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VISION - PowerPoint PPT Presentation

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VISION. 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.

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part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponsefunctions. 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.


  • 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, andsaturation.

  • The color or hue of light depends on itswavelength, the distance between the peaks of its waves.

  • Brightness depends on lightwave 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

Thecorneais the transparent, protective outer membrane of the eye.

Theiris, the colored part of the eye, is a ring of muscle.

The iris surrounds an opening called thepupil, 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.

Thelens, 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


Light passing through the cornea, pupil, and lens falls onto the retina at the back of the eye. Theretinais a thin layer of neural tissue. The image that falls on the retina is always upside down.

Rodsare responsible for vision at low light levels (scotopic vision). They do not mediate color vision, and have a low spatial acuity.

Conesare active at higher light levels (photopic vision), are capable of color vision and are responsible for high spatial acuity.

The center of the retina, thefovea, 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 corneas 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 theeye.

  • 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 eyes 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 eyes 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


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-adaption to light

    Scotopia-adaption to dark

  • Anafterimageorghost imageorimage burn-inis anoptical illusionthat 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'seyesafter looking into a light source for a few seconds.

Eye Trouble

  • Nearsightedness is the inability to clearly see distant 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 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.




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 testagain.

Yourdistance visionis 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 havemyopiaor have a myopic tendency.Conversely, if you see the O blacker or sharper in the green part, you are definitelyhyperopeand 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 AstigmatismCover 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 anastigmatism- consult your eye care specialist

Test your eyes

TheIshihara testis commonly used to detectDaltonism(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 aIshihara testbut instead requires a 100 Hue test, which needs to be carried out by youreye 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 QualityWavelenght(mu)





    Blue 492-433



  • The average blink lasts for about 1/10th of a second.

  • While it takes some time for most parts of your body to warm 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 dont produce tears. They make crying sounds, but the tears dont 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 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 dont notice the hole in your vision because your eyes work together to fill in each others 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.

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