Which description is more closely related to points within object space?

• A. Panum's fusional space
• B. Panum's fusional area
• C. Horopter
• D. Conjugate points

Answer: (A)

In binocular vision, there is a region in space where the brain can fuse the two eyes' images into a single percept even when the retinal images are not exactly corresponding. This region is Panum's fusional space, describing how far from the exact corresponding point in space a stimulus can be and still be fused. The horopter, by contrast, is the precise locus in object space where points project to corresponding retinal points, producing zero disparity. So, when we talk about points within object space that can still be fused despite small disparities, Panum's fusional space is the best match. Conjugate points relate to corresponding retinal locations and vergence, not to the tolerance region for fusion in space.