Vision
part of the central nervous system which
enables organisms to process visual detail, as
well as enabling several non-image forming
photoresponse functions. It interprets
information from visible light to build a
representation of the surrounding world
A. Stimulus : Light Waves
 Sun emits radiant energy or electromagnetic waves. If
this enters the eyes, it becomes a visual stimulus.
Light
 is electromagnetic radiation that travels in the form of
waves
 is emitted from the sun, stars, fire, and lightbulbs.
Most other objects just reflect light.
 People experience light as having three
features: color,brightness, and saturation.
 The color or hue of light depends on its wavelength,
the distance between the peaks of its waves.
 Brightness depends on light wave amplitude, the
height of light waves. Brightness is also somewhat
influenced by wavelength. Yellow light tends to look
brighter than reds or blues.
 Saturation or colorfulness depends on light
complexity, the range of wavelengths in light. Light of
mixed wavelengths looks duller or paler than pure
light.
Wavelength ——> Color
Amplitude ——> Brightness
Complexity ——> Saturation
There are essentially four steps to vision.
 First we have to gather light into our eye.
 The light has to be channeled to the back of the eye.
 Transduction occurs.
 The information goes to our brain where we interpret
it.
The Eyes
 the organ of vision
 detect light and convert it into electro-chemical
impulses in neurons
 complex optical system which collects light from the
surrounding environment, regulates its intensity
through a diaphragm, focuses it through an adjustable
assembly of lenses to form an image, converts this
image into a set of electrical signals, and transmits
these signals to the brain through complex neural
pathways that connect the eye via the optic nerve to
the visual cortex and other areas of the brain
The cornea is
the transparent,
protective outer
membrane of
the eye.
The iris,
the colored
part of the
eye, is a
ring of
muscle.
The iris surrounds
an opening called
the pupil, which
can get bigger or
smaller to allow
different amounts
of light through the
lens to the back of
the eye. In bright
light, the pupil
contracts to restrict
light intake; in dim
light, the pupil
expands to increase
light intake.
The lens, which lies
behind the pupil
and iris, can adjust
its shape to focus
light from objects
that are near or far
away. This process
is called
accommodation.
Light passing
through the
cornea, pupil, and
lens falls onto the
retina at the back
of the eye.
The retina is a
thin layer of
neural tissue. The
image that falls on
the retina is
always upside
down.
Rods are
responsible for
vision at low light
levels (scotopic
vision). They do
not mediate color
vision, and have a
low spatial acuity.
Cones are active at
higher light levels
(photopic vision),
are capable of
color vision and
are responsible for
high spatial acuity.
The center of the
retina, the fovea,
is where vision is
sharpest. This
explains why
people look
directly at an
object they want
to inspect. This
causes the image
to fall onto the
fovea, where
vision is clearest.
 Light rays enter the eye through the cornea, the clear front
“window” of the eye. The cornea’s refractive power bends the
light rays in such a way that they pass freely through the
pupil the opening in the center of the iris through which light
enters the eye.
 The iris works like a shutter in a camera. It has the ability to
enlarge and shrink, depending on how much light is entering
the eye.
 After passing through the iris, the light rays pass thru the
eye’s natural crystalline lens. This clear, flexible structure
works like the lens in a camera, shortening and lengthening
its width in order to focus light rays properly.
 Light rays pass through a dense, transparent gel-like
substance, called the vitreous that fills the globe of the
eyeball and helps the eye hold its spherical shape.
 In a normal eye, the light rays come to a sharp focusing
point on the retina. The retina functions much like the film in
a camera. It is responsible for capturing all of the light rays,
processing them into light impulses through millions of tiny
nerve endings, then sending these light impulses through
over a million nerve fibers to the optic nerve.
 Because the keratoconus cornea is irregular and cone shaped,
light rays enter the eye at different angles, and do not focus on
one point the retina, but on many different points causing a
blurred, distorted image.
 In summary, the cornea is the clear, transparent front covering
which admits light and begins the refractive process. It also keeps
foreign particles from entering the eye.
 The pupil is an adjustable opening that controls the intensity of
light permitted to strike the lens. The lens focuses light through
the vitreous humor, a clear gel-like substance that fills the back of
the eye and supports the retina.
 The retina receives the image that the cornea focuses
through the eye’s internal lens and transforms this image
into electrical impulses that are carried by the optic nerve
to the brain. We can tolerate very large scars on our
bodies with no concern except for our vanity. This is not
so in the cornea. Even a minor scar or irregularity in the
shape can impair vision. No matter how well the rest of
the eye is functioning, if the cornea is scarred, clouded or
distorted, vision will be affected.
 In keratoconus, the irregular shape of the cornea does
not allow it to do its job correctly, leading to distortion of
the image it passed to the retina and transmitted to the
brain.
Visual Phenomena

Inverted Retinal Image
 Visual Acuity
- this deals with the ability of the retina
distinguish differences in stimulation.
