https://nabeelbeeran.blogspot.com/ Adaptation,Color vision & Errors of Refraction

1. ADAPTATION, COLOR VISION & ERRORS
OF REFRACTION
Dr. Nabeel Beeran
MBBS, MD
Department of Physiology
28/03/2020
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2. Specific Learning Objectives
At the end of this class, student should be able to:
• Explain dark and light adaptation.
• Explain night blindness.
• Describe briefly about color blindness.
• Define and explain myopia, hypermetropia, astigmatism, presbyopia.
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3. ADAPTATION
• Getting accustomed or used to a new condition.
• Dark Adaptation (going from light to dark)
• Light Adaptation (going from dark to light)
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4. Dark Adaptation
• The process by which the person is able to see the objects in dim light.
• If a person enters a darkroom from a bright lighted area, he is blind for
sometime.
• After sometime his eyes get adapted and he starts seeing the objects
slowly.
• Maximum duration is about 20 minutes.
Changes occuring during dark adaptation:
1. Resynthesis of Rhodopsin.
2. Photoreceptor function, Cones-> Rods.
3. Dilatation of pupil.
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5. Light Adaptation
• The process in which eyes get adapted to bright light.
• When a person enters a bright lighted area from a dim lighted area, he feels
discomfort due to the dazzling effect of bright light.
• After sometime, when the eyes become adapted to light, he sees the
objects around him without any discomfort.
• Maximum period is about 5 minutes.
Changes occuring during light adaptation:
1. Photo pigments are bleached and their concentration decreases.
2. Photoreceptor function, Rods-> Cones.
3. Pupil constrict.
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6. Night Blindness
• Blindness in dim light
• Nyctalopia or Scotopic vision
• Deficiency of vitamin A
• Degeneration of neural layers of retina
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7. COLOR VISION
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8. SPECTRAL COLORS
• Visibility range of vision is 400nm-750nm.
• When the sunlight or white light is passed through a glass prism, it is
separated into different colors.
• The series of colored light produced by the prism is called the visible
spectrum and the colors that form the spectrum are called the
spectral colors.
• The spectral colors are red, orange, yellow, green, blue, indigo and
violet (ROYGBIV or VIBGYOR).
• Red has got the maximum wavelength of about 8000 A.
• Violet has got the minimum wavelength of about 3000 A.
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9. 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.
Primary colors:
• Red, Green and Blue.
Complementary colors:
• Pair of 2 colors which produce white when mixed or combined in proper
proportion.
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10. Theories of color vision
1. Thomas Young Trichromatic Theory
2. Helmholtz Trichromatic Theory
3. Granit Dominator- Modulator Theory
4. Hartridge Polychromatic Theory
5. Hering’s Theory Of Opposite Colors
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11. YOUNG- HELMHOLTZ
TRICHROMATIC THEORY
• Retina has 3 types of cones and each
cone is supplied by a separate fiber of
optic nerve.
• Each cone has its own photosensitive
pigment and gives response to one of
the primary colors (red, green and blue).
• Different color sensations are produced
by the stimulation of various
combinations of 3 types of cones.
• For sensation of white light, all the 3
types of cones are stimulated equally.
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12. COLOR BLINDNESS
• Color blindness is the failure to appreciate one or more colors.
• It is common in 8% of males and only in 0.4% of females.
• Mostly the color blindness is an inherited sex-linked recessive character.
• Color blindness occurs due to acquired conditions also such as ocular
diseases or injury or disease of retina.
• The term ‘color blind’ does not mean that objects are seen only in black
and white.
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13. CLASSIFICATION OF COLOR BLINDNESS
• Based on Young- Helmholtz trichromatic theory, color blindness is
classified into three types:
1. Monochromatism
2. Dichromatism
3. Trichromatism
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14. Monochromatism
• Total inability to perceive color.
• Also called total color blindness or achromatopsia.
• Very rare.
• Persons with monochromatism are called monochromats.
