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Visual Acuity, accommodation
& accommodative convergence

It is complex integration of light sense, from sense, sense of
contrast and colour sense
1.The Light sense
Its is awareness of the light. The minimum brightness
required to evoke a sensation of light is called the light
minimum. It should be measured when the eye is dark
adapted for atleast 20-30 minutes.
The Process of visual adaptaion primarly involves:
1. Dark adaption (adjustment in dim illumination) and
2. Light adaption (adjustment to bright illumination)
Visual Perception

It is the ability of the eye to adapt itself to decreasing illumination.
The time taken to see in dim illumination is called ‘Dark
adaptation time’
The rods are much more sensitive to low illumination than the
cones.
Rods are used more in dim light (scotopic vision) and cones in
bright light (photopic vision)
Dark adaptation curve consists of two parts: the initial small curve
represents the adaptation of cones and the remainder of the
curve represents the adaptation of rods.
Delayed dark adaptation occurs in diseases of rods e.g. retinitis
pigmentosa and vitamin A deficiency
DARK ADAPTATION

The Process by mean of which retina adapts itself
to bright light is called light adaptation process is
very quick and occurs over a period of 5 minutes
Result in :
•Increased spatial acuity
•Increased temporal acuity
•Decreased sensitivity
LIGHT ADAPTATION

•It is ability to discriminate between the shapes of
the objects.
•
•The from sense is most acute at the fovea, where
there are maximum number of cones and decrease
very rapidly towards the periphery.
•Visual acuity recorded by snellen’s test chart is a
measure of Form sense.
The Form sense

Minimum visible
•Determine presence or absence of target in visual field without
resolving them.
•Threshold is 1 second of arc.
Components of Visual Acuity

Minimum resolvable ( ordinary visual acuity)
• Determines presence of identifying/ distinguishing feature in visible
target
• Threshold 60 sec of arc.
• Snellen’s visual acuity measure Minimum resolvable ( ordinary
visual acuity)
• The test targets may either consists of letters (Snellen’s Chart) or
broken circle (Landolt’s ring)
Minimum discriminable(hyper acuity/vernier acuity)
• Determines relative location of 2 or more visible features with
respect to each other
• Determines whether or not two parallel and straight lines are aligned
in the frontal plane.
• Thresold 3-5 sec of arc.

•It consists of a series of black capital letters on a
white board, arranged in lines, Diminishing in
size,
•The each part of the letters subtend an angle of
1 min at the nodal point
•Thus, at the given distance, each letter subtends
as angle of 5 min at the nodal point of the eve.
SNELLEN’ S TEST TYPES
•The Chart should be properly illuminated
•The numerator being the distance of the patient
from the letters, and the denominator being the
smallest letters accurately read.

•Simple picture chart: used for children
•Landolt’s C-chart used for illiterate
patients
•E-chart: used for illiterate patients
OTHER TESTS BASED ON
SNELLEN’S TEST TYPES

Tested by asking the patient to read the near version
chart kept at a distance of 35 cm in good illumination,
with each eye separately.
Visual acuity for near
Commonly used near vision charts:
1. Jaeger’s chart: Prints are marked
from 1 to 7 and accordingly patient’s
acuity is labelled as J1 to J7.
2. Roman test types: According to this
chart, the near vision is recorded as
N5, N8, N10, N12 and N18
3. Snellen’s near vision test types

Methods of visual acuity
testing in preverbal
children

VISUAL MILESTONES :
• Very soon after birth - Can fix and follow a light source, face or large, colorful
toy.
• 1 months - Fixation is central, steady and maintained, can follow a slow target,
and converge, preference of looking at face.
• 3 months - binocular vision and eye cordination, eyes follow a moving light or
face, responsive smile.
• 6 months - Reaches out accurately for toys.
• 9 months – look for hidden toys.
• 2 years - Picture matching
• 3 years - Letter matching of single letters (e.g., Sheridan Gardiner)
• 5 years - Snellen chart by matching or naming

VISUAL ACUITY OF INFANT EYES
Test 2Months 4Months 6Months 1Year Attainment
(months)
Opticokinetic
nystagmus
test
20/400 20/400 20/200 20/80 24–30
Forced
choice
preferential
looking test
20/400 20/200 20/200 20/50 18–24
Visual
evoked
response test
20/200 20/80 20/60–20/20 20/40–20/20 6–12

Tests for indirect assessment of vision :
a) Historical and observational tests,
b) Binocular fixation preference and fixation
targets,
c) CSM method.

TECHNIQUES FOR VISUAL ACUITY QUANTITATION

Examples of recognition acuity. A. Kay pictures
B. LEA symbols.

An assessment of visual Acuity is made by varying the Width of stripes or the
distance From the drum.

