An edge-simplicity bias in the visual input to young infants

The development of sparse edge coding in the mammalian visual cortex depends on early visual experience. In humans, there are multiple indicators that the statistics of early visual experiences has unique properties that may support these developments. However, there are no direct measures of the edge statistics of infant daily-life experience. Using head-mounted cameras to capture egocentric images of young infants and adults in the home, we found infant images to have distinct edge statistics relative to adults. For infants, scenes with sparse edge patterns-few edges and few orientations-dominate. The findings implicate biased early input at the scale of daily life that is likely specific to the early months after birth and provide insights into the quality, amount, and timing of the visual experiences during the foundational developmental period for human vision.

How Do Most Young Moderate Hyperopes Avoid Strabismus?

Purpose

Uncorrected hyperopic children must overcome an apparent conflict between accommodation and vergence demands to focus and align their retinal images. This study tested hypotheses about simultaneous accommodation and vergence performance of young hyperopes to gain insight into ocular motor strategies used to maintain eye alignment.

Methods

Simultaneous eccentric photorefraction and Purkinje image tracking were used to assess accommodative and vergence responses of 26 adult emmetropes (AE) and 94 children (0-13 years) viewing cartoons. Children were habitually uncorrected (CU) (spherical equivalent refractive error [SE] -0.5 to +4 D), corrected and aligned (CCA), or corrected with a history of refractive esotropia (CCS). Accommodative and vergence accuracy, dissociated heterophoria, and vergence/accommodation ratios in the absence of retinal disparity cues were measured for 33- and 80-cm viewing distances.

Results

In binocular viewing, median accommodative lags for 33 cm were 1.0 D (AE), 1.33 D (CU), 1.25 D (CCA), and 1.0 D (CCS). Median exophorias at 80 and 33 cm were 1.2 and 4.5 pd (AE), 0.8 and 2.5 pd (CU), and 0 and 1.2 pd (CCA), respectively. Without disparity cues, most response vergence/accommodation ratios were between 1 and 2 meter angle/D (∼5-10 pd/D) (69% of AE, 44% of CU, 60% of CCA, and 50% of CCS).

Conclusions

Despite apparent conflict in motor coupling, uncorrected hyperopes were typically exophoric and achieved adultlike accuracy of both vergence and accommodation simultaneously, indicating ability to compensate for conflicting demands rather than bias to accurate vergence while tolerating inaccurate accommodation. Large lags and esophoria are therefore atypical. This analysis provides normative guidelines for clinicians and a deeper mechanistic understanding of how hyperopes avoid strabismus.

Prescribing patterns for paediatric hyperopia among paediatric eye care providers

PURPOSE

To survey paediatric eye care providers to identify current patterns of prescribing for hyperopia.

METHODS

Paediatric eye care providers were invited, via email, to participate in a survey to evaluate current age-based refractive error prescribing practices. Questions were designed to determine which factors may influence the survey participant's prescribing pattern (e.g., patient's age, magnitude of hyperopia, patient's symptoms, heterophoria and stereopsis) and if the providers were to prescribe, how much hyperopic correction would they prescribe (e.g., full or partial prescription). The response distributions by profession (optometry and ophthalmology) were compared using the Kolmogorov-Smirnov cumulative distribution function test.

RESULTS

Responses were submitted by 738 participants regarding how they prescribe for their hyperopic patients. Most providers within each profession considered similar clinical factors when prescribing. The percentages of optometrists and ophthalmologists who reported considering the factor often differed significantly. Factors considered similarly by both optometrists and ophthalmologists were the presence of symptoms (98.0%, p = 0.14), presence of astigmatism and/or anisometropia (97.5%, p = 0.06) and the possibility of teasing (8.3%, p = 0.49). A wide range of prescribing was observed within each profession, with some providers reporting that they would prescribe for low levels of hyperopia while others reported that they would never prescribe. When prescribing for bilateral hyperopia in children with age-normal visual acuity and no manifest deviation or symptoms, the threshold for prescribing decreased with age for both professions, with ophthalmologists typically prescribing 1.5-2 D less than optometrists. The threshold for prescribing also decreased for both optometrists and ophthalmologists when children had associated clinical factors (e.g., esophoria or reduced near visual function). Optometrists and ophthalmologists most commonly prescribed based on cycloplegic refraction, although optometrists most commonly prescribed based on both the manifest and cycloplegic refraction for children ≥7 years.

CONCLUSION

Prescribing patterns for paediatric hyperopia vary significantly among eye care providers.

Discrimination of paediatric acuity test optotypes by 6-year-old children

PURPOSE

To compare the discrimination performance of 6-year-old children for optotypes from six paediatric visual acuity tests and to fit Luce's Biased Choice Model to the data to estimate the relative similarities and bias for each optotype.

METHODS

Full data sets were collected from 20 typically developing 6-year-olds who had passed a vision screening. They were presented with single optotypes labelled 6/12 at a distance of 9 m and were asked to identify the optotype using a matching task containing all optotypes from the relevant test. The data were combined to form a confusion matrix for each test and a biased choice model was fitted to the data.

RESULTS

Median correct performance varied from 40% to 100% across optotypes, with the HOTV test having the highest values. Estimates of the similarity of each pair of optotypes indicated equal values for all pairs in the Landolt C, HOTV, Lea numbers and Tumbling E tests. The values differed for the picture tests, that is Lea Symbols and Allen figures. The estimates of bias for each individual optotype also indicated different values with the picture tests.

CONCLUSIONS

Previous studies of the threshold acuity of young children and adults have indicated differences in acuity estimates across paediatric tests. A recognition acuity task typically requires resolving the difference information between optotypes. The performance of the 6-year-olds here reveals variance in similarity and bias values for picture tests, particularly for the Allen figures when compared with the Lea Symbols. Ideally, this analysis should be performed when designing new tests, and these results motivate progression from the use of current picture tests to well calibrated letter or number tests at the earliest possible age.

Vergence and accommodation responses in the control of intermittent exotropia

PURPOSE

Individuals with different types of intermittent exotropia (IXT) may use neurally coupled accommodation and vergence responses differently from those without exotropia to achieve eye alignment. This study examined the relationship between simultaneously recorded accommodation and vergence responses in children and young adults with a range of types of IXT while aligned and deviated.

METHODS

Responses of 29 participants with IXT (4-31 years) and 24 age-matched controls were recorded using simultaneous eye-tracking and eccentric photorefraction while they watched a movie in binocular or monocular viewing at varying viewing distances. Gradient response AC/A ratios and fusional vergence ranges were also assessed. Eight participants had divergence or pseudo-divergence excess type IXT, 5 had convergence insufficiency and 16 had basic IXT.

