horopter

The horopter is the locus in space of points that project to corresponding points on the two retinas, such that they are seen as a single image with zero binocular disparity when the eyes are fixated at a given point. Points on the horopter are fused by binocular vision, while points off it generally produce disparity that is interpreted as depth or, if the disparity is large, may appear double.

In a simplified geometric model with symmetrical eyes, the horopter is represented by a circle known as the Vieth-Müller circle. This circle passes through the fixation point and the eye nodal points, and points on it produce zero disparity for that fixation. Points outside the circle exhibit nonzero disparity and require binocular fusion with depth cues; their perception depends on the brain’s interpretation of disparity.

In actual human vision, the horopter is not a perfect circle. The empirical horopter is shaped by the geometry of the eyes, vergence, and accommodation, and tends to deviate from the idealized circle, forming a curved surface that can shift with fixation distance. Panum’s fusional areas describe the range of small disparities around the horopter that can still be fused into a single image.

The horopter is a central concept in studies of stereopsis and depth perception, and it has practical relevance in diagnosing and understanding binocular vision conditions such as strabismus and anisometropia.