Function Of Vergence Research Articles

Vergence in mild traumatic brain injury: A pilot study

Vergence dysfunction in individuals with mild traumatic brain injury (mTBI) may have a negative effect on quality of life, functional abilities, and rehabilitative progress. In this study, we used a range of dynamic and static objective and subjective measures of vergence to assess 21 adult patients with mTBI and nearwork symptoms. The results were compared with 10 control adult subjects. With respect to dynamic parameters, responses in those with mTBI were slowed, variable, and delayed. With respect to static parameters, reduced near point of convergence and restricted near vergence ranges were found in those with mTBI. The present results provide evidence for the substantial adverse effect of mTBI on vergence function.

A Comparison of Self-Reported and Measured Autostereogram Skills with Clinical Indicators of Vergence and Accommodative Function

Failure to perceive the disparity-defined form in autostereograms by those with clinically normal stereoacuity can occur when achieving or maintaining the precise vergence angle required to place the intended left and right images on corresponding areas of the two retinas is difficult. Since vergence and accommodation must be maintained at different depth planes to permit sensory fusion of an autostereogram, poor autostereogram skill has been suggested by different investigators to be related either to the presence of a binocular vision anomaly (ie a poorly tuned binocular system) or to a binocular system that is well-coordinated. The purpose of this study was to clarify the relationship between binocular visual performance and autostereogram skill.

Contact lenses vs spectacles in myopes: is there any difference in accommodative and binocular function?

Theoretically, the accommodative and vergence demands are different between single-vision contact lenses and spectacle lenses. The aim of the present study was to determine whether these differences exist when these two correction methods are used in clinical practice. For this, different visual parameters that characterize the accommodative (accommodation amplitude, accommodative facility, and accommodative response) and binocular function (near and distance horizontal and vertical dissociated phorias, near and vertical associated phorias, near and distance negative and positive fusional vergence, vergence facility, near point of convergence, negative and positive relative accommodation, stimulus AC:A ratio and stereoacuity) were evaluated in a student population when their myopia was corrected with either spectacles or soft contact lenses (SCL). All parameters were measured on two separate occasions in 30 myopic habitual contact lens and spectacle wearers of mean age 19 ± 2.4years. Some parameters such as accommodation amplitude, accommodative response, and stimulus AC:A ratio were measured using two measurement methods which are commonly used in clinical practice. Three measurements were taken for each parameter and averaged. For the comparative statistical analysis, we used the Student's t-test (p value < 0.05). The following statistically significant differences were found with the use of SCL in comparison to spectacles: higher accommodative lags, higher negative relative accommodation, more esophoric near horizontal dissociated phoria, and lower negative fusional vergence in near vision. The results found in this study show a definite trend towards poorer accommodative and vergence function with the use of contact lenses in comparison to glasses. This downward trend, though not statistically significant in accommodative function (lower PRA values and less lens amplitude of accommodation) might suggest that temporal insufficiency in the accommodation process could be occurring while contact lenses are used, thereby possibly creating a lag in accommodation to reduce associated overconvergence. This would be manifested in more esophoric values being found in the vergence function. The higher accommodative lags found in this study with SCL could indicate that prolonged use of SCL in near tasks may provoke a continuous hyperopic retinal defocus, a risk factor for the onset and progression of myopia, as indicated in numerous studies.

Relationship between accommodative and vergence dysfunctions and academic achievement for primary school children

The purpose of this study was to investigate the prevalence and types of non-strabismic accommodative and/or vergence dysfunctions in primary school children, and to determine the relationship of these dysfunctions to academic achievement. A total of 1031 parents and their children aged 9-13 years responded to the College of Optometrists in Vision Development Quality of Life (COVD-QOL) questionnaire. Of these, 258 children whose visual symptom scores were > or =20 were identified for further evaluation. Comprehensive eye and vision examinations were provided to the children who met the eligibility criteria (114 of 258): eligible symptomatic children were those without amblyopia, strabismus, ocular and systemic pathology, and contact lens wear. Children were also excluded if they had visual acuity poorer than 20/25 in either eye or vertical phoria >1 prism diopter. The results showed that 82 of 114 (71.9%) of criteria-eligible symptomatic primary school children had non-strabismic accommodative and/or vergence dysfunctions. In addition, a significant relationship was found between these dysfunctions and academic scores in every academic area (reading, mathematics, social science and science) in the total sample. Therefore, accommodative and vergence functions should be tested for all school children who have visual symptoms and/or academic difficulties. Additional study is needed to determine if improvements of accommodative and vergence functions also improve academic achievement.