RESULTS

Control and IXT participants accommodated similarly both in monocular and binocular-aligned conditions to visual targets at 80 and 33 cm. When deviated in binocular viewing, most participants with IXT exhibited changes in accommodation <0.5D relative to alignment. Gradient response AC/A ratios were similar for control [0.56 MA/D (IQR: 0.51 MA/D)] and IXT participants [0.42 MA/D (0.54 MA/D); p  = 0.60]. IXT participants showed larger vergence to accommodation ratios with changes from distance to near fixation [1.19 MA/D (1.45 MA/D)] than control participants [0.78 MA/D (0.60 MA/D); p = 0.02], especially among IXT participants with divergence or pseudo-divergence excess. Participants with IXT exhibited typical fusional divergence ranges beyond their dissociated position [8.86 Δ (7.10 Δ)] and typical fusional convergence ranges from alignment [18 Δ (15.75 Δ)].

CONCLUSIONS

This study suggests that control of IXT is typically neither driven by accommodative convergence alone nor associated with over-accommodation secondary to fusional convergence efforts. These simultaneous measurements confirmed that proximal vergence contributed significantly to IXT control, particularly for divergence or pseudo-divergence excess type IXT. For IXT participants in this study, achieving eye alignment did not conflict with having clear vision.

Detection of Significant Hyperopia in Preschool Children Using Two Automated Vision Screeners

SIGNIFICANCE

Moderate to high uncorrected hyperopia in preschool children is associated with amblyopia, strabismus, reduced visual function, and reduced literacy. Detecting significant hyperopia during screening is important to allow children to be followed for development of amblyopia or strabismus and implementation of any needed ophthalmic or educational interventions.

PURPOSE

This study aimed to compare the sensitivity and specificity of two automated screening devices to identify preschool children with moderate to high hyperopia.

METHODS

Children in the Vision in Preschoolers (VIP) study were screened with the Retinomax Autorefractor (Nikon, Inc., Melville, NY) and Plusoptix Power Refractor II (Plusoptix, Nuremberg, Germany) and examined by masked eye care professionals to detect the targeted conditions of amblyopia, strabismus, or significant refractive error, and reduced visual acuity. Significant hyperopia (American Association for Pediatric Ophthalmology and Strabismus definition of hyperopia as an amblyopia risk factor), based on cycloplegic retinoscopy, was >4.00 D for age 36 to 48 months and >3.50 D for age older than 48 months. Referral criteria from VIP for each device and from a distributor (PediaVision) for the Power Refractor II were applied to screening results.

RESULTS

Among 1430 children, 132 children had significant hyperopia in at least one eye. Using the VIP referral criteria, sensitivities for significant hyperopia were 80.3% for the Retinomax and 69.7% for the Power Refractor II (difference, 10.6%; 95% confidence interval, 7.0 to 20.5%; P = .04); specificities relative to any targeted condition were 89.9 and 89.1%, respectively. Using the PediaVision referral criteria for the Power Refractor, sensitivity for significant hyperopia was 84.9%; however, specificity relative to any targeted condition was 78.3%, 11.6% lower than the specificity for the Retinomax. Analyses using the VIP definition of significant hyperopia yielded results similar to when the American Association for Pediatric Ophthalmology and Strabismus definition was used.

DISCUSSION

When implementing vision screening programs for preschool children, the potential for automated devices that use eccentric photorefraction to either miss detecting significant hyperopia or increase false-positive referrals must be taken into consideration.

The effect of spherical defocus on vergence and diplopia in adults and children

Anisometropic children (with unequal refractive error in the two eyes) are at risk for strabismus or amblyopia. We simulated unilateral and bilateral spherical defocus to clarify the link between anisometropia, vergence eye movements and the perception of single vision. Adults (N=20, 15-51 yrs) and typically developing children (N=9, 5-8 yrs) viewed a dichoptic cartoon character presented in the fixation plane. Step changes in disparity of between 0 and 8 deg were introduced for 320 ms, followed by a blank screen. Vergence responses were recorded with an Eyelink 1000 and subjects reported whether they perceived a single (fused) or double (diplopic) character. Spherical defocus from 0.5 to 4D was applied to one or both eyes' stimuli using convolution of the point spread function for the subject's pupil size. Open loop vergence amplitude was used to estimate the reflex response (without feedback). For no defocus, open loop vergence peaked at +/-2 deg disparity with amplitudes of 0.29 deg convergence and -0.44 deg divergence for adults, and 0.56 deg and -0.46 deg for children. Bilateral defocus had no effect on this reflex response; however, 4D unilateral defocus significantly reduced vergence amplitudes in both age groups with no corresponding change in diplopia thresholds. Our results confirm that anisometropic blur is particularly disruptive to reflex vergence responses, which could limit the accurate realignment of the eyes and lead to atypical binocular development.

The Impact of Hyperopia on Academic Performance Among Children: A Systematic Review

PURPOSE

To assess the impact of uncorrected hyperopia and hyperopic spectacle correction on children's academic performance.

DESIGN

Systematic review and meta-analysis.

METHODS

We searched 9 electronic databases from inception to July 26, 2021, for studies assessing associations between hyperopia and academic performance. There were no restrictions on language, publication date, or geographic location. A quality checklist was applied. Random-effects models estimated pooled effect size as a standardized mean difference (SMD) in 4 outcome domains: cognitive skills, educational performance, reading skills, and reading speed. (PROSPERO registration: CRD-42021268972).

RESULTS

Twenty-five studies (21 observational and 4 interventional) out of 3415 met the inclusion criteria. No full-scale randomized trials were identified. Meta-analyses of the 5 studies revealed a small but significant adverse effect on educational performance in uncorrected hyperopic compared to emmetropic children {SMD -0.18 [95% confidence interval (CI), -0.27 to -0.09]; P < 0.001, 4 studies} and a moderate negative effect on reading skills in uncorrected hyperopic compared to emmetropic children [SMD -0.46 (95% CI, -0.90 to -0.03); P = 0.036, 3 studies]. Reading skills were significantly worse in hyperopic than myopic children [SMD -0.29 (95% CI, -0.43 to -0.15); P < 0.001, 1 study]. Qualitative analysis on 10 (52.6%) of 19 studies excluded from meta-analysis found a significant (P < 0.05) association between uncorrected hyperopia and impaired academic performance. Two interventional studies found hyperopic spectacle correction significantly improved reading speed (P < 0.05).

CONCLUSIONS

Evidence indicates that uncorrected hyperopia is associated with poor academic performance. Given the limitations of current methodologies, further research is needed to evaluate the impact on academic performance of providing hyperopic correction.

