Vergence Adaptation Research Articles

Heterophoria and monocular occlusion

Prism adaptation responses in orthophoric, exophoric and esophoric subjects are compared and found to be significantly different. Following a prolonged period of monocular occlusion the prism adaptation responses were compared, after occlusion the differences are no longer significant. This supports the theory that vergence (prism) adaptation is responsible for the existence of heterophoria. This paper shows that changes in heterophoria with monocular occlusion are caused by desaturation of prism adaptation.

Selective nonconjugate binocular adaptation of vertical saccades and pursuits

Hering's law describes the equal and symmetrical rotation of the two eyes. It is possible to calibrate the binocular yoking of the two eyes in response to disparate size and/or motion of the two ocular images. It is unclear if the ratio of movements by the two eyes is modified by selective adaptation of each versional system (i.e. saccades and pursuits) or if there is an apparent adaptation of Hering's law that results from a single underlying process. The latter could be accomplished by vergence (prism) adaptation, which could interact with all versional systems. In this investigation, binocularly stimulated saccades and pursuits were adapted separately for 2hr to unequal vertical target displacements. Three adaptation paradigms were used; each included a 10% binocular gradient disparity. The adapting stimulus for the pursuit system was 0.25 Hz vertical triangular motion of 20 deg, peak to peak. Two saccade adaptation paradigms included one which emphasized correcting vertical disparity during the pulse component of the saccade, the other minimized the influence of disparity prior to, during and immediately after saccades (vergence paradigm). Yoking ratios (YRs) for vertical pursuits and saccades were compared before and after adaptation. The pursuit paradigm produced marked adaptation of the pursuit YRs while it had negligible effect on saccade YRs. The pulse saccade paradigm adapted the saccade YRs twice as much as the pursuit YRs whereas the vergence paradigm resulted in little adaptation of YRs for either saccades or pursuits. Pursuits adapted to the first paradigm in 15–30 min whereas saccades adapted to the second paradigm in 1.5–2 hr. These results indicate that there is not a single common nonconjugate adaptation mechanism for vertical pursuits and saccades. Results of the vergence paradigm demonstrate that feedback during or immediately after eye movements is necessary in order to stimulate the binocular versional adaptation mechanism. Versional adaptation may be considered as a calibration of Hering's law.

Adaptive disorders of accommodation and vergence in binocular dysfunction.

Disturbances of binocular vision are described clinically by the Duane-White classification in terms of the magnitude of the accommodative-convergence ratio (AC/A). Convergence excess and convergence insufficiency are assumed to result from high and low AC/A ratios respectively. It is assumed that the abnormal AC/A ratio is an independent variable that underlies abnormal phorias. However, recent studies have demonstrated that the AC/A ratio is inversely related to the adaptability of tonic accommodation (lens adaptation) and directly related to adaptability of tonic vergence (prism adaptation). We have tested whether clinical categories of convergence excess and convergence insufficiency are associated with insufficient and excessive adaptation of tonic accommodation and tonic vergence. Results demonstrate greater amplitude and duration of accommodative after-effects (lens adaptation) in the convergence insufficiency than the convergence excess group. Vergence after-effects (prism adaptation) had the reverse trend for the two groups. These results indicate that adaptive disorders of accommodation and vergence may underlie binocular disorders in symptomatic patients categorized as convergence excess and convergence insufficiency.

Influence of accommodative and vergence adaptation on binocular motor disorders.

Tait described four categories of binocular disorders including convergence excess, convergence insufficiency, divergence excess, and divergence insufficiency. These disorders are defined by the distance where the largest heterophoria occurs (distance or near), and the amplitude of the accommodative vergence ratio (AC/A). Insufficiency corresponds to a low AC/A ratio, whereas excess corresponds to a high AC/A. The magnitude of the AC/A ratio, which may be influenced by the adaptability of the accommodation and vergence systems, has been shown to be reciprocally related to adaptability of accommodation. Likewise, the degree of vergence accommodation has been shown to be related reciprocally to adaptability of vergence to prism. An imbalance of adaptability of accommodation and vergence systems produces abnormal cross-coupling between the two motor systems. When accommodation is more adaptable than vergence, the AC/A ratio is low and the CA/C ratio is high. Conversely, when vergence is more adaptable than accommodation, the AC/A ratio is high and the CA/C ratio is low. A method is reported for temporarily restoring moderate amplitudes of abnormal AC/A and CA/C ratios by reducing excessive adaptation with fatigue. Finally, new clinical procedures for measuring adaptation of accommodation and the CA/C ratio are presented. Taken together with current measures of vergence adaptation and AC/A ratio these procedures will permit a more complete evaluation of mutual interactions between accommodation and vergence in patients diagnosed as having excessive and insufficient vergence.

Imbalanced adaptation of accommodation and vergence produces opposite extremes of the AC/A and CA/C ratios.

