How is binocularity maintained during convergence and divergence?

Examination of Near Point of Convergence Scores in High-School Athletes: Implications for Identifying Binocular Vision Dysfunction After Concussion Injury

OBJECTIVE

To identify normative near point of convergence (NPC) data for healthy high-school-aged athletes (13-19 years old) and determine the percentage of individuals with NPC scores that fall outside the currently accepted clinical cutoff value of 5 cm. Another objective was to determine the relationship between sex, concussion history, and attention deficit disorder/attention deficit hyperactivity disorder (ADD/ADHD) with NPC scores, if any.

DESIGN

Case series.

SETTING

High-school sports medicine clinic.

PATIENTS OR PARTICIPANTS

Near point of convergence was assessed in 718 high-school student athletes (141 females and 577 males) with an average age of 15.96 ± 1.16 years.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

An accommodation convergence ruler was used to measure NPC. Near point of convergence scores were repeated a total of 3 times and the mean used for all statistical analyses.

RESULTS

The NPC scores for all participants averaged 3.58 cm, and the intraclass correlation coefficient for the 3 repeated measurements was 0.956. Approximately 20% of mean NPC scores were above the accepted upper limit of 5 cm. Although a statistically significant effect for sex was identified, the difference between them was considered clinically insignificant. No relationship between NPC and history of concussion or ADD/ADHD was identified.

CONCLUSIONS

Results indicate that in high-school-aged subjects, approximately 20% of individuals may have NPC values that fall outside the current critical cutoff value and may lead to incorrect diagnosis of ocular dysfunction. In addition, NPC does not seem to be affected by the history of concussion or a diagnosis of ADD/ADHD.

Test-retest of a phoria adaptation stimulus-induced functional MRI experiment

This study was designed to identify the neural substrates activated during a phoria adaptation task using functional magnetic resonance imaging (MRI) in young adults with normal binocular vision and to test the repeatability of the fMRI measurements for this protocol. The phoria adaptation task consisted of a block protocol of 90 seconds of near visual crossed fixation followed by 90 seconds of far visual uncrossed fixation, repeated three times; the data were collected during two different experimental sessions. Results showed that the oculomotor vermis, cuneus, and primary visual cortex had the greatest functional activity within the regions of interest studied when stimulated by the phoria adaptation task. The oculomotor vermis functional activity had an intraclass correlation coefficient (ICC) of 0.3, whereas the bilateral cuneus and primary visual cortex had good ICC results of greater than 0.6. These results suggest that the sustained visual fixation task described within this study reliably activates the neural substrates of phoria adaptation. This protocol establishes a methodology that can be used in future longitudinal studies investigating therapeutic interventions that may modify phoria adaptation.

Vergence and accommodation disorders in children with vertigo: A need for evidence-based diagnosis

BACKGROUND

Previous clinical evaluations have demonstrated a difference in eye movements in healthy children compared to children with vertigo without vestibular pathology. It has also been previously shown that accommodation and vergence responses can be measured with remote haploscopic photo refractor (RHP) devices. We have developed a method, called REMOBI (patent US8851669, WO2011073288) that allows us to test eye movements in three-dimensional space without decoupling vergence and accommodation.[1].

METHODS

We compared standard clinical testing of vergence and accommodation responses separately, with laboratory simultaneous measurement of vergence and accommodation in healthy children, 31 with vertigo (mean age 11 SD +/- 3.02), and 53 without (mean age 10 SD +/- 3.29). Children diagnosed with vertigo then underwent orthoptic rehabilitation for vergence and accommodation disorders and were re-evaluated twice using laboratory testing: once after 12 sessions and once 3-months after completing the sessions.

FINDINGS

Using the clinical tests, significant differences were found between the vertigo and healthy groups: D' (break point of divergence near), D2 (second measurement of divergence after convergence far), D2' (second measurement of divergence after convergence near), C (break point of convergence far), and C' (break point of convergence near). However, no significant differences in accommodation or vergence were seen between the two groups using laboratory tests (RHP and REMOBI). Further, there was no difference in laboratory measurements in children with vertigo before, after, and 3 months after clinical rehabilitation.

