Upload
manoj-aryal
View
373
Download
0
Embed Size (px)
Citation preview
DEVELOPMENT O F VISION
MANOJ ARYAL
Institute Of MedicineMaharajgunj
Medical College
Introduction
The human visual system is not fully developed at birth :rather it matures over the first several years of life.
It includes: Development of anatomical structures Development of refractive errors Development of grating acuity Development of other visual attributes.
Development of anatomical structures
At birth axial length :17mm ,70 % of adult size.
Volume of orbit is only 50% of adult. Cornea is flat at birth ,becomes
steepes as age increases. Lens accommodation occurs at 1 month
of age ,becomes more regular at 2-3 months ,almost adult like range by 6 months 14-16D at birth
Muscle insertion and their relationships to the limbus and equator change dramatically within 1st yr of life.
Differentiation of fovea occurs relatively late than other parts of retina -incomplete until 4mths. after birth
Optic nerve head relatively full size after birth.
Myelination of Visual pathway uncompleted until 2yrs.of age
Peripheral retinal development: Between 8 to 9 month of gestation development of
temporal retinal region complete. Indentation of peripheral retina in other regions
of globe continues to develop after birth. Zone between ora -serrata and equator enlarges in
size until about 2 year of age.Retinal vascularization:
Proceed from centre to periphery .Mature pattern of vascularization - 3 months after birth.
Visual cortex developmentocular dominance
Most neurons in visual cortex are binocular, receiving input from both eyes.
However , most neurons do not receive equal input from two eyes:
one eye tend to dominate a given cortical cell
ocular dominance.Can be illustrated with an ocular
dominance histogram
Cells in categories 1 and 7 are monocular
Category 1 cells receive input from only the contralateral eye
. Whereas
category 7 receive input from only the ipsilateral eye.
Cells in categories 1 and 7 are monocular Category 1 cells receive input from only
the contralateral eye. Whereas category 7 receive input from
only the ipsilateral eye. Neurons in category 4 are
binocular and receive input from both eyes.
Neurons in category 2 and 3 are dominated by the contralateral eye and those in categories 5 and 7are dominated by ipsilateral eye.
David Hubel and Torston Wiesel Experiment
Experiment on kitten:
They sutured one of a kittens eye lids closed at birthand recorded from striate cortex after animal has fully matured
Striate cortex of monocularly deprived animal is very different from that of a normal animal.
2 3 4 5 6 7
Ocular dominance
Number of cells
contralateral ipsilateral
Virtually all cells are monocular and responsive only to the nondeprived eye
Conclusion: For striate cortex to develop a
normal complement of binocular cells , it is necessary for both eyes to provide input during development.
Hubel and Wiesel work suggest that :
During critical period the two eyes compete with each other to dominate cortical neurons. If both eyes have equal retinal image ,then most of cortical neurons becomes binocular.
When one eye wins out in competition as a consequence ocular dominance.
Critical period
Synaptic connection in cortex is strengthened by neural activity.
The geniculate neurons with input from non deprived eye will stimulate cortical cell more than from deprived eye.
There is strengthening of synapses for non deprived eye relative to deprived eye.
The period during which the visual system can be influenced by environmental manipulation is referred as the critical period or sensitive period.
The human critical period is over by about 7 to 9 years of age.(vaegan and Taylor 1980)
Developmental Plasticity:monocular deprivation Visual system is plastic
early in life,it becomes hard wired later in life
From birth to about 12 year the visual system is still flexible to change
Development of refractive errors The average newborn infant to be
hypermetropic with a mean refractive error of around 2D .
A rapid decline in hypermetropia occurs between six months and two years in normally developing eyes.
A further, decrease towards emmetropia is then seen up until the age of six years
Later, in teenage years there is a tendency for the number of children with myopia to increase.
The correction of refractive error in infants and toddlers is controversial because
Lenses could be potentially interfere with emmetropization.
The prescription of minus lenses for myopia lead to near defocus ,there by promoting the development of additional amount of myopia.
Spectacle correction of clinically significant amount of hyperopia in infants does not interfere with emmetropization.
