Vergence Adaptation Research Articles

Topical Review: Methodological Variables in Clinical and Laboratory Measurements of Fixation Disparity.

Fixation disparity is a small vergence error that does not disrupt fusion. Fixation disparity measures correlate with binocular symptoms. This article covers methodological differences between clinical fixation disparity measurement devices, findings when objective and subjective fixation disparities are compared, and the potential impact of binocular capture on fixation disparity measurements. Fixation disparity is a small vergence error that occurs in nonstrabismic individuals and does not disrupt fusion. This article reviews clinical fixation disparity variables and their clinical diagnostic value. Clinical devices that are used to measure these variables are described, as are studies in which the output from these devices has been compared. Methodological differences between the devices such as the location of the fusional stimulus, the rate at which judgments of dichoptic alignment are made, and the strength of the accommodative stimulus are all considered. In addition, the article covers theories of the neural origins of fixation disparity and control system models incorporating fixation disparity. Studies in which objective fixation disparities (oculomotor portion of fixation disparity assessed with an eye tracker) and subjective fixation disparities (sensory portion of fixation disparity assessed psychophysically with dichoptic Nonius lines) have been compared are also examined, and consideration is given to why some investigators find differences in these measures, whereas other investigators do not. The conclusion thus far is that there are likely complex interactions between vergence adaptation, accommodation, and the location of the fusional stimulus that lead to differences in objective and subjective fixation disparity measures. Finally, capture of the visual direction of monocular stimuli by adjacent fusional stimuli and the implications for fixation disparity measures are considered.

How Lingering Fusional Adaptation Influences the Bielschowsky Head Tilt Test in Superior Oblique Paresis.

The lack of a positive Bielschowsky head tilt test (BHTT) is commonly seen as an indicator that superior oblique paresis (SOP) is not present. This study investigated the influence of fusion on the BHTT in unilateral SOP. We analyzed vertical fusional vergence using our eye-tracking haploscope and the value of BHTT difference (BHTTD) in 11 patients who were diagnosed with congenital unilateral SOP and able to fuse. Patients used one of three different mechanisms of vertical vergence to achieve fusion. The three fusional mechanisms were associated with a significantly different BHTTD (p < 0.05). Seven of the eleven patients used vertical recti-mediated fusion and had a mean BHTTD ± SD of 21.7 ± 6.3 prism diopters (PD). Three of these patients whom we measured after a patch test for at least 30 min showed a decreased BHTTD (12.7 ± 3.8 PD). Three of the eleven patients used a mixed (oblique/rectus) fusional mechanism and had a mean BHTTD ± SD of 9.3 ± 8.6 PD. Of these patients, the one whom we measured after patching showed an increase of 11 PD in BHTTD. The remaining patient used oblique muscle-mediated fusion and had a BHTTD of only 3 PD that increased to 21 PD after patching. One explanation for this BHTT behavior in the latter patient involves lingering vergence adaptation of the "paretic" superior oblique muscle (SOM) and contralateral inferior oblique muscle, which makes these muscles more effective when activated, as is the case on ipsilateral head tilt (part of the ocular counter-roll mechanism), lessening the expected increase in hyperdeviation. Similarly, in our patients with mixed fusion, the vergence-adapted "paretic" SOM and contralateral superior rectus muscle are activated on ipsilateral and contralateral tilt, respectively, lessening the hyperdeviation in both directions. In the other seven patients, however, the vergence-adapted ipsilateral inferior rectus muscle and contralateral superior rectus muscle are activated on contralateral tilt, accentuating the BHTTD. Depending upon the specific muscles used for vertical fusion, the BHTTD is decreased or increased. The presence of a large BHTTD points to lingering or persisting fusional tonus involving the vertical rectus muscles. The lack of a positive BHTT does not rule out the diagnosis of SOP, but rather may be caused by lingering or persevering fusional tonus involving the oblique muscles. Performing the BHTT after a patch test for a minimum of 30 minutes may be necessary to reveal the BHTTD, supporting the diagnosis of SOP.

Effect of Vergence Facility Testing on Horizontal Fusional Reserves.

