The development of crowding and interocular interactions in a resolution acuity task.

PURPOSE To investigate the impact of interocular similarities of a surround stimulus on foveal resolution acuity in the normally developing visual system. METHODS Liquid crystal shutter goggles synchronized with the monitor frame rate were used to present a Landolt C and surround bars to one or both eyes, in monocular, dichoptic, half-binocular, and binocular viewing conditions. Resolution acuity was measured under each condition in 56 normally sighted children (7 to 14 years of age) and 22 adults (21 to 38 years of age). The effect of the surround bars (crowding) was tested in a subgroup of nine children, and 10 adults. RESULTS Across all age groups resolution acuity was significantly better in the binocular condition than in the other three viewing conditions (binocular summation), and was significantly better in the half-binocular (with target presented to the test eye and bars presented to both eyes) than in the dichoptic condition (target presented to test eye and bars presented to the nontested eye only). In children, but not in adults, resolution acuity was significantly better without than with bars. CONCLUSIONS The interocular similarities may explain the better visual resolution in the half-binocular condition than in the dichoptic condition for all age groups tested. The results suggest that interocular interactions underpinning resolution acuity under these viewing conditions are developed in early childhood. The foveal crowding effect was found to be apparent at the beginning of school age, and diminished with maturation.