The development of binocular summation in human infants.

The difference in pupil diameter between monocular and binocular viewing conditions was used as a measure of binocular luminance summation in 97 infants aged 0 to 12 months. No significant difference in pupil diameter under monocular versus binocular viewing conditions was found until the end of the fourth month of life. The age of onset was positively correlated with the age of onset for stereopsis as assessed by preferential looking. By the end of the 6th month, the difference in pupil diameter under the two viewing conditions was adult-like in magnitude. The procedure provides an objective measure of binocular function that is not affected by fixation error. The data suggest that the visual pathways by which signals from the two eyes converge are not functional until the 4th month of life.