The effect of vergence adaptation on test sequences should be considered in clinical practice. This study aimed to compare the effect of vergence adaptation after different vergence facility (VF) testing methods on following horizontal fusional reserve measurement at near. This cross-sectional study was performed on 50 healthy subjects with a mean ± standard deviation age of 21.18 ± 1.41 years. The baseline horizontal fusional reserves were measured using base-in (BI) and base-out (BO) prisms (∆) at near. Then, VF was measured using 3∆BI/12∆BI flip prism at near, and then the measurement of fusional reserves was repeated. In the next step, fusional reserves were measured after VF testing using 8∆BI/8∆BO flip prism. The values of fusional reserves were compared with their baseline findings. The results of negative fusional reserves after VF measurement using the 3∆BI/12∆BO prisms were significantly lower than the baseline values (blur [P = .03], break [P = .03], and recovery [P = .03]). There were no significant differences between the values of fusional reserves before and after VF testing using the 8∆BI/8∆BO prisms (P > .05). No correlation was observed between fusional reserves and VF responses (P > .05); however, the results revealed a positive correlation between the values of fusional reserves before and after VF testing using both methods (all P < .05). There was no significant difference in VF findings between the methods (P = .23). The measurements of horizontal fusional reserves followed by VF testing were not affected except for the responses of negative fusional reserves, which decreased after VF determination using the 3∆BI/12∆BO method. Although the vergence adaptation does not seem clinically significant, it is recommended to evaluate horizontal fusional reserves before VF measurement or long-enough time after VF testing for the vergence system to return to its nonadapted state.

Inferior Oblique Muscle "Overaction" Caused by Inferior Oblique Muscle Shortening, Not by Hypertonicity.

To determine whether fusional vergence adaptation in patients who can fuse in at least some gaze positions can cause curvature of the non-fixing eye movement paths of patients with apparent "overaction" or "underaction" of the oblique muscles, yielding possibly erroneous evidence of hypertonicity as the cause of the overaction or underaction. The authors retrospectively studied Lancaster red-green plots of patients with oblique muscle "overaction/underaction." If fusion was present, the plot had usually been repeated after monocular occlusion for at least 30 minutes. Fundus torsion and fusion status were also recorded. After a patch test in patients displaying fusion, the non-fixing eye's movement path became more linear. Although it has been argued that true overaction of the oblique muscles would show curved eye movement paths on side gazes, in the study patients displaying fusion in at least some directions of gaze, the eye movement paths became more linear after patch testing, favoring the more mechanical explanation. Illustrated cases were consistent with the hypothesis that short inferior oblique muscles simply hold the globes in extorted positions, and the appearance of inferior oblique muscle "overaction" arises from the eyes' following their extorted movement paths on side gazes, not from hypertonicity of the inferior oblique muscle in the adducting eye. [J Pediatr Ophthalmol Strabismus. 2022;59(1):28-34.].

Changes in corneal astigmatism and near heterophoria after smartphone use while walking and sitting.

Smartphone use has become an indispensable part of our daily life. The handy design and powerful processor allow smartphone users to perform diversified tasks even when walking. This study aimed to investigate and compare the optical aftereffect and vergence adaptation of using a smartphone while walking and sitting. Twenty-nine young healthy adults (aged 19 to 24 years old) with normal binocular and accommodative functions were recruited. Participants were asked to watch a movie for 30 minutes using a smartphone while either walking on a treadmill or sitting on a chair. Corneal aberrations and near heterophoria were measured before and after smartphone use by a corneal topographer and modified Thorington heterophoria test, respectively. Using the smartphone while walking induced a change in corneal H/V astigmatism, becoming 0.11±0.03 μm less negative (two-way ANOVA repeated measures, Bonferroni post-hoc test, p = 0.001). This optical aftereffect was significantly higher than after smartphone use while sitting by 0.10±0.03 μm (paired t-test, p = 0.003). Although smartphone use did not result in a significant change in near heterophoria (Bonferroni post-hoc test, p > 0.15), the vergence adaptation showed relatively more eso- or less exo-deviation by 0.79±0.36Δ in the walking than the sitting condition (paired t-test, p = 0.037). Eyecare practitioners should be cautious of the potential optical after effect and vergence adaptation after prolonged smartphone usage.

Effects of aligning prisms on the objective and subjective fixation disparity in far distance.

