Characteristics of Binocular Single Vision

Question 1

What does development of BSV depend on?

Answer 1

• The development of BSV depends on coordination of sensory aspects & motor responses of visual processes
  o Sensory – appreciate images. Motor – moving eyes around.
• This coordination is not present in new-born but develop from birth to ~18months in a series of reflexes called binocular reflexes
  o Although visual apparatus is intact soon after birth the possession & consolidation of BV is not in newborn but must be acquired gradually during 1st few yrs of life
  o Need coordinated responses – within 18mths
  BSV depends on straight eyes, good VA in each eye, normal brain anatomy

Question 2

Describe the sensory mechanism?

Answer 2

• Light sense
• Form sense – good VA
• Colour sense
• Spatial relationship – projection & localisation
• Normal Development depends on:
  o Clear refractive media – no corneal scarring, no cong cataract, no opactities
  o Normal retina
  o Cones – form & colour
  o Rods – light & movement
  o Good VA in either eye
  o Normal retinal correspondence
  o Normal visual pathways from retina to cortex
  o Normal proprioceptive receptors
   Need these receptors in inner ear organs in neck to tell us when to move head, move eyes in opposite direction – to keep image stable
   Incom – abnormal head posture if one eye higher than other
  o Reinforcement of stimulus
  o Vestibular apparatus otolith & endolymph
   Otolith inner ear gravity & acceleration
   Endolymph fluid inner ear coordinate balance
  o Nerve endings in neck muscles – so they know where you head is in space to where your eyes should be following

Question 3

What happens if you develop amblyopia in relation to the sensory mechanism and BSV?

Answer 3

If develop amblyopia will interrupt sensory mechanism for normal BSV. If one eye is 6/6 & other is not then will not be able to be binocular.
If not using BSV you will lose it
Px starts to develop cataract – vision drops, contrast sensitivity changes, may have XOP could control whole life – 6/6 in one eye and dense cataract 6/36 in other eye – lost binocularity – squinting

Question 4

Describe the motor mechanism?

Answer 4

• To put object of attention onto each fovea – central fixation – want central fixation regardless of what looking at
• Maintain it there when object moves
• To allow change of fixation from one object of attention to another
• Ensure proper alignment of both eyes
  o If one eye is lagging & needs to catch up w/ other eye – will start getting dip – one eye stops & other keeps moving
  o If px reports w/ diplopia – something has happened to them, interrupted something
• Normal Development depends on:
  o Reinforcement of stimulus
  o Normal EOM muscles & adnexae (CN supply)
  o Normal oculo-motor nerve nuclei & pathways
  o Inter-nuclear & supra-nuclear pathways
   Childs eyes look straight but problems with saccadic system – oculomotor apraxia
  o Lower motor neuron pathways controlling vestibular mechanisms
   So ears and vestibular working together

Question 5

What are sensory & motor functions linked by?

Answer 5

The sensory & motor functions are linked by postural & psycho-optical (binocular) reflexes

Question 6

Describe postural reflexes?

Answer 6

• These are innate (or unconditioned) & are controlled by lower motor neuron pathways they are independent of visual stimuli (occur in blind)
  o Even if move blind persons head, their eyes will move – innate development that if head moves then eyes move other way
  o If px with abnormal head posture – if want px to move eyes aways from problem they will move their head towards the problem
• To maintain posture despite changes in position of head in relation to space
• To maintain posture despite changes in position of head in relation to trunk
• To maintain orientation of the eyes despite changes in position of the head
• Postural reflexes really facilitate fluid movement balance posture & stability i.e. combat effects of gravity
• Static postural reflex:
  o Tonic labyrinthine – stimulation of otolith organs when head falls forward eyes move up
  o Tonic neck – stimulation of proprioceptive nerve endings in the neck muscles when head is inclined to right shoulder eyes make a compensatory movement to the left
• Stato-kinetic postural reflex
  o Stimulation of endolymph in semi-circular canals when head & body are rotated results in physiological vestibular nystagmus. The slow movement of which is to maintain the eyes as long as possible in same position in relation to visual field.
• These reflexes are present at birth but as visual function develops the control of posture becomes more dependent on eyes
  Inner ear infection – whole postural reflex may be swayed – may get labyrinthitis – vertigo, nystagmus, oscillopsia

Question 7

Describe binocular reflexes?

