Fig. 4 Development of stereoacuity. Proportion of infants reaching 75% correct performance on a depth discrimination of 1 arc-minute of crossed (filled circles) or uncrossed (open circles) disparity. Note that the crossed disparity performance precedes the uncrossed by approximately 1 month. (Birch EE, Gwiazda J, Held R: Stereoacuity development for crossed and uncrossed disparities in human infants. Vision Res 22:507, 1982)