Erin Babinsky

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.

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.

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 2 pd exophoric) and 5.6 pd of exophoria (SD ±4.7) in adults (69% of adults > 2pd exophoric) (a non-significant difference), with no effect of age between two and six 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.

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

Visual information about the hand, the reach space, and a target can all contribute to the control of a reaching movement. When visual information is removed, both feedforward mechanisms (involved in planning the movement) and feedback mechanisms (involved in correcting errors) may be affected. This study looks at how 4- to 5-year-old children use visual information to guide reaching movements. Children reached for a toy object in four conditions—in the light, in the dark while the toy was glowing, and in complete darkness after a 0-s delay and a 4-s delay. When a reach in the glowing condition was compared with a reach in the light, reaches were more curved, had a longer duration, and earlier time-to-peak-velocity than a reach in the light but the number of grasping responses were comparable to in the light condition. When a reach in the two dark conditions (0- and 4-s) was compared with a reach in the light, the number of grasping responses decreased and 14 and 31 % of reaches resulted in a miss, that is, no contact was made with the object. While we did not find any significant kinematic differences between the 0- and 4-s dark conditions, there was a significantly larger number of misses in the 4-s dark condition, suggesting that memory of target position may decay over time. Overall, removing vision of the hand and reach space in the glowing condition appears to affect the planning of a reach (as vision of the hand was not available at reach initiation) and feedback control, while removing vision of the object in the dark conditions has an effect on endpoint response as we found that children experience difficulty retrieving the object in the dark. While young children demonstrate more adult-like reach control (i.e., relatively longer deceleration time, increased reach duration) under reduced feedback conditions, they have difficulty retrieving the object in the dark, particularly after a 4-s delay, and it is possible that mechanisms guiding predictive control and visual memory are still developing.

It has been shown that infants over the age of 6 months will reach for an object in complete darkness. This experiment measured the reaching movements of 9- to 16-month-old infants and adults under several different conditions of illumination to investigate the role of vision and stored visual representations in reach control. In one condition, participants reached for an object with the lights on. In a second condition, participants reached for an object glowing in the dark (glowing condition). This allowed us to measure the effects of vision of the arm and vision of the reach space. We also looked at the effect of removing vision of the object on reach control: in the final two conditions, participants reached for an object in complete darkness (0-s dark) and in complete darkness after a 4-s delay (4-s dark). We compared the kinematics of a reach (e.g. average speed, reach straightness) between the four illumination conditions. The results showed that infants reached faster and decelerated for a shorter period of time in the dark (0- and 4-s dark) than in the light. By comparison, adults reached slower and decelerated for a longer period of time in the dark (0- and 4-s dark) than in the light. We did not find any effect of the glowing condition compared to full vision on infant reaching movements. These results suggest that infant reaching movements only become compromised when the target is not visible, whereas vision of the hand and the reach space are less significant. Without online visual feedback, an infant reach in the dark appears to be driven by feedforward mechanisms and control may be affected by an immature ability to form and/or retain visual spatial memory.