Learning objectives – Vision- 2 Learning objectives – Vision- 2 Describe the basis of color vision and defects in color vision Describe the basis of color vision and defects in color vision The Optics of the eye ; Refractive medias of the eye, Principles of optics, Power of accomodation The Optics of the eye ; Refractive medias of the eye, Principles of optics, Power of accomodation Concept of reduced eye & image formation Concept of reduced eye & image formation Refractory errors ; cause & correction Refractory errors ; cause & correction To describe Pupillary light reflex To describe Pupillary light reflex To describe Accomodation reflex To describe Accomodation reflex

Learning outcomes Learning outcomes At the end of this lecture students should be able to At the end of this lecture students should be able to Explain the characteristics of color Explain the characteristics of color Explain the Young-Helmholtz theory of colour Explain the Young-Helmholtz theory of colour vision vision Describe the types of colour blindness Describe the types of colour blindness Explain the principles of optics, Explain the principles of optics, refractive medias of the eye and refractive medias of the eye and power of accommodation. power of accommodation Describe the concept of reduced eye Describe the concept of reduced eye and image formation. and image formation Define and explain the common Define and explain the common defects of the optical system of the defects of the optical system of the eye: eye: Hyperopia, Myopia, Astigmatism, Hyperopia, Myopia, Astigmatism, Presbyopia, and Strabismus. Presbyopia, and Strabismus.

COLOR VISION Is the ability to descriminate a light stimulus as a function of it’s wave length Is the ability to descriminate a light stimulus as a function of it’s wave length Primary colors: red, green, blue Primary colors: red, green, blue Trichromatic theory; Power of the eye to discriminate stimuli on the basis of their wavelength Trichromatic theory; Power of the eye to discriminate stimuli on the basis of their wavelength

Young-Helmholtz Theory 3 primary colours due to 3 types of cones. 3 primary colours due to 3 types of cones. So the color sensation depends on the relative stimulation of 3 primary color cone systems Equal simultaneous stimulation of three receptors produce white colour. Equal simultaneous stimulation of three receptors produce white colour. Young-Helmoltz tricolor theory postulates existence of 3 kinds of cones, each containing a diff photo pigment & that are maximally sensitive to one of the 3 primary colors Young-Helmoltz tricolor theory postulates existence of 3 kinds of cones, each containing a diff photo pigment & that are maximally sensitive to one of the 3 primary colors

3 ATTRIBUTES OF COLOR: 3 ATTRIBUTES OF COLOR: HUE: proportion in which 3 cone systems are activated HUE: proportion in which 3 cone systems are activated SATURATION: degree of dilution by grayness SATURATION: degree of dilution by grayness BRIGHTNESS: total effect of 3 cone mechanisms BRIGHTNESS: total effect of 3 cone mechanisms For any color Complementary color that, when properly mixed with it produces sensation of white For any color Complementary color that, when properly mixed with it produces sensation of white

PHOTOPIGMENTS PHOTOPIGMENTS ERYTROLABE(red)-PEAK ABSORPTION WAVE LENGTH: 560nm ERYTROLABE(red)-PEAK ABSORPTION WAVE LENGTH: 560nm CHLOROLABE(green)-PEAK ABSORPTION WAVE LENGTH: 530nm CHLOROLABE(green)-PEAK ABSORPTION WAVE LENGTH: 530nm CYANOLABE(blue)-PEAK ABSORPTION WAVE LENGTH- 420nm CYANOLABE(blue)-PEAK ABSORPTION WAVE LENGTH- 420nm

COLOR VISION

Tests for color vision: Tests for color vision: 1. Holmgran’s coloured wool test 2. Ishihara Chart Protanopia (lack of red cones) Protanopia (lack of red cones) Deuteranopia (lack of green cones) Deuteranopia (lack of green cones) Tritanopia (lack of blue cones) Tritanopia (lack of blue cones)