Vision Screening, Vision Disorders, and Impacts of Hyperopia in Young Children: Outcomes of the Vision in Preschoolers (VIP) and Vision in Preschoolers - Hyperopia in Preschoolers (VIP-HIP) Studies

ABSTRACT

This review summarizes clinically relevant outcomes from the Vision in Preschoolers (VIP) and VIP-Hyperopia in Preschoolers (VIP-HIP) studies. In VIP, refraction tests (retinoscopy, Retinomax, SureSight) and Lea Symbols Visual Acuity performed best in identifying children with vision disorders. For lay screeners, Lea Symbols single, crowded visual acuity (VA) testing (VIP, 5-foot) was significantly better than linear, crowded testing (10-foot). Children unable to perform the tests (<2%) were more likely to have vision disorders than children who passed and should be referred for vision evaluation. Among racial/ethnic groups, the prevalence of amblyopia and strabismus was similar while that of hyperopia, astigmatism, and anisometropia varied. The presence of strabismus and significant refractive errors were risk factors for unilateral amblyopia, while bilateral astigmatism and bilateral hyperopia were risk factors for bilateral amblyopia. A greater risk of astigmatism was associated with Hispanic, African American, and Asian race, and myopic and hyperopic refractive error. The presence and severity of hyperopia were associated with higher rates of amblyopia, strabismus, and other associated refractive error. In the VIP-HIP study, compared to emmetropes, meaningful deficits in early literacy were observed in uncorrected hyperopic 4- and 5-year-olds [≥+4.0 diopter (D) or ≥+3.0 D to ≤+6.0 D associated with reduced near visual function (near VA 20/40 or worse; stereoacuity worse than 240")]. Hyperopia with reduced near visual function also was associated with attention deficits. Compared to emmetropic children, VA (distance, near), accommodative accuracy, and stereoacuity were significantly reduced in moderate hyperopes, with the greatest risk in those with higher hyperopia. Increasing hyperopia was associated with decreasing visual function.

Recent understanding of binocular vision in the natural environment with clinical implications

Technological advances in recent decades have allowed us to measure both the information available to the visual system in the natural environment and the rich array of behaviors that the visual system supports. This review highlights the tasks undertaken by the binocular visual system in particular and how, for much of human activity, these tasks differ from those considered when an observer fixates a static target on the midline. The everyday motor and perceptual challenges involved in generating a stable, useful binocular percept of the environment are discussed, together with how these challenges are but minimally addressed by much of current clinical interpretation of binocular function. The implications for new technology, such as virtual reality, are also highlighted in terms of clinical and basic research application.

The relationship between reflex eye realignment and the percept of single vision in young children

Effective binocular vision is dependent on both motor and perceptual function. Young children undergo development of both components while interacting with their dynamic three-dimensional environment. When this development fails, eye misalignment and double vision may result. We compared the range of image disparities over which young children display reflex motor realignment of their eyes with the range over which they report a single versus double percept. In response to step changes in the disparity of a 2.2° wide stimulus, 5-year-olds generated an adult-like reflex vergence velocity tuning function peaking at 2° of disparity, with a mean latency of 210 ms. On average, they reported double vision for stimulus disparities of 3° and larger, compared to 1° in adult reports. Three-year-olds also generated reflex vergence tuning functions peaking at approximately 2° of disparity, but their percepts could not be assessed. These data suggest that, by age 5, reflex eye realignment responses and percepts driven by these brief stimuli are tightly coordinated in space and time to permit robust binocular function around the point of fixation. Importantly, the plastic neural processes maintaining this tight coordination during growth control the stability of visual information driving learning during childhood.

The Importance of the Interaction Between Ocular Motor Function and Vision During Human Infancy

Numerous studies have demonstrated the impact of imposed abnormal visual experience on the postnatal development of the visual system. These studies have provided fundamental insights into the mechanisms underlying neuroplasticity and its role in clinical care. However, the ocular motor responses of postnatal human infants largely define their visual experience in dynamic three-dimensional environments. Thus, the immature visual system needs to control its own visual experience. This review explores the interaction between the developing motor and sensory/perceptual visual systems, together with its importance in both typical development and the development of forms of strabismus and amblyopia.

Calibration of the PlusOptix PowerRef 3 with change in viewing distance, adult age and refractive error

Purpose

The PowerRef 3 is frequently used in studying the near triad of accommodation, vergence and pupil responses in normal and clinical populations. Within a range, the defocus measurement of the PowerRef 3 is linearly related to the eye's defocus. While the default factory‐calibrated slope of this relation (calibration factor) is 1, it has been shown that the slope can vary across individuals. Here, we addressed the impact of changes in viewing distance, age and defocus of the eye on the calibration factor.

Methods

We manipulated viewing distance (40 cm, 1 m and 6 m) and recruited participants with a range of accommodative capabilities: participants in their 20s, 40s and over 60 years old. To test whether any effect was larger than the range of measurement reliability of the instrument, we collected data for each condition four times: two in the same session, another on the same day, and one on a different day.

Results

The results demonstrated that viewing distance did not affect the calibration factor over the linear range, regardless of age or uncorrected refractive error. The largest proportion of the variance was explained by between‐subject differences.

Conclusions

Calibration data for the PowerRef 3 were not sensitive to changes in viewing distance. Nevertheless, our results re‐emphasise the relevance of calibration for studies of individual participants.

Longitudinal Evaluation of Accommodation During Treatment for Unilateral Amblyopia

Purpose

Retinal image quality is dependent on accommodative performance. This longitudinal observational study of children with unilateral amblyopia evaluated the accommodative performance of the amblyopic eye during treatment.

Methods

Twenty-six participants with unilateral amblyopia and 10 participants with typical vision aged 3 to 10 years participated. Accommodative response was measured using modified Nott retinoscopy in monocular and binocular viewing conditions for target distances of 50, 33, and 25 cm, at enrollment and each follow-up visit.

Results

Participants with amblyopia accommodated less accurately when viewing with their amblyopic eye in monocular than in binocular conditions. Over the course of amblyopia treatment, accommodative performance improved with amblyopic eye visual acuity (VA) improvement, although this was not consistent across individual participants. A linear mixed model showed that accommodative error worsened with increasing depth of amblyopia for monocular viewing with the amblyopic eye (0.14 diopter [D] per line of acuity loss, P = 0.001), with an interaction between VA and stimulus demand (0.09 D of additional lag per diopter of stimulus, per line of acuity loss, P < 0.001). Participant age, patching duration, length of time in the study, history of strabismus, and stereoacuity were not significant predictors of accommodative performance.

Conclusions

Overall, poor monocular accommodative performance of the amblyopic eye was associated with worse amblyopia and improved simultaneously with VA improvement, although there was variability across the study cohort. Further research is needed to determine the causal relationship between amblyopic eye VA and accommodation and its impact on amblyopia treatment.