Fixation disparity (FD) refers to a suboptimal condition of binocular vision. The oculomotor aspect of FD refers to a misadjustment in the vergence angle between the two visual axes that is measured in research with eye trackers (objective fixation disparity, oFD). The sensory aspect is psychophysically tested using dichoptic nonius lines (subjective fixation disparity, sFD). Some optometrists use nonius tests to determine the prisms for constant wear aiming to align the eyes. However, they do not (yet) use eye trackers. We investigate the effect of aligning prisms on oFD and sFD for 60 sec exposure duration of prisms determined with the clinically established Cross test in far distance vision. Without prisms, both types of FD were correlated with the aligning prism, while with prisms the FD was close to zero (these analyses included all base-in and base-out cases). The effect of base-in prisms on oFD was proportional to the amount of the aligning prism for the present 60 sec exposure, similar as for the 2- 5 sec exposure in Schmid et al. (2018). Thus, within 1 minute of prism exposure, no substantial vergence adaptation seems to occur in the present test conditions. Further studies may investigate intra-individual responses to different exposure times of aligning prisms in both prism directions.

A differential role for the posterior cerebellum in the adaptive control of convergence eye movements

IntroductionThe vergence oculomotor system possesses two robust adaptive mechanisms; a fast “dynamic” and a slow “tonic” system that are both vital for single, clear and comfortable binocular vision. The neural substrates underlying these vergence adaptive mechanisms in humans is unclear. MethodsWe investigated the role of the posterior cerebellum in convergence adaptation using inhibitory continuous theta-burst repetitive transcranial magnetic stimulation (cTBS) within a double-blind, sham controlled design while eye movements were recorded at 250hz via infrared oculography. ResultsIn a preliminary experiment we validated our stimulation protocols by reproducing results from previous work on saccadic adaptation during the classic double-step adaptive shortening paradigm. Following this, across a series of three separate experiments we observed a clear dissociation in the effect of cTBS on convergence adaptation. Dynamic adaptation was substantially reduced while tonic adaptation was unaffected. Baseline dynamic fusional vergence response were also unaffected by stimulation. ConclusionsThese results indicate a differential role for the posterior cerebellum in the adaptive control of convergence eye movements and provide initial evidence that repetitive transcranial magnetic stimulation is a viable tool to investigate the neurophysiology of vergence control. The results are discussed in the context of the current models of implicit motor adaptation of vergence and their application to clinical populations and technology design in virtual and augmented head mounted display architectures. Significance statementThe cerebellum plays a critical role in the adaptive control of motor systems. Vergence eye movements shift our gaze in depth allowing us to see in 3D and exhibit two distinct adaptive mechanisms that are engaged under a range of conditions including reading, wearing head-mounted displays and using a new spectacle prescription. It is unclear what role the cerebellum plays in these adaptive mechanisms. To answer this, we temporarily disrupted the function of the posterior cerebellum using non-invasive brain stimulation and report impairment of only one adaptive mechanism, providing evidence for neural compartmentalization. The results have implications for vergence control models and applications to comfort and experience studies in head-mounted displays and the rehabilitation of clinical populations exhibiting vergence dysfunctions.

Vergence adaptation in hyperopic children with and without a strabismic history

Vergence adaptation in hyperopic children with and without a strabismic history

Adaptation of reflexive fusional vergence is directionally biased

Divergence is known to differ from convergence across a wide range of clinical parameters. We have postulated that a limited neural substrate results in reduced fusional divergence velocities and subsequently a reduced capacity to adapt tonic vergence to uncrossed disparities. We further investigated this hypothesis by characterizing the degree of plasticity in reflexive fusional vergence to repetitive end-point errors using a disparity-based double-step paradigm. 10 adults completed 4 study visits where reflexive fusional convergence or divergence was measured (250 Hz infrared oculography) to a 2° disparity step and then lengthened or shortened via a repeated double-step (2° ± 1.5°). Stimuli were presented dichoptically at 40 cm. Adaptive modification of vergence responses was similar between directions for the shortening conditions, suggesting a common neural mechanism responds to overshooting errors. In comparison, adaptive lengthening of convergence was slower, but of equal magnitude, suggesting a second neural mechanism with a longer time constant for undershooting errors. Divergence response velocities were slower at baseline and did not increase after adaptive lengthening. Instead, increases in divergence response amplitudes were a result of increased response duration, implying saturation of the reflexive, preprogrammed response. Adaptive responses serving to increase or decrease reflexive fusional vergence recruitment were asymmetric. Adaptive lengthening of convergence and divergence identified further directional asymmetries. The results support the hypothesis that the neural substrate underlying divergence is attenuated, resulting in reduced reflexive plasticity when compared to convergence. The clinical and technological implications of these results are discussed.

