pengantar penglihatan mdw 2011

8/13/2019 PENGANTAR PENGLIHATAN MDW 2011

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Pengantar Fisologi Penglihatan

M. Djauhari Widjajakusumah

Departemen Fisiologi

Fakultas Kedokteran Unversitas Indonesia



Dee Silverthorn



Penglihatan



INTRODUCTION

o The eyes are complex sense organs that have evolvedfrom primitive light-sensitive spots on the surface ofinvertebrates.

o Within its protective casing, each eye has

o a layer of receptors

o a lens system that focuses light on these receptors

o a system of nerves that conducts impulses from thereceptors to the brain.

o The way these components operate to set up consciousvisual images is the subject of this chapter.

Ganong’s Review of Med Physiol, 23rd

ed



ANATOMIC CONSIDERATIONS

Ganong’s Review of Med Physiol 22nd

ed

Figure 8 – 1.

Fig 8-1



o The sclera is modified anteriorly to form the transparent cornea, through which light rays enter the eye.

o The choroid, is a layer inside the sclera that contains many of theblood vessels that nourish the structures in the eyeball.

o The retina is lining the posterior two thirds of the choroid, the neuraltissue containing the receptor cells.

o The crystalline lens is a transparent structure held in place by a

circular lens suspensary ligament (zonule). • The zonule is attached to the thickened anterior part of the

choroid, the ciliary body that contains circular muscle fibers andlongitudinal muscle fibers that attach near the corneoscleral

junction.

o In front of the lens is the pigmented and opaque iris, the coloredportion of the eye.

• Contains circular muscle fibers that constrict and radial fibersthat dilate the pupil.



ed




o The space between the lens and the retina is filled

primarily with a clear gelatinous material called the vitreous(vitreous humor)

o Aqueous humor, a clear liquid that nourishes the corneaand lens

o Produced in the ciliary body by diffusion and activetransport from plasma.

o Flows through the pupil and fills the anterior chamber ofthe eye.

o Reabsorbed through a network of trabeculae into thecanal of Schlemm, a venous channel at the junctionbetween the iris and the cornea (anterior chamberangle). Obstruction of this outlet leads to increasedintraocular pressure

Ganong’s Review of Med Physiol, 23rd ed



Physical Principles of Optics

Refraction of Light Refractive Index of a Transparent Substance

Light rays travel through air at a velocity of about 300,000km/sec, (the refractive index of air is 1.00), but they travel

much slower through transparent solids and liquids.

The refractive index of a transparent substance is the ratio ofthe velocity of light in air to the velocity in the substance.

If light travels through a particular type of glass at a velocityof 200,000 km/sec, the refractive index of this glass is300,000 divided by 200,000, or 1.50.

Guyton & Hall Textbook of Med Physiol, 12e, 2011



Physical Principles of Optics

Figure 49-1 Light rays entering a glass surface perpendicular to the light rays ( A ) and aglass surface angulated to the light rays (B ). This figure demonstrates that the distance

between waves after they enter the glass is shortened to about two-thirds that in air. It alsoshows that light rays striking an angulated glass surface are bent.




Application of Refractive Principles to LensesConvex Lens Focuses Light Rays

Figure 49-2 Bending of light rays at each surface of a convex spherical lens, showing that

parallel light rays are focused to a focal point.




Convex Lens Focuses Light Rays

► Parallel light rays passing through the center of the convex lensstrike the lens exactly perpendicular to the lens surface and,

therefore, pass through the lens without being refracted

► Toward either edge of the lens, the light rays strike a progressively

more angulated interface

The outer rays bend more toward the center, convergence

of the rays.

Half the bending occurs when the rays enter the lens, and half

as they exit from the opposite side.

Parallel light rays passing through each part of the lens will be

bent that all the rays will pass through the focal point.




Application of Refractive Principles to LensesConcave Lens Diverges Light Rays

Figure 49-3 Bending of light rays at each surface of a concave spherical lens, showing that

parallel light rays are diverged. Guyton & Hall Textbook of Med Physiol, 12e, 2011



Concave Lens Diverges Light Rays

The parallel light rays that enter the center of the concave lens strikeinterface that is perpendicular to the beam and, therefore, do notrefract.

