For the "healthy" group, the visual acuity of both eyes was in the 95% referenced range with no structural abnormalities.

The referenced range could be found in the following publication:

Mayer, DL., et al. Monocular acuity norms for the Teller Acuity Cards between ages one month and four years. Investigative Ophthalmology & Visual Science. 36(3):671 (1995)

The "mildly impaired" group was defined as a VA out of the 95% reference range in at least 1 eye, but the VA of both eyes was in the 99% referenced range with structural abnormalities.

For the "severely impaired" group, the VA of both eyes was out of the 99% referenced range or worse than light perception with structural abnormalities.

A standardized apparatus, scenario, and procedure was applied to record all the behavioral phenotypes with minimized background interference and stimulation.

For each standardized procedure, the guardian sits in the chair, holding the infant facing the stage. Each infant is given a few minutes to adapt to the surroundings and to be calm before recording. No hints or simulations are permitted during the entire process. The recording process lasted for over 5 minutes to ensure that behavioral phenotypes could be completely recorded.

Eyeball movement (strabismus, nystagmus, and incongruous binocular movement); hand-related behaviors (eye rubbing, pressing, and poking); fixation-related behaviors (compulsive light gazing, compensatory head position, motionless fixation, and poor fixation); and eyelid reaction (frequent blinking, squint, and frown). Five experienced ophthalmologists identified the behaviors independently, and 2 professors with over 10 years of experience in pediatric ophthalmology department were consulted in cases of disagreement.