Accommodation responses to the dynamic stimuli were recorded with a commercially available video-based eccentric photorefractor (PowerRefractor; Multi Channel Systems, Reutlingen, Germany). Data were gathered remotely from 1 m, enabling the infant to be placed in a relatively natural setting while binocular refraction data were collected along the vertical meridian at 25 Hz.
24 25 The manufacturer calibrated the commercial image analysis algorithm empirically using adult eyes, and this adult calibration has been tested by other groups.
26 27 28 29 30 The validity of the calibration was recently assessed by Blade and Candy
31 for infant eyes that did not undergo cycloplegia. This study found that a 0-D reading from this photorefractor typically corresponded to between 0 and 1 D of myopia for infants and adults, determined using simultaneous retinoscopy. Blade and Candy
31 also recorded the photorefractor reading as a function of known defocus (induced anisometropia) for subjects. The mean slope of 13 infant defocus calibration functions was 1.06 (median, 1.07; range, 0.84–1.33) with no significant change with age between 4 and 24 weeks. The mean slope of the 13 adult functions was 0.90 (median, 0.90; range, 0.55–1.14). Thus, structural differences between infant and adult eyes resulted in relatively small mean inaccuracies in the refraction estimates provided by the photorefractor, though the inaccuracies did vary across subjects.
The adults were seated on a stool and the infants, who were given no optical correction, were seated in an infant car seat or on their parent’s lap with their heads gently supported. The axis of the photorefractor camera was aligned with the bridge of the subject’s nose, and the target was centered between the subject’s eyes in the real-time image from the photorefractor. The room was kept in dim illumination to attract the subject’s attention to the task.