Disparity limit for binocular fusion in fovea

In the human visual system, binocular disparity limit must be limited within a certain fusional area, called "Panum's fusional area"; otherwise, unsuitable disparity could cause double vision. The previous studies on Panum's fusional area have focused only on the horizontal and vertical meridians. In order to measure this area in more directions, we determined its limits in 16 different directions from 0 to 360° at 22.5° steps in fovea. It was found that: (1) the horizontal disparity limit (about 32-40 arcmin) is larger than the vertical limit (about 19.2-25.6 arcmin); (2) Panum's fusional area is approximately symmetrical around the horizontal meridian; (3) however, it is not symmetrical around the vertical meridian; the nasalward disparity limits are obviously larger than temporalward disparity limits; (4) therefore, the form of Panum's fusional area in fovea could be suggested to be an ellipse off-centered toward the nasal side on the horizontal meridian.