Animal studies on monkeys, cats and chickens have shown that: Eyes in which the retina is allowed to
receive light, but no form vision (form deprivation), tend to become highly myopic
i.e. the disruption of normal visual experience leads to a breakdown in the emmetropisation process.
This myopia can be reversed if normal viewing conditions are resumed, as long as this occurs within a critical period” of development
Human infants born with ocular pathology, e.g. cataract, tend to develop high myopia and have a much wider spread of refractive error.
Development of grating acuity
Resolution acuity of 1 month old infant as measured behaviorally with spatial grating is on the order of 20/600.
Adult levels are reached by about 3 to 5 years of age.
Procedures used to assess grating acuity in infants include: Optokinetic nystagmus Preferential looking Visually evoked potential.
Optokinetic nystagmus:
A moving grating produces nystagmus.
Consist of slow following movement followed by fast compensating eye movements(saccade)
Depend upon the ability to resolve the gratings.
Used to assess visual capabilities in uncooperative children, including infants, malingers and mentally retarded.
Preferential looking:
When given a choice between patterned and non patterned stimulus infants prefer to view the patterned stimulus ,this behavior is known as preferential looking.
Used to determine infants grating acuity.
Both patterned and non patterned stimuli have same average luminance
Eliminating luminance as cue.
If examiner is required to guess which side the pattern is on ,procedure is referred as Forced Choice Preferential Looking.
Alternative of preferential looking involves the use of Tellers grating acuity cards.
Studies using Tellers acuity cards reveals: Healthy 1 month infants have
acuities of about 20/600. Resolution acuity improves rapidly
during first year of life. 1 year child manifesting acuities
of about 20/100 Adult level of 20/20 acuity are not
reached until 3 to 5 year of age
Visually evoked potential
FPL suggest that adult level of resolution acuity are reached between 3 to 5 year of age .
VEP show adult level at 6 to 8 months. Monocular visual acuity as a function of age as
determined by tellers acuity cards
Technique Birth 2 months 4 months 6 months 1 yr Age for 20/20
OKN 20/400 20/400 20/200 20/100 20/60 20-30 months
FPL 20/400 20/400 20/200 20/150 20/50 18-24 months
VEP 20/800 20/150 20/60 20/40 20/20 6-12 months
OKN – Optokinetic nystagmus FPL – Forced preferential looking
VEP – Visual evoked potential
Cause of decreased visual acuity in the infant
Foveal cone immaturities cone attain adult density & size of cones by 4 years age
Cortical immaturities
Incomplete myelination of the optic pathways
complete myelination of the optic nerve & optic pathway takes >2 years
Development of other visual attributes
Contrast sensitivity:
CS for 1 month old infants does not have band pass form suggesting that lateral interconnections within retina have not yet fully developed.
As infants mature .CSF assumes a band pass form and shifts to the right and upward indicating
Increased contrast sensitivity for most spatial frequencies and improved visual acuity.
Peak of CSF is at adult location at about 4 years and overall function is adult by 9 years.
CSF shifts upward and to the right as infant matures reaching adult form and location at about 9 year of age
1 month2 month
3 month
Vernier acuity: Form of hyper acuity Matures rapidly during the first
year Reaching adult level at slightly
older age (6-8 year of age)than grating acuity
Depend on cortical processing ,it reaches adult level later in life.
Development of binocular vision Becomes establish during 1st few year of life. Binocular cortical function 1st emerges at 3-
5 months. Anatomically-After birth
Retina &fovea are not fully developed-visual perception -poor
Ciliary muscle not fully developed .until 3 years. Medial rectus more developed than other muscles. By age of 6 months ,str.dev. enough to establish
BSV.
Physiologically :
At birth Compensatory reflex present
At 2-3 months Orientation reflex ,refixation reflex ,pupillary reflex
vergene reflex are present
At 2-3 yrs Accomodation reflex, fusional vergene reflex
present
Stereopsis:
Rapid onset between 3 and 6 month Sensitivity to crossed disparities appears 3 weeks
earlier than uncrossed disparities In 1-3 months ,infants do not alternately suppress
each eye but instead superimpose images At 3 month ,begin to show binocular fusion . Reaching 1 minutes of arc by 6 months
Color vision: By 2 year ,can match colors Infants (younger than 3 month) are less
sensitive to blue than adults Infants preferred red, green, and yellow
pattern By 2-3 month color vision close to
adults.