The rays at the edge of the lens enter the lens ahead of the rays in the

center. This is opposite to the effect in the convex lens, and it causesthe peripheral light rays to diverge from the light rays that passthrough the center of the lens.

Thus, the concave lens diverges light rays, but the convex lensconverges light rays.




Cylindrical Lens Bends Light Rays in Only One Plane-Comparison

with Spherical Lenses

Figure 49-4

A Point focus of parallel light rays by a

spherical convex lens.

B, Line focus of parallel light rays by a

cylindrical convex lens.




► Figure 49-6

The effect of parallel versus diverging rays on the focal distance is shown.

► The bottom lens has far more refractive power than either of the other two

lenses (i.e., has a much shorter focal length), demonstrating that the stronger

the lens is (the more refractive power tha lens has), the nearer to the lensthe point focus is.

► The relation of focal length of the lens, distance of the point source of light,

and distance of focus is expressed by the following formula:

1 = 1 + 1

f a b f is the focal length of the lens for parallel rays,

a is the distance of the point source of light from the lens,

b is the distance of focus on the other side of the lens.

The relation of focal length of the lens, distance of the point source

of light, and distance of focus




Figure 49-8 Effectof lens strength

on the focal

distance

Measurement of the Refractive Power of a Lens-"Diopter"




► The more a lens bends light rays, the greater is its "refractive power,“

measured in terms of diopters. ► Figure 49-8: a spherical lens that converges parallel light rays to a focal point

1 meter beyond the lens has a refractive power of +1 diopter,. I

► If the lens is capable of bending parallel light rays twice as much as a lens

with a power of +1 diopter, it is said to have a refractive power of +2

diopters, the light rays come to a focal point 0.5 meter beyond the lens.

► A lens capable of converging parallel light rays to a focal point only 10

centimeters (0.10 meter) beyond the lens has a refractive power of +10

diopters. T

► The refractive power of concave lenses cannot be stated in terms of the focal

distance beyond the lens because the light rays diverge

► If a concave lens diverges light rays at the same rate that a 1-diopter convex

lens converges them, the concave lens is said to have a dioptric strength of -

1. Likewise, if the concave lens diverges light rays as much as a +10-diopter

lens converges them, this lens is said to have a strength of -10 diopters.





Optics of the Eye

The Eye as a Camera

► The eye, has a lens system, a variable aperture system (the pupil), anda retina.

► The lens system of the eye is composed of four refractive interfaces:

(1) the interface between air and the anterior surface of the cornea,

(2) the interface between the posterior surface of the cornea and theaqueous humor,

(3) the interface between the aqueous humor and the anterior surface

of the lens of the eye,

(4) the interface between the posterior surface of the lens and the

vitreous humor.

► The internal index of air is 1; the cornea, 1.38; the aqueous humor,

1.33; the crystalline lens (on average), 1.40; and the vitreous humor,

1.34.




► If all the refractive surfaces of the eye considered to be one single lens,("reduced eye“), it has a total refractive power of 59 diopters when the

lens is accommodated for distant vision.

► + 2/3 of the 59 diopters of refractive power of the eye is provided by the

anterior surface of the cornea (not by the eye lens).

The refractive index of the cornea is markedly different from that of

air, whereas the refractive index of the eye lens is not greatly

different from the indices of the aqueous humor and vitreous humor

► The total refractive power of the internal lens of the eye is only 20

diopters, + 1/3 the total refractive power of the eye.

The importance of the internal lens is that, in response to nervous

signals from the brain, its curvature can be increased markedly to

provide "accommodation,"

Refractive Surfaces of the Eye as a Single Lens-

The "Reduced" Eye




Mechanism of "Accommodation"

Figure 49-10 Mechanism of accommodation (focusing).




