627- Exam 4
development and maturation of vision - ANS-- HVS is developmentally incomplete at birth
- matures over the first few years of life
- normal visual environment is imperative to the process of visual development
- normal progression of development is an expression of inborn, genetic factors and is
significantly influenced by the environment
critical period - ANS-- development period in which the environment has a powerful effect on the
structure and function of the HVS
- most vulnerable to abnormal visual experience during the critical period
visual deprivation - ANS-- one or both eyes are deprived of a normal visual environment during
the critical period
- due to degraded image in one or both eyes due to optical factors, disease, or formation of
non-identical images on corresponding retinal points
- in animals, artificially achieved by suturing lids of one eye of the infant animal
ocular dominance columns - ANS-- neurons all driven in varying degrees by the same eye
- located in VI
- shifted when visual deprivation occurs during development
ocular dominance histogram - ANS-- 7 categories of cortical cells
- determined using extracellular recordings while only stimulating one eye
- normal adult cortex has greatest number of cells and are driven equally by both eyes
- category 1 are driven by contralateral eye
- category 7 are driven by ipsilateral eye
ocular dominance change with monocular deprivation during critical period - ANS-- created in
kitten by suturing its eyelid closed at birth
- virtually all cells in the striate cortex exclusively responsive to the open unsutured eye
- equivalent to total congenital cataract where all details are filtered out but light gets to retina
- retina and LGN are relatively normal
- deficit occurs at level of VI and higher up
- striate cortex has wider columns for normal eye than for deprived eye
ocular dominance change with monocular deprivation outside critical period - ANS-- no affect on
the architecture of ocular dominance column
- visual system has lost its plasticity and is increasingly hardwired with age
- pattern of neurons in VI maintained
- neurons are still responsive when a developed cataract is removed
,competitive interaction during critical period - ANS-- not simply result of atrophy of weak inputs
- inputs from the normal eye actively replace and take over cortical territory that formerly
belonged to the deprived eye
- result of competitive binocular interaction between the two eyes during the critical period
- visual deprivation during the critical period results in deprived eye losing some of its cortical
territory with corresponding expansion of the cortical territory belonging to the normal eye
binocular competition - ANS-- at birth, the cortical representation of the two eyes is equal
- if the visual environment of each eye is similar this balance is retained
- monocular deprivation results in the normal eye getting a competitive advantage
- leads the normal eye to replace many of the synaptic inputs from the deprived eye
changes in individual neurons after monocular deprivation - ANS-- reduction in the size and
complexity of the arborizations
- occurs very quickly
- effect can be reduced by treatment/management
effect of binocular deprivation on ocular dominance - ANS-- distribution that appears roughly
normal
- these neurons have abnormal spatial properties to orientation, disparity, and spatial features
- both eyes retain cortical territory because there is no imbalance in visual stimulation
development of binocular vision - ANS-- at birth, retinal correspondence is near normal
- oculomotor system is not developed
- inaccuracy in binocular coordination can create intermittent diplopia and confusion
- if critical period started at birth, majority of infants would develop binocularity problems
- critical period begins about 2 months after birth
development of accommodation - ANS-- some accommodative capacity present at two weeks
- response is inaccurate and does not match the accommodative demand due to poor spatial
resolution and a large depth of focus
- accommodation is accurate and consistent by 3-5 months of age
development of vergence - ANS-- infants exhibit some vergence response by 1 month but it is
mismatched with speed of the target
- accurate vergence movements develop by about 6 months of age
- development of vergence response depends on development oculomotor control, relative
depth judgement, and spatially tuned receptive fields that can detect fine disparities
- large crude receptive fields result in large PFS early on making detecting movement difficult
motor fusion - ANS-- young infants frequently demonstrate ocular misalignments (under or over
convergence)
- under convergence is more common
- majority of infants have intermittent exotropia
, - small number have intermittent esotropias
- vergence response becomes very accurate by six months
sensory fusion - ANS-- retinal correspondence is pre-programmed before birth
- visual system has capacity to register images in corresponding locations and achieve sensory
fusion
- oculomotor control required for motor fusion is th elimitation
development of the three grades of fusion - ANS-- between 1-3 months of age, infants
demonstrate simultaneous perception of images (worth grade I fusion)
- by 3 months of age, most infants have flat fusion (worth grade II fusion)
- between 6 months of age, stereopsis develops (worth grade III fusion)
the visual cliff - ANS-- infant placed on elevated glass surface with checkerboard pattern
beneath on one side with steep drop-off on other side
- infant placed on border of two sides and called to the edge
- most infants avoid crossing over which indicates depth perception
- does not necessarily prove that infant can perceive disparity as the visual cliff experiment does
not eliminate monocular cues to depth
preferential looking technique - ANS-- infant wears polaroid filters
- presented with a random dot stereogram on one side and a random dot pattern on the other
side
- assume that the infant will prefer to look at the stereogram if it perceives binocular disparity
and thus stereoscopic depth
- two alternative forced choice test
random dot stereogram in motion - ANS-- infant is presented with a moving random dot
stereogram
- ability of infant to track the movement is tracked
visually evoked potential - ANS-- electrical response of visual cortex is measured as the infant is
presented targets with binocular disparity
- significant electrical activity signifies ability to perceive disparity
development of binocular disparity - ANS-- VEP and PLT studies show the ability to detect depth
based on binocular disparity alone develops at about 3 months
- improves rapidly and has adult-like thresholds of <1 arc minute is achieved by 6 months
- ability to perceive crossed disparities sooner than uncrossed
- female infants develop stereopsis a month ahead of males
development of spatial vision - ANS-- objective estimates of the visual acuity of an infant can be
obtained with the sweep VEP
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