Which statement is true about the horopter?

• A. It is the spatial representation of all points imaged on corresponding retinal points in both eyes
• B. It is the area around the horopter
• C. It is identical to Panum's fusional space
• D. It is only relevant to monocular vision

Answer: (A)

The horopter represents the spatial locus of all points in space that, for a given fixation, cause images to fall on corresponding retinal points in both eyes. When images land on corresponding points, the brain fuses them into a single percept, so objects on the horopter are seen with single vision. Panum's fusional space surrounds this locus and contains points that produce small disparities still small enough to be fused; it is not the horopter itself, which is about exact correspondence. The horopter is a binocular concept, not monocular. So the statement describing the horopter as the spatial representation of all points imaged on corresponding retinal points in both eyes is the correct one.