The cards were used by a trained ophthalmologist, under standard settings in the infant's room/neonatal intensive care unit (NICU) or the examination room prior to any other ocular tests. Testing was performed monocularly and both eyes were tested. The 0.10 cyc/deg vertical grating cards of varying contrast threshold were used. The cards were used in a random order based on the method of constant stimuli described in Brown et al.
17 As per this method, the first card was always the 1.0 contrast card, or the “easy card.” This allowed us to determine whether the infant was seeing “anything at all” and allowed us to record how the infant reacted to a clearly visible card. Then, stimuli at contrasts 1.0, 0.71, 0.50, 0.35, and 0.25 were used in random order. We chose the method of constant stimuli approach, as this yielded better outcomes than the conventional descending method of limits, detailed in Brown et al.
17 Each card was placed along the infant's line of sight, at 38 cm, while watching the infant's eyes through the peephole (
Fig. 2). If the infant responded to the stimulus, the card was quickly displaced to the right or left, by approximately 15 cm (at roughly 22 degrees/second relative to the infant's retina), attempting to induce the infant to follow the stimulus with his/her eye or head movements (
Supplementary Video). The contrast threshold value measured was the lowest contrast card to which the infant showed a response, irrespective of any higher contrast cards not inducing a response from the infant. A pilot study with 68 infants, including preterm and term infants, was conducted first to assess the feasibility of contrast threshold testing. The results from the pilot cohort are reported in
Supplementary Table S1 and
Supplementary Figure S1. To test the intragrader and intergrader reproducibility for contrast threshold values, we enrolled 13 and 10 participants, respectively. The contrast threshold was measured 3 times, with an interval of 5 minutes between each measurement for the intragrader measurement. For the intergrader measurements, the contrast threshold was measured by 2 independent observers within a time interval of 10 minutes. The intraclass correlation coefficients and 95% confidence intervals for the intragrader and intergrader measurements were 0.92 (0.82–0.97) and 0.95 (0.82–0.98), respectively.