The Influence of the Vergence System on Strabismus Diagnosis and Management

The vergence system is an important element in human eye movement control. It comprises horizontal, vertical, and torsional components, the largest of which is convergence. Vergence performance is largely involuntary, although voluntary effort can influence convergence. Vergence function can be damaged by trauma or organic disease and, because convergence can be exerted voluntarily, it is susceptible to purposeful subjective disturbances. Vergence function is important in the maintenance of binocular control, being synonymous with motor fusion. The most common vergence dysfunction affects convergence and may be exhibited as an excess or weakness such as convergence spasm or paralysis. Divergence dysfunction generally presents with diminished function such as divergence weakness; vertical and cyclovergence disturbances are rare. An update is given on the neuroanatomical areas that subserve the vergence system. Clinical examples of the most common vergence disorders are presented. The influence of the vergence system on the diagnosis and management in common and less well-known clinical entities is described and discussed.

Magnification and accommodation with phakic intraocular lenses

The calculation of phakic lenses (PL) was described by van der Heijde et al. [Klin. Monatsbl. Augenheilkd (1988) Vol. 193, pp. 99-102], but a formalism for estimating relative magnification compared with spectacle correction and accommodation effects are not yet published. The purpose of this study was to describe a mathematical strategy for calculating PL and relative magnification as a function of object vergence (phakic accommodation). Parameters used for the calculations are the spectacle refraction before and after (target refraction) surgery, the vertex distance, corneal refraction, and the predicted position of the phakic intraocular lens. The lens power is determined as the difference in vergences between the spectacle-corrected eye and the uncorrected eye at the reference plane of the predicted lens position. If we simplify the crystalline lens to a single refracting surface located at the principal plane of the crystalline lens, the vergence of the eye with spectacle correction and with PL is determined as a function of object distance [object vergence 0 D (infinity) to 10 D (object at a distance of 10 cm)] to evaluate accommodation effects of the crystalline lens. The method was applied to two clinical examples. In example 1 we calculated the power of a PL for correction of a 10-D myopia and determined the relative magnification and the vergence at the principal plane of the crystalline lens as a function of object vergence. Magnification gain increases with objects at near from 17% to 26%, whereas the vergence at the principal plane of the crystalline lens changes by 3.04 D less than in the spectacle-corrected eye. In example 2, a 20-D myopia was corrected with a PL. The gain in magnification changed from 33% to 58% with nearer objects. The change in vergence at the principal plane of the crystalline lens with objects at near was much higher with the PL compared with the spectacle correction, which implies that the refractive change necessary for focusing objects at near distance is much higher in the PL correction. Even if the predictability of postoperative refraction with PL is comparable or better than in other methods of correcting high or excessive ametropia, the effects of lateral magnification change and accommodation have to be considered to avoid image-size disparities (aniseikonia) and to maintain binocular vision, especially with monocular PL implantation and anisometropia.

Mechanical interdependence of version and vergence eye movements

In this paper, the authors investigate whether the idea of independent control of version and vergence eye movements is compatible with the mechanics of the eye plant. By computing the change in the axes of action of the eye muscles as a function of ocular vergence, they prove that, regardless of the muscle pulley locations, the required muscle activity for vertical version depends on the initial vergence angle. The binocular extension of Listing’s law (‘L2’) describes how the torsional orientation of the eye depends on both gaze direction and ocular convergence. The authors show that for each vergence angle there is a range of possible muscle pulley locations that would cause independent control of version and vergence to result in L2. They also show that this mechanical explanation of L2 requires that the muscle pulleys move as a function of vergence.

Perceived depth and direction as a function of ocular vergence

Perceived depth and direction as a function of ocular vergence

Comparison of fixation disparities obtained by objective and subjective methods.

Fixation disparities (FD) were measured as a function of forced vergence using binocular scleral search coils and simultaneously with nonius lines. The slope of the objective FD curve was significantly greater than the subjective FD curve for three of five subjects. This indicates an alteration in retinal correspondence of up to one degree, that shifts Panum's area to avoid the diplopia normally present with large disparities. This process allows for fusion in the presence of large objective fixation disparities which would normally cause diplopia. The shift in correspondence enhances the range of forced vergence, since the larger objective FDs serve as more effective stimuli to fusional vergence. The remaining subjects who lacked this effect had "flat" FD curves indicative of high vergence adaptation.

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

We 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.