If all the cone receptors work, but one type does not work as well as the other two, an “ anomalous trichromatism ” results. If all the cone receptors work, but one type does not work as well as the other two, an “ anomalous trichromatism ” results. A weakness in long wave length “ red ” or “ L ” cones causes “ Protoanomaly, ” A weakness in long wave length “ red ” or “ L ” cones causes “ Protoanomaly, ” A weakness in medium wavelength “ green ” or “ M ” cones causes “ Deuteranomaly, ” A weakness in medium wavelength “ green ” or “ M ” cones causes “ Deuteranomaly, ” A weakness in the short wavelength ( “ blue ” or “ S ” ) cones causes Tritanomaly, ” A weakness in the short wavelength ( “ blue ” or “ S ” ) cones causes Tritanomaly, ”

Normal: reads as “74” Red-green color blind reads as “21” Normal person reads as “42”. Red color blind reads as “2” Green color blind reads as “4”

When light rays pass from one medium to another having a different refractive index, it suffers refraction (bending). When light rays pass from one medium to another having a different refractive index, it suffers refraction (bending). Greater the difference of refractive indices (between 2 media), greater is the refraction. Greater the difference of refractive indices (between 2 media), greater is the refraction. At which interface maximum refraction will occur ? Why ?

Converging & Diverging lens Converging & Diverging lens Optical center or Nodal point Optical center or Nodal point Principal axis Principal axis Focal point Focal point Focal length Focal length Calculation of strength or power of lens Calculation of strength or power of lens

Listing’s Concept of Reduced eye Mathematical model of the eye, having 24 mm length in antero posterior diameter Mathematical model of the eye, having 24 mm length in antero posterior diameter Algebraic sum of multiple refractive surfaces, simplified & represented as single refractive surface Algebraic sum of multiple refractive surfaces, simplified & represented as single refractive surface Nodal point (optical center) : center of the optical system, rays passing through nodal point suffers no refraction (doesn’t bend) Nodal point (optical center) : center of the optical system, rays passing through nodal point suffers no refraction (doesn’t bend)

Nodal point present at 17mm in front of the retina while focused at distant object (relaxed) Nodal point present at 17mm in front of the retina while focused at distant object (relaxed) Focal length: distance between optical center of lens and principal focus Focal length: distance between optical center of lens and principal focus Power of eye = reciprocal of focal length in meters represented as diopters. (1/focal length in meters) Power of eye = reciprocal of focal length in meters represented as diopters. (1/focal length in meters)

From distant object >6Ms From near object <6Ms

D (Diopters) =1 ∕Focal length in meters D (Diopters) =1 ∕Focal length in meters If, F=100cm then D=1 ∕1m If, F=100cm then D=1 ∕1m D=1 D=1 If, F=50cm then D=1 ∕0.5m If, F=50cm then D=1 ∕0.5m D=2 D=2 If, F = 10cm then D=1 ∕0.1m If, F = 10cm then D=1 ∕0.1m D=10 D=10

Far & Near Point To focus an object clearly on the retina processes involved are 1.Refraction of light rays 2.Accomodation

Near point: Nearest point to the eye at which an object can be brought into clear focus with accomodation. Near point: Nearest point to the eye at which an object can be brought into clear focus with accomodation. 9cm at the age of 10 9cm at the age of 10 Recedes with the advancing age Recedes with the advancing age 80 cm at the age of cm at the age of 60 Due to hardening of lens resulting in loss of accomodation (can’t increase the curvature of lens) Due to hardening of lens resulting in loss of accomodation (can’t increase the curvature of lens)

Power of accomodation Healthy eye can assume small & large focal length & increase power from 59 to 63 D that is while shifting gaze from distant object ( far point- infinity) to near object Healthy eye can assume small & large focal length & increase power from 59 to 63 D that is while shifting gaze from distant object ( far point- infinity) to near object Power of accomodation of young healthy adult is 4D. Power of accomodation of young healthy adult is 4D. Power of accomodation may go up to 12D also in young children Power of accomodation may go up to 12D also in young children

Distance or depth perception Looking at an object, ability to estimate roughly how far the object is situated. Looking at an object, ability to estimate roughly how far the object is situated. By 3 means: By 3 means: 1.known object by knowing the size 1.known object by knowing the size 2. motion parallax (distant object move slower when observer moves) 2. motion parallax (distant object move slower when observer moves) 3.stereopsis or binocular disparity- the difference between the images on the retinas- disparity between the retinas produce double vision-diplopia 3.stereopsis or binocular disparity- the difference between the images on the retinas- disparity between the retinas produce double vision-diplopia