► In children, and a young person the refractive power of the lens of theeye can be increased voluntarily from 20 diopters to about 34 diopters; "accommodation" of 14 diopters.

the shape of the lens is changed from that of a moderately convexlens to that of a very convex lens.

the lens is composed of a strong elastic capsule filled with viscous,proteinaceous, transparent fluid.

When the lens is in a relaxed state with no tension on its capsule, itassumes an almost spherical shape, owing mainly to the elasticretraction of the lens capsule. However, as shown in Figure 49-10,

about 70 suspensory ligaments attach radially around the lens,pulling the lens edges toward the outer circle of the eyeball. Theseligaments are constantly tensed by their attachments at theanterior border of the choroid and retina. The tension on theligaments causes the lens to remain relatively flat under normalconditions of the eye.

Mechanism of "Accommodation"



Formation of an Image by a Convex Lens

Figure 49-7 A, Two point sources of light focused at two separate points on

opposite sides of the lens. B, Formation of an image by a convex spherical

lens. Guyton & Hall Textbook of Med Physiol, 12e, 2011



Figure 49-7 A

► Light rays pass through the center of a convex lens without beingrefracted in either direction, the light rays from each point source of

light come to a point focus on the opposite side of the lens directly in

line with the point source and the center of the lens.

Figure 49-7B ► Each point source of light on the object comes to a separate point

focus on the opposite side of the lens in line with the lens center.

► The image of the object, is upside down with respect to the original

object, and the two lateral sides of the image are reversed.

► This is the method by which the lens of a camera focuses images on

film.

Formation of an Image by a Convex Lens




► The more a lens bends light rays, the greater is its "refractive power.“ ,

measured in terms of diopters. ► The refractive power in diopters of a convex lens is equal to 1 meter divided

by its focal length.

A spherical lens that converges parallel light rays to a focal point 1 meter

beyond the lens has a refractive power of +1 diopter, (Figure 49-8).

A lens capable of bending parallel light rays twice as much as a lens witha power of +1 diopter, it is said to have a strength of +2 diopters, and the

light rays come to a focal point 0.5 meter beyond the lens.

A lens capable of converging parallel light rays to a focal point only 10

centimeters (0.10 meter) beyond the lens has a refractive power of +10

diopters.

► Concave lenses "neutralize" the refractive power of convex lenses. Thus,

placing a 1-diopter concave lens immediately in front of a 1-diopter convex

lens results in a lens system with zero refractive power



Errors of Refraction

Figure 49-12 Parallel light

rays focus on the retina

in emmetropia, behind

the retina in hyperopia,

and in front of the

retina in myopia



Correction of Myopia and Hyperopia by Use of Lenses

Figure 49-13 Correction of myopia with a concave lens, and correction of

hyperopia with a convex lens.

Accommodation



Accommodation

FIGURE 4.11

The eye as an optical device. During fixation the center of the image falls on the fovea. A, With

the lens flattened, parallel rays from a distant object are brought to a sharp focus. B, Lens

curvature increases with accommodation, and rays from a nearby object are focused.

A d ti



Accommodation

The solid lines represent the shape of the lens, iris, and ciliary body at rest, and the dashed

lines represent the shape during accommodation. When gaze is directed at a near object,

ciliary muscles contract. This decreases the distance between the edges of the ciliary body

and relaxes the lens ligaments, and the lens becomes more convex.



Refractive Errors Corections

Focusing point sources of light



Focusing point sources of light.

Common Defects of the Image-Forming



Co o e ects o t e age o gMechanism

Ganong’s Review of Med

Physiol, 23rd ed

Common defects of the optical system of the eye. In myopia (nearsightedness), the eyeball

is too long and light rays focus in front of the retina. Placing a biconcave lens in front of the eye

causes the light rays to diverge slightly before striking the eye, so that they are brought to a

focus on the retina.





Mechanism

Common defects of the optical system of the eye. In hyperopia (farsightedness), the

eyeball is too short and light rays come to a focus behind the retina. A biconvex lens corrects

this by adding to the refractive power of the lens of the eye.

