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DEVELOPMENT O F VISION MANOJ ARYAL Institute Of Medicine Maharajgunj Medical College

Development Of Vision

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Page 1: Development Of Vision

DEVELOPMENT O F VISION

MANOJ ARYAL

Institute Of MedicineMaharajgunj

Medical College

Page 2: Development Of Vision

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.

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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

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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

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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.

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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

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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.

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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.

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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

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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.

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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.

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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.

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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)

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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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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

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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

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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

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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.

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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

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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.

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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.

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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

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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

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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.

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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.

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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

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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.

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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.

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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.

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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

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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

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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.

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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

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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

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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)

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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)

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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.

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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)

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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)

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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)

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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)

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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)

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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)

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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

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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.

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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.

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Abnormalities in visual development Abnormalities such as:

Strasbismus Ptosis Congenital cataract

Can lead to AMBLYOPIA

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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.

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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.

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Subtypes of amblyopia

Form Deprivation Amblyopia:

Caused by a physical obstruction e.g., congenital or traumatic

cataract, corneal opacities, prolonged uncontrolled occlusion

therapy.

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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

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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

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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

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Available Treatment Options: Optical correction

(spectacles and/or contact lenses)

Occlusion (part-time or full-time)

Active vision therapy

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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.

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Excessive rubbing of the eyes. Tilting of the head. Poor eye hand co-ordination. Abnormally white pupil. Dropping of lids.

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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.

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references

Visual perception Adler’s physiology of eye internet