The gaze stability of 4- to 10-week-old human infants

The relationship between gaze stability, retinal image quality, and visual perception is complex. Gaze instability related to pathology in adults can cause a reduction in visual acuity (e.g., Chung, LaFrance, & Bedell, 2011). Conversely, poor retinal image quality and spatial vision may be a contributing factor to gaze instability (e.g., Ukwade & Bedell, 1993). Though much is known about the immaturities in spatial vision of human infants, little is currently understood about their gaze stability. To characterize the gaze stability of young infants, adult participants and 4- to 10-week-old infants were shown a dynamic random-noise stimulus for 30-s intervals while their eye positions were recorded binocularly. After removing adultlike saccades, we used 5-s epochs of stable intersaccade gaze to estimate bivariate contour ellipse area and standard deviations of vergence. The geometric means (with standard deviations) for infants' bivariate contour ellipse area were left eye = -0.697 ± 0.534 log(°2), right eye = -0.471 ± 0.367 log(°2). For binocular vergence stability, the infant geometric means (with standard deviations) were horizontal = -1.057 ± 0.743 log(°), vertical = -1.257 ± 0.573 log(°). These values were all not significantly different from those of the adult comparison sample, suggesting that gaze instability is not a significant limiting factor in retinal image quality and spatial vision during early postnatal development.

Human infants can generate vergence responses to retinal disparity by 5 to 10 weeks of age

Vergence is defined as a binocular eye movement during which the two eyes move in opposite directions to align to a target in depth. In adults, fine vergence control is driven primarily by interocular retinal image disparity. Although infants have not typically been shown to respond to disparity until 3 to 5 months postpartum, they have been shown to align their eyes from hours after birth. It remains unclear what drives these responses in young infants. In this experiment, 5- to 10-week-old human infants were presented with a dynamic random noise stimulus oscillating in disparity at 0.1 Hz over an amplitude of 2° for 30 s. Fourier transforms of the horizontal eye movements revealed significant disparity-driven responses at the frequency of the stimulus in over half of the tested infants. Because the stimulus updated dynamically, this experiment precluded the possibility of independent monocular fixations to a sustained target. These data demonstrate cortical binocular function in humans by five weeks, the youngest age tested here, which is as much as two months younger than previously believed.

Two-dimensional simulation of eccentric photorefraction images for ametropes: factors influencing the measurement

PURPOSE

Eccentric photorefraction and Purkinje image tracking are used to estimate refractive state and eye position simultaneously. Beyond vision screening, they provide insight into typical and atypical visual development. Systematic analysis of the effect of refractive error and spectacles on photorefraction data is needed to gauge the accuracy and precision of the technique.

METHODS

Simulation of two-dimensional, double-pass eccentric photorefraction was performed (Zemax). The inward pass included appropriate light sources, lenses and a single surface pupil plane eye model to create an extended retinal image that served as the source for the outward pass. Refractive state, as computed from the luminance gradient in the image of the pupil captured by the model's camera, was evaluated for a range of refractive errors (-15D to +15D), pupil sizes (3 mm to 7 mm) and two sets of higher-order monochromatic aberrations. Instrument calibration was simulated using -8D to +8D trial lenses at the spectacle plane for: (1) vertex distances from 3 mm to 23 mm, (2) uncorrected and corrected hyperopic refractive errors of +4D and +7D, and (3) uncorrected and corrected astigmatism of 4D at four different axes. Empirical calibration of a commercial photorefractor was also compared with a wavefront aberrometer for human eyes.

RESULTS

The pupil luminance gradient varied linearly with refractive state for defocus less than approximately 4D (5 mm pupil). For larger errors, the gradient magnitude saturated and then reduced, leading to under-estimation of refractive state. Additional inaccuracy (up to 1D for 8D of defocus) resulted from spectacle magnification in the pupil image, which would reduce precision in situations where vertex distance is variable. The empirical calibration revealed a constant offset between the two clinical instruments.

CONCLUSIONS

Computational modelling demonstrates the principles and limitations of photorefraction to help users avoid potential measurement errors. Factors that could cause clinically significant errors in photorefraction estimates include high refractive error, vertex distance and magnification effects of a spectacle lens, increased higher-order monochromatic aberrations, and changes in primary spherical aberration with accommodation. The impact of these errors increases with increasing defocus.

Objective Measurement of Fusional Vergence Ranges and Heterophoria in Infants and Preschool Children

Purpose

Binocular alignment typically includes motor fusion compensating for heterophoria. This study evaluated heterophoria and then accommodation and vergence responses during measurement of fusional ranges in infants and preschoolers.

Methods

Purkinje image eye tracking and eccentric photorefraction (MCS PowerRefractor) were used to record the eye alignment and accommodation of uncorrected infants (n = 17; 3–5 months old), preschoolers (n = 19; 2.5–5 years), and naïve functionally emmetropic adults (n = 14; 20–32 years; spherical equivalent [SE], +1 to −1 diopters [D]). Heterophoria was derived from the difference between monocular and binocular alignments while participants viewed naturalistic images at 80 cm. The presence or absence of fusion was then assessed after base-in (BI) and base-out (BO) prisms (2–40 prism diopters [pd]) were introduced.

Results

Mean (±SD) SE refractions were hyperopic in infants (+2.4 ± 1.2 D) and preschoolers (+1.1 ± 0.6 D). The average exophoria was similar (P = 0.11) across groups (Infants, −0.79 ± 2.5 pd; Preschool, −2.43 ± 2.0 pd; Adults, −1.0 ± 2.7 pd). Mean fusional vergence range also was similar (P = 0.1) for BI (Infants, 11.2 ± 2.5 pd; Preschool, 8.8 ± 2.8 pd; Adults, 11.8 ± 5.2 pd) and BO (Infants, 14 ± 6.6 pd; Preschool, 15.3 ± 8.3 pd; Adults, 20 ± 9.2 pd). Maximum change in accommodation to the highest fusible prism was positive (increased accommodation) for BO (Infants, 1.69 ± 1.4 D; Preschool, 1.35 ± 1.6 D; Adults, 1.22 ± 1.0 D) and negative for BI (Infants, −0.96 ± 1.0 D; Preschool, −0.78 ± 0.6 D; Adults, −0.62 ± 0.3 D), with a similar magnitude across groups (BO, P = 0.6; BI, P = 0.4).

Conclusions

Despite typical uncorrected hyperopia, infants and preschoolers exhibited small exophorias at 80 cm, similar to adults. All participants demonstrated substantial fusional ranges, providing evidence that even 3- to 5-month-old infants can respond to a large range of image disparities.

