On the typical development of stereopsis: fine and coarse processing.

Stereoscopic depth perception may be obtained from small retinal disparities that can be fused for single vision (fine stereopsis), but reliable depth information is also obtained from larger disparities that produce double vision (coarse stereopsis). While there is some evidence that stereoacuity improves with age, little is known about the development and maturation of coarse stereopsis. Here we address this gap by assessing the maturation of stereoscopic depth perception in children (4-14 years) and adults over a large range of disparities from fused (fine) to diplopic (coarse). The observer's task was to indicate whether a stereoscopic cartoon character was nearer or farther away than a zero-disparity reference frame. The test disparities were grouped into fine (0.02, 0.08, 0.17, 0.33, 0.68, 1.0 deg) and coarse (2.0, 2.5, 3.0, 3.5 deg) ranges based on an initial determination of the diplopia threshold for each observer. Next, percent correct depth direction was determined as a function of disparity. In the coarse range, accuracy decreased slightly with disparity and there were no differences as a function of age. In the fine range, accuracy was constant across all disparities in adults and increased with disparity in children of all ages. Performance was immature in all children at the finest disparity tested. We conclude that stereopsis in the coarse range is mature at 4 years of age, but stereopsis in the fine range, at least for small disparities, continues to mature into the school-age years.