BV LEC

• Corresponding Retinal Points (CRP)

  • Points on the retina that give rise to the same visual direction
  • Every retinal point has a partner in the fellow retina with which they share relative subjective visual direction
  • Principal Visual Direction from object of regard to the Fovea
  • Fovea Centralis is the point of keenest vision
• Examples of CRP


  • Fovea of OD and OS
  • Nasal retinal point of an eye, Temporal retinal point of an eye or Vice Versa
  • aL and aR, bL and bR
• CRP


  • Principal elements of the two retinas that give rise to binocular single vision
  • Single vision is the hallmark of retinal correspondence
• Non-Corresponding Retinal Points or Disparate Retinal Points


  Points on the two retina that give rise to different visual directions (Double vision)
• Examples of Non-Corresponding Retinal Points


  • Temporal retinal points of both eyes bL and bR or Nasal retinal points of both eyes aL and aR
• The Horopter


  • Locus of points in space that stimulates corresponding retinal points
  • An imaginary surface centered on the fixation point that moves with the eyes and leads to single vision
  • Vieth Muller Circle is formed by symmetrically placed corresponding retinal points for the two eyes
  • Panum's Fusional Area is the region where fusion is possible even when images are not on corresponding retinal points
• Vieth Muller Circle


  Shape of the horopter when corresponding retinal points for the two eyes are symmetrically placed, passing through the fixation point and the centers of the entrance pupil of the two eyes
• Panum's Fusional Area


  Region in space surrounding a horopter where images that appear at different points on the two retinas can be fused into single images
• 3 Aspects of Binocular Single Vision

  • Sensory aspect: ability to perceive an image from each eye simultaneously
  • Motor aspect: ability of both eyes to maintain sensory fusion through vergence movements
  • Stereopsis: perception of depth based on binocular disparity
• Sensory Fusion


  • Process of combining images from the retinas of the two eyes into a single percept
  • Cortical process of blending the images into a single binocular stereoscopic image
• Physiologic Diplopia
• Double vision is the hallmark of diplopia
• Object of regard at near
• Diplopia: The result of simultaneous stimulation of non-corresponding or disparate retinal elements by an object point causes this point to be localized in two different subjective visual directions (either crossed or uncrossed)
• Physiologic Diplopia: Diplopia occurring in normal binocular vision for non-fixated objects whose images stimulate disparate points on the retina outside of the Panum’s area
• Motor Aspect of Binocular Vision


  • The ability of both the eyes to maintain sensory fusion through a range of vergence movements
• Conjugate Movements


  • The ability of the eyes to work together or in unison
• Saccadic Eye Movement


  The fast, abrupt movement initiated by a sudden increase in innervation to the muscle concerned. This increase in innervation is followed by a steady firing of nerve impulses to the agonist muscle and an accompanying reduction in firing of the antagonist muscle. Saccades are used to move the fovea to the next object/region of interest
• Saccadic Eye Movement can occur as

  • Voluntary refixation movements
  • Involuntary or random movements, or in response to visual and auditory stimuli
  • The fast phase in nystagmus
  • Rapid eye movements (REM) occurring in REM sleep
  • Microsaccades
  • Pursuit movements faster than 45 degrees per second
• Smooth Pursuit Eye Movement

  A slower conjugate movement made to keep the image of a moving object on or near the fovea or to keep the image of a stationary object on or near the fovea as the head moves
• Sensory Fusion


  The process by means of which the images formed on the retinas of the two eyes are combined into a single percept
• Motor Fusion


  Refers to the version/vergence movements made by the eyes in response to retinal disparity and having the result of obtaining images on corresponding retinal points so that sensory fusion may take place
• Worth’s Level of Binocular Vision: 1st Degree - Simultaneous Perception
• Synoptophore or Amblyoscope or Haploscope: 1st Degree - Simultaneous Perception
• Worth’s Level of Binocular Vision: 2nd Degree - Fusion/Flat Fusion
• 2 Types of Fusion: Sensory Fusion, Motor Fusion
• Sensory Fusion Requirements: 1. Stimulation of CRPs 2. Images should have the same size (aniseikonia-free) 3. Images should be similar
• Sensory Fusion


  The ability of both eyes to perceive two similar images, one formed on each retina, when interpreted as one single visual image
• Sensory Fusion Requirements


  1. Stimulation of CRPs

  2. Images should have same size (aniseikonia-free)

  3. Images should be similar/almost similar in clarity/acuteness

  4. Images should have the same shape (distortion free)

  5. Intact cortical fusion mechanism capability
• Aniseikonia is associated with anisometropia as both anisometropia itself and the optical correction for anisometropia can cause aniseikonia
• Aniseikonia is a difference in the perceived size or shape of images between eyes
• Causes of Aniseikonia


  • Differences in size of dioptric images formed by the retina
  • Difference in the distribution of retinal elements (rods and cones)
• Our brain can tolerate aniseikonia brought about by a difference of 1.50 D eyeglass prescription between the 2 eyes
• Vertex distance


  Distance between the spectacle plane and cornea
• Metamorphopsia Symptoms


  1. Straight objects, like a signpost, appear wavy

  2. Flat things, such as the sign itself, look rounded

  3. Shapes, such as a face, can appear distorted

  4. Objects appear smaller than they are (micropsia) or larger than they are (macropsia)
• Motor Fusion


  1. The ability to align both eyes in such a way that sensory fusion can be maintained

  2. Needs vergence/version movements of the eyes to allow sensory fusion to happen

  3. Retinal disparity formed outside Panum’s area and the eyes moving in opposite direction which may be horizontal, vertical, or cyclovergence is the stimulus for these fusional eye movements
• Motor Fusion Requirements


  1. Eyes must be able to align themselves so that the image of the fixated object can be placed and maintained at the fovea of the 2 eyes

  2. Yoke muscles should function properly
• 3rd Degree of Stereopsis is the highest level of binocular vision function where a smaller amount of retinal disparity doesn’t produce diplopia but a sense of points closer/farther from the fixation point (3D)