Teller and Cardiff acuity cards

Ability of the eye to perceive slight changes in the
luminance between regions which are not separated by
definite borders.
Contrast sensitivity is affected by various factors like age,
refractive errors, glaucoma, amblyopia, diabetes, optic
nerve disease and lenticular changes.
Impaired even in the presence of normal visual acuity.
Snellen’s chart is a measure of visual acuity under 100%
contrast.
Clinical measurement targets at various spatial
frequencies or at various peak contrast
Sense of contrast

•Ability of the eye to discriminate between
different colour excited by light of different
wavelenghts
•Is a function of the cones
•In dim light all colours are seen as grey
(purkinje shift).
Colour sense

Trichromatic theory (young Helmholtz)
•Existence of three kinds of cones, each containing a different
photopigment which is maximallly sensitive to one of the three
primary colours viz. red, green and blue.
•Each having different absorption spectrum as below
•Red sensitive cone pigment, long wave length sensitive (LWS)
cone pigment, absorbs maximally with a peak at 565 nm.
•Green sensitive cone pigment, also known medium wavelength
sensitive (MWS) cone with a peak at 535 nm.
•Blue Sensitive cone pigment, short wave length sensitive (SWS)
cone pigment absorbs maximally in the blue-violet with a peak
at440 nm.
•The gene for human rhodopsin is located on chromosome 3, and
the gene for the blue-sensitive cone is located on choromosome 7
•The Red and green sensitive cones q arm of the X choromosomes
Theories of colour vision

Opponent colour theory of Hering some
colours appear to be ‘ mutually exclusive’
Two main types of colour opponent
ganglion Cells:
•Red –green opponent colour cells
•Blue –yellow opponent colour cells
THEORIES OF COLOUR VISION

ACCOMMODATION
•In an emmetropic eye, parallel rays of light coming
from infinity are brought to focus on the retina, with
accommodation being at rest. The mechanism to
focus diverging rays coming from a near object on the
retina, is called accommodation . In this there occurs
increase in the power of crystalline lens due to
increase in the curvature of its surface
•At rest the anterior radius of curvature of lens is 10
mm and in accommodation changes to 6 mm and
posterior radius of curvature of lens is 6 mm and in
accommodation changes to 6 mm
.

The nearest point at which small objects can be seen
clearly is near point and the farthest point at which
they are seen clearly is the far point
Far Point and near point of the eye vary with the static
refraction of the eye
1. In an emmetropic eye far point is infinity and near
point varies with age.
2. In hypermetropic eye far point is virtual and lies
behind the eye.
3. In myopic eye, it is real and lies in front of the eye
Far Point and near point

RANGE AND
AMPLITUDE OF
ACCOMMODATION
•Range – Distance between near
point and Far point
•Amplitude – Difference between
dioptric power for near focus and
distance focus

ANOMALIES OF ACCOMMODATION
These include:
1.Presbyopia
2.Insufficiency
3.Paralysis
4.Spasm

Presbyopia – condition of falling near vision due to age
related decrease in the amplitude of
accommodation or increase in near point.
Causes
1. Age related change in the lens
i. Decrease in the elasticity of lens capsule,
ii. Progressive, increase in size and hardness of
lens substance.
2. Age related decrease in ciliary muscle power
Symptoms
i. Difficulty in near vision
ii. Asthenopia

Treatment
i. Prescription of convex glasses
Basic principle for presbyopic correction
i. Correct distance refractive error
ii. Find near correction for each eye separately and add
to the distance correction
iii. Consider patient’s profession for fixing the near point
iv. Prescribe the weakest lens

Insufficiency of Accoomdation
Accomondative power is significantly less than the normal limits
for the patients age
Causes
1. Premature sclerosis of lens
2. Weakness of ciliary muscle due to systemic causes
3. Weakness of ciliary muscle associated with POAG
Symptoms
1. Asthenopia
2. Blurring of vision
Treatment
1. Treat the systemic cause
2. Near vision spectacles
3. Accomondation exercises

PARALYSIS OF ACCOMMODATION
Complete absence of accommodation
Causes
1. Drug induced cycloplegia
2. Internal ophthalmoplegia
3. Third nerve paralysis
Symptoms
1. Blurring of near vision
2. Photophobia
3. Abnormal receding of near point and decreased range of
accommodation
Treatment
1. Self recovery
2. Dark Glasses
3. Convex lenses for near vision

SPASM OF ACCOMMODATION
Abnormally excessive accomodation
Causes
1. Drug Induced
2. Spontaneous spasm
Symptoms
1. Defective vision due to induced myopia
2. Asthenopia
Diagnosis
1. Refraction under atropine
Treatment
1. Relaxtation of ciliary muscle by atropine
2. Prohibition of near work
3. Correction of associated causative factors
4. Assurance and psychotherapy

CONVERGENCE
Simultaneous adduction (Inward turning)
1. Voluntary
2. Reflex
Reflex Convergence
1. Tonic – Inherent innervational tone to the MR
2. Proximial – Psychological awareness of near object
3. Fusional – Maintains BSV by insuring that similar images
are projected on to corresponding retinal areas of each
eye.
4. Accommodative – Induced by act of accommodation as
part of synkinetic near reflex

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Lecture on vision

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