Ganong’s Review of Med

Physiol, 23rd ed



RETINA



RETINA

o Extends anteriorly almost to the ciliary body

o Organized in 10 layers and contains the rods and cones, whichare the visual receptors, plus four types of neurons: bipolar cells, ganglion cells, horizontal cells, and amacrine cells

o The rods and cones synapse with bipolar cells, and the bipolarcells synapse with ganglion cells

o The axons of the ganglion cells converge and leave the eye asthe optic nerve

o Horizontal cells connect receptor cells to the other receptor cellsin the outer plexiform layer

o Amacrine cells connect ganglion cells to one another in the innerplexiform layer via processes of varying length and patterns

o Gap junctions also connect retinal neurons to one another, andthe permeability of these gap junctions is regulated.




RETINA

o The receptor layer of the retina rests on the pigment

epithelium next to the choroid light rays must pass

through the ganglion cell and bipolar cell layers to reach

the rods and cones

o The pigment epithelium absorbs light rays, preventing the

reflection of rays back through the retina. Such reflection

would produce blurring of the visual images.


RETINA



RETINA

The optic nerve leaves the eye and the retinal blood vessels enter it at a point 3 mm medial to and

slightly above the posterior pole of the globe. This region is visible through the ophthalmoscope as

the optic disk (Figure 12 –3). There are no visual receptors over the disk, and consequently this

spot

A photograph of the optic fundus

Ganong’s Review of Med Physiol, 23rd e

RETINA



RETINA

The optic nerve leaves the eye and the retinal blood vessels enter it at a point 3 mm medial to and

slightly above the posterior pole of the globe. This region is visible through the ophthalmoscope as

the optic disk (Figure 12 –3). There are no visual receptors over the disk, and consequently this

spot

An illustration of the optic fundus




Tahap-tahap persepsi penglihatan

o cahaya kornea susunan optik mata penajamanrangsang di retina (fovea sentralis)

o di retina cahaya diubah menjadi listrik oleh fotoreseptor(transduksi) potensial reseptor

o potensial aksi n. optikus (N. II) tr. optikus kortekspenglihatan di proses persepsi

RETINA



RETINA

Neural components of the

extrafoveal portion of theretina. C, cone; R, rod; MB

(miget bipolara), RB (rod

bipolar), and FB (flat bipolar)

cells; DG (diffuse ganglion)

and MG (midget ganglion)

cells; H, horizontal cells; A,

amacrine cells.


Organization of the human retina.

A Ch id



A, Choroid.

B, Pigment epithelium.

C, Photoreceptor layer.

D, Neural network layer.

E, Ganglion cell layer.

r, rod;

c, cone;

h, horizontal cell;

b, bipolar cell;

a, amacrine cell;

g, ganglion cell.

(Modified from Dowling JE, Boycott

BB. Organization of the primate retina:

Electron microscopy.

Proc Roy Soc Lond 1966:166:80 –111.



Guyton Textbook of Med Physiol

11 th ed

Th R ti d It Ph t t



The Retina and Its Photoreceptors

o The retina is a multilayered (10 layers ) structurecontaining the photoreceptor cells and a complex web ofseveral types of nerve cells

o A simpler four-layer scheme:o pigment epithelium,

o photoreceptor layer

o neural network layer

o ganglion cell layervisual signal light entering the

retina



Th R ti d It Ph t t




o Pigment Epithelium

o consists of cells with high melanin content

o opaque material

o also extends between portions of individual rods and

coneso prevents the scattering of stray light greatly sharpening

the resolving power of the retina

o ensures that a tiny spot of light (or a tiny portion of an

image) will excite only those receptors on which it fallsdirectly

o albinism lack this pigment blurred vision

o phagocytose bits of cell membrane that are constantlyshed from the outer segments of the photoreceptors.

