure version; conjugate saccades) or in both distance and direction (disjunctive saccades). A wide range of changes in vergence (0–25 deg) and version (0–65 deg) was recorded to study the dynamics of disjunctive saccades, described until now for limited ranges, throughout the horizontal oculomotor range within manual working space, and to study the velocity-duration-amplitude relations (“main sequence”) of disjunctive vs conjugate saccades. Pure vergence was almost never observed; divergence, especially, was always associated with saccades. Likewise, horizontal saccades were never strictly conjugate, they always contained a transient divergence-convergence sequence. The amplitude and velocity of these transient components varied systematically with saccadic size. In combined version-vergence movements, vergence was, in general, accelerated and shortened as a function of increasing version. This effect was fairly uniform for divergence, which appeared to increase in velocity by about as much as the transient peak divergent velocity of the version saccade. The intrasaccadic fraction of divergence increased from about 50% to close to 100% as a function of increasing version. For convergence, saccades up to about 20 deg were also accelerating; in this case it appeared as if the transient peak convergent velocity of the version saccade was added to the basic convergence velocity. For larger saccades this effect was partly counteracted by the penetration of an initial divergence associated with the saccade. This initial divergence delayed and slowed down convergence. The intrasaccadic fraction of convergence varied between about 40% and 70%. In disjunctive saccades the individual eyes did not follow the main-sequence parameters of conjugate saccades of comparable sizes, except for the eye that moved with the combination “abduction and divergence”. For all other combinations of vergence and version, disjunctive saccades had lower peak velocities and longer durations than conjugate saccades. As a consequence, disjunctive version was also slower than conjugate version. Thus, while version accelerates vergence, vergence slows down version: in the generalized case of three-dimensional gaze-shifts, peak velocities and durations are in between those of the limiting cases of pure version and pure vergence. We conclude that, within manual working space, binocular gaze-shifts are effected by the highly integrated action of conjugate and disjunctive mechanisms, both of which are expressed preferentially in fast, saccadic movements.

Oculomotor functions in a Swedish population of dyslexic and normally reading children

Eighty-six nine-year-old dyslexic children were matched to control children with regard to age, sex, class in school, and intelligence. Orthoptic and eye movement analysis were performed on all children. It was concluded that the dyslexic pupils did not differ significantly from control children in terms of strabismus, accommodation, stereo acuity, vergence function or ocular dominance. Eye movement recordings did not show any qualitative differences between the groups in vergence dynamics during synoptophore investigations.

Vergence control of central and peripheral disparities

The results of previous studies using either small or large target configurations suggest that open-loop (OL) vergence response dynamics may depend on the peripheral extent of the target. To investigate systemically the effect of peripheral target extent on dynamic vergence control, OL vergence responses to central, peripheral, and central-plus-peripheral targets were recorded. Open-loop disparity stimuli included steps, ramps, and sinusoids. It was found that the OL step and ramp responses to the central target were more rapid and exhibited significantly more step and multiple-step movements than those for either the peripheral or the central-plus-peripheral targets. During OL ramp stimulation, the maximum disparity for which tracking could be maintained was largest for the peripheral target. Sinusoidal responses showed the greatest gain for the central target. For all three types of stimuli, responses to the central-plus-peripheral target showed characteristics that were somewhat between those for the central and the peripheral targets. These results resolve some previously conflicting findings about open-loop vergence dynamics and suggest that large peripheral disparity stimulation initiates slow fusion of a visual scene, whereas small central disparity stimulation produces more rapid movements for precise binocular tracking of targets moving in depth.

Ocular vergence under natural conditions. I. Continuous changes of target distance along the median plane

Horizontal binocular eye movements of four subjects were recorded with the scleral sensor coil--revolving magnetic field technique while they fixated a natural target, whose distance was varied in a normally illuminated room. The distance of the target relative to the head of the subject was changed in three ways: (a) the target was moved manually by the experimenter; (b) the target was moved manually by the subject; (c) the target remained stationary while the subject moved his upper torso towards and away from the target. The rate of change of target distance was varied systematically in four levels, ranging from 'slow' to 'very fast', corresponding to changes in target vergence from about 10 degrees s-1 to about 100 degrees s-1. The dynamics of ocular vergence with regard to delay and speed were, under all three conditions, considerably better than could be expected from the literature on ocular vergence induced by disparity and/or blur. When 'very fast' changes in the distance of the target were made, subjects achieved maximum vergence speeds of up to about 100 degrees s-1. Delays of these fast vergence responses were generally smaller than 125 ms. Negative delays, i.e. ocular vergence leading the change in target distance, were observed. The eyes led the target (i.e. predicted target motion) by about 90 ms on average, when the subject used his hand to move the target. Vergence tracking was almost perfect when changes in distance were produced by moving the upper torso. In this condition, the eye led the target by about 5 ms. In the 'slow' and 'medium' conditions (stimulus speeds about 10-40 degrees s-1) tracking was accurate to within 1-2 degrees, irrespective of the way in which the target was moved. In the 'fast' and 'very fast' conditions (stimulus speeds about 40-100 degrees s-1), the accuracy of vergence tracking was better for self-induced than for experimenter-induced target displacements, and accuracy was best during voluntary movements of the upper torso. In the last case, ocular vergence speed was within about 10% of the rate of change of the vergence angle formed by the eyes and the stationary target. The dynamics of convergent and divergent vergence responses varied considerably. These variations were idiosyncratic. They were consistent within, but not between, subjects. Ocular vergence associated with attempted fixation of an imagined target, changing distance in darkness, could only be made by two of the four subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Disparity vergence dynamics and fixation disparity.

The initial velocity of the vergence response to step changes in disparity was examined in two subjects as a function of the retinal locus and spatial frequency of fusion stimuli. Vergence velocity decreased for both subjects as retinal eccentricity increased and it was unaffected by spatial frequency. The same subjects had very different amounts of fixation disparity (FD). The flat forced duction FD curve for one subject who had rapid vergence adaptation remained unaffected by these two stimulus variables. In contrast, the steep forced duction FD curve for the other subject became steeper as retinal eccentricity and target coarseness increased. These results suggest that prism adaptation overshadows the effects of disparity vergence dynamics upon FD, but in the absence of adaptation, two dynamic properties of vergence (velocity and decay during occlusion) determine the slope of the peripheral limits of the forced duction FD curves. The results also suggest that the vergence system is more sensitive to small disparities subtended by high than low spatial frequency detail. We recommend that the slope of the extreme limits of the forced duction curve be used clinically to discriminate between normal and abnormal disparity vergence dynamics.