The HOROPTER is an imaginary. surface whose points are all at the. same distance as the fixation point. Points on the horopter project to. corresponding locations on the temporal and nasal retinas, respectively. These corresponding locations. exhibit zero retinal disparity. i.e., D = dtemporal – dnasal = 0.

A horopter is an imaginary plane made up from an infinite number of points in space projecting from corresponding retinal areas in the two eyes. An object placed on the horopter will stimulate exactly corresponding points on the two retinae

  • Vieth-Müller circle
  • The Vieth-Müller circle (theoretical horopter) is based on certain geometric assumptions about the eyes. These are:
    • Each retina may be represented by a perfect circle
    • Corresponding points are evenly spaced across the nasal & temporal retinas of each eye
    • Both retinas are the same size & corresponding points are perfectly matched for their locations in the two eyes
  • Empirical Horopter
  • The empirical Horopter is much flatter than the theoretical horopter that forms part of the Vieth-Müller circle
  • Nasal & Temporal corresponding points differ in their distance to the fixation point
  • Each individual has their own individual empirical horopter (cannot calculate this, have to measure it for each individual)
  • Objects located exactly on the horopter are seen as fused, but what happens if the object is very slightly off the horopter, either closer or farther away?
  • FUSION – align the eyes & facilitates binocular vision
  • (when the two images are just outside horopter, fusion tries to align two eyes together make a single image; give binocular vision)
  • Motor fusion: (eye actually changes position to correct double vision, brain is aligning the two eyes)
    • A “closed loop” response to small disparities
    • It changes the vergence & reduces the disparity
  • Sensory fusion: (doesn’t change position of eyes, brain just overlaps images)
    • An “open loop” response to small disparities
    • Does not change the vergence – disparity analysis within Panum’s Areas (constant – feedback reaction)
  • Panum’s area
    • Within a small distance, either side of the horopter, objects can still be fused & seen as single. Strictly speaking, they fall on non-corresponding retinal points & there will be a small disparity
    • The zone on either side of the horopter within which it is still possible to see objects singly is known as Panum’s area
    • At periphery, Panum’s area is large, at fovea it is small
  • Panum’s Fusional Space indicates that:
    • Retinal correspondence is not just between pairs of points but between retinal areas centred on corresponding points
  • Panum’s Areas:
    • The receptive fields of cortical binocular neurons
    • Objects far enough from the horopter to be outside Panum’s area produce very large retinal disparities, so cannot be fused
    • They are seen as double

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