It is customary to describe abnormal interactions between accommodation and convergence according to the Duane-White classification of convergence excess and insufficiency or divergence excess and insufficiency. However, recent studies indicate that these extremely high and low categories of accommodative vergence may result from adaptive disorders of accommodation and convergence. The AC/A ratio is shown in this paper to vary inversely with adaptability of accommodation and to vary directly with adaptability of vergence. Extremely high AC/A ratios can result from an imbalance in which vergence adapts readily to prism, whereas accommodation does not adapt to minus lenses. Similarly, extremely high values for convergence accommodation (the CA/C ratio) are observed when there is robust adaptation of accommodation to lenses and little adaptation of vergence to prism. Furthermore, in these extreme cases the AC/A and CA/C ratios tend to be inversely related. For example, abnormally high AC/A ratios are often accompanied by extremely low CA/C ratios and vice versa. These observations suggest that equalizing the adaptability of accommodation and vergence with orthoptics may prove to be an effective means of restoring the AC/A and CA/C ratios to normal values.

The effect of vergence adaptation on convergent accommodation.

This study investigates whether, during a sustained near-vision task, the convergent accommodation (CA) derived initially from the fast (reflex) vergence response is subsequently maintained by tonic (adaptive) vergence. CA was examined during the course of a 3 min near-vision task (viewing distance 33 cm) in 10 emmetropic subjects. Vergence was stimulated by the introduction of a 6 delta base-out prism before the right eye while the open-loop accommodative response of the left eye was measured at approximately 1 s intervals using an objective infra-red optometer. In a control condition the subject viewed the target through a plano lens (0 delta). The accommodation loop was opened by the subject viewing near-type through 0.5 mm diameter pinholes. Pre- and post-task measurement of heterophoria was made using a Maddox rod and prism bar. Regression of post-task heterophoria to the pre-task value indicated that adaptation to the vergence stimulus had occurred. However, there was no significant reduction in induced CA during the task. As the onset of adaptation does not produce any fall in CA, it is concluded that both reflex and adaptive vergence act as a stimulus to CA. This finding is discussed with regard to models of accommodation-vergence interaction.

NEAR VISION STRESS: VERGENCE ADAPTATION AND ACCOMMODATIVE FATIGUE*

Changes in the visual system following prolonged near work were investigated. Fifteen young, normal subjects undertook a severe, two hour long, binocular near visual task at 20 cm without any breaks. Fusional stress was assessed by near 'phoria change. Refractive change was measured with an autorefractometer to investigate whether transient myopia occurred and its subsequent recovery. The near task caused vergence adaptation which was primarily due to the fusional stress of the task (accounting for 67% of its variance). The 'phoria change was to a lesser extent (40%) dependent on the accommodative "stress" of the task. Fatigue of the accommodative system resulted in increased accommodative innervation to maintain the same accurate response. Increased innervation can continue after the task on subsequent distance viewing, resulting in transient myopia (mean 0.29 DS). This transient myopia was found to be due to a transient regression of the far point towards the subject's tonic accommodation level. This can be accounted for by a shift of the tonic level as well as an increased bias towards the pre-task tonic level.

Adaptation of ocular vergence to stimulation with large disparities.

Ocular vergence movements were measured with a scleral coil technique under stabilized viewing conditions for disparity. Crossed disparity steps, ranging between 0.25 and 10 deg, of three different targets were imposed. Ocular vergence responses consisted of converging movements with an initially constant velocity. This velocity increased with the magnitude of disparity up to about 4 deg and decreased for larger disparities. For disparities up to 2 deg the responses saturated at the limit of convergence. For larger disparities responses were transient, i.e. after large converging movements the angle of convergence gradually declined to about its initial value. For disparities larger than 5 deg amplitudes of the transient responses decreased and occasionally responses were completely absent. The transient character of responses was apparently due to adaptation of the vergence system to a specific disparity, since responses to different disparities could still be induced. Probing of the vergence system with two successive disparity steps of different magnitudes showed that adaptation was selective for a limited range of disparities around the adapting disparity stimulus. Stabilized disparity pulses with durations ranging from 100 to 800 ms induced ocular vergence movements following the time integral of disparity rather than momentary disparity. This indicated that the part of the vergence system sensitive to disparity has mainly integrative properties for large as well as small disparities. Stimulation with trains of shortlasting disparity pulses showed that adaptation also occurred under normal viewing conditions. They further revealed that adaptation is erased during a short period after each blink.

Near vision stress: Vergence adaptation and accommodative fatigue

Abstract— Changes in the visual system following prolonged near work were investigated. Fifteen young, normal subjects undertook a severe, two hour long, binocular near visual task at 20 cm without any breaks. Fusional stress was assessed by near ‘phoria change. Refractive change was measured with an auto-refractometer to investigate whether transient myopia occurred and its subsequent recovery. The near task caused vergence adaptation which was primarily due to the fusional stress of the task (accounting for 67% of its variance). The ’phoria change was to a lesser extent (40%) dependent on the accommodative “stress” of the task. Fatigue of the accommodative system resulted in increased accommodative innervation to maintain the same accurate response. Increased innervation can continue after the task on subsequent distance viewing, resulting in transient myopia (mean 0.29 DS). This transient myopia was found to be due to a transient regression of the far point towards the subject's tonic accommodation level. This can be accounted for by a shift of the tonic level as well as an increased bias towards the pre-task tonic level.