The heterophoria of 3-5 year old children as a function of viewing distance and target type

PURPOSE

Heterophoria is the misalignment of the eyes in monocular viewing and represents the accuracy of vergence driven by all classical cues except disparity. It is challenging to assess restless children using clinical cover tests, and phoria in early childhood is poorly understood. Here we used eye tracking to assess phoria as a function of viewing distance and target in adults and young children, with comparison to clinical cover tests.

METHODS

Purkinje image tracking (MCS PowerRefractor) was used to record eye alignment in adults (19-28 years, N = 24) and typically developing children (3-5 years, N = 24). Objective unilateral and alternating cover tests were performed using an infrared filter while participants viewed a pseudo-randomised sequence of Lea symbols (0.18 logMAR; Snellen: 20/30 or 6/9) and animated cartoon movies at distances of 40 cm, 1 m, and 6 m. For the unilateral cover test, a 10 s binocular period preceded and followed 30 s of occlusion of the right eye. For the alternating cover test, a 10 s binocular period preceded and followed alternate covering of right and left eyes for 3-s each. Phoria was derived from the difference in weighted average binocular and monocular alignment. A masked prism-neutralised clinical cover test was performed for each of the conditions for comparison.

RESULTS

Closer viewing distance resulted in greater exophoria for both children and adults (p < 0.001). Phorias were similar for adults and children for each viewing distance and target, with mean differences of less than 2 prism dioptres (pd). Overall, the average PowerRefractor phorias (pooled across protocols) for adults were 1.3, 2.3 and 3.8 pd exophoria and for children were 0.1 pd esophoria, 0.94 and 3.8 pd exophoria for the 6 m, 1 m and 40 cm distances respectively. The corresponding clinical cover test values were 0.7, 1.9, and 4.1 pd exophoria for adults and 0, 1.5 and 3.3 pd exophoria for the children. Refractive states were also similar (≤0.5 D difference) for viewing the Lea symbols or movie for any protocol tested.

CONCLUSIONS

Phoria estimation can be challenging for a pre-school child. These data suggest that by 3-5 years of age objective eye-tracking measures in a typically developing group are adult-like at the range of distances tested, and that use of an animated movie produces similar average results to a small optotype (0.18 logMAR; Snellen 20/30 or 6/9).

Visual Function of Moderately Hyperopic 4- and 5-Year-Old Children in the Vision in Preschoolers - Hyperopia in Preschoolers Study

PURPOSE

To compare visual performance between emmetropic and uncorrected moderately hyperopic preschool-age children without strabismus or amblyopia.

DESIGN

Cross-sectional study.

METHODS

setting: Multicenter, institutional. patient or study population: Children aged 4 or 5 years. intervention or observation procedures: Visual functions were classified as normal or reduced for each child based on the 95% confidence interval for emmetropic individuals. Hyperopic (≥3.0 diopters [D] to ≤6.0 D in the most hyperopic meridian; astigmatism ≤1.50 D; anisometropia ≤1.0 D) and emmetropic status were determined by cycloplegic autorefraction.

MAIN OUTCOME MEASURES

Uncorrected monocular distance and binocular near visual acuity (VA); accommodative response; and near random dot stereoacuity.

RESULTS

Mean (± standard deviation) logMAR distance visual acuity (VA) among 248 emmetropic children was better than among 244 hyperopic children for the better (0.05 ± 0.10 vs 0.14 ± 0.11, P < .001) and worse eyes (0.10 ± 0.11 vs 0.19 ± 0.10, P < .001). Mean binocular logMAR near VA was better in emmetropic than in hyperopic children (0.13 ± 0.11 vs 0.21 ± 0.11, P < .001). Mean accommodative response for emmetropic children was lower than for hyperopic subjects for both Monocular Estimation Method (1.03 ± 0.51 D vs 2.03 ± 1.03 D, P < .001) and Grand Seiko (0.46 ± 0.45 D vs 0.99 ± 1.0 D, P < .001). Median near stereoacuity was better in emmetropic than in than hyperopic children (40 sec arc vs 120 sec arc, P < .001). The average number of reduced visual functions was lower in emmetropic than in hyperopic children (0.19 vs 1.0, P < .001).

CONCLUSIONS

VA, accommodative response, and stereoacuity were significantly reduced in moderate uncorrected hyperopic preschool children compared to emmetropic subjects. Those with higher hyperopia (≥4 D to ≤6 D) were at greatest risk, although more than half of children with lower magnitudes (≥3 D to <4 D) demonstrated 1 or more reductions in function.

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.

Vergence Adaptation to Short-Duration Stimuli in Early Childhood

Purpose

To investigate whether nonstrabismic typically developing young children are capable of exhibiting vergence adaptation.

Methods

Fifteen 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.

Results

The 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.

Conclusions

For 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.

Sensitivity of vergence responses of 5- to 10-week-old human infants

Infants have been shown to make vergence eye movements by 1 month of age to stimulation with prisms or targets moving in depth. However, little is currently understood about the threshold sensitivity of the maturing visual system to such stimulation. In this study, 5- to 10-week-old human infants and adults viewed a target moving in depth as a triangle wave of three amplitudes (1.0, 0.5, and 0.25 meter angles). Their horizontal eye position and the refractive state of both eyes were measured simultaneously. The vergence responses of the infants and adults varied at the same frequency as the stimulus at the three tested modulation amplitudes. For a typical infant of this age, the smallest amplitude is equivalent to an interocular change of approximately 2° of retinal disparity, from nearest to farthest points. The infants' accommodation responses only modulated reliably to the largest stimulus, while adults responded to all three amplitudes. Although the accommodative system appears relatively insensitive, the sensitivity of the vergence responses suggests that subtle cues are available to drive vergence in the second month after birth.

Uncorrected Hyperopia and Preschool Early Literacy: Results of the Vision in Preschoolers-Hyperopia in Preschoolers (VIP-HIP) Study

PURPOSE

To compare early literacy of 4- and 5-year-old uncorrected hyperopic children with that of emmetropic children.

DESIGN

Cross-sectional.

PARTICIPANTS

Children attending preschool or kindergarten who had not previously worn refractive correction.

METHODS

Cycloplegic refraction was used to identify hyperopia (≥3.0 to ≤6.0 diopters [D] in most hyperopic meridian of at least 1 eye, astigmatism ≤1.5 D, anisometropia ≤1.0 D) or emmetropia (hyperopia ≤1.0 D; astigmatism, anisometropia, and myopia <1.0 D). Threshold visual acuity (VA) and cover testing ruled out amblyopia or strabismus. Accommodative response, binocular near VA, and near stereoacuity were measured.

MAIN OUTCOME MEASURES

Trained examiners administered the Test of Preschool Early Literacy (TOPEL), composed of Print Knowledge, Definitional Vocabulary, and Phonological Awareness subtests.