INTERPRETATION

We postulate the difference in these two tests is because the laboratory tests are more accurate and more realistic because they measure accommodation and vergence simultaneously, as it incorporates a stronger binocular coordination response not appreciated by current clinical measurements. Further studies should be conducted to evaluate whether clinicians should consider adding objective measurements, such as using a RHP device, when diagnosing patients with vergence and accommodation disorders, to avoid prescribing costly and timely rehabilitation programs that do not improve accommodative and vergence movements.

FUNDING

We thank the Fulbright Foundation, along with the University of California, San Francisco, for the research fellowship to Lindsey M Ward. This study is part of the PHRC VERVE, hospital research program, run at the hospital Robert Debré and supported by Direction de la Recherche Clinique, Assistance Publique, France. The funding sources had no involvement in the study design; collection, analysis, and interpretation of data; writing of the manuscript; and in the decision to submit the manuscript for publication.

Comparison of Two Wavefront Autorefractors: Binocular Open-Field versus Monocular Closed-Field

Purpose

To evaluate the agreement and repeatability between a new commercially available binocular open-field wavefront autorefractor, as part of the Eye Refract system, and a monocular closed-field wavefront autorefractor (VX110).

Methods

A cross-sectional, randomized, and single-masked study was performed. Ninety-nine eyes of 99 healthy participants (37.22 ± 18.04 years, range 8 to 69 years) were randomly analyzed. Three measurements with the Eye Refract and the VX110 were taken on three different days, under noncycloplegic conditions. Mean spherical equivalent (MSE), cylindrical vectors (J0 and J45), and binocular corrected distance visual acuity (BCDVA) were compared between both autorefractors. An intersession repeatability analysis was done considering the values of repeatability (S_r) and its 95% limit (r).

Results

The VX110 showed more negative values (P < 0.001) in terms of MSE in comparison with the Eye Refract (0.20 D). Regarding cylindrical vectors, J45 showed statistically significant differences (P=0.001) between both wavefront autorefractors, but they were not clinically relevant (<0.05 D). In BCDVA, there were no statistically significant differences (P=0.667) between both wavefront autorefractors. Additionally, the Eye Refract was more repeatable than the VX110 in terms of both MSE (S_{rEYE REFRACT} = 0.21 D, S_rVX110 = 0.53 D) and J0 (S_{rEYE REFRACT} = 0.12 D, S_rVX110 = 0.35 D).

Conclusions

The Eye Refract provided enough accuracy and reliability to estimate refractive errors in different age groups, achieving better results than the VX110. Therefore, the Eye Refract proved to be a useful autorefractor to be incorporated into clinical practice.

Functional anatomy of extraocular muscles during human vergence compensation of horizontal heterophoria

We employed magnetic resonance imaging to quantify human extraocular muscle (EOM) contractility during intermittent convergent and divergent strabismus with each eye viewing monocularly at 20 cm compared with centered target fusion. Contractility, indicated by posterior partial volume change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments. In five subjects with intermittent esotropia, abduction of the deviated eye to monocular target fixation was associated with significant whole lateral rectus (LR) contraction, but with medial rectus (MR) relaxation that was significantly greater in the superior than inferior compartment. Esotropic eye abduction to binocular fusion was associated with similar relaxation in the two MR compartments, but with greater contraction in the LR's superior than inferior compartment. The whole diverging eye SO muscle relaxed. In three subjects with intermittent exotropia, converging eye fusional adduction was associated with significant whole LR relaxation and with MR contraction attributable to significantly greater contraction in the superior than inferior compartment. In adduction of the exotropic eye to monocular target fixation but not fusional adduction, the whole SO exhibited significant relaxation. Rectus pulley positions were not significantly altered by fusion of either form of intermittent strabismus. Globe rotational axis was eccentric in intermittent strabismus, rolling the eye so that rectus EOM lever arms facilitated vergence. These results confirm, and extend to fusion of intermittent horizontal strabismus, differential compartmental function in horizontal rectus EOMs and suggest a novel role for the SO in compensation of both intermittent esotropia and exotropia. NEW & NOTEWORTHY Disjunctive eye movements normally permit binocular fixation in near visual space but also compensate for mechanical imbalances in binocular alignment developing over the life span. Magnetic resonance imaging of the extraocular muscles demonstrates important differential function in muscle compartments during compensation of large-angle intermittent convergent and divergent strabismus in humans. Eye translation during rotation also enhances vergence compensation of intermittent strabismus.