Behavioral studies of local stereopsis and disparity vergence in monkeys

Investigations on macaque monkeys have provided much of our knowledge of the neural mechanisms of binocular vision, but there is little psychophysical data on the accuracy of vergence responses or the precision of stereoscopic depth perception in these primates. We have conducted comparative behavioral studies of binocular disparity processing in rhesus monkeys and humans via measurements of prism-induced fixation disparities (disparity vergence) and relative depth discrimination for spatially localized stimuli (local stereopsis). The results of these studies demonstrated a remarkable similarity in both the oculomotor and the sensory aspects of binocular vision in the two species when the stimulus dimensions were specified in visual angels, which were independent of interocular separation. The disparity vergence functions for the two species revealed fusion responses over the same range of prism-induced vergence and comparable vergence errors for stimuli near their fusional limits. Disparity vergence responses were independent of the spatial frequency of the binocular fusion stimulus. Stereothresholds as a function of the spatial frequency of the difference-of-Gaussian stimuli were of the same form, with equivalent stereoacuities, in monkey and human observers. The presence of substantial vergence errors had only a small effect on the precision of stereoscopic depth perception. We conclude that, after compensation for the differences in the lateral separation of their eyes, the operating characteristics of disparity vergence and stereoscopic vision are virtually identical in rhesus monkeys and humans and, consequently, the performance limits for these visual functions must be determined by anatomical and/or neural constraints that are similar in both species.

Hyperchromatic lenses as potential aids for the presbyope

It has been suggested that a ‘hyperchromatic’ lens which enhanced the longitudinal chromatic aberration (LCA) of the eye would increase its depth of focus (DOF) and hence might prove a useful aid to presbyopes who have lost their natural ability to accommodate for different distances. Experiments are described in which high- and low-contrast visual acuity, and contrast sensitivity were measured as a function of object vergence using the naked eye and various levels of effective lens—eye LCA. It was found that doubling the effective lens—eye LCA increased total DOF in the high-contrast acuity task by about 0.5 D, while correcting the ocular LCA reduced DOF by about 0.3 D. On the other hand, enhanced LCA led to some loss in performance in the low-contrast acuity task and in contrast sensitivity.

Comparison of the depths of focus with the naked eye and with three types of presbyopic contact lens correction

Experiments are described in which monocular, high-contrast, letter acuity was measured as a function of target vergence (effective distance) for six simulated presbyopic subjects, who were placed under cycloplegia with a 4mm diameter artificial pupil. Measurements were made with the naked eye or single-vision spectacle lens corrected eye, and with three types of simultaneous-vision (simultaneous image) ‡ presbyopic contact lens correction: a centre-near biofocal; a centre-distance varifocal or progressive power lens; and a centre-mear varifocal. Results show that all designs usefully improve the range of distances over which an adequate standard of vision is achieved, the depth of focus typically being increased by about 1–2 dioptres, depending upon the criterion used to define depth of focus. Peak visual acuity at optimal focus is, however, slightly worse with the presbyopic contact lens corrections than with the naked or spectacle-lens corrected eye. Intersubject differences suggest that individual lens characteristics, the aberrations of the individual eye and lens centration may all play a role in determining the optical effectiveness of any particular design.

Hyperchromatic lenses as potential aids for the presbyope

SummaryIt has been suggested that a ‘hyperchromatic’ lens which enhanced the longitudinal chromatic aberration (LCA) of the eye would increase its depth of focus (DOF) and hence might prove a useful aid to presbyopes who have lost their natural ability to accommodate for different distances. Experiments are described in which high‐ and low‐contrast visual acuity, and contrast sensitivity were measured as a function of object vergence using the naked eye and various levels of effective lens‐eye LCA. It was found that doubling the effective lens‐eye LCA increased total DOF in the high‐contrast acuity task by about 0.5 D, while correcting the ocular LCA reduced DOF by about 0.3 D. On the other hand, enhanced LCA led to some loss in performance in the low‐contrast acuity task and in contrast sensitivity.

