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Specific learning objectives:
Functional anatomy of eye
Photoreceptor mechanism
Image-forming mechanism
Visual pathway
Visual Acuity, light and dark adaptations
Color Vision
eye
• EYE has two major parts:
• Optical system
Helps to focus and form an image on the
receptor cells-light rays fall.
• Neural system.
Transmits optical signal in the form of
Aps along the optic nerve to visual cortex .
• Eye ball-hollow spherical organ
• Optical system –Anterior part of the eye ball
• Visual receptors –posterior surface-optic
nerve arises and proceeds towards occipital
cortex.
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Interior of eyeball is divided into three spaces:
Anterior chamber
Posterior chamber
Posterior cavity
• Sclera
• White avascular fibrous coat-composed of
collagen fibers.
• Shape to eyeball
• Protective function
• Extraocular muscles-regulates the eyeball
movement.
• Central portion of eye-transparent cornea
• Lacrimal gland
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Lateral corner of eye.
Secretes tear
Keeps cornea moistened.
Prevent infection
Drained through naso-lacrimal duct.
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Choroid-posterior 2/3 rd of eyeball
Numerous blood vessels
Nourishes structure of eyeball
Front thickened portion-ciliary body-absorb
extra amount of light.
• Ciliary body
• Attached to suspensory ligament at one end.
• Other end-crystalline lens
• Two types of smooth muscle:
• Circular and longitudinal .
• Accommodation for near vision.
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Iris
Pigmented and opaque muscular structure.
Colour to eye.
Center-aperture-pupil-light enters the eye.
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Two types of muscles:
Sphincter and dilator pupillae
Determines size of pupil.
Functions
Regulates intensity of light.
Absorbs extra amount of light.
Prevents entry of light through periphery of lens.
Increases the depth of focus by constriction of
pupil.
• Retina
• Outer pigmented layer-attached to inner
surface of choroid.
• Inner layer-nerve cells and nerve fibersphotoreceptors.
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Crystalline lens
Circular biconvex
Formation of image on retina.
Has no blood supply.
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All the nerve fibers-retina-optic nerve-brain
Optic disc-optic nerve leaves eye.
Blind spot
Macula lutea-posterior pole of eye.
Marks the location of fovea centralis.
Absence of rods and densely packed cones.
Greatest visual acuity.
• Line joining the anterior pole-posterior pole of
the eyeball-optical axis
• Line joining the fixation point to fovea
centralis-visual axis.
Fluids in the eye:
• Aqueous Humor-anterior cavity
• Vitreous Humor-posterior cavity.
• Aqueous humor-protein –free clear fluid
• Formed- Ciliary processes-posterior chamberpupil-anterior chamber.
• Aqueous pressure-15-18 mm Hg higher than
intracranial pressure.
• Helps to maintain shape of eye.
• Composition
• High content of NaCl, vitamin C, lactic acid and
hyaluronic acid.
Outflow of Aqueous Humor:
Aqueous Humor is formed in ciliary processes
Through the pupil
In to the anterior chamber
Anterior to the lens
The angle b/w cornea and the iris
Canal of Schlemm
Extra ocular veins
• Functions
• It provides nutrition to all avascular structures
of eye.
• It maintains intraocular pressure.
• It maintains shape of the eye.
• Vitreous Humour
• Interior between the lens and retina is filled
with albumin and hyaluronic acid.
Functions of vitreous humor:
• Prevent the wall of eyeball from
collapsing.
• It maintain the intraocular pressure and
keep the intraocular structures in
position.
• Acts as a refractive medium.
Glucoma/Ocular Hypertension:
Pressure increase to 60-70 mm of Hg.
Normal :15mm of Hg(12-20mm of Hg)
Pressure above 25-30 mm of Hg can
cause blindness if maintained for long
duration.
Causes:
• Blockage of canal of Schlemm
• Excessive production of the fluid
2 types:
Primary:
• Open angle: after 40yrs
• Closed angle: after 60yrs.
Secondary :
Cataracts, Trauma, Intraocular haemorrage.