Functional anatomy of human extraocular muscles during fusional divergence

We employed magnetic resonance imaging to quantify human extraocular muscle contractility during centered target fusion and fusional divergence repeated with each eye viewing monocularly at 20 cm through 8Δ and at 400 cm through 4Δ base in prism. Contractility, indicated by posterior partial volume (PPV) change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments and by cross-sectional area change in the inferior oblique (IO). At 20 cm, 3.1 ± 0.5° (SE) diverging eye abduction in 10 subjects was associated with 4.2 ± 1.5% whole lateral rectus (LR) PPV increase ( P < 0.05) and 1.7 ± 1.1% overall medial rectus (MR) PPV decrease attributable to 3.1 ± 1.8% reduction in the superior compartment ( P < 0.025), without change in its inferior compartment or in muscles of the aligned eye. At 400 cm, 2.2 ± 0.5° diverging eye abduction in nine subjects was associated with 6.1 ± 1.3% whole LR PPV increase ( P < 10^-5) but no change in MR, with compartmentally similar relaxation in the LR and MR of the aligned eye. Unlike convergence, there were no IO or SO contractile changes for divergence to either target nor any change in rectus pulley positions. Results confirm and extend to proximal divergence the unique role of the superior MR compartment, yet no MR role for far divergence. Corelaxation of aligned eye LR and MR combined with failure of MR relaxation during divergence is consistent with the limited behavioral range of divergence. NEW & NOTEWORTHY Magnetic resonance imaging shows that the lateral rectus muscle must overcome continued contraction by its opponent the medial rectus when humans diverge their visual axes to achieve single, binocular vision. While the upper but not lower compartment of the medial rectus assists by relaxing for near targets, it does not do so when targets are far away. This behavior violates Sherrington's law of reciprocal action of antagonists and conventional assumptions about the ocular motor system.

Three-dimensional binocular eye-hand coordination in normal vision and with simulated visual impairment

Sensorimotor coupling in healthy humans is demonstrated by the higher accuracy of visually tracking intrinsically-rather than extrinsically-generated hand movements in the fronto-parallel plane. It is unknown whether this coupling also facilitates vergence eye movements for tracking objects in depth, or can overcome symmetric or asymmetric binocular visual impairments. Human observers were therefore asked to track with their gaze a target moving horizontally or in depth. The movement of the target was either directly controlled by the observer's hand or followed hand movements executed by the observer in a previous trial. Visual impairments were simulated by blurring stimuli independently in each eye. Accuracy was higher for self-generated movements in all conditions, demonstrating that motor signals are employed by the oculomotor system to improve the accuracy of vergence as well as horizontal eye movements. Asymmetric monocular blur affected horizontal tracking less than symmetric binocular blur, but impaired tracking in depth as much as binocular blur. There was a critical blur level up to which pursuit and vergence eye movements maintained tracking accuracy independent of blur level. Hand-eye coordination may therefore help compensate for functional deficits associated with eye disease and may be employed to augment visual impairment rehabilitation.

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.

Dynamics of the near response under natural viewing conditions with an open-view sensor

We have studied the temporal dynamics of the near response (accommodation, convergence and pupil constriction) in healthy subjects when accommodation was performed under natural binocular and monocular viewing conditions. A binocular open-view multi-sensor based on an invisible infrared Hartmann-Shack sensor was used for non-invasive measurements of both eyes simultaneously in real time at 25Hz. Response times for each process under different conditions were measured. The accommodative responses for binocular vision were faster than for monocular conditions. When one eye was blocked, accommodation and convergence were triggered simultaneously and synchronized, despite the fact that no retinal disparity was available. We found that upon the onset of the near target, the unblocked eye rapidly changes its line of sight to fix it on the stimulus while the blocked eye moves in the same direction, producing the equivalent to a saccade, but then converges to the (blocked) target in synchrony with accommodation. This open-view instrument could be further used for additional experiments with other tasks and conditions.

