Intro to Binocular Vision

Question 1

Describe Binocular Single Vision (BSV)?

Answer 1

• Ability to use both eyes together
• Each eye receives a separate retinal image which is interpreted by the brain as a single common perception
• In normal usage the individual sees as one

Question 2

Describe how laterally placed eyes see and how frontal eyes see?
What are the advantages of frontal eyes?

Answer 2

Laterally placed eyes = total decussation = no overlap in fields
Frontal eyes = partial decussation = overlap in fields

+ves:
Messages from overlapping areas can be combined at early stages of visual processing i.e. command to move eyes may be sent out quickly
Early combination of visual input may prevent diplopia i.e. fusion of the 2 visual inputs
Coordinated eye movements

Question 3

What are the advantages of binocular single vision?

Answer 3

• Main advantage of having binocular single vision is ability to perceive in depth
• This ability is termed stereopsis
  o Measuring how good 3D VA is
  o Hyperacuity – doesn’t matter how straight your eyes are – if you don’t have adequate vision in both eyes then won’t have stereo
• Individuals w/o BSV use monocular cues to appreciate depth
• Beyond a few metres monocular cues are more important, stereopsis contributing little to depth perception
  o Monocular cues e.g. smaller tree is further away than larger tree
  o If never had 3D then don’t know what missing
• Spare eye
• Large field of vision
• Binocular summation (square root of 2)
  o E.g. visual acuity, contrast sensitivity, motion
• Enhanced visual-motor skills

Question 4

What happens when one eye not working with other?

Answer 4

• Two eyes need to work together
• Child densely amblyopic – want good vision in both eyes in case something happens to poorer eye but also to have BSV
• Earlier something happens to BV system – the more detrimental – pick it up as early as possible so these children can see BV
• 80% chance of getting squint if have congenital cataract
• Patching can delay binocularity – but need the other eye to catch up to get good vision
• Patching – roughly half an hour per each week of life
• Not just about vision – need eyes to work together too

Question 5

Describe perception of space?

Answer 5

• Visual system tells us what is seen & where it is seen
• Basic visual function is sense of direction
• When we look directly at an object image falls on fovea, this enables us to see it clearly & also tells us where object is located in visual space
• Bifoveal projection – foveas talking to each other – should only see one image

Question 6

Describe visual direction?

Answer 6

• Projection is term for object location in space & it depends on the part of retina being stimulated
• Each part of retina has a given projection for a given distance of fixation
• This is termed visual sign or visual direction
  o Projection: allows up to know where something is - & is it left or right, up or down
  o Proprioception: tells us where something is moving to
• Any system that specifies location or direction needs a reference point
• The main reference point for projection is the fovea which is described as having the principal visual direction; independent of direction of gaze
• All other retinal points/areas have a secondary visual direction

Question 7

Describe oculocentric visual direction?

Answer 7

• Visual direction of an object can be represented by a line that joins object & fovea called Principal Visual Direction or visual axis
• Based on principal visual direction, the direction of all other objects in subjects visual field is determined
  o This is the oculocentric visual direction
• Therefore, each point on retina can be considered to have its own sense of direction
• Oculocentric: we know from visual perception that if fovea is stimulated then object is straight ahead

Question 8

Describe egocentric visual direction?

Answer 8

• Refers to direction of an object in space relative to one self rather than the eyes
• Determine by retinal position, proprioceptive info about eye, head & body position & vestibular apparatus
• All this info allows us to determine if a change in retinal position is due to object movement or due to eye or head movement object has remained stationary

Question 9

Describe normal binocular viewing?

Answer 9

• Certain retinal elements in each eye share a common visual direction
• Our fixation object stimulates each fovea simultaneously & each image is projected back to same location in space
• The 2 foveae are described as corresponding points
• If normal development – right fovea & left fovea should work together
  o Partnerships in nasal & temporal retina that work together too
• Bifoveal – both foveas working together, seeing well enough to see straight ahead

Question 10

Describe peripheral retinal correspondence?

Answer 10

• Same visual projection in both eyes
• If an object in right part of visual field stimulates a temporal point in LE it will also fall on a nasal point in RE
• These are corresponding points which project back to same point in space
• Corresponding retinal areas – straight forward when px has straight eyes & BV
  o The 2 connecting points aren’t linked together when px squinting
• This correspondence principal applies to all retinal areas
• This is the basis of normal retinal correspondence
• When foveae have a common visual direction & retinal elements nasal to one fovea correspond w/ those temporal to other foveae
• Normal retinal correspondence – R&L eye talk to each other, fovea have principal visual direction. This px may go on to develop intermittent squint
  o Convergence excess esotropia – when look at near they squint, high AC:A ratio, when look in distance no longer squinting
  o Just squinting doesn’t mean have abnormal retinal correspondence e.g. intermittent squint
  o If squinting thenn may get diplopia or will get suppression

Question 11

What is cyclopean projection?

Answer 11

• Normal retinal correspondence (NRC) is necessary for BSV
• In BSV an object fixated by the 2 foveae appears to be located centrally by a cyclopean eye
• Binocular projection can be considered as if from this imaginary cyclopean single eye
• Cyclopean projection diagrams are always used to describe BSV

Question 12

Describe the horopter?

Answer 12

• Imaginary plane/structure existing in BV
• It represents locus of all points in space whose images stimulate corresponding retinal points
• Horopter seen in a cyclopean projection is known as Vieth-Muller circle is passes through fixation point & nodal point of BEs
• This is how we get single vision
• Images outwith horopter – cannot acknowledge them singly – either have to ignore it or get diplopia
• Any object placed on horopter, will have a constant image size on retina, no matter where it is located on horopter
• If image size is constant (angular size on retina) & binocular disparity remains zero (object stays on horopter) then any object that is moved along horopter should always appear to be same distance from observer
• 2D binocular vision i.e. no depth perception
• If on horopter & normal development – get 3D, if not on horopter only getting 2D