Modulation of oculomotor control &amp; adaptation with cerebellar TMS: effects on slow-tonic vergence adaptation.

Modulation of oculomotor control &amp; adaptation with cerebellar TMS: effects on slow-tonic vergence adaptation.

The influence of age on adaptation of disparity vergence and phoria

A paucity of research exists to investigate whether the normal aging process influences the ability to adapt disparity vergence and phoria. Vergence eye movements and dissociated phoria were recorded from 49 healthy subjects (ages 20–70years) using an objective eye movement tracking system. Four-degree vergence responses were modified using a double-step protocol. Dynamics of vergence were quantified via peak velocity. The phoria adaptation experiment measured the magnitude (net change in phoria level) and rate (magnitude divided by the time constant) of phoria adaption during 5min of sustained fixation on a binocular target (40cm/8.44° from midline). The magnitude of phoria adaptation decreased as a function of age (r=−0.33; p=0.04). The ability to adapt vergence peak velocity and the rate of phoria adaptation showed no significant age-related influence (p>0.05). The data suggest that the ability to modify the disparity vergence system and the rate of phoria adaptation are not dependent on age; whereas, the magnitude of phoria adaptation decreases as part of the normal adult aging process. These results have clinical and basic science implications because one should consider age when assessing the changes in the magnitude of phoria adaptation which can be abnormal in those with oculomotor dysfunctions.

Adaptation of horizontal eye alignment in the presence of prism in young children.

Young children experience decreased convergence and increased accommodation demands relative to adults, as a result of their small interpupillary distance and hyperopic refraction. Those with typical amounts of hyperopic refractive error must accommodate more than an emmetrope to achieve focused retinal images, which may also drive additional convergence through the neural coupling. Adults and older children have demonstrated vergence adaptation to a variety of visual stimuli. Can vergence adaptation help younger children achieve alignment in the presence of these potentially conflicting demands? Purkinje image eye tracking and eccentric photorefraction were used to record simultaneous vergence and accommodation responses in adults and young children (3–6 years). To assess vergence adaptation, heterophoria was monitored before, during, and after adaptation induced by both base-in and base-out prisms. Adaptation was observed in both adults and young children with no significant effect of age, F(1, 34) = 0.014, p = 0.907. Changes in accommodation between before, during, and after adaptation were less than 0.5 D in binocular viewing. Typically developing children appear capable of vergence adaptation, which might play an important role in the maintenance of eye alignment under their changing visual demands.

Unmasking the linear behaviour of slow motor adaptation to prolonged convergence.

Adaptation to changing environmental demands is central to maintaining optimal motor system function. Current theories suggest that adaptation in both the skeletal-motor and oculomotor systems involves a combination of fast (reflexive) and slow (recalibration) mechanisms. Here we used the oculomotor vergence system as a model to investigate the mechanisms underlying slow motor adaptation. Unlike reaching with the upper limbs, vergence is less susceptible to changes in cognitive strategy that can affect the behaviour of motor adaptation. We tested the hypothesis that mechanisms of slow motor adaptation reflect early neural processing by assessing the linearity of adaptive responses over a large range of stimuli. Using varied disparity stimuli in conflict with accommodation, the slow adaptation of tonic vergence was found to exhibit a linear response whereby the rate (R(2) =0.85, P<0.0001) and amplitude (R(2) =0.65, P<0.0001) of the adaptive effects increased proportionally with stimulus amplitude. These results suggest that this slow adaptive mechanism is an early neural process, implying a fundamental physiological nature that is potentially dominated by subcortical and cerebellar substrates.

Vergence Adaptation to Short-Duration Stimuli in Early Childhood.

