Binocular alignment and vergence in early infancy

Vergence to static targets presented at five distances between 25 and 200 cm from the subject was measured in 631 infants aged between 17 and 120 days. Photographic images of the eyes were magnified and measured to yield information on the monocular and binocular eye positions for each target. Vergence data were fit by a linear function and compared to the vergence calculated from target distance and each infant's measured interpupillary distance. Differences in vergence across targets were also evaluated for each subject by calculating the change in angle of rotation for each eye. Many of even the youngest infants showed good ocular alignment both monocularly and binocularly, although the youngest infants showed the greatest variability in vergence. However, the median difference in vergence angle between the eyes for even the youngest group was < 4 deg (6.8 prism D), and some of this difference is attributed to versional eye movements and to slightly off-axis head position across trials. The average infant of 1-2 months showed substantially better vergence than has been reported in some recent studies. Apparently, oculomotor constraints are not a significant barrier to the development of the higher forms of binocularity that begin to emerge in the months immediately following the interval studied here, and may form the substrate for later developments in binocular vision.