A Case Report of Nystagmus with Acute Comitant Esotropia Secondary to Heroin Withdrawal: A Novel Presentation

Background

Acute comitant esotropia secondary to heroin withdrawal is a rarely reported phenomenon that has never been described with nystagmus. Adverse effects of heroin on eye alignment were first reported in soldiers returning from Vietnam, yet no theory is generally accepted as the cause of these abnormalities.

Method

We present a case of a 22-year-old female who developed 40 prism diopters of alternating comitant esotropia with nystagmus 8 days after abrupt heroin cessation, review the existing literature, and propose a novel hypothesis for this phenomenon.

Results

After 76 days, her esotropia resolved, and she was left with 7 prism diopters of esophoria.

Conclusion

This case demonstrates that acquired nystagmus can present in addition to acute-onset esotropia after abrupt heroin cessation. We compare and contrast the theories of this mechanism and review the literature.

Disparity-driven vs blur-driven models of accommodation and convergence in binocular vision and intermittent strabismus

PURPOSE

To propose an alternative and practical model to conceptualize clinical patterns of concomitant intermittent strabismus, heterophoria, and convergence and accommodation anomalies.

METHODS

Despite identical ratios, there can be a disparity- or blur-biased "style" in three hypothetical scenarios: normal; high ratio of accommodative convergence to accommodation (AC/A) and low ratio of convergence accommodation to convergence (CA/C); low AC/A and high CA/C. We calculated disparity bias indices (DBI) to reflect these biases and provide early objective data from small illustrative clinical groups that fit these styles.

RESULTS

Normal adults (n = 56) and children (n = 24) showed disparity bias (adult DBI 0.43 [95% CI, 0.50-0.36], child DBI 0.20 [95% CI, 0.31-0.07]; P = 0.001). Accommodative esotropia (n = 3) showed less disparity-bias (DBI 0.03). In the high AC/A-low CA/C scenario, early presbyopia (n = 22) showed mean DBI of 0.17 (95% CI, 0.28-0.06), compared to DBI of -0.31 in convergence excess esotropia (n=8). In the low AC/A-high CA/C scenario near exotropia (n = 17) showed mean DBI of 0.27. DBI ranged between 1.25 and -1.67.

CONCLUSIONS

Establishing disparity or blur bias adds to AC/A and CA/C ratios to explain clinical patterns. Excessive bias or inflexibility in near-cue use increases risk of clinical problems.

Clinical test responses to different orthoptic exercise regimes in typical young adults

Purpose

The relative efficiency of different eye exercise regimes is unclear, and in particular the influences of practice, placebo and the amount of effort required are rarely considered. This study measured conventional clinical measures following different regimes in typical young adults.

Methods

A total of 156 asymptomatic young adults were directed to carry out eye exercises three times daily for 2 weeks. Exercises were directed at improving blur responses (accommodation), disparity responses (convergence), both in a naturalistic relationship, convergence in excess of accommodation, accommodation in excess of convergence, and a placebo regime. They were compared to two control groups, neither of which were given exercises, but the second of which were asked to make maximum effort during the second testing.

Results

Instruction set and participant effort were more effective than many exercises. Convergence exercises independent of accommodation were the most effective treatment, followed by accommodation exercises, and both regimes resulted in changes in both vergence and accommodation test responses. Exercises targeting convergence and accommodation working together were less effective than those where they were separated. Accommodation measures were prone to large instruction/effort effects and monocular accommodation facility was subject to large practice effects.

Conclusions

Separating convergence and accommodation exercises seemed more effective than exercising both systems concurrently and suggests that stimulation of accommodation and convergence may act in an additive fashion to aid responses. Instruction/effort effects are large and should be carefully controlled if claims for the efficacy of any exercise regime are to be made.

