Transcript SS3 Eye- Applied aspects

Eye: Retina and Neural
Mechanisms.
Prof. K. Sivapalan
Critical fusion frequency.
• Time resolving ability of the eye.
• Rate at which stimuli can be presented
and perceived as separate stimulus.
Movies- 16 frames per second.
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Color vision
• Three different cones with different pigments.
• Colors have three attributes:
– Hue- shade of color
– Intensity- concentration
– Saturation- degree of freedom from dilution with white.
• Every color has a complementary color that produces
sensation of white when properly mixed.
• It is said that black is a sensation produced by absence of
light. It probably is a positive sensation otherwise it cannot
be seen in white or colored screens.
• Sensation of white or any color can be produced by mixing
blue, green and red- primary colors.
• Color perceived is related to color of other objects in the field.
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Color Blindness.
• Rhodopsin is coded in chromosome 3, blue
sensitive pigment in chromosome 7, red and
green pigments are in X chromosome.
• Red green blindness is sex linked recessive.
• Males have only one X. If it happens to be
defective gene, color blindness is expressed.
• Females express defect only if both sex
chromosomes have defective gene.
• [Males suffer, females carry]
• Yarn-matching test and Ishihara charts are used
to identify color blindness.
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Ishihara Charts
• Normal person reads
74 but blue green
color blind person
reads 21.
• Normal person reads
42, red blind person
reads 2 and green
blind person reads 4.
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Eye Movements.
• 4 recti and 2 oblique muscles.
• High order of coordination is necessary to maintain images in
the corresponding points of the retina.
• Involuntary eye movements can be classified into four types:
• Saccades- sudden jerky movements to bring new objects of
interest into fovea, to scan the visual field and to avoid
adaptation that occurs when focused on the same object for
long. Coordinated by superior colliculi, secondary visual cortex.
• Smooth Pursuit Movements- follow moving objectsCoordinated by cerebellum.
• Vestibular Movements- movements initiated by vestibular
stimuli- maintain gaze fixed while head moves. Coordinated by
vestibular nuclei.
• Convergence Movements- to bring visual axis towards each
other on focusing on near object. Coordinated by visual
association area in the cortex.
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Binocular vision.
• The central parts of visual fields of both eyes
coincide.
• The images on the retina are fused at the cortical
level.
• Fusion occurs when same image falls on the
corresponding points in the retina- if not diplopia.
• Perception of depth• Monocular- relative size of the image of known
object, movement relative to another object.
• Binocular vision - adds appreciation of depth and
proportion accurately.
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Strabismus- squint.
• When image does not fall on
corresponding points.
• ?congenital defects.
• When image falls on non-corresponding
points before the age of 6, one is
suppressed and diplopia disappears.
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Near Response
• Eye response when changing gaze from a
distant object to a near object.
• Comprises convergence, pupillary
constriction and accommodation.
• Coordinated by visual association cortex.
Efferents go through pretectal nucleus in
superior colliculus
• Effectors are ocular muscles, ciliary
muscles and pupil.
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Light reflex- Pupillary Reflex
• Papillary response to light into one eye.
• Fibers in optic nerve leave near the lateral
geniculate body, enter the midbrain,
terminate in the pretectal nucleus.
• Second order neurons project to ipsilateral
and contralateral Edinger-Westphal nuclei.
• Causes contraction of both pupils.
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Vision in Light and Dark.
• Variation from
darkness to
bright sun light is
10 log units
[factor of 10
billion]
• Pupillary
constriction can
reduce light
intensity by a
factor of 16.
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Dark Adaptation.
• As soon as a person moves from brightly lit area to dimly lit
area nothing can be seen.
• Slowly vision returns. It takes 20 minutes for near maximal
adaptation.
• In moderate illuminatio cone adaptation occurs by reduction of
threshold.
• In dark adaptation of rods occurs.
• Time taken for adaptation is the time taken to build rhodopsin
stores.
• Use of red glasses before going into dark area allows rods to
dark-adapt beforerhand. Used by radiologists and pilots.
• When the person moves from dark area to lit area, the light
appears intensely bright until eyes adapt.
• It is light adaptation [reverse of dark adaptation]. Takes about 5
minutes.
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Vitamin A deficiency.
• Nyctalopia- night blindness. Severe
deficiency- damage to rods and cones.
• Also other effects as dry cornea.
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Visual acuity.
• The degree to which the details and contours of objects
are perceived.
• Retinal factors: state of receptors, concentration,
convergence, and central factors.
• Image forming mechanism: refraction, adequate
illumination, brightness of the stimulus, contrast and the
adequate time should be provided.
• Visual acuity is greatest in fovea centralis.
• Measured by minimum separable- shortest distance two
lines are separated and perceived as two lines.
• Snellen Charts are read from 20 ft. (6 m). The Visual
angle of the letters is 5 minutes and the angle of the
lines 1 minute at the distance mentioned below them.
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Field of Vision.
• Portion of the world
visible through one eye.
• It should be circular but
nose, roof of the orbit and
zygoma obstruct.
• Perimetry: One eye is
focused in the central
point, other eye covered.
• Small target is moved
towards the central point
along selected meridians.
• As the target is first seen,
the position is plotted as
the angle of the arc.
• Increased fieldexophthalmia.
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Field of Vision ctd.
• A target screen can
be used to map the
blind spot and
objective scotomas.
• A white object moved
in a black screen in
the visual field.
• The points where the
object disappears and
reappears is plotted.
• Lesions:– Optic nerve- blindness.
– Optic tracthomonymous
hemianopia.
– Optic chiasmheteronymous
hemianopia.
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Visual Perception
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Il
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Ila
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Ilango
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Illam
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