THE DEVELOPMENT OF EYE ALIGNMENT, CONVERGENCE, AND SENSORY BINOCULARITY IN YOUNG INFANTS

Purpose. To measure and compare the development of ocular alignment, s ensory binocularity, and convergence in infants during the first 5 mon ths of life. Methods. Healthy infants were tested between 2 and 21 wee ks of age. Ocular alignment was measured by the Hirschberg test; conve rgence was determined by visual examination as an illuminated toy appr oached an infant's face; and sensory binocularity was measured by pref erential looking for fusible versus rivalrous gratings. In experiment 1, we compared the proportion of infants at different ages demonstrati ng orthotropic ocular alignment with those showing convergence. In exp eriment 2, we compared the age of onset of convergence to that of sens ory binocularity. Results. Experiment 1: Most infants were orthotropic during the first month, and almost all of the others showed small amo unts of exotropia. None of the infants showed accurate convergence unt il 6 weeks of age. By 4 months of age virtually all were orthotropic a nd had good convergence. Experiment 2: The onset of sensory binocular fusion occurred at 12.8 +/- 3.3 weeks. Full convergence did not occur until 13.7 +/- 3.2 weeks, although the first signs of convergence occu rred slightly earlier. For individual infants there was a high correla tion between the age of onset of sensory binocularity and convergence, and both onsets occurred earlier in girls than in boys. Conclusions. Ocular alignment did not require the development of binocularity mecha nisms, and the development of binocularity mechanisms did not await th e onset of good ocular alignment. The relatively sudden onset of binoc ularity, both sensory (preference for fusion and stereopsis) and motor (convergence) at about 3 months of age and the high correlation betwe en these measures indicate a common causal mechanism that probably inv olves refinements in striate cortex circuitry.