RESULTS

A total of 492 children (244 hyperopes and 248 emmetropes) participated (mean age, 58 months; mean ± standard deviation of the most hyperopic meridian, +3.78±0.81 D in hyperopes and +0.51±0.48 D in emmetropes). After adjustment for age, race/ethnicity, and parent/caregiver's education, the mean difference between hyperopes and emmetropes was -4.3 (P = 0.01) for TOPEL overall, -2.4 (P = 0.007) for Print Knowledge, -1.6 (P = 0.07) for Definitional Vocabulary, and -0.3 (P = 0.39) for Phonological Awareness. Greater deficits in TOPEL scores were observed in hyperopic children with ≥4.0 D than in emmetropes (-6.8, P = 0.01 for total score; -4.0, P = 0.003 for Print Knowledge). The largest deficits in TOPEL scores were observed in hyperopic children with binocular near VA of 20/40 or worse (-8.5, P = 0.002 for total score; -4.5, P = 0.001 for Print Knowledge; -3.1, P = 0.04 for Definitional Vocabulary) or near stereoacuity of 240 seconds of arc or worse (-8.6, P < 0.001 for total score; -5.3, P < 0.001 for Print Knowledge) compared with emmetropic children.

CONCLUSIONS

Uncorrected hyperopia ≥4.0 D or hyperopia ≥3.0 to ≤6.0 D associated with reduced binocular near VA (20/40 or worse) or reduced near stereoacuity (240 seconds of arc or worse) in 4- and 5-year-old children enrolled in preschool or kindergarten is associated with significantly worse performance on a test of early literacy.

Near heterophoria in early childhood

PURPOSE

The purpose of this study was to measure near heterophoria in young children to determine the impact of early growth and development on the alignment of the eyes.

METHODS

Fifty young children (≥2 and <7 years of age; range of spherical equivalent refractive error -1.25 diopters [D] to +3.75 D) and 13 adults participated. Their eye position and accommodation responses, in the absence of optical correction, were measured using simultaneous Purkinje image tracking and photorefraction technology (MCS PowerRefractor, PR). The resulting heterophorias, and both accommodative convergence/accommodation (AC/A) and convergence accommodation/convergence (CA/C) ratios were then computed as a function of age, refractive error, and an alternating cover test.

RESULTS

The mean heterophoria after approximately 60 seconds of dissociation at a 33-cm viewing distance was 5.0 prism diopters (pd) of exophoria (SD ± 3.7) in the children (78% of children > 2 pd exophoric) and 5.6 pd of exophoria (SD ± 4.7) in adults (69% of adults > 2pd exophoric; a nonsignificant difference), with no effect of age between 2 and 6 years. In these children, heterophoria was not significantly correlated with AC/A (r = 0.25), CA/C (r = 0.12), or refractive error (r = 0.21). The mean difference between heterophoria measurements from the PR and the clinical cover test was -2.4 pd (SD = ±3.4), with an exophoric bias in the PR measurements.

CONCLUSIONS

Despite developmental maturation of interpupillary distance, refractive error, and AC/A, in a typical sample of young children the predominant dissociated position is one of exophoria.

Accommodative performance of children with unilateral amblyopia

PURPOSE

The purpose of this study was to compare the accommodative performance of the amblyopic eye of children with unilateral amblyopia to that of their nonamblyopic eye, and also to that of children without amblyopia, during both monocular and binocular viewing.

METHODS

Modified Nott retinoscopy was used to measure accommodative performance of 38 subjects with unilateral amblyopia and 25 subjects with typical vision from 3 to 13 years of age during monocular and binocular viewing at target distances of 50, 33, and 25 cm. The relationship between accommodative demand and interocular difference (IOD) in accommodative error was assessed in each group.

RESULTS

The mean IOD in monocular accommodative error for amblyopic subjects across all three viewing distances was 0.49 diopters (D) (95% confidence interval [CI], ±1.12 D) in the 180° meridian and 0.54 D (95% CI, ±1.27 D) in the 90° meridian, with the amblyopic eye exhibiting greater accommodative errors on average. Interocular difference in monocular accommodative error increased significantly with increasing accommodative demand; 5%, 47%, and 58% of amblyopic subjects had monocular errors in the amblyopic eye that fell outside the upper 95% confidence limit for the better eye of control subjects at viewing distances of 50, 33, and 25 cm, respectively.

CONCLUSIONS

When viewing monocularly, children with unilateral amblyopia had greater mean accommodative errors in their amblyopic eyes than in their nonamblyopic eyes, and when compared with control subjects. This could lead to unintended retinal image defocus during patching therapy for amblyopia.

Why do only some hyperopes become strabismic?

Children with hyperopia greater than +3.5 diopters (D) are at increased risk for developing refractive esotropia. However, only approximately 20% of these hyperopes develop strabismus. This review provides a systematic theoretical analysis of the accommodation and vergence oculomotor systems with a view to understanding factors that could either protect a hyperopic individual or precipitate a strabismus. The goal is to consider factors that may predict refractive esotropia in an individual and therefore help identify the subset of hyperopes who are at the highest risk for this strabismus, warranting the most consideration in a preventive effort.

The relationship between anisometropia and amblyopia

This review aims to disentangle cause and effect in the relationship between anisometropia and amblyopia. Specifically, we examine the literature for evidence to support different possible developmental sequences that could ultimately lead to the presentation of both conditions. The prevalence of anisometropia is around 20% for an inter-ocular difference of 0.5D or greater in spherical equivalent refraction, falling to 2-3%, for an inter-ocular difference of 3D or above. Anisometropia prevalence is relatively high in the weeks following birth, in the teenage years coinciding with the onset of myopia and, most notably, in older adults starting after the onset of presbyopia. It has about one-third the prevalence of bilateral refractive errors of the same magnitude. Importantly, the prevalence of anisometropia is higher in highly ametropic groups, suggesting that emmetropization failures underlying ametropia and anisometropia may be similar. Amblyopia is present in 1-3% of humans and around one-half to two-thirds of amblyopes have anisometropia either alone or in combination with strabismus. The frequent co-existence of amblyopia and anisometropia at a child's first clinical examination promotes the belief that the anisometropia has caused the amblyopia, as has been demonstrated in animal models of the condition. In reviewing the human and monkey literature however it is clear that there are additional paths beyond this classic hypothesis to the co-occurrence of anisometropia and amblyopia. For example, after the emergence of amblyopia secondary to either deprivation or strabismus, anisometropia often follows. In cases of anisometropia with no apparent deprivation or strabismus, questions remain about the failure of the emmetropization mechanism that routinely eliminates infantile anisometropia. Also, the chronology of amblyopia development is poorly documented in cases of 'pure' anisometropic amblyopia. Although indirect, the therapeutic impact of refractive correction on anisometropic amblyopia provides strong support for the hypothesis that the anisometropia caused the amblyopia. Direct evidence for the aetiology of anisometropic amblyopia will require longitudinal tracking of at-risk infants, which poses numerous methodological and ethical challenges. However, if we are to prevent this condition, we must understand the factors that cause it to develop.