Developmental changes in the balance of disparity, blur, and looming/proximity cues to drive ocular alignment and focus

Accurate coordination of accommodation and convergence is necessary to view near objects and develop fine motor coordination. We used a remote haploscopic videorefraction paradigm to measure longitudinal changes in simultaneous ocular accommodation and vergence to targets at different depths, and to all combinations of blur, binocular disparity, and change-in-size ('proximity') cues. Infants were followed longitudinally and compared with older children and young adults, with the prediction that sensitivity to different cues would change during development. Mean infant responses to the most naturalistic condition were similar to those of adults from 6-7 weeks (accommodation) and 8-9 weeks (vergence). Proximity cues influenced responses most in infants of less than 14 weeks of age, but sensitivity declined thereafter. Between 12 and 28 weeks of age infants were equally responsive to all three cues, while in older children and adults manipulation of disparity resulted in the greatest changes in response. Despite rapid development of visual acuity (thus increasing availability of blur cues), responses to blur were stable throughout development. Our results suggest that, during much of infancy, vergence and accommodation responses are not dependent on the development of specific depth cues, but make use of any cues available to drive appropriate changes in response.

The clinical near gradient stimulus AC/A ratio correlates better with the response CA/C ratio than with the response AC/A ratio

AIM

To provide evidence that a near clinical gradient AC/A ratio could instead reflect the CA/C relationship (the accommodation driven by response to disparity).

DESIGN

Case control study.

METHODOLOGY

27 emmetropic participants with heterophoria <4 PD, 19 with intermittent distance exotropia, and 17 with near exophoria >6 PD were tested. A remote haploscopic photorefractor, which can measure simultaneous convergence and accommodation to a range of targets containing all combinations of presence or absence of binocular disparity, blur, and proximal (looming) cues, was used to assess response AC/A and CA/C relationships. These were compared with clinical gradient AC/A ratios at near and distance fixation using alternate prism cover test and plus or minus lenses.

RESULTS

Although the near and distance clinical AC/A ratios correlated weakly with each other (p = 0.03), neither clinical method correlated with the more accurate response AC/A ratio from the laboratory method (p = 0.88 and p = 0.93, respectively). The laboratory CA/C ratio correlated strongly with the near clinical AC/A ratio (p = 0.004) but only very weakly with the distance ratio (p = 0.16).

CONCLUSIONS

The "near gradient AC/A ratio" may actually reflect the CA/C linkage as the dissociation of the prism cover test disrupts vergence accommodation. If the near deviation diverges more with plus lenses, it may be because the lenses allow clear near vision without needing to recruit convergence accommodation to achieve it.

Receding and disparity cues aid relaxation of accommodation

PURPOSE

Accommodation can mask hyperopia and reduce the accuracy of non-cycloplegic refraction. It is, therefore, important to minimize accommodation to obtain a measure of hyperopia as accurate as possible. To characterize the parameters required to measure the maximally hyperopic error using photorefraction, we used different target types and distances to determine which target was most likely to maximally relax accommodation and thus more accurately detect hyperopia in an individual.

METHODS

A PlusoptiX SO4 infra-red photorefractor was mounted in a remote haploscope which presented the targets. All participants were tested with targets at four fixation distances between 0.3 and 2 m containing all combinations of blur, disparity, and proximity/looming cues. Thirty-eight infants (6 to 44 weeks) were studied longitudinally, and 104 children [4 to 15 years (mean 6.4)] and 85 adults, with a range of refractive errors and binocular vision status, were tested once. Cycloplegic refraction data were available for a sub-set of 59 participants spread across the age range.

RESULTS

The maximally hyperopic refraction (MHR) found at any time in the session was most frequently found when fixating the most distant targets and those containing disparity and dynamic proximity/looming cues. Presence or absence of blur was less significant, and targets in which only single cues to depth were present were also less likely to produce MHR. MHR correlated closely with cycloplegic refraction (r = 0.93, mean difference 0.07 D, p = n.s., 95% confidence interval +/-<0.25 D) after correction by a calibration factor.