PurposeTo investigate whether nonstrabismic typically developing young children are capable of exhibiting vergence adaptation.MethodsFifteen adults (19.5–35.8 years) and 34 children (2.5–7.3 years) provided usable data. None wore habitual refractive correction. Eye position and accommodation were recorded using Purkinje image eye tracking and eccentric photorefraction (MCS PowerRefractor). Vergence was measured in three conditions while the participant viewed naturalistic targets at 33 cm. Viewing was monocular for at least 60 seconds and then binocular for either 5 seconds (5-second condition), 60 seconds (60-second), or 60 seconds through a 10-pd base-out prism (prism 60-second). The right eye was then occluded again for 60 seconds and an exponential function was fit to these data to assess the impact of adaptation on alignment.ResultsThe 63% time constant was significantly longer for the prism 60-second condition (mean = 11.5 seconds) compared to both the 5-second (5.3 seconds; P = 0.015) and the 60-second conditions (7.1 seconds; P = 0.035), with no significant difference between children and adults (P > 0.4). Correlations between the 63% time constant (prism 60-second condition) and age, refractive error, interpupillary distance (IPD), or baseline heterophoria were not significant (P > 0.4). The final stable monocular alignment, measured after binocular viewing, was similar to the baseline initial alignment across all conditions and ages.ConclusionsFor a limited-duration near task, 2- to 7-year-old children showed comparable levels of vergence adaptation to adults. In a typically developing visual system, where IPD and refractive error are maturing, this adaptation could help maintain eye alignment.

Gaze-dependent phoria and vergence adaptation

Incomitance is a condition with gaze-dependent deviations of ocular alignment and is common in strabismus patients. The physiological mechanisms that maintain equal horizontal ocular alignment in all gaze directions (concomitance) in healthy individuals are poorly explored. We investigate adaptive processes in the vergence system that are induced by horizontal incomitant vergence stimuli (stimuli that require a gaze-dependent vergence response in order to re-establish binocular single vision). We measured horizontal vergence responses elicited after healthy subjects shifted their gaze from a position that required no vergence to a position that required convergence. Repetitive saccades into a position with a convergence stimulus rapidly decreased phoria (defined as the deviation of ocular alignment in the absence of a binocular stimulus). This change of phoria was present in all viewing directions (from 0° to 0.86° ± 0.40°, p < 0.001) but was more pronounced in the gaze direction with a convergence stimulus (from 0.26° ± 0.13° to 1.39° ± 0.33°, p < 0.001). We also found that vergence velocity rapidly increased (p = 0.015) and vergence latency promptly decreased (p < 0.001). We found gaze-dependent modulation of phoria in combined saccade-vergence eye movements and also in pursuit-vergence eye movements. Thus, acute horizontal, gaze-dependent changes of vergence, such as may be encountered in new onset strabismus due to paralysis, can rapidly increase vergence velocity and decrease latency. Gaze-specific (concomitant) and gaze-independent (incomitant) phoria levels will adapt. These early adaptive processes increase the efficacy of binocular vision and maintain good ocular alignment in all directions of gaze.

The influence of lingering fusional adaptation on the Bielschowsky head tilt test in superior oblique paresis

PurposeTo investigate how fusion influences the Bielschowsky head tilt test (BHTT) in unilateral superior oblique paresis (SOP).MethodsIn eight fusing patients, we correlated haploscopic‐determined fusion mechanisms with BHTT differences (BHTTD).ResultsFive patients used the vertical recti for vertical fusional vergence and had a mean±SD BHTTD of 22 ± 8PD. After a 30‐minute patch test one of those, in whom the test was performed, showed a decrease of 10PD. Two patients used the “paretic” superior oblique muscle (SOM) and the contralateral superior rectus muscle (SRM) to fuse, and had a mean ± SD BHTTD of 6 ± 8PD. The BHTTD of one, in whom a patch test was performed, increased by 11PD. The remaining patient used the “paretic” SOM and contralateral inferior oblique muscle (IOM) to fuse, and had a BHTTD of only 3PD, increasing to 21PD after patching.One explanation for this behavior in the last patient involves lingering vergence adaptation of the “paretic” SOM and contralateral IOM, which makes these muscles more effective when activated on ipsilateral head tilt, lessening the expected increase in hyperdeviation. Similarly, in our patients with oblique/rectus‐mediated fusion, the vergence‐adapted “paretic” SOM and contralateral SRM are activated on ipsilateral and contralateral tilt respectively, lessening the hyperdeviation in both directions. In the other five patients, however, the vergence‐adapted ipsilateral IRM and contralateral SRM are activated on contralateral tilt, accentuating the BHTTD.ConclusionsFusion influences the BHTTD, either decreasing or increasing it depending on the particular muscles used for fusion. The absence of a positive BHTT should not be relied upon to rule out the diagnosis of SOP. In suspected SOP patients with fusion, performing the BHTT after a patch test may be necessary to bring out the BHTTD supporting the diagnosis.