Retina
LAYERS OF RETINA:
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Pigmented epithelium
Layers of rods and cones
External limiting membrane
Outer nuclear layer
Outer synaptic layer
Inner nuclear layer
Inner synaptic layer
ganglion cell layer
Optic nerve
Internal limiting membrane.
1. Outer pigmented epithelial layer.
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Rich in melanocytes.
Prevents scattering of light.
Phagocytosis.
Storage of Vit A.
• Rods and cones-Photoreceptors
• Each rod and cone is divided into outer
segment, inner segment and a synaptic zone.
• Outer and inner segments form the layer of
rods and cones.
outer segment
Modified cilia-piles of flattened discs.
Discs - Rhodopsin and Iodopsin
• Rods-thin ,rod like appearance
• Old discs-shed and removed by pigmented
epithelium.
• New discs-inner edge of outer segment.
• Synapse with several rod bipolar cells
• Cones
• Conical outer segment
• Renewal –diffused-multiple sites at outer
segment.
• Inner segment:
Rich in mitochondria
In cones-thick, oval and is larger
• Synaptic Zone.
Synaptic vesicles –glutamate.
• External limiting membrane.
Glial tissues.
• Outer nuclear layer
Nucleus of rods and cones
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Outer synaptic layer
synapse between ends of rods and
cones with dendrites of bipolar cells and
horizontal cell processes.
• Inner nuclear layer
o Bipolar cells
o Horizontal cells
o Amacrine cells-synaptic contacts with
dendrites of both ganglion and bipolar cells.
• Inner synaptic layer
Site of processing of visual image.
• Ganglion cell
single layer of cell containing round cells
• Optic nerve
axons of ganglion cells
• Internal limiting membrane
separates retina from vitreous humour
• How light rays reach photoreceptors
Light rays
Ganglion cell
Bipolar cell
photoreceptors
• Functions of Rods
• sensitive to light .
• dim light vision or night vision or scotopic
vision.
• Low threshold
• Functions of cones.
• High threshold .
• bright light vision or daylight vision or
photopic vision.
• Cones are also responsible for acuity of vision
and the color vision .
Rods :
1. 120 million.
2. Mainly in peripheral
retina.
3. Slender ,Elongated,
Rod like.
4. More pigments/rod.
5. Only one type of
pigment-Rhodopsin.
6. Cannot detect color.
7. Functions better in
dim light.
8. Loss- Night blindness
Cones:
1. 6million
2. Mainly in the central
retina.
3. Conical shape.
4. Less pigments/cone.
5. Three types:
blue colour(cyanolabe)
Green (chlorolabe)
red (erythrolabe)
6. Can detect color.
7. Functions better in day
light
8. Loss-functional
blindness.
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Chemistry of Rhodopsin
Rhodopsin is a conjugated protein .
Protein called opsin and a chromophore.
Opsin -scotopsin.
Chromophore - retinal. Retinal is the aldehyde
of vitamin A or retinol.
Retinal is present in the form of 11-cis retinal
known as retinine 1
Rhodopsin –Retinal
visual cycle
LIGHT ENERGY
RHODOPSIN
BATHORODOPSIN
LUMIRHODOPSIN
METARHODOPSIN 1
SCOTOPSIN
11-CIS RETINAL
11-CIS RETINOL
ISOMERASE
METARHODOPSIN 2
ALL-TRANS RETINAL
ISOMERASE
ALL-TRANS RETINOL
(Vit A)
• Rhodopsin --metarhodopsin I -metarhodopsin
II Resynthesis of Rhodopsin
• all-trans retinal - 11-cis retinal ( enzyme
retinal isomerase).
• 11-cis retinal immediately combines with
scotopsin to form rhodopsin.
PHOTOTRANSDUCTION
The process by which light energy is converted
into receptor potential in visual receptor.
• In darkness-visual receptors -resting
membrane potential is about –40 mV .
• sodium ions leak back into the rod cellsreduce the electronegativity inside rod cell
• Dark current.
Maintenance of dark current in outer segment of rod cell
Functions of retina:
1. Visual functions:
• Light sense: Help to perceive light.