Th R ti d It Ph t t




o Photoreceptor Layer

o the rods and cones are packed tightly side-by-sideo density of thousands per square millimeter, depending on the

region of the retina

o each eye contains about 125 million rods and 5.5 million cones

o the photoreceptor cells occupy a deep layer of the retina lightmust pass through several overlying layers to reach them

o divided into two classes

o the cones are responsible for photopic (daytime) vision, whichis in color (chromatic)

o the rods are responsible for scotopic (nighttime) vision, whichis not in color.

o their functions are basically similar

o they have structural and biochemical differences.

Th R ti d It Ph t t




o Cones

Three different photopigments are associated with cone cells thatdiffer in the wavelength of light that optimally excites them. Thepeak spectral sensitivity for

the red-sensitive pigment is 560 nm in red photoreceptors

the green-sensitive pigment is 530 nm in greenphotoreceptors

the blue-sensitive pigment, is 420 nm in blue photoreceptors

At wavelengths away from the optimum, the pigments still absorblight but with reduced sensitivity

Colorblind individuals

loss of a single color system produces dichromatic vision lack of two of the systems causes monochromatic vision.

all three are lacking, vision is monochromatic and dependsonly on the rods.

Th R ti d It Ph t t




o Rods

Long, slender, and cylindrical and is larger than a cone cell. Its outer segment contains photoreceptor disks composed of

cellular membrane in which the molecules of the photopigment

rhodopsin are embedded

The lamellae near the tip are regularly shed and replaced withnew membrane synthesized at the opposite end of the outersegment.

The inner segment, connected to the outer segment by amodified cilium, contains the cell nucleus, many mitochondriathat provide energy for the phototransduction process, and othercell organelles.

At the base of the cell is a synaptic body that makes contactwith one or more bipolar nerve cells and liberates a transmittersubstance in response to changing light levels.



Pathways to the Cortex



Pathways to the Cortex

Ganglion cell projections from

the right hemiretina of each eye

to the right lateral geniculatebody and from this nucleus to

the right primary visual cortex.

Note the six layers of the

geniculate. P ganglion cells project

to layers 3 –6, and M ganglion cells

project to layers 1 and 2. The

ipsilateral (I) and contralateral (C)

eyes project to alternate layers.

Not shown are the interlaminar

area cells, which project via a

separate component of the Ppathway to blobs in the visual

cortex.



Neural Pathways



Neural Pathways

o The axons of the ganglion cells pass caudally in the optic nerve and optictract to end in the lateral geniculate body, a part of the thalamus

o The fibers from each nasal hemiretina decussate in the optic chiasm. Inthe geniculate body, the fibers from the nasal half of one retina and thetemporal half of the other synapse on the cells whose axons form thegeniculocalcarine tract. This tract passes to the occipital lobe of thecerebral cortex

o The primary visual receiving area (primary visual cortex, Brodmann's area17; also known as V1), is located principally on the sides of the calcarinefissure (Figure 8-5).

Medial view of the human right

cerebral hemisphere showing

projection of the retina on theoccipital cortex around the

calcarine fissure.

Ganong’s Review of Med Physiol, 21st

ed

Neural Pathways



Neural Pathways

o Some ganglion cell axons pass from the optic tract to the pretectal region of

the midbrain and the superior colliculus, where they form connections thatmediate pupillary reflexes and eye movements.

o Pupillary Reflexes

o When light is directed into one eye, the pupil constricts (pupillary lightreflex)

o The pupil of the other eye also constricts (consensual light reflex)

o The impulses leave the optic nerves near the lateral geniculate bodies enter the midbrain via the brachium of the superior colliculus terminate in the pretectal nucleus the second-order neurons projectto the ipsilateral Edinger-Westphal nucleus and the contralateralEdinger-Westphal nucleus the third-order neurons pass to the ciliaryganglion in the oculomotor nerve the fourth-order neurons pass fromthis ganglion to the ciliary body. This pathway is dorsal to the pathwayfor the near response.

Ganong’s Review of Med Physiol, 21st ed

Neural Pathways



Neural Pathways

o The frontal cortex is also concerned with eye movement, and

especially its refinement

o The bilateral frontal eye fields in this part of the cortex areconcerned with control of saccades

o An area just anterior to these fields is concerned with vergence

and the near responseo The frontal areas concerned with vision probably project to the

nucleus reticularis tegmentalis pontinus, and from there to theother brain stem nuclei mentioned above.

