The heterophoria of 3-5 year old children as a function of viewing distance and target type
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Updated: Mar 10, 2026
A Method to Quantify Visual Information Processing in Children Using Eye Tracking
Published on: July 9, 2016
Mary E Troyer1, Vidhyapriya Sreenivasan1, T J Peper1
1Indiana University School of Optometry, Bloomington, Indiana, USA.
Insights
Eye tracking accurately measures heterophoria in young children, showing adult-like results by age 3-5. This method offers a reliable alternative for assessing eye alignment in children, even with engaging visual targets like cartoons.
Area of Science:
- Ophthalmology and Vision Science
- Pediatric Optometry
- Human Factors and Ergonomics
Background:
- Heterophoria, the misalignment of eyes during monocular viewing, is crucial for assessing vergence accuracy.
- Clinical cover tests are challenging for assessing phoria in young, restless children, leading to a poor understanding of early childhood phoria.
- Objective eye-tracking offers a potential solution for accurate phoria assessment in pediatric populations.
Purpose of the Study:
- To utilize eye tracking to assess heterophoria as a function of viewing distance and target type in adults and young children.
- To compare eye-tracking measurements of phoria with traditional clinical cover tests.
- To investigate the feasibility of using engaging visual stimuli, such as animated movies, for phoria assessment in children.
Main Methods:
- Purkinje image tracking (MCS PowerRefractor) was employed to record eye alignment in adults (N=24) and children (N=24, ages 3-5).
- Participants viewed Lea symbols and animated movies at 40 cm, 1 m, and 6 m distances.
- Objective unilateral and alternating cover tests were performed, with phoria derived from binocular and monocular alignment differences; clinical cover tests served as a comparison.
Main Results:
- Closer viewing distances were associated with increased exophoria in both children and adults (p < 0.001).
- Phoria measurements were comparable between adults and children across viewing distances and target types, with mean differences under 2 prism diopters (pd).
- Eye-tracking and clinical cover test results showed good agreement, with average adult phorias ranging from 1.3 to 3.8 pd exophoria and children's from 0.1 pd esophoria to 3.8 pd exophoria.
Conclusions:
- Objective eye-tracking measures provide adult-like phoria assessments in typically developing 3-5-year-old children at various distances.
- Animated movies are as effective as standard optotypes for obtaining similar average phoria results in this age group.
- Eye tracking offers a viable and objective method for assessing heterophoria in pre-school children, overcoming challenges associated with traditional clinical tests.
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).
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