NBEO Binocular Vision Practice Test

The area around the horopter where the brain can still fuse two slightly different images into one percept is Panum's fusional space. This region provides the “wiggle room” that allows objects not exactly on the horopter to be seen as a single image and still give you depth information, because the disparity is small enough for fusion to occur. If disparity falls outside this space, fusion breaks and diplopia appears. The term Panum's fusional space specifically captures this tolerance zone; the other terms either refer to the horopter itself (zero disparity) or are not the standard way to describe this fusion-tolerance region.