Increased onset of vergence adaptation reduces excessive accommodation during the orthoptic treatment of convergence insufficiency

This research tested the hypothesis that the successful treatment of convergence insufficiency (CI) with vision-training (VT) procedures, leads to an increased capacity of vergence adaptation (VAdapt) allowing a more rapid downward adjustment of the convergence accommodation cross-link. Nine subjects with CI were recruited from a clinical population, based upon reduced fusional vergence amplitudes, receded near point of convergence or symptomology. VAdapt and the resulting changes to convergence accommodation (CA) were measured at specific intervals over 15min (pre-training). Separate clinical measures of the accommodative convergence cross link, horizontal fusion limits and near point of convergence were taken and a symptomology questionnaire completed. Subjects then participated in a VT program composed of 2.5h at home and 1h in-office weekly for 12–14weeks. Clinical testing was done weekly. VAdapt and CA measures were retaken once clinical measures normalized for 2weeks (mid-training) and then again when symptoms had cleared (post-training). VAdapt and CA responses as well as the clinical measures were taken on a control group showing normal clinical findings. Six subjects provided complete data sets. CI clinical findings reached normal levels between 4 and 7weeks of training but symptoms, VAdapt, and CA output remained significantly different from the controls until 12–14weeks. The hypothesis was retained. The reduced VAdapt and excessive CA found in CI were normalized through orthoptic treatment. This time course was underestimated by clinical findings but matched symptom amelioration.

Vergence adaptation in young children

Vergence adaptation in young children

Modeling and optimizing eye vergence response to stereoscopic cuts

Sudden temporal depth changes, such as cuts that are introduced by video edits, can significantly degrade the quality of stereoscopic content. Since usually not encountered in the real world, they are very challenging for the audience. This is because the eye vergence has to constantly adapt to new disparities in spite of conflicting accommodation requirements. Such rapid disparity changes may lead to confusion, reduced understanding of the scene, and overall attractiveness of the content. In most cases the problem cannot be solved by simply matching the depth around the transition, as this would require flattening the scene completely. To better understand this limitation of the human visual system, we conducted a series of eye-tracking experiments. The data obtained allowed us to derive and evaluate a model describing adaptation of vergence to disparity changes on a stereoscopic display. Besides computing user-specific models, we also estimated parameters of an average observer model. This enables a range of strategies for minimizing the adaptation time in the audience.

Can current models of accommodation and vergence predict accommodative behavior in myopic children?

Investigations into the progression of myopia in children have long considered the role of accommodation as a cause and solution. Myopic children show high levels of accommodative adaptation, coupled with accommodative lag and high response AC/A (accommodative convergence per diopter of accommodation). This pattern differs from that predicted by current models of interaction between accommodation and vergence, where weakened reflex responses and a high AC/A would be associated with a low not high levels of accommodative adaptation. However, studies of young myopes were limited to only part of the accommodative vergence synkinesis and the reciprocal components of vergence adaptation and convergence accommodation were not studied in tandem. Accordingly, we test the hypothesis that the accommodative behavior of myopic children is not predicted by current models and whether that departure is explained by differences in the accommodative plant of the myopic child. Responses to incongruent stimuli (-2D, +2D adds, 10 prism diopter base-out prism) were investigated in 28 myopic and 25 non-myopic children aged 7-15 years. Subjects were divided into phoria groups - exo, ortho and eso based upon their near phoria. The school aged myopes showed high levels of accommodative adaptation but with reduced accommodation and high AC/A. This pattern is not explained by current adult models and could reflect a sluggish gain of the accommodative plant (ciliary muscle and lens), changes in near triad innervation or both. Further, vergence adaptation showed a predictable reciprocal relationship with the high accommodative adaptation, suggesting that departures from adult models were limited to accommodation not vergence behavior.