Answer 7

• The stimulus is visual but interest & attention needed, these reflexes are conditioned & are basically involuntary (in some instances can be voluntary inhibited)
• Fixation reflex places object of attention on fovea – present very soon after birth
  o Nystagmus – this reflex doesn’t develop – eyes moving all time
• Re-fixation reflex
  o Passive enables eye to maintain fixation on a moving object ‘smooth pursuit’
  o Active enables eye to change fixation from one object to another ‘saccadic’
• Conjugate fixation reflex links 1st two reflexes above in binocular fixation demonstratable at 6mths
  o Test this by assessing convergence – testing pxs eyes moving in different directions
• Fusional vergence reflex maintains bifoveal fixation in presence of a heterophoria demonstratable at 6wks established by 6mths
• Vergence reflex maintains bi-foveal fixation on an object as it approaches eyes demonstratable at 1mth well established by 6mths
• Near reflex or near synkinesis links vergence reflex with pupillary reflex (unconditioned) & accommodation reflex
  o They are linked but can be demonstrated separately under certain conditions
  o E.g. Parinauds Syndrome – no longer stimulating Edinger Westphal Nucleus – light-near dissociation – they had it but then lost it.

Question 8

What are the 4 main characteristics of BSV?

Answer 8

• Fusion
  o Need simultaneous perception – see with both eyes -
• Retinal rivalry
  o If one eye slightly better than other – eyes start competing with each other
  o Anisometropia – one eye slightly fuzzier than other will not have true fusion
• Stereopsis
• Physiological diplopia

Question 9

Describe fusion?

Answer 9

• 2 components:
  o Sensory
   Ability to perceive 2 similar images, one formed on each retina & interpret them as one
• Images must be located on corresponding retinal areas
• Similar in size, brightness, sharpness – don’t have to be exactly same but must be similar in order to fuse them
  o Motor
   Ability to maintain sensory fusion through range of vergence movements
   May be horizontal, vertical or cyclovergence
   Obstacles to fusion may be important factors in aetiology of strabismus
• If poor fusion, that’s when develop a squint
• Everyone has potential to have a strabismus – size of phoria & amount of fusional ability to control it is what prevents it
• More convergence range than abduction range
• Need an angle of deviation that isn’t too large that you can control it
• Traumatic loss of fusion – may not be able to fuse & then cannot control 8 dioptres esophoria they’ve been able to control whole life

Question 10

Describe retinal rivalry?

Answer 10

• When dissimilar images are presented to corresponding retinal areas fusion becomes impossible
• Dissimilar images localised in same place give rise to conflict and confusion one or other is temporarily suppressed
  o Luminance
  o Colour
  o Contour
• A bit of this is good but don’t want too much
  o Need a bit to appreciate depth
  o Too much is a barrier to the 2 eyes working together
• Image: brain tries to fuse these but they are too different so cannot
  o Brain saying is right eye different, is left eye different – cannot fuse them
• Amblyopia is barrier to BSV even if eyes straight – because of this rivalry

Question 11

Describe stereopsis?

Answer 11

• Occurs when similar images are presented to non-corresponding (disparate) retinal areas
  o & are situated close to horopter in an area termed Panum’s fusional space they will be fused with appreciation of depth
• Panum’s fusional space is a narrow band around horopter within which object points give rise to BSV
  o Wiggle room for horopter – allows us to appreciate depth – stereopsis
• Stimulating slightly connected points but not on fovea – that’s when appreciate depth
• Anything outwith Panum’s fusional space will be seen as double

Question 12

Describe Panum’s fusional space?

Answer 12

• Elliptical in shape narrowest at fixation & increasing in depth towards periphery
• Consistent with larger receptive fields and poorer visual acuity in periphery
• 6-10 seconds of arc at centre and 30-40 second of arc at periphery
• Does not have fixed size, but varies depending on stimulus conditions e.g. Panum’s areas in fovea are larger with low spatial & temporal frequencies

Question 13

Describe Panum’s fusional area?

Answer 13

• Area surrounding corresponding retinal points within each disparity of correspondence can occur whilst maintaining BSV
  In presence of binocular vision, objects outside of Panum’s fusional space (area) cause very large disparites on retinas & they cannot be fused. They are seen in diplopia.

Question 14

Which type of diplopia is seen in exotropia? and in esotropia?

Answer 14

Crossed (heteronymous) diplopia seen in exotropia
Uncrossed (homonymous) diplopia is seen in esotropia