Development of binocular motion processing
Magnocellular neurons appears earlier in dev. than parvocellular neurons.
Magnocellular pathway at birth is biased so as to respond preferentially to target that move in temporal to nasal direction in the visual field.
Development of vergence
Vergences becomes remarkably accurate by age of 6 months
Vestibulo-ocular reflex -present at birth ,which stabilizes the eye when the head assumes different static position or the body turns
Temporal vision: critical flicker fusion frequency 40 Hz at 1 month of age Reaches adult level of
about 55hz by 3 monthsRetinal and cortical
immaturities that slow the development of grating and vernier acuity apparently have little effect on the maturation of temporal resolution.
Scotopic sensitivity: Adult like at 1 month of age. The absolute sensitivity of the
scotopic system(i.e. the sensitivity for a stimulus of 507 nm presented under conditions that maximize scotopic sensitivity) apparently reaches adult levels by about 6 month of age.
Face processing: Capable of discriminating between two
monkey faces as well as two human faces by 6 month
Older infants and adults are less capable of discriminating between the two monkey faces.revealing that the ability to distinguish among faces become more specialized as an infant matures.
Visual Development Milestones Pupillary light reaction – 30 weeks
gestation
Saccades well developed – 1-3 months
Ocular alignment stabilized – 1 month
Smooth pursuit well developed- 6-8 weeks
Blink response to visual threat – 2-5 month
Fixation well developed – 2 months
Accommodation appropriate to target – 4 months
Foveal maturation – 4 months
Stereopsis well developed – 3-7 months
Contrast sensitivity function well developed -7 months
Optic nerve complete myelination – 7 months to 2 years
Summery of visual development CFF and the forms of scotopic and
photopic sensitivity functions showing adult like characteristics with in the first 6 month.
Stereopsis and color vision emerges with in 3 month of life and reaches adult level within the 1st year .
Grating and vernier acuity improve rapidly during the first year, but then slowly mature until the child is 3 to 5 and 6 to 8 year old respectively.
The different rates of maturation for the various visual functions are consistent with notion that each has a different critical period.
Time frame for develipment of various visual attributes.Dot line: period over which adult or near adult level of performance are obtainedDashed line: red/green color vision may continue to develop until aldolescence
Expected visual performances
Birth to 6 weeks of age:· Stares at surrounding
when awake · Momentarily holds gaze
on bright light or bright object
· Blinks at camera flash · Eyes and head move
together · One eye may seem
turned in at times
Expected visual performances
8 weeks to 24 weeks:· Eyes begin to move more
widely with less head movement
· Eyes begin to follow moving objects or people (8-12 weeks)
· Watches parent's face when being talked to (10-12 weeks)
· Begins to watch own hands (12-16 weeks)
Expected visual performances
8 weeks to 24 weeks:· Eyes move in active
inspection of surroundings (18-20 weeks)
· While sitting, looks at hands, food, bottle (18-24 weeks)
· Now looking for, and watching more distant objects (20-28 weeks)
Expected visual performances30 weeks to 48 weeks:
· May turn eyes inward while inspecting hands or toy (28-32 weeks)
· Eyes more mobile and move with little head movement (30-36 weeks)
· Watches activities around for longer periods of time (30-36 weeks)
· Looks for toys s/he drops (32-38 weeks) Contd.