• Retina of monochromats is totally insensitive to color and they see the
whole spectrum in only black, white and different shades of grey
(vision is similar to black and white photography).
• Monochromatism is divided into two types:
i. Rod monochromatism
ii. Cone monochromatism
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15. Dichromatism
• Can appreciate only two colors.
• Persons with this defect are called dichromats.
• They can match entire spectrum of colors by only two primary colors
because the receptors for third color are defective.
• The defects are classified into three groups:
i. Protanopia- red color cannot be appreciated).
ii. Deuteranopia- cannot appreciate green color.
iii. Tritanopia- cannot appreciate blue color.
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16. Trichromatism
• Intensity of one of the primary colors cannot be appreciated correctly
though the affected persons are able to perceive all the three colors.
• Persons with this defect are called trichromats.
• Even the dark shades of one particular color look dull for them.
• Trichromatism is classified into three types:
i. Protanomaly- perception for red is weak.
ii. Deuteranomaly- perception for green is weak.
iii. Tritanomaly- weak perception for blue.
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18. TESTS FOR COLOR BLINDNESS
1. Ishihara color charts
2. Holmgren colored wool
3. Edridge-Green lantern.
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19. ERRORS OF REFRACTION
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20. • The eye with normal refractive power is called emmetropic eye and
the condition is called emmetropia.
• Any deviation in the refractive power from normal condition,
resulting in inadequate focusing on retina is called ametropia and the
eye is called ametropic eye.
• The defect is due to the change in shape of the eyeball.
• Ametropia is of two types:
1. Myopia
2. Hypermetropia.
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21. MYOPIA OR SHORT SIGHTEDNESS
• Inability to see the distant object.
• Can see near objects clearly but not the distant objects.
• Common in young adults.
Cause:
• Increase in anteroposterior diameter of the eyeball.
• Image is brought to focus a little in front of retina.
• Light rays, after coming to a focus, disperse again so, a blurred image is
formed upon retina.
Correction:
• Biconcave lens.
• Surgical correction
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22. HYPERMETROPIA OR LONG SIGHTEDNESS
• Inability to see near object.
• Can see the distant objects clearly but not the near objects.
• In this defect, distant vision is normal but, near vision is affected.
Cause:
• Due to decreased anteroposterior diameter of the eyeball.
• Light rays are brought to a focus behind retina causing a blurred image of
near objects.
• Occurs in childhood, if the eyeballs fail to develop the correct size.
• Common in old age people.
Correction:
• Biconvex lens.
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24. ANISOMETROPIA
• The condition in which the two eyes have unequal refractive power.
• It is corrected by using different appropriate lens for each eye
(biconcave or biconvex).
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25. ASTIGMATISM
• Light rays are not brought to a sharp point upon retina.
• It is the common optical defect.
Cause:
• Due to the faulty curvature of the cornea or lens curvature is not uniform.
• Refractory power of lens is different in different meridians.
Types:
1. Regular Astigmatism- refractive power of lens is unequal in different
meridians, but uniform in one single meridian.
2. Irregular Astigmatism- refractive power of lens is unequal in different
meridians as well as in different points of same meridian.
Correction:
• Cylindrical lens. 25

26. PRESBYOPIA
• Progressive diminished ability of eyes to focus on near objects with age.
• Due to the gradual reduction in the amplitude of accommodation.
• Starts developing after middle age, progresses as the age.
• Distant vision is unaffected, only the near vision is affected.
• Anterior curvature of lens does not increase during near vision, so the light
rays from near objects are not brought to focus on retina.
Causes:
1. Decreased elasticity of lens.
2. Decreased convergence of eyeballs.
Correction:
• Bifocal lens.
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27. References
1. Review of Medical Physiology by Ganong 24th edition.
2. Textbook of Medical Physiology by Guyton and Hall 13th edition.
3. Essentials of Physiology for Dental Students by K Sembulingam and
Prema Sembulingam 2nd edition.
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28. Summary
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29. THANK YOU