Lecture 5

• Amblyopia and the effect of visual deprivation

  • Compared with the relatively dark environment within the uterus, the newborn is bombed with visual stimuli of differing light intensity and contours within the first few months of life
  • This encourages the development of the lateral geniculate nucleus (LGN) and striate cortex
  • Visual acuity (VA) improves rapidly as a result
• Critical period


  Time where the visual system is still plastic and also susceptible to any interruptions, so the effect of experience on the brain is mainly strong. Beyond this period, disruption to visual function is thought to be permanent.

  
• The critical period for visual development is in the first 3 months of life
• Critical ages for decisions about treatment of anomalies of binocular vision


  • 12 weeks
  • 24 months
  • 8 years
• 0-12 weeks (the critical period for fixation)


  Operation for congenital monocular cataracts gives a better prognosis if performed before 12 weeks
• The first 24 months (the critical period for acuity)


  Early onset squint has a better binocular function if surgery is completed before the first 24 months
• The (plastic period) where the fixation reflex (foveal fixation in each eye) and vergence control can be lost, up to the age of approximately 8 years


  Treatment of amblyopia and ARC
• Peak onset of squint occurs between 2 and 3 years old, due to the increasing visual demands made by a longer active day
• From 8 years old the response to a sudden change in motor or sensory input may be diplopia rather than adaptations such as suppression, anomalous correspondence or eccentric fixation
• After 8 years old either a traumatic squint or surgery for a long-standing squint may each produce diplopia
• More rarely, excessive amblyopia treatment can also produce diplopia when carried out after this age, where it also produces a change in the squint angle, particularly in deep ARC
• Physiological and anatomical changes that result from optical and motor deficits


  • Monocular Deprivation
  • One Eye Out of Focus
  • Strabismus
• Monocular Deprivation


  The eyelids of one eye are sutured shut for a period of time, resulting in light perception but allowing very little patterned image (shape and form are difficult to see) on the retina. However the direction of movement of an object can be detected.

  
• If a cat or monkey is monocularly deprived for a period of 3 months from the time of eye opening, it becomes severely amblyopic in that eye
• When recording cells in the visual cortex after opening both eyes for the experiment very few cells can be activated by the eye that was sutured

  
• The retina in the sutured eye is normal
• Physiological properties of cells in the lateral geniculate driven by the sutured eye


  The arbors of X (fine detail) cells are smaller than normal, and the arbors of Y (movement) cells are smaller than normal. As a result, fewer Y cells are recorded, and the spatial contrast sensitivity of the X cells is reduced.

  
• Lateral geniculate nucleus


  • The four superficial layers have neurons with small cell bodies and are called the parvocellular layers. The two deep layers have neurons with large cell bodies and are called the magnocellular layers.
• Major changes occur in the projections from the lateral geniculate nucleus to the visual cortex

  
• Ocular dominance columns


  Because one eye is closed the ocular dominance columns from that eye will not be active and will hence have less metabolic activity. The ocular dominance columns from the other eye will be active and will have greater activity leading to a darker stain in these areas for cytochrome oxidase.

  
• When one eye is sutured from an early age, the stripes from the sutured eye are narrower than normal, and the stripes from the open eye are wider
• The terminal arborization in the cortex coming from cells in the lateral geniculate nucleus driven by the sutured eye is shrunken
• As a result, the cells in the lateral geniculate for this eye are smaller than usual because they have a smaller terminal arbor to support
• The ocular dominance changes occur in a particular sequence. At the critical period, monocular deprivation produces a saturating ocular dominance shift.

  
• In the case of long-term monocular deprivation from an early age, almost no cells in the visual cortex can be driven by the deprived eye.

  
• In the case of long-term binocular deprivation, up to one third of the cells can be driven by one or both eyes.

  
• Binocular deprivation is not the sum of two monocular deprivations.
• The projections from the lateral geniculate nucleus to the visual cortex from the two eyes overlap each other at birth, then they segregate into eye specific columns around the time that stereopsis develops.

  
• In the case of monocular deprivation, the terminals from the deprived eye retract until they cover a small fraction of the space, and the terminals from the normal eye do not retract.

  
• One can close the left eye until nearly all cells are dominated by the right eye, and then open the left eye and close the right eye until nearly all cells are dominated by the left eye, a process known as reverse suture.
• Binocular recovery following monocular deprivation


  For full recovery of acuity and binocular vision, one needs monocular vision through the previously deprived eye to bring up the acuity in that eye as well as binocular vision to get the two eyes to work together and prevent vision in the previously non-deprived eye from declining. The ideal procedure is to patch the non deprived eye for one half to three quarters of the time, reflecting clinical experience with infants. Unfortunately, even the best procedure does not yield normal depth perception.

  
• The major effects of monocular deprivation occur within the visual cortex and involve competition between left and right eye inputs.

  
• If the left eye is sutured closed, then there is a retraction of terminals in the visual cortex coming from the left eye and an expansion of terminals coming from the right eye.

  
• Binocular recovery following monocular deprivation

  • Requires monocular vision through the previously deprived eye to bring up the acuity in that eye
  • Requires binocular vision to get the two eyes to work together and prevent vision in the previously non-deprived eye from declining

  
• Ideal procedure for binocular recovery


  Patch the non deprived eye for one half to three quarters of the time
• Even the best procedure does not yield normal depth perception
• Effects of monocular deprivation


  • Occur within the visual cortex
  • Involve competition between left and right eye inputs

  
• Effects of monocular deprivation

  1. Retraction of terminals in the visual cortex coming from the left eye

  2. Expansion of terminals coming from the right eye

  3. Cells in the visual cortex are driven almost exclusively by the right eye

  4. Right eye becomes exclusively dominant

  
• Effects of putting one eye out of focus (atropinization)


  • Cells driven by the atropinized eye have reduced contrast sensitivity
  • Reduction in the percentage of binocularly driven neurons
  • Small shift in ocular dominance toward the normal eye

  
• Cells in the layers driven by the atropinized eye


  • Are smaller
  • This is seen in the parvocellular layers, which deal with high spatial frequencies, and not in the magnocellular layers, which deal primarily with movement