Expected visual performances· Visually inspects toys
s/he can hold (38-40 weeks)
· Creeps after favorite toy when seen (40-44 weeks)
· Sweeps eyes around room to see what's happening (44-48 weeks)
· Visually responds to smiles and voice of others (40-48 weeks)
Expected visual performances
12 months to 18 months:· Now using both hands and visually steering hand activity (12-14 months)
· Visually interested in simple pictures (14-16 months)
· Often holds objects very close to eyes to inspect (14-18 months)
Expected visual performances
12 months to 18 months:
· Points to objects or people using words "look" or "see" (14-18 months)
· Looks for and identifies pictures in books (16-18 months)
Expected visual performances
24 months to 36 months:· Occasionally visually inspects without needing to touch (20-24 months)
· Smiles, facial brightening when views favorite objects and people (20-24 months)
· Likes to watch movement of wheels, etc. (24-28 months)
· Watches own hand while scribbling (26-30 months)
Expected visual performances
24 months to 36 months:· Visually explores and steers own walking and climbing (30-36 months)
· Watches and imitates other children (30-36 months)
· Can now begin to keep coloring on the paper (34-38 months)
· "Reads" pictures in books (34-38 months)
Expected visual performances
40 months to 48 months:· Brings head and eyes close to page of book while inspecting (40-44 months)
· Draws and names circle and cross on paper (40-44 months)
· Can close eyes on request, and may be able to wink one eye (46-50 months)
Expected visual performances
4 years to 5 years:· Copies simple forms and some letters
· Can place small objects in small openings
· Visually alert and observant of surroundings
· Tells about places, objects, or people seen elsewhere
School –Age children's:
Clear near vision for reading and comfortably viewing close objects.
Binocular vision or the ability to use both eyes.
Eye movement skills in order to accurately aims the eyes.
Focusing ability to keep both eyes clearly focused at various distances.
School –Age children's: Peripheral vision to be aware of
objects located out of direct view Eye hand co-ordination to
accurately use the eyes and hand together.
Eye-body co-ordination to visually guide body movements.
Abnormalities in visual development Abnormalities such as:
Strasbismus Ptosis Congenital cataract
Can lead to AMBLYOPIA
Amblyopia
“lazy eye” Relatively common developmental
disorder Reduced visual acuity in an otherwise
healthy and properly corrected eye. Associated with interruption of
normal early visual experience. Most common cause of vision loss in
cchildrens.
Visual deficits in amblyopia
Reduced visual acuity defining factor
usually 20/30 -20/60 Impaired contrast sensitivity.
prominent at high spatial frequencies
Central visual field is generally most affected.
Moderate deficit in object recognition and spatial localization.
Severe deficit in binocular interaction.
Subtypes of amblyopia
Form Deprivation Amblyopia:
Caused by a physical obstruction e.g., congenital or traumatic
cataract, corneal opacities, prolonged uncontrolled occlusion
therapy.
Refractive Amblyopia: Isoametropic amblyopia is caused by high,
but equal, uncorrected refractive error (e.g., astigmatism > 2.50 D; hyperopia > than 5.00 D; myopia > 8.00D)
Anisometropic amblyopia is caused by unequal, uncorrected refractive error (e.g., astigmatism > 1.50 D; hyperopia > 1.00 D; myopia > than 3.00 D
Strabismic Amblyopia: Caused by early onset of constant
unilateral strabismus
Risk factors
Strabismus uncorrected significant refractive error Physical obstruction along the line of
sight Prematurity/low birth weight Retinopathy of prematurity Cerebral palsy mental retardation strabismus, or congenital cataracts Extraocular muscle surgery for early-
onset of esotropia
Management Basis for Treatment:
Improving vision in the amblyopic eye
Decreasing the risk of blindness in the fellow eye
Facilitating fusion and maintaining eye alignment
Developing normal binocular vision
Available Treatment Options: Optical correction
(spectacles and/or contact lenses)
Occlusion (part-time or full-time)
Active vision therapy
Signs of abnormal visual problem Frequent inward, upward, or
downward eye turning. Excessive tearing of eyes. Squinting or frequent closing of
one eye. Covering of one eye with hands
when looking a object. Drifting of one eye when looking at
objects.
Excessive rubbing of the eyes. Tilting of the head. Poor eye hand co-ordination. Abnormally white pupil. Dropping of lids.
Clinical consideration of visual development Conditions such as anisometropia ,congenital
cataract, strasbismus and ptosis may produce visual deprivation that can lead to permanent visual loss in the form of amblyopia.
Bcz the visual system becomes hard wiring early in life,it is important that these conditions be diagnosed and treated earliest as possible.
This necessitates eye examination early in life.
references
Visual perception Adler’s physiology of eye internet