Schematic diagrams of normal image formation & Errors of refraction

Correction for Myopia & Hyperopia

Hyperopia (hypermetropia) or long sightedness: Hyperopia (hypermetropia) or long sightedness: Parallel rays from infinity are focused behind the retina Parallel rays from infinity are focused behind the retina Even to see an distant object he has to accommodate Even to see an distant object he has to accommodate So when the person attempts to see nearer objects, the ability to accommodate has already reached it’s maximum capacity, so no clarity for near objects, due to tiring of ciliary muscle may produce head ache. Prolonged convergence of visual axes may lead to strabismus. So when the person attempts to see nearer objects, the ability to accommodate has already reached it’s maximum capacity, so no clarity for near objects, due to tiring of ciliary muscle may produce head ache. Prolonged convergence of visual axes may lead to strabismus. Cause: AP diameter of eye ball may be shorter Cause: AP diameter of eye ball may be shorter Correction: biconvex lens Correction: biconvex lens

Myopia or short sightedness: Myopia or short sightedness: Cause: Cause: Parallel rays from infinity focussed in front of the retina, so he can’t see distant object clearly, but near vision is alright Parallel rays from infinity focussed in front of the retina, so he can’t see distant object clearly, but near vision is alright AP diameter of eye ball longer than normal AP diameter of eye ball longer than normal Correction: concave (diverging) lens Correction: concave (diverging) lens

By convention power of concave & convex lenses are symbolized by –ve and +ve signs respectively. By convention power of concave & convex lenses are symbolized by –ve and +ve signs respectively. -10D denotes concave lens of 10D power -10D denotes concave lens of 10D power +10D denotes convex lens of 10D power +10D denotes convex lens of 10D power

Astigmatism: Astigmatism: Corneal curvature varies from meridian to meridian, so refractive power in diff angles will be different Corneal curvature varies from meridian to meridian, so refractive power in diff angles will be different Correction: cylindrical lens Correction: cylindrical lens Rarely due to lens problems. Rarely due to lens problems.

Visual pathway

LIGHT REFLEX PTN OPTIC TRACT LGB EWN CIL G Light into rt eye Direct : constriction of Rt pupil Consensual: constriction of Lt pupil Visual cortex Ciliary m Sphincter P Red- afferent Black- efferent Short ciliary nerve

Light reflex (flow chart) Light into one eye Light into one eye Impulses in optic nerve Impulses in optic nerve Optic chiasma Optic chiasma Rt Optic tracts Lt optic tract Rt Optic tracts Lt optic tract Rt pretectal N Lt pretectal Rt pretectal N Lt pretectal Rt EWN Lt EWN Rt EWN Lt EWN Rt Ciliary gn Lt ciliary gn Rt Ciliary gn Lt ciliary gn Rt Short ciliary N Lt short ciliary N Rt Short ciliary N Lt short ciliary N Rt sphincter pup Lt sphincter pup Rt sphincter pup Lt sphincter pup Pupillary constriction of both eyes Pupillary constriction of both eyes

ACCOMODATION REFLEX Mechanism of each change, pathway by which it occurs and purpose of each change. Accommodation- ability of the eye to adjust the power of the lens & thereby see both far & near objects clearly. Accommodation- ability of the eye to adjust the power of the lens & thereby see both far & near objects clearly. amplitude ? amplitude ?

Accommodation reflex(Near reflex or near response) Accommodation reflex(Near reflex or near response) Sudden shift of gaze from distant to near object reflex changes observed in the eye 3 components of accommodation reflex are: 3 components of accommodation reflex are: - 1. Convergence of the eye ball-image forms in the corresponding points of retina Lens becomes more convex (accommodation)- increases power of lens - 3. Pupillary constriction- increases depth of focus - Argyll-Robertson pupil- accommodation reflex present but Light reflex absent in CNS syphilis or midbrain lesions

cont

Motor n of IIICN EWN of IIICN Medial recti ciliary gn 1.convergence of eyes short cilia N ciliaris M sphincter p ciliaris M sphincter p 2.lens bulges 3.pup constric 2.lens bulges 3.pup constric