Empirical variability in the calibration of slope-based eccentric photorefraction

Refraction estimates from eccentric infrared (IR) photorefraction depend critically on the calibration of luminance slopes in the pupil. While the intersubject variability of this calibration has been estimated, there is no systematic evaluation of its intrasubject variability. This study determined the within subject inter- and intra-session repeatability of this calibration factor and the optimum range of lenses needed to derive this value. Relative calibrations for the MCS PowerRefractor and a customized photorefractor were estimated twice within one session or across two sessions by placing trial lenses before one eye covered with an IR transmitting filter. The data were subsequently resampled with various lens combinations to determine the impact of lens power range on the calibration estimates. Mean (±1.96 SD) calibration slopes were 0.99±0.39 for North Americans with the MCS PowerRefractor (relative to its built-in value) and 0.65±0.25 Ls/D and 0.40±0.09 Ls/D for Indians and North Americans with the custom photorefractor, respectively. The ±95% limits of agreement of intrasubject variability ranged from ±0.39 to ±0.56 for the MCS PowerRefractor and ±0.03 Ls/D to ±0.04 Ls/D for the custom photorefractor. The mean differences within and across sessions were not significantly different from zero (p>0.38 for all). The combined intersubject and intrasubject variability of calibration is therefore about ±40% of the mean value, implying that significant errors in individual refraction/accommodation estimates may arise if a group-average calibration is used. Protocols containing both plus and minus lenses had calibration slopes closest to the gold-standard protocol, suggesting that they may provide the best estimate of the calibration factor compared to those containing either plus or minus lenses.

The accommodative lag of the young hyperopic patient

PURPOSE

To determine the accommodative accuracy of infants and young children before they had had any form of clinical intervention or treatment, in an attempt to determine the difference between 'normal' and 'abnormal' visual experience for these individuals.

METHODS

Nott retinoscopy was performed on 111 subjects in binocular viewing conditions at a viewing distance of 50 cm. The target was a naturalistic cartoon image with a broadband spatial frequency amplitude spectrum.

RESULTS

Accommodative accuracy was not related to age (4-90 months). In the group found to have no apparent clinical abnormality (n = 71), the mean lag in the more hyperopic meridian of the least ametropic eye was 0.34 diopters (D). When considering the group as a whole, those with less than approximately 4 D of hyperopia demonstrated similar lags, while those with higher hyperopia, amblyopia, or strabismus had more variable lags. An ROC analysis designed to detect hyperopia >5 D in any meridian, amblyopia and/or strabismus had an area under the curve of 0.90 (95% confidence interval [CI], 0.82 to 0.95), and for a lag criterion of 1.3 D had a sensitivity of 83.3% and a specificity of 96.5%.

CONCLUSIONS

These data from a relatively small but broad sampling of age and clinical status suggest that clinically normal young infants and children with low amounts of hyperopia have similar lags of accommodation from the first few months after birth. Subjects with greater than 4 D of hyperopia, or amblyopia or strabismus, have more variable lags and therefore evidence of abnormal visual experience.

Adult discrimination performance for pediatric acuity test optotypes

PURPOSE

To compare adult discrimination performance on nine pediatric visual acuity tests to determine the consistency of optotype design.

METHODS

After their binocular acuity was measured with each test, eight adult observers (mean age, 27 years ± 6.3 SD; three emmetropes and five corrected myopes) were shown isolated single optotypes from the Allen figures, HOTV, Landolt C, Lea Numbers, Lea Symbols, Lighthouse, Patti Pics, Precision Vision numbers, and Tumbling E tests. A one-interval, two-alternative forced-choice protocol was used at a single distance, and each optotype was paired with all optotypes from the same chart. Confusion matrices were generated for each test and Luce's (1963) biased-choice model was fit to each matrix to derive measures of pairwise similarity between the optotypes.

RESULTS

The acuities from the Allen figures (P < 0.001) and HOTV (P = 0.029) were the only ones to differ significantly from the reference Landolt C. The choice-model analyses of the confusion matrices revealed that the Allen figures, HOTV, Lighthouse, Patti Pics, and Precision Vision numbers tests all had significant differences in discriminability of optotypes within the test.

CONCLUSIONS

Pediatric acuity test optotypes are not all equally discriminable to adult observers with normal vision and no ocular disorders. The current data suggest that care must be taken when presenting limited numbers of optotypes, as is done with young patients.

The effect of lens-induced anisometropia on accommodation and vergence during human visual development

PURPOSE

Clear and single binocular vision, a prerequisite for normal human visual development, is achieved through accommodation and vergence. Anisometropia is associated with abnormal visual development, but its impact on accommodation and vergence, and therefore on the individual's visual experience, is not known. This study determined the impact of transiently induced anisometropia on accommodative and vergence performance of the typically developing human visual system.

METHODS

One hundred eighteen subjects (age range, 2.9 months to 41.1 years) watched a cartoon movie that moved between 80 and 33 cm under six different viewing conditions: binocular and monocular, and with ±2 diopters (D) and ±4 D of lens-induced anisometropia. Twenty-one subjects (age range, 3.1 months to 12.1 years) also watched the movie with 11% induced aniseikonia. Accommodation and vergence were recorded in both eyes using a videoretinoscope (25 Hz).

RESULTS

The main effect of viewing condition was statistically significant for both accommodation and vergence (both P < 0.001), with monocular accommodative and vergence gains statistically significantly smaller than the binocular and four induced anisometropia conditions (P < 0.001 for both accommodation and vergence). The main effect of age approached significance for accommodation (P = 0.06) and was not significant for vergence (P = 0.32). Accommodative and vergence gains with induced aniseikonia were not statistically significantly different from the binocular condition (both P > 0.5).

CONCLUSIONS

Accommodative and vergence gains of the typically developing visual system deteriorated marginally (accommodation more than vergence) with transiently induced anisometropia (up to ±4 D) and did not deteriorate significantly with induced aniseikonia of 11%. Some binocular cues remained with ±4 D of induced anisometropia and 11% induced aniseikonia, as indicated by the accommodative and vergence gains being higher than in monocular viewing.