- is influenced by three factors:
a. Size
b. Illumination
c. proximity
a. SIZE
b. Illumination
c. Proximity
 Visual Accommodation
-process of adjusting the lenses of the
eyes to varying distances within the visual
field to obtain a clear vision
 Visual Adaptation
- this due to the difference in the action
between the rods and the cones
Photopia
Scotopia
-
adaption to light
adaption to dark
 An afterimage or ghost image or image burn-in is
an optical illusion that refers to an image continuing to
appear in one's vision after the exposure to the original
image has ceased. A common form of afterimages is
the bright glow that seems to float before
one's eyes after looking into a light source for a few
seconds.
Eye Trouble
 Nearsightedness is the inability to clearly see distant
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objects. (Myopia)
Farsightedness is the inability to clearly see close objects.
(Hyperopia)
A cataract is a lens that has become opaque, resulting in
impaired vision.
Astigmatism due to uneven curvature of the cornea or the
lenses.
Old-sightedness is a condition where the lenses harden and
lose the ability to accomodate,. (Presbyopia)
 There is a blindness to one side of the visual field due
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to lesions on the optic tract. (Hemianopsia)
Scotoma is a condition where there are blind spots in
the visual field.
Visual paresthesia I a condition where there are optic
sensations without an object.
Color blindness. Certain colors are seen as black or
green.
Night Blindness is a defect where there is inability to
see well at night due to the lack of vitamin A.
-trichromat
-dichromat
-monochromat
Test your eyes
Eye test for myopia and hyperopia
Cover one eye with your hand, without pressing on the lid,
and take the test. Cover the other eye and begin the test
again.
Your distance vision is deficient if you see
the O sharper or blacker, either in the red or
the green color.
Generally, if you see the O blacker or sharper
in the red part, you probably have myopia or
have a myopic tendency.
Conversely, if you see the O blacker or
sharper in the green part, you are
definitely hyperope and have hyperopia.
In any case, an eyesight check by an eye care
specialist would appear necessary.
Test your eyes
Eye test for
Distance vision
Cover one eye with
your hand, without
pressing on the lid,
and read the test
below, letter by
letter.
Cover the other
eye and begin the
test again.
If you have read the test as successfully
with the right eye as with the left eye, you
probably have good central acuity.
If you do not manage to read all the letters,
either with one eye or with two, an eyesight
check by an eye care specialist would appear
necessary.
Test your eyes
Eye test for Astigmatism
Cover one eye with your hand, without pressing on the
lid, and take the test. Cover the other eye and begin the
test again.
If some of the lines appear grayer and some blacker,
you probably have an astigmatism - consult your
eye care specialist
Test your eyes
The Ishihara test is commonly used to
detect Daltonism (color blindness of basic colors: red, blue
and green).
A distinction is made between:
•protanopes, who do not see red;
•deuteranopes, who do not see green;
•tritanopes, who do not see either blue or yellow.
Tritanopes cannot be identified using a Ishihara test but
instead requires a 100 Hue test, which needs to be carried out
by your eye care specialist.
Rarer are
people with
complete color
blindness, who
only perceive
levels of gray.
They are said
to suffer from
achromatopsia.
What can you
see inside
each of these
circles?
If you do not manage to read the
figures 7, 16, 57 and 42, an eyesight
check by an eye care specialist would
appear necessary.
 Color Vision
- originated by the famous English Scientist
Sir Isaac Newton.
The wave lenghts to which the human eye is sensitive
to range from 780-380 m.u.
Color Quality
Red
Orange
Yellow
Green
Blue
Violet
Wavelenght(mu)
760-647
647-588
588-550
550-492
492-433
433-390
SOME FUN FACTS YOU MAY NOT KNOW
ABOUT EYES:
 The average blink lasts for about 1/10th of a second.
 While it takes some time for most parts of your body to warm
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up to their full potential, your eyes are on their “A game” 24/7.
Eyes heal quickly. With proper care, it only takes about 48
hours for the eye to repair a corneal scratch.
Seeing is such a big part of everyday life that it requires about
half of the brain to get involved.
Newborns don’t produce tears. They make crying sounds, but
the tears don’t start flowing until they are about 4-13 weeks old.
Around the world, about 39 million people are blind and
roughly 6 times that many have some kind of vision
impairment.
Doctors have yet to find a way to transplant an eyeball. The
optic nerve that connects the eye to the brain is too sensitive to
reconstruct successfully.
 The cells in your eye come in different shapes. Rod-shaped
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cells allow you to see shapes, and cone-shaped cells allow you
to see color.
You blink about 12 times every minute.
Your eyes are about 1 inch across and weigh about 0.25 ounce.
Some people are born with two differently colored eyes. This
condition is heterochromia.
Even if no one in the past few generations of your family had
blue or green eyes, these recessive traits can still appear in
later generations.
Each of your eyes has a small blind spot in the back of the
retina where the optic nerve attaches. You don’t notice the
hole in your vision because your eyes work together to fill in
each other’s blind spot.
Out of all the muscles in your body, the muscles that control
your eyes are the most active.
80% of vision problems worldwide are avoidable or even
curable.