VISUAL DISORDERSOF THE EYE

B.) Structural Description of the Eyes: The Process of Seeing Light rays enter the eye, pass through the glassy corneas, then the watery liquid called the aqueous humor, the pupil, and the lens. The pupils contract or dilate depending upon the light intensity.

The eyeball is covered by 3 coats: 1. The sclerotic coat is the outer tough covering2. The chloroid coat is the heavily-pigmented layer which keeps out all light except

the pupil3. The retina is the photoreceptor that translates light stimulus into nerve impulses C. Visual Phenomena4. Inverted retinal image: Light waves that fall obliquely on a surface two media of different densities refract and converge at the pasterior focal point. The rays coming from the upper part of the stimuli are refracted to the lower portion of retina: light waves from the lower part of the stimuli are refracted to the upper part of the retina an