• Form sense: Appreciate the shape.
• Color sense: Perceive and recognize
different color and different
intensities of the same color
2. Reflexes:
• Light reflex
• Accommodation reflex.
3.Help to maintain the Tone ,Posture and
Equlilibrium .
IMAGE FORMING MECHANISMS
• Light rays-falls on eye-image formation on
retina:
• Refraction of light by cornea and lens
• Ciliary muscle activity-accommodation of lens
• Change in pupil size –iris muscles.
• Principles of optics
• Refraction-Change in the direction of light rays
• Lens-transparent glass-two spherical surfaces3 types:
Convex
Concave
Cylindrical
• Optical centre/nodal point-centre point of
lens
• Principal axis-line joining centre of two
spherical part lens surfaces.
• Principal focus
convex lens-point on principal axis-light rays
–converges
Concave lens-point on principal axis-light rays
–diverges
• Focal length-distance between optic centre
and principal focus of the lens.
• Light rays-distant object(>6m)-parallel and
object closer(<6m)-diverging.
• Power
• Reciprocal of focal length.
• S.I-dioptre.
• Convex lens-positive focal length and concave
lens-negative.
• Reduced /schematic Eye.
• Differences in refractive indices of eye
structure
• Reduced eye –single spherical surface –single
principal and nodal point.
• Nodal point-7mm anterior surface of cornea.
• Human eye-24mm in length-focal length17mm.
• Refractive power of reduced eye-59D
• Normal eye(59D)-behaves as the reduced eye.
ACCOMMODATION:
The ability of the eye ball by which the near
objects are clearly focused on the retina.
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Changing the curvature of lens.
Ciliary muscle contracts
Suspensory ligaments relax
Tension on lens decreases.
Lens becomes convex.
Near point:
The nearest point to the eye at which an
object can be brought in to clear focus by
accommodation.
Far point:
The farthest point which can be brought to
focus.
• Near vision
• Nearest point to the eye, at which object seen
clearly.
• Accommodation is maximum.
Changes associated with near vision:
• Change in the anterior curvature of the lens.
• Constriction of pupil.
• Convergence of the eye ball.
Far vision:
Near vision:
• Ciliary muscles
relax.
• Ciliary muscles
contract.
• Lens is held under
tension by the lens
ligament.
• Lens ligaments are
relaxed.
• Lens flattens.
• Lens convex shape.
• Defects of image forming mechanisms
Emmetropia= Normal eye.
Myopia/Near sightedness:
Objects are focused infront of retina.
Cause:
• Too long an eye ball.
• Stronger curvature of the eye.
Seen in:
• 5 % of newborns and infants.
Correction :
• Biconcave lens
• Surgical correction.
Refractive power +ive
Hypermetropia/Farsightedness.
Object is focused beyond the retina.
Cause:
• Too short an eye ball.
• Weak curvature of the eye.
Seen in:
• New born and infants 80%
• Rare in young adults.
Correction :
• Biconvex lens.
Refractive power : -ive
Astigmatism:
Failure to focus light at one point- blurring of
the image.
Cause: due to the faulty curvature of the
cornea.
Correction:
• Cyclindrical lens
Presbyopia:
Lens looses its elastic nature and becomes
relatively solid mass( Denaturation of
proteins).
• Accommodation power decreases.
• Far point not affected.
• Eye remains focused at a constant distance.
Correction:
Bifocal lens
Normal aberrations of vision:
Spherical Aberration:
• Crystalline lens of the eye is not regularly
formed.
• Non uniform refractive index of lens.
• Light rays-periphery refracted more.
• Light of single wavelength gets focused at
different points on retina
Chromatic aberration:
Light –longer wavelength-refracted more
Light rays passing through the periphery of the lens is
affected most- color fringes appear.
Color fringes are ignored by the brain.
Light of different colors get focused at different points on
retina.
Adaptation:
Getting accustomed or used to a new condition.
Dark adaptation: the decrease in the visual
threshold or increase in the sensitivity of
the eye to light.
