Horopter

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

  • Fusion & Panum’s Areas
    • (You can only fuse images for so long before getting tired)
    • (As you move towards the periphery, the number of neurons sending info to the brain decrease, there are more neurons in the centre of the eye sending info to the brain – less space is dedicated in visual cortex to collect info from periphery)
    • Fusional stress – effect of horizontal shear (eye moving horizontally) – central diplopia before peripheral diplopia (fuses much worse horizontally than vertically, & centrally than peripherally. Receptive fields are larger for peripheral areas than central)
    • Fusional stress – effect of vertical shear: central diplopia before peripheral diplopia (fuses much worse vertically than horizontally, & centrally than peripherally)
    • Fusional stress – effect of cyclo shear: peripheral diplopia before central diplopia
  • Fusional eye movements (extremely quick)
    • Reaction time ~ 200 msec
    • Slow movement ~ 30°/sec
    • (cf. Saccades ~ 600°/sec) – saccades = how quickly eyes can move from one side to the other
    • Affected by alcohol, anoxia (lack of O₂)
  • Strabismus
    • The retinal images in each eye do not fall onto corresponding points
    • Adults who develop strabismus often have double vision because the brain is already trained to receive images from both eyes & cannot ignore the image from the turned eye
    • In a young child, (before the corresponding points are formed) the brain learns to ignore the image from one eye & sees only the image from the straight or better-seeing eye. The child then develops amblyopia
  • http://webeye.ophth.uiowa.edu/eyeforum/tutorials/bhola-binocularvision.htm