Ganong’s Review of Med Physiol, 21st ed

The Receptors



The Receptors



The Receptors

o Rods and cones are divided into

o an outer segment

o made up of regular stacks of flattened saccules ordisks containing the photosensitive compounds thatreact to light, initiating action potentials in the visualpathways

o an inner segment that includes a nuclear region

o rich in mitochondria

o a synaptic zone



THE PHOTORECEPTOR MECHANISM

• Electrical Responses

The action of light on photosensitive compounds in the rods andcones light is absorbed structural changes electricalpotential changes initiate action potentials in the retina

• The receptor potentials of the photoreceptors and the

electrical responses of most of the other neural elements inthe retina are local, graded potentials

• Only in the ganglion cells that all-or-none action potentialstransmitted over appreciable distances are generated

• The responses of the rods, cones, and horizontal cells arehyperpolarizing

• The responses of the bipolar cells are either hyperpolarizingor depolarizing

• Amacrine cells produce depolarizing potentials and spikesthat may act as generator potentials for the propagatedspikes produced in the ganglion cells.



Intracellularly recorded responses of

cells in the retina to light. The synaptic

connections of the cells are also indicated.

The eye is unique in that the receptor

potentials of the photoreceptors and the

electrical responses of most of the otherneural elements in the retina are local,

graded potentials. The rod (R) on the left is

receiving a light flash, whereas the rod on

the right is receiving steady, low-intensity

illumination. The responses of rods and

horizontal cells (H) are hyperpolarizing,

responses of bipolar cells (B) are either

hyperpolarizing or depolarizing, and

amacrine (A) cells produce depolarizing

potentials and spikes that may act as

generator potentials for propagated spikes

of ganglion cells (G).

Figure 12 –

11

Ionic Basis of Photoreceptor Potentials



Ionic Basis of Photoreceptor Potentials

o Na+ channels in the outer segments of the rods and cones

are open in the dark, so current flows from the inner to theouter segment

o Current also flows to the synaptic ending of thephotoreceptor. The Na+ –K+ pump in the inner segmentmaintains ionic equilibrium.

o Release of synaptic transmitter is steady in the dark.

o When light strikes the outer segment, the reactions that areinitiated close some of the Na+ channels, and the result isa hyperpolarizing receptor potential.

o The hyperpolarization reduces the release of synaptictransmitter, and this generates a signal in the bipolar cellsthat ultimately leads to action potentials in ganglion cells.

o The action potentials are transmitted to the brain.



Effect of light on current flow in visual receptors. In the dark, Na+ channels in the outer

segment are held open by cGMP. Light leads to increased conversion of cGMP to 5'-GMP,

and some of the channels close. This produces hyperpolarization of the synaptic terminal

of the photoreceptor.



Initial steps in phototransduction in rods. Light activates rhodopsin, which

activates transducin to bind GTP. This activates phosphodiesterase, which

catalyzes the conversion of cGMP to 5'-GMP. The resulting decrease in the

cytoplasmic cGMP concentration causes cGMP-gated ion channels to close.



Sequence of events involved in phototransduction in rods and cones.Figure 8 –19

EYE MOVEMENTS



o A very high order of coordination of the movements of the two eyesis necessary if visual images are to fall at all times on

corresponding points in the two retinas and diplopia is to beavoided.

o Four types of eye movements, each controlled by a different neuralsystem but sharing the same final common path, the motor neuronsthat supply the external ocular muscles

o Saccades, sudden jerky movements, occur as the gaze shifts

from one object to another, programmed in the frontal cortexand the superior colliculi, bring new objects of interest onto thefovea and reduce adaptation in the visual pathway that wouldoccur if gaze were fixed on a single object for long periods.

o Smooth pursuit movements are tracking movements of theeyes as they follow moving objects, programmed in the

cerebellumo Vestibular movements, adjustments in response to stimuli

initiated in the semicircular canals, maintain visual fixation as thehead moves.

o Convergence movements bring the visual axes toward eachother as attention is focused on objects near the observer.