• Causes for Dark Adaptation
• Increased sensitivity of rods as a result of
resynthesis of rhodopsin.
• Dilatation of pupil
• Dark Adaptation Curve
• Demonstrates the relationship between
threshold of light stimulus and time spent in
dark.
• Dark adaptation curve is biphasic.
• First part of the curve represents threshold of
photopic vision-cone adaptation.
• Second part of the curve represents threshold
of scotopic vision-rod adaptation
Cone adaptation:
• This first phase is rapid and it is completed in 8 to
10 minutes.
• During this period the threshold decreases by 2 to
3 log units
Rod-cone break
• After the first phase, there is a sudden change in
slope of the curve -rod-cone break.
• rod sensitivity begins to exceed cone sensitivity.
continuing adaptation of rods.
• During this phase the threshold decreases further
by 5 to 6 log units.
LIGHT ADAPTATION
Rod adaptation
• Second phase of the curve is slow.
• gradual decrease in the threshold.
• completed in 20 to 30 minutes.
• Light adaptation is the process in which eyes
get adapted to increased illumination.
• Reduced sensitivity of rods
• Constriction of pupil
Changes occurring during dark adaptation:
• Dilation of pupil.
• Photoreceptor function, Cones-> Rods.
• Resynthesis of Rhodopsin.
Changes occurring during light adaptation.
• Pupil constrict.
• Photoreceptor function ,Rods-> Cones.
• Photo pigments are bleached and their
concentration decrease.
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Night blindness:
Blindness in dim light
Vitamin A deficiency
Degeneration of neural layers of retina
Field of vision
• Part of the external world seen by one eye• fixed in one direction is called field of vision or
visual field of that eye.
• Binocular vision in which both the eyes are
used together for vision.
• Monocular vision is the vision in which each
eye is used separately.
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Visual field
Area visualized –gaze is fixed at an object.
Temporal field
Nasal field
Upper field
Lower field
• Visual field of each eye -divided into outer or
temporal field and the inner or nasal field, by
a vertical line passing through the fixation
point.
• Visual field of each eye -divided into an upper
field and a lower field by a horizontal line
passing through the fixation point.
• Light rays from different halves of each visual
field do not fall on the same halves of the
retina.
• Light rays from temporal part of visual field of
an eye fall on the nasal half of retina of that
eye.
• Light rays from nasal part of visual field fall on
the temporal half of retina of the same side.
• The shape and extent of visual field is mapped
out by means of an instrument called
Goldmann perimeter and this technique is
called perimetry.
Visual pathway
• Nervous pathway –impulses from retina to visual
center in cerebral cortex
• First order neurons
• Primary neurons
• Dendrites of bipolar cells synapse with
photoreceptors
• Axons from bipolar cells synapse with dendrites
of ganglionic cells.
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Second order neurons
Ganglionic cells
Axons of ganglionic cells –optic nerve
Leaves eye and terminates in lateral geniculate
body .
• Third order Neurons
• Lateral geniculate body
• Fibers-visual cortex.
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Optic nerve
Optic chiasma
Optic tact
Lateral geniculate body
Optic radiation
Visual cortex
Visual pathway
Optic Radiations
• OPTIC NERVE
• Optic nerve is formed by the axons of
ganglionic cells .
• Optic nerve leaves the eye through optic disk.
Optic chiasma
• Partial crossing of fibers .
• Fibers-temporal side of retinae-uncrossed.
nasal side cross to opposite side.
Optic tract
Left optic tract –fibers from left halves of both
retinae.
Right optic tract-fibers from right halves of
both retinae.
Lateral geniculate body.
• Majority of the fibers of optic tract terminate
here
• Forms the subcortical center for visual
sensation.
Few fibers terminate in any of these centers
• Superior colliculus: It is concerned with reflex
movements of eyeballs in response to optic
stimulus.
• Pretectal nucleus: It is concerned with light
reflexes .
• Supraoptic nucleus of hypothalamus: It is
concerned with the retinal control of pituitary
in animals.