Subjective and Objective Assessment of Soft Bifocal Contact Lens Performance

Subjective and objective techniques were used to assess the on-eye performance of soft bifocal contact lenses. In the subjective technique a young observer whose accommodation had been paralyzed with a cycloplegic agent was fitted with the contact lens type under investigation and aligned with a Maxwellian view Badal optometer using a bite-bar. Visual acuity was measured as a function of both target vergence (0.00 to -4.00 D) and pupil size (1 to 5 mm). Aspheric, concentric, and diffractive soft bifocal contact lens designs were investigated. Diffractive and concentric bifocal designs with +2.00 D near additions showed "twin peaks" of visual acuity with one peak at 0.00 D target vergence (equivalent to distance viewing) and the second at -2.00 D target vergence (equivalent to a target at 50 cm). Some aspheric designs produced a relatively constant visual acuity across this vergence range, whereas others gave poor acuity at near. Visual acuity with the Echelon diffractive lens was relatively unaffected by pupil size, confirming theoretical predictions. Visual acuity with concentric designs was also relatively free of pupil size effects. In the objective technique, the variation in surface power across the lens was assessed using video-keratography. This technique provided an elegant means of visualizing the power profile of the lens. The location of the zones of increased power and the magnitude of power variations allowed an accurate prediction of the visual performance measured subjectively. Temporal displacement of lenses may explain the occasions where visual performance did not vary with pupil size.

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.

The relationship between absolute disparity and ocular vergence.

The relationship between disparity and ocular vergence was investigated under closed-loop as well as under open-loop viewing conditions. First we examined whether vergence responded similarly to disparity presented under open-loop and closed-loop conditions. Similar response were observed in both conditions. The direct relationship between disparity and vergence was examined by presenting constant disparities between 0.2 degrees and 4 degrees under open-loop viewing conditions. Such vergence responses are described as the outputs of first-order low-pass filters with different filter characteristics for each amplitude of disparity. By analyzing the latency of vergence responses induced by constant disparities with help of the transfer function of disparity-controlled vergence, the time delay of disparity processing in the vergence loop was estimated. We suggested that the time delay was approximately between 80 and 120 ms instead of 160 ms as is generally assumed. The relationship between the rate of disparity change and vergence was examined by comparing responses to ramp and stepwise changes in target vergence. From the similar responses to ramp and staircase changes in disparity we concluded that vergence is not sensitive to the velocity of target vergence as such. On the basis of these findings we developed a model of disparity-controlled vergence. In this model disparity is processed through several parallel, imperfect integrators with slightly different low-pass filter characteristics, each of them susceptible to a limited range of disparities. Gains as well as phase lags of vergence responses to sinusoidal disparities are accurately simulated by this model.(ABSTRACT TRUNCATED AT 250 WORDS)

Convergence insufficiency in brain-injured patients

Disorders of eye movements are relatively common in brain-injured patients. Some of these disturbances are caused by direct trauma to the orbital content, cranial nerves and other brain areas. Convergence, which is a part of the near-vision complex and depends on the integrative function of the cortical and subcortical areas, is commonly affected by traumatic brain injury. Intact vergence needs the integrative function of brain structures associated with acquisition of information on one side and intact mesencephalic function, including function of a vergence integrator, and their interconnections. Clinical investigation of vergence function in 26 traumatically brain-injured patients examined as early as possible after trauma revealed disturbances in over a third of the examinees. A follow-up of 72 patients some 3 years after injury revealed vergence insufficiency in 42%. Convergence insufficiency was associated with longer periods of coma (p less than 0.001), presence of cognitive disturbances (p less than 0.005) and patients' failing to find work in the open market (p less than 0.01). It is suggested that the presence of disturbed vergence several years after trauma, and its close association with prolonged coma and cognitive disturbances, is an expression of permanent damage to mesencephalic and cortical brain structures and can serve as an important aid in evaluation of these patients. This paper describes the close relationship between vergence state and over-all rehabilitation outcome as evaluated by patients' occupational status.

A Continuation of the Screen Test

SUMMARY A method is offered as a continuation of the usually perfomned screen test, which aids in definining the offending mechanism in conditions of variation from accepted normal balance. Used with other musele tests for near, these tests afford information regarding the values of the most important contributors in the combined function of vergence. The objcctive nature of the tests permit use upon the uuco-operative patient as well as the very young. Defining the mechanism to which treatment should be directed offers a safeguard against inducing anomalies of opposite nature, in the attempt to achieve balance.

A Continuation of the Screen Test

SUMMARY A method is offered as a continuation of the usually perfomned screen test, which aids in definining the offending mechanism in conditions of variation from accepted normal balance. Used with other musele tests for near, these tests afford information regarding the values of the most important contributors in the combined function of vergence. The objcctive nature of the tests permit use upon the uuco-operative patient as well as the very young. Defining the mechanism to which treatment should be directed offers a safeguard against inducing anomalies of opposite nature, in the attempt to achieve balance.