Panum's fusional area corresponds to which description?

• A. The area around a corresponding retinal point where objects are seen as single and in depth
• B. The area immediately around the horopter
• C. The line of sight through the fovea
• D. The entire retina

Answer: (A)

Panum's fusional area is the region of disparity that the visual system can fuse into a single image. For a given corresponding retinal point in the two eyes, small misalignments from object points land within this zone and are merged by the brain, producing a single perception that can also convey some depth. The horopter marks where zero disparity occurs in space, but Panum's area describes the tolerance around those corresponding retinal points for fusion, not points around the horopter itself. When disparities exceed this area, diplopia or suppression can occur. The other descriptions don’t capture this tolerance or fusion process—the line of sight through the fovea is just gaze direction, and the entire retina is simply the visual surface, not where fusion stability is defined.