OPTIC RADIATION
• Fibers from lateral geniculate body pass - internal
capsule and form optic radiation.
• The fibers between lateral geniculate body and
visual cortex - geniculocalcarine fibers.
• Optic radiation ends in visual cortex
VISUAL CORTEX
• Primary cortical center .
• Three areas are present:
• Primary visual area (area 17)-perception of visual
impulses.
• Secondary visual area or visual association area
(area 18)- interpretation of visual impulses
• Occipital eye field (area 19)- movement of eyes
EFFECTS OF LESION AT DIFFERENT LEVELS
OF VISUAL PATHWAY
• Injury to any part of optic pathway causes visual
defect.
• Loss of vision in one visual field is known as anopia.
• Loss of vision in one half of visual field is called
hemianopia.
Hemianopia -two types
Homonymous hemianopia
• Loss of vision in the same halves of both the visual
fields.
Heteronymous hemianopia
• Loss of vision in opposite halves of visual field.
binasal heteronymous hemianopia loss of vision nasal half of both visual fields.
• Bitemporal heteronymous hemianopia is the loss of
sight in temporal half of both visual fields
Types of hemianopia
A. Lesion of right optic nerve – blindness in right eye.
B. Lesion at optic chiasma – bitemporal hemianopia.
C. Lesion of right optic tract – left homonymous
hemianopia.
D. Lesion of right optic radiation – left homonymous
hemianopia with macular sparing.
1. Lesion of optic nerve:
cuts off impulse transmission –blindness in that
visual field.
2. Lesion at optic chiasma :
Damages nasal fibers (both eyes)-bitemporal
hemianopia.
3. Lesion of one side temporal fibers:
Blindness of that nasal field
4. Lesion of right optic tract:
Damages ipsilateral temporal fibers and
contralateral nasal fibers resulting in homonymous
hemianopia.
5. Lesion of right optic radiation :
Damages ipsilateral temporal fibers and
contralateral nasal fibers resulting in homonymous
hemianopia with macular sparing.
Processing of visual signal
• Signals from photoreceptors -transmitted
vertically - layers of retina by bipolar and ganglion
cells.
• Ganglion cells-brain through optic nerve.
• Horizontal and amacrine cells transmit signals
horizontally between cells in inner and outer
synaptic layers.
Responses of retinal neurons
• Photoreceptors
Threshold –rods-lower than cones
Human eye-wide range of light intensities.
Response of rods are slower than cones
Light –receptive field-photoreceptorhyperpolarizing response-transmitted –bipolar
cell.
• Bipolar cells
 Links photoreceptor-ganglion cells.
 Two types-`on´ and `off´ cells.
 Provides opposing excitatory and inhibitory
signals in visual pathway.
• Horizontal cells
Gets input from photoreceptors
Respond with hyperpolarizing membrane
potential.
• Link between photoreceptors and laterally
produce inhibition of bipolar cells on which they
synapse.
• Amacrine cells
• Depolarizing response
• Synapse on ganglion cell –excitatory input.
Processing of Visual information takes place in
neural network layer.
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Ganglion cells
Integrate information-bipolar and amacrine cells
Respond with depolarizing potential
Two types-`on´ and `off´ cells (depending on the
responses)
• Contrast detectors
• Depending on size-M cells and P cells.
• M cells-detecting movement in stimulus.
• P cells-send information-color, texture and shape
of object
Important features of retinal neurons.
• Photoreceptors- hyperpolarized-input cells.
• Ganglion cells-output cells.
• Processing of visual signals-neuronal layer
• Ganglion cells-Action potentials
• Visual information converges on ganglion cells
• Decrease the contrast of image and improves
sensitivity.
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Lateral geniculate body
Point to point transmission of visual inputs
from retina-visual cortex.
Visual cortex
On –center and off- center
Three types of cell
Simple
Complex
Hyper complex.
Visual cortex receives information-orientation,
movement, depth, velocity and color.
Visual reflexes
Light reflex
• Light is focused on one eye, the pupils of both eyes
constrict – light reflex
• Direct reflex
• Indirect reflex
Accomodation reflex:
• When the subject is asked to focus at distant object
and then asked to shift the gaze to near object,
 Pupilary constriction of both eyes.
