Binocular function in human infants: correlation of stereoptic and fusion-rivalry discriminations.

Studies of stereopsis in infants have shown that the average age of onset is 3.5 months. This is the same age at which infants first show evidence of another binocular function, namely, preference for binocularly fusible patterns over rivalrous ones. We tracked the development, using two-alternative forced-choice preferential looking, of the two forms of binocular function in 17 infants, 11 male and six female. They were tested at regular intervals until they showed preferences for 1) a fusible pattern (vertical stripes presented to each eye) over a rivalrous one (vertical stripes presented to one eye, horizontal stripes to the other), and 2) a line stereogram of 32 min crossed disparity over a comparable stereogram with zero disparity. The correlation between the age of onset of the fusion preference (mean 12.4 weeks) and the age of onset of stereopsis (mean 11.0 weeks) was r = 0.79. Female infants showed a preference for the fusible stimulus at a mean age of 9.9 weeks, significantly earlier than the males at a mean age of 13.8 weeks. Similarly, females also showed evidence of stereopsis at an earlier age (9.1 weeks compared with 12.1 weeks for males).