EYE MOVEMENTS

EYE



MOVEMENTS

Extraocular muscles subserving the six cardinal positions of gaze. The eye is

adducted by the medial rectus and abducted by the lateral rectus. The adducted

eye is elevated by the inferior oblique and depressed by the superior oblique;

the abducted eye is elevated by the superior rectus and depressed by the

inferior rectus. (Reproduced, with permission, from Greenberg DA, Aminoff MJ,

Simon RP: Clinical Neurology, 5th ed. McGraw-Hill, 2002.)

EYE



MOVEMENTS

Figure 8-8. The six extraocular muscles, viewed from the top. (Modified from Dox I,

Melloni BJ, Eisner GM: Melloni's Illustrated Medical Dictionary. Williams & Williams,

1979.)

EYE



MOVEMENTS

Extraocular muscles subserving the six cardinal positions of gaze. The eye is adducted by

the medial rectus and abducted by the lateral rectus. The adducted eye is elevated by the

inferior oblique and depressed by the superior oblique; the abducted eye is elevated by the

superior rectus and depressed by the inferior rectus.

(From Squire LR, et al [editors]:Fundamental Neuroscience,

3rd ed. Academic Press,2008.)

Figure 12 –22

Figure 12 –22

Fig 12-22

Vis al Ac it



Visual Acuity

o Visual acuity is the degree to which the details andcontours of objects are perceived.

o Visual acuity is usually defined in terms of the

minimum separable—ie, the shortest distance by

which two lines can be separated and still be

perceived as two lines.

o Visual threshold (!) is the minimal amount of light

that elicits a sensation of light

Visual Acuity



Clinically, is often determined by the use of the Snellen letter chartsviewed

at a distance of 20 ft (6 m).

The individual being tested reads aloud the smallest linedistinguishable.

The results are expressed as a fraction.

The numerator of the fraction is 20, the distance at which thesubject reads the chart.

The denominator is the greatest distance from the chart atwhich a normal individual can read the smallest line the subjectcan read.

Normal visual acuity is 20/20; a subject with 20/15 visual acuityhas better than normal vision (not farsightedness); and one with20/100 visual acuity has subnormal vision.

The Snellen charts are designed so that the height of the letters inthe smallest line a normal individual can read at 20 ft subtends avisual angle of 5 minutes.

Each of the lines in the letters are separated by 1 minute of arc.Thus, the minimum separable in a normal individual corresponds to

a visual angle of about 1 minute.



Vi l Fi ld



Monocular and binocular visual fields. The dashed line encloses the visual

field of the left eye; the solid line, that of the right eye. The common area

(heart-shaped clear zone in the center) is viewed with binocular vision. The

colored areas are viewed with monocular vision.

Visual Fields

& Binocular Vision



Dark Adaptation



p

The decline in visual threshold is known as dark adaptation. It isnearly maximal in about 20 minutes, although some further decline

occurs over longer periods.

The dark adaptation response actually has two components

o The first drop in visual threshold, rapid but small in magnitude, isknown to be due to dark adaptation of the cones because whenonly the foveal, rod-free portion of the retina is tested, thedecline proceeds no further.

o In the peripheral portions of the retina, a further drop occurs as aresult of adaptation of the rods. The total change in thresholdbetween the light-adapted and the fully dark-adapted eye is verygreat.

This rise in visual threshold is known as light adaptation. occursover a period of about 5 minutes. It is merely the disappearance ofdark adaptation.

Dark



Range of luminance to which the

human eye responds, with the

receptive mechanisms involved.

(Reproduced, with permission, bycourtesy of Campbell FW, from Bell

GH, Emslie-Smith D, Paterson CR:

Textboo k of Physio logy and

Biochemis t ry , 9th ed. Churchill

Livingstone, 1976.)

Adaptation

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