 Medial convergence of eye balls.
 Increase in anterior curvature of lens.
Purkinje-Sanson images
• change in convexity of lens during
accommodation for near vision.
ARGYLL ROBERTSON PUPIL
• Light reflex is lost but the accommodation
reflex is present.
• lesion in Edinger-Westphal nucleus,diabetes
and alcoholic neuropathy.
Color vision
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Visibility range of vision-400nm-750nm
Wavelength of spectral colours(VIBGYOR)
Violet-400nm
Blue-450nm
Bluish –green-500nm
Green-550 nm
Red -650-750nm
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Maximum sensitivity –scotopic vision-500nm
Photopic -560nm
Purkinje shift
Shifting of sensitivity.
Seen towards evening.
Achromatic vision:
It is the sensation of white color and no color has
been assigned to it.
Chromatic vision:
Spectral colors vision and extra spectral color vision
(carbon blue).
Primary colors:
Red Green blue.
Complimentary color: when two colors are mixed
an appropriate amounts= white.
Theories of color vision;
1. Thomas young and Von –Helmholtz's theory(
Trichromatic color theory).
Three different types of cones each containing a
different photosensitive pigment and maximum
sensitivity to one type of primary color.
2. Mullers doctrine of specific nerve energy theory:
There are specific nerve fibers with specific ganglion
cells responding to three primary colors.
After image:
After one stops looking at a color he may continue to
see it for a short time(+ive after image)
Or he may see its complimentary color(-ive after
image)-RED+BLUISH GREEN-WHITE
Granites dominator and modulator theory.
Two types of ganglion cells:
• Dominators: they respond to the whole visual
spectrum
Detects intensity of the light but not the color.
Y ganglion.
Modulators: respond maximum to a narrow wave
length of light.
Blue: 450-470nm(Peak=445nm )-cyanolabe
Green: 520-540nm(Peak=535nm)-cholorolabe
Red:500-600nm(Peak=570nm) –erytrolabe
they respond to the entire visual spectra to a varied
degree. Hence they are responsible for color vision
X ganglions
Hering ‘s opponent color theory
• Extension of first theory.
• 4 primary color: Red Blue Green Yellow
• Photo chemical substances give one sensation on
breakdown and other on resynthesis.
Color blindness:
This is based on the young Helmholtz theory of color
vision:
8%of males, 0.4% females
Red and green color blindness is X linked.
Types of color
blindness:
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Deficiency of color vision.
Causes
Chronic diseases
Drugs
Toxins
Alcohol
aging
Monochromatism
• Total inability to perceive color.
• It is also called total color blindness.
• Very rare.
• Monochromats
• They can see black ,white and shades of grey.
Dichromatism
• Appreciate only two colors.
• Dichromats.
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Trichromatism
Have three cone system.
One cone system is defective.
trichromats
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Anomaly-colour weakness
Anopia- colour blindness.
Prot-red
Deuter-green
defective cone
Trit-blue
EYE MOVEMENT
Muscles in ocular movements
• Eye movements
• Conjugate Movement-in same directioncontraction of MR of one eye and LR of other eye.
• Saccadic movement-jerky movement
• Smooth pursuit movements-eyeball along object
• Convergence and divergence-towards nose and
temporal side.
• Vestibular movements-compensating head
movement.
• Nystagmus-jerky movement when the gaze at
stationary object
Visual acuity:
The degree to which the details and contours
of objects are perceived/Minimal separable
distance b/w two objects to see them as two
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Functions of cone.
Factors affecting visual acuity:
1. Optical factors:
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Curvature of the Cornea and Lens.
Elasticity of the eye.
Conditions of ciliary muscles and lens ligaments.
2.Retinal factors:
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Functional status of retina.
Highest in the Fovea and decreases towards the
periphery.
3.Stimulus factors:
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Size of the object.
Distance from the eye.
Color of the object.
Shape, brightness, duration for which the object is viewed.
Age.