CONCLUSIONS

Maximum relaxation of accommodation occurred for binocular targets receding into the distance. Proximal and disparity cues aid relaxation of accommodation to a greater extent than blur, and thus non-cycloplegic refraction targets should incorporate these cues. This is especially important in screening contexts with a brief opportunity to test for significant hyperopia. MHR in our laboratory was found to be a reliable estimation of cycloplegic refraction.

Human vergence eye movements to oblique disparity stimuli: evidence for an anisotropy favoring horizontal disparities

Binocular disparities applied to large-field patterns elicit vergence eye movements at ultra-short latencies. We used the electromagnetic search coil technique to record the horizontal and vertical positions of both eyes while subjects briefly viewed (150 ms) large patterns that were identical at the two eyes except for a difference in position (binocular disparity) that was varied in direction from trial to trial. For accurate alignment with the stimuli, the horizontal and vertical disparity vergence responses (HDVRs, VDVRs) should vary as the sine and cosine, respectively, of the direction of the disparity stimulus vector. In a first experiment, using random-dots patterns (RDs) with a binocular disparity of 0.2 degrees , this was indeed the case. In a second experiment, using 1-D sine-wave gratings with a binocular phase difference (disparity) of 1/4-wavelength, it was not the case: HDVRs were maximal when the grating was vertical and showed little decrement until the grating was oriented more than approximately 65 degrees away from vertical, whereas VDVRs were maximal when the grating was horizontal and began to decrement roughly linearly when the grating was oriented away from the horizontal. We attribute these complex directional dependencies with gratings to the aperture problem, and the HDVR data strongly resemble the stereothresholds for 1-D gratings, which are minimal when the gratings are vertical and remain constant for orientations up to approximately 80 degrees away from the vertical when expressed as spatial phase disparities [Morgan, M. J., & Castet, E. (1997). The aperture problem in stereopsis. Vision Research, 37, 2737-2744.]. To explain this constancy of stereothresholds, Morgan and Castet (1997) postulated detectors sensitive to the phase disparity of the gratings seen by the two eyes (rather than their linear separation along some fixed axis, such as the horizontal). However, because (1) our VDVR data with gratings did not show this constancy and (2) the available evidence strongly suggests that there are no major differences in the disparity detectors mediating the initial HDVR and VDVR, we sought an alternative explanation for our data. We show that the dependence of the initial HDVR and VDVR on grating orientation can be successfully modeled by a bias in the number and/or efficacy of the detectors that favors horizontal disparities.

The use of cues to convergence and accommodation in naïve, uninstructed participants

A remote haploscopic video refractor was used to assess vergence and accommodation responses in a group of 32 emmetropic, orthophoric, symptom free, young adults naïve to vision experiments in a minimally instructed setting. Picture targets were presented at four positions between 2 m and 33 cm. Blur, disparity and looming cues were presented in combination or separately to asses their contributions to the total near response in a within-subjects design. Response gain for both vergence and accommodation reduced markedly whenever disparity was excluded, with much smaller effects when blur and proximity were excluded. Despite the clinical homogeneity of the participant group there were also some individual differences.

Human vergence eye movements initiated by competing disparities: evidence for a winner-take-all mechanism

Vergence eye movements were elicited in human subjects at short latencies (approximately 70 ms) by applying binocular disparities briefly (200 ms) to large grating patterns (46 degrees wide, 35 degrees high). The positions of both eyes were recorded with the electromagnetic search coil technique. Using a dichoptic viewing arrangement (Wheatstone stereoscope), each eye viewed two overlapping 1-D sine waves that had the same orientation but different spatial frequencies. These two sine waves each had a binocular disparity that was 1/4 of its wavelength and the effect of varying their relative contrasts was examined (15 contrast ratios ranging from 0.125 to 8). The first experiment used horizontal gratings and recorded the vertical vergence responses when the two sine waves had spatial frequencies in the ratio 3:5 and vertical disparities of opposite sign. Initial vergence responses showed a highly nonlinear dependence on the contrast ratio. On average, when the contrast of one sine wave exceeded that of the other by a factor of >2.2, the sine wave with the higher contrast dominated responses and the sine wave with the lower contrast had almost no influence: winner-take-all. A second experiment, which used vertical gratings and recorded the horizontal vergence responses when the two sine waves had spatial frequencies in the ratio 3:5 and horizontal disparities of opposite sign, also uncovered nonlinear interactions but these were much more variable from one subject to another and, on average, one sine wave did not achieve complete dominance until its contrast exceeded that of the other by a factor of >4.5. When these two experiments were repeated with grating patterns in which the two sine waves had spatial frequencies in the ratio 3:7 and disparities of the same sign, similar nonlinear interactions were apparent. We attribute the nonlinear dependence on relative contrast to mutual inhibition between the neural elements processing the disparities of the two sine waves. We further suggest that this interaction will help to maintain binocular alignment on the objects in the plane of regard because the retinal images of those objects will tend to be better focused-and hence tend to have higher contrasts-than the images of objects in other depth planes.

Short-latency disparity vergence eye movements: a response to disparity energy

Vergence eye movements were elicited in human subjects by applying disparities to square-wave gratings lacking the fundamental ("missing fundamental", mf). Using a dichoptic arrangement, subjects viewed gratings that were identical at the two eyes except for a phase difference of 1/4 wavelength so that, based on the nearest-neighbor matches, the features and the 4n+1 harmonics (5th, 9th, etc.) all had binocular disparities of one sign, whereas the 4n-1 harmonics (3rd, 7th, etc.) all had disparities of the opposite sign. Further, the amplitude of the ith harmonic was proportional to 1/i. Using the electromagnetic search coil technique to record the positions of both eyes indicated that the earliest vergence eye movements elicited by these disparity stimuli had ultra-short latencies (minimum, <65 ms) and were always in the direction of the most prominent harmonic, the 3rd, but their magnitudes fell short of those elicited when the same disparities were applied to pure sinusoids whose spatial frequency and contrast matched those of the 3rd harmonic. This shortfall was evident in both the horizontal vergence responses recorded with vertical grating stimuli and the vertical vergence responses recorded with horizontal grating stimuli. When the next most prominent harmonic, the 5th, was removed from the mf stimulus (creating the "mf-5" stimulus) the vertical vergence responses showed almost no shortfall-indicating that it had been almost entirely due to that 5th harmonic-but the horizontal vergence responses still showed a small shortfall, at least with higher contrast stimuli. This small shortfall might represent a very minor contribution from higher harmonics and/or distortion products and/or a feature-based mechanism. We conclude that the earliest disparity vergence responses-especially vertical-were strongly dependent on the major Fourier components of the binocular images, consistent with early spatial filtering of the monocular visual inputs prior to their binocular combination as in the disparity-energy model of complex cells in striate cortex [Ohzawa, I., DeAngelis, G. C., & Freeman, R. D. (1990). Stereoscopic depth discrimination in the visual cortex: neurons ideally suited as disparity detectors. Science, 249, 1037-1041].

Orthoptic status before and immediately after heroin detoxification

AIM

To determine whether changes in orthoptic status take place during withdrawal from heroin and/or methadone.

METHOD

A prospective study of patients, using a repeated measures design, attending a 5 day naltrexone compressed opiate detoxification programme.

RESULTS

83 patients were seen before detoxification (mean age 27.1 (SD 4.6) years) and 69 after detoxification. The horizontal angle of deviation became less exo/more eso at distance (p<0.001) but no significant change was found at near (p = 0.069). Stereoacuity, visual acuity, and convergence were found to be reduced in the immediate post-detoxification period. Prism fusion range, refractive error, subjective accommodation, and objective accommodation at 33 cm did not reduce but a small decrease was found in objective accommodation at 20 cm.

CONCLUSIONS

The eso trend found in these patients may be responsible for the development of acute concomitant esotropia in some patients undergoing heroin detoxification. However, the mechanism for this trend does not appear to be caused by divergence insufficiency or sixth nerve palsy.

Development of the response AC/A ratio over the first year of life

This study investigated the development of the link from accommodation to vergence in infants by occluding one eye thus removing binocular cues. Occluded adults continue to converge partially demonstrating that the accommodative drive to vergence (the AC/A link) and proximal cues are sufficient to drive vergence. For infants of all ages, AC/A ratios were found to be in the normal adult range. We conclude that infants can use monocular cues to drive vergence and that this occurs before the age when there is a substantial increase in the accuracy of oculomotor processes. There is flexibility in the developing visual system which is able to produce early vergence responses by relying upon alternative cues.