Orthophorization after Occlusion-Induced Exotropia

A 36-year-old male had been constantly wearing a patch to avoid diplopia since an operation on the right parietal area of the brain. Postsurgical visual field evaluation indicated a complete bilateral left hemianopia with macular splitting. The patient had a constant alternating comitant exotropia with no suppression. By terminating the patching, establishing the appropriate fixing eye, and applying relieving prisms, diplopia was removed. Once binocularity was thus established, it was found (over the course of 7 months) that binocular function was possible with diminishing amounts of prism power. During this 7-month period, measurements of the patient's heterophoria, motor fusion, and sensory fusion revealed that he adapted from his occlusion-induced exotropia toward an habitual heterophoria. Such vergence adaptation has been defined as orthophorization. The occurrence of vergence adaptation after sensory fusion indicates that sensory fusion plays an important role in the etiology of prism adaptation.

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.

Vergence adaptation: a review.

The present review aims to discuss the current researches on the ability of the vergence system to adjust to new information. A concomitant or non-concomitant binocular retinal disparity caused by a prism or an anisometropic prescription can be adapted to such that the induced phoria of fixation disparity reduces back to the original value. The features and mechanisms of this normal adaptation and also the response in subjects with abnormal binocular vision are discussed. The less extensive work on the adaptive ability of the accommodative vergence system is also reviewed.

Dynamic interactions between accommodation and convergence are velocity sensitive

Aftereffects of accommodation and vergence occur following approximately 1 min of adaptation to lenses and prisms respectively. This observation can be interpreted to mean that accommodation and vergence responses have phasic and tonic components. We have examined the role that these proposed subcomponents play in mutual interactions between accommodation and vergence. Both accommodative vergence (AV) and vergence accommodation (VA) were unresponsive to low temporal frequency variations (< 0. 1 Hz) in defocus and disparity respectively. However, both AV and VA were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system was cancelled electronically, both AV and VA become responsive to low temporal frequency stimuli. The ratio or gain of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) increased nonlinearly with stimulus amplitude. Vergence aftereffects resulted from stimulation of AV and accommodative aftereffects resulted from stimulation of VA. These results are interpreted to mean there could be a complementary relationship between the amplitude of the AC/A ratio and proposed tonic adaptation of accommodation, and between the amplitude of the CA/C ratio and proposed tonic adaptation of vergence. A low saturation limit or stimulus window for tonic adaptation may account for the amplitude dependent nonlinearities of the AC/A and CA/C ratios.

Vergence adaptation maintains heterophoria in normal binocular vision.

This investigation sought to determine whether vergence adaptation serves to maintain orthophoria or heterophoria in normal binocular vision. While wearing prisms which neutralized their nearpoint (40 cm) heterophorias, 27 of 30 nonstrabismic subjects adapted from their prismatically created orthophoria toward their habitual heterophoria. Three subjects showed no adaptive response. A symptom questionnaire distributed to these subjects determined that the maintenance of heterophoria by vergence adaptation is: characteristic of normal and comfortable binocular vision, and neither an entirely necessary nor sufficient condition for normal and comfortable binocular vision.

Clinical factors in proximal vergence.

Using forced vergence fixation disparity curves, proximal vergence was analyzed in two asymptomatic patient samples; 20 young adults (average age 25.2 +/- 4.71 years) and 20 presbyopes (average age 61.6 +/- 7.8 years). Total proximal vergence was significantly larger when measured under binocular (closed loop) conditions than monocular (open loop) conditions for pre-presbyopes (binocular 10.51 delta +/- 4.7 vs. 6.4 delta +/- 3.84 monocular) and presbyopes (binocular 11.59 delta +/- 3.13 vs. 4.71 delta +/- 2.15 monocular). Associated proximal vergence was shown to predict associated phoria magnitude in presbyopes and in pre-presbyopes when vergence adaptation and convergence accommodation (CA/C) interactions with accommodative convergence (AC/A) are included. A vergence/accommodation model is presented which includes influences of proximal vergence. Clinical implications of the findings relating to diagnosis and therapy are discussed.

Parameters of fixation disparity.

This paper reviews the definition, causation, and significance of fixation disparity and suggests that the magnitude and character of fixation disparity depend on several major parameters: individual differences in the efficiency of motor and sensory processes of binocularity; natural or prism-induced demands on motor fusion; the presence or absence of foveal or parafoveal fusion clues; and the presence or absence of vergence adaptation. The clinical and research usefulness of fixation disparity as an indicator of binocular stress is emphasized.