Pupil responses to near visual demand during human visual development

Pupil responses of adults to near visual demands are well characterized but those of typically developing infants and children are not. This study determined the following pupil characteristics of infants, children and adults using a PowerRefractor (25 Hz): i) binocular and monocular responses to a cartoon movie that ramped between 80 and 33 cm (20 infants, 20 2-4-yr-olds and 20 adults participated) ii) binocular and monocular response threshold for 0.1 Hz sinusoidal stimuli of 0.25 D, 0.5 D or 0.75 D amplitude (33 infants and 8 adults participated) iii) steady-state stability of pupil responses at 80 cms (8 infants and 8 adults participated). The change in pupil diameter with viewing distance (Δpd) was significantly smaller in infants and 2-4-yr-olds than in adults (p < 0.001) and significantly smaller under monocular than binocular conditions (p<0.001). The 0.75 D sinusoidal stimulus elicited a significant binocular pupillary response in infants and a significant binocular and monocular pupillary response in adults. Steady-state pupillary fluctuations were similar in infants and adults (p = 0.25). The results suggest that the contribution of pupil size to changes in retinal image quality when tracking slow moving objects may be smaller during development than in adulthood. Smaller monocular Δpd reflects the importance of binocular cues in driving near-pupillary responses.

Orientation tuning in the visual cortex of 3-month-old human infants

Sensitivity to orientation is critical for making a whole and complete picture of the world. We measured the orientation tuning of mechanisms in the visual cortex of typically developing 3-month-olds and adults using a nonlinear analysis of the two-input steady-state Visually Evoked Potential (VEP). Two gratings, one a fixed test and the other a variable orientation masker were tagged with distinct temporal frequencies and the corresponding evoked responses were measured at the harmonics of the test and masker frequencies and at a frequency equal to the sum of the two stimulus frequencies. The magnitude of the sum frequency component depended strongly on the relative orientation of the test and masker in both infants and adults. The VEP tuning bandwidths of the 3-month-olds measured at the sum frequency were similar to those of adults, suggesting that behavioral immaturities in functions such as orientation discrimination and contour integration may result from other immaturities in long-range lateral projections or feedback mechanisms.

The sensitivity of the 2- to 4-month-old human infant accommodation system

PURPOSE

The goal of this study was to compare objectively the sensitivity of the accommodation system in human infants and adults under binocular and monocular viewing conditions.

METHODS

Full-term infants from 2 to 4 months of age and pre-presbyopic adults were presented with a high-contrast cartoon stimulus moving sinusoidally in diopters around a mean position of 2 D (50 cm). Three stimulus amplitudes were used in one trial (0.25, 0.50, and 0.75 D), with unpredictable stimulus motion during each amplitude change. Eccentric photorefraction was used to record accommodative responses at 25 Hz. The stimulus was made monocular by placing an infrared filter over the right eye, to block visible light but pass the near-infrared wavelength of the photorefractor and allow responses to be recorded from both eyes.

RESULTS

Fourier analysis was used to determine the accommodative response at the frequency of the stimulus. Significant signal-to-noise ratios indicated that, on average, the 2- to 4-month-old infants generated an accommodative response to at least the 0.75 D amplitude monocular stimulus and the 0.75 and 0.50 D binocular stimuli. Adults responded to the 0.25 D amplitude both binocularly and monocularly.

CONCLUSIONS

In infants 2 to 4 months of age, the developing visual system compensates for small changes in defocus relative to the typical amounts of hyperopic refractive error found at that age.

Accommodative and vergence responses to conflicting blur and disparity stimuli during development

Accommodative and vergence responses of the typically developing visual system are generated using a combination of cues, including retinal blur and disparity. The developmental importance of blur and disparity cues in generating these motor responses was assessed by placing the two cues in conflict with each other. Cue-conflicts were induced by placing either -2 D lenses or 2 MA base-out prisms before both eyes of 140 subjects (2.0 months to 40.8 years) while they watched a cartoon movie binocularly at 80 cm. The frequency and amplitude of accommodation to lenses and vergence to prisms increased with age (both p < 0.001), with the vergence response (mean +/- 1 SEM = 1.38 +/- 0.05 MA) being slightly larger than the accommodative response (1.18 +/- 0.04 D) at all ages (p = 0.007). The amplitude of these responses decreased with an increase in conflict stimuli (1 to 3 D or MA) (both p < 0.01). The coupled vergence response to -2 D lenses (0.31 +/- 0.06 MA) and coupled accommodative response to 2 MA base-out prisms (0.21 +/- 0.02 D) were significantly smaller than (both p < 0.001) and poorly correlated with the open-loop vergence (r = 0.12; p = 0.44) and open-loop accommodation (r = -0.08; p = 0.69), respectively. The typically developing visual system compensates for transiently induced conflicts between blur and disparity stimuli, without exhibiting a strong preference for either cue. The accuracy of this compensation decreases with an increase in amplitude of cue-conflict.

Cues for the control of ocular accommodation and vergence during postnatal human development

Accommodation and vergence help maintain single and focused visual experience while an object moves in depth. The relative importance of retinal blur and disparity, the primary sensory cues to accommodation and vergence, is largely unknown during development; a period when growth of the eye and head necessitate continual recalibration of egocentric space. Here we measured the developmental importance of retinal disparity in 192 typically developing subjects (1.9 months to 46 years). Subjects viewed high-contrast cartoon targets with naturalistic spatial frequency spectra while their accommodation and vergence responses were measured from both eyes using a PowerRefractor. Accommodative gain was reduced during monocular viewing relative to full binocular viewing, even though the fixating eye generated comparable tracking eye movements in the two conditions. This result was consistent across three forms of monocular occlusion. The accommodative gain was lowest in infants and only reached adult levels by 7 to 10 years of age. As expected, the gain of vergence was also reduced in monocular conditions. When 4- to 6-year-old children read 20/40-sized letters, their monocular accommodative gain reached adult-like levels. In summary, binocular viewing appears necessary under naturalistic viewing conditions to generate full accommodation and vergence responses in typically developing humans.

Longitudinal chromatic aberration of the human infant eye

Although the longitudinal chromatic aberration (LCA) of the adult eye has been studied, there are no data collected from the human infant eye. A chromatic retinoscope was used to measure cyclopleged infant and adult refractions with four pseudomonochromatic sources (centered at 472, 538, 589, and 652 nm) and with polychromatic light. The LCA of the infant eyes between 472 and 652 nm was a factor of 1.7 greater than the LCA found in the adult group: infant mean=1.62 D, SD+/- 0.14 D; adult mean=0.96 D, SD+/- 0.17 D. The elevated level of LCA in infant eyes is consistent with the greater optical power of the immature eye and indicates similar chromatic dispersion in infant and adult eyes. The implications for visual performance, defocus detection, and measurement of refraction are discussed.