Variations in accommodation and convergence responses in a minimally controlled photorefractive setting

PURPOSE

A remote haploscopic photorefractor, designed for assessment of accommodation and convergence in infants and clinical groups, was used to determine heterophoria accommodative convergence/accommodation (AC/A) ratios in normal naïve adults. These were compared with conventional clinical measures.

METHODS

Twenty-one naïve subjects were used to compare occluded and unoccluded prism cover test responses with the remote haploscopic photorefractor using a text and picture target.

RESULTS

Although luminance was generally low for both targets, binocular vergences were appropriate for target demand in both studies. Binocular accommodation showed greater lag for the highest target accommodative demand and the less demanding target. Occlusion not only reduced vergence response, but also frequently caused a marked reduction in accommodation, especially to the picture target. Normal mean AC/A values were found, but with wide variations between individual subjects.

CONCLUSIONS

Although mean accommodation, vergence, and AC/A values were comparable with published data, we suggest that in these conditions using naïve subjects, accommodation is frequently inaccurate, especially on occlusion, without concomitant loss of vergence, at least at low light levels. Accommodative convergence may play a less important part in, and other cues contribute more to, the near reflex than has been previously suggested.

Heroin withdrawal as a possible cause of acute concomitant esotropia in adults

PURPOSE

To report the possible effects of heroin withdrawal on binocular vision.

METHODS

A case series of patients is presented in whom esotropia developed on cessation of heroin use.

RESULTS

In each case the esotropia was concomitant and prismatic correction restored binocular single vision. Intermittent spontaneous control occurred in one patient, the deviation resolved in one and one patient was lost to follow-up.

CONCLUSIONS

Heroin withdrawal should be considered as a cause of acute concomitant esotropia. However, an accurate history of other medication is needed to ensure that this is not the cause of decompensation.

Binocularity in infancy

A variety of behavioural and electrophysiological studies agree that the onset of functional binocular interaction in human visual cortex normally occurs between 10 and 16 weeks of age. Measures of sensitivity to binocular correlation and to disparity agree closely, and behavioural and visual evoked potential measures on the same infant show onset of binocularity within about a 2 week range. Beyond the initial onset, the maximum disparity to which infants are sensitive increases steadily and stereoacuity is found to increase very rapidly. The initial development of binocularity does not appear to be a consequence of improving alignment of the eyes and occurs even in the presence of strabismus. However, the connections subserving binocularity are plastic in early childhood; they can be disrupted by unilateral strabismus, although in some strabismic children who use both eyes for fixation, they can adapt to serve stereo function at the angle of deviation and re-adapt, albeit temporarily, to the surgical alignment of the eyes. These findings allow us to pose some as yet unanswered questions about the development of binocularity, including: How is the infant's visual system organised before the establishment of binocularity? How does the pre-binocular infant maintain vergence? And what neural changes underlie the increase in performance for small and large disparities following the initial onset of binocular function?

What happens to binocularity in primate strabismus?

Normal humans for whom the positions and movements of the two eyes are constrained to be yoked together are able to extract rich binocular sensory information from the environment. Humans with strabismus are deficient in extracting some of this information. Studies of strabismus in non-human primates can augment what has been learned from humans about relationships between strabismus and sensory binocular function. For example, speculation about the role of binocular vision in primate evolution can help us understand why it is that the advantages of sensory binocular function outweigh the disadvantages of having the positions of the two eyes yoked together. Physiological optics assessments of fixation patterns and accommodative responses in monkeys provide information about how the brain accomplishes and coordinates motor and sensory binocular functions, and sets the stage for determining underlying neural mechanisms responsible for this coordination. Finally, a developmental perspective, concerned with events that occur during an early sensitive period in the life span of an infant primate, can help us understand how nature and nurture interact to set up this complex neural system in normal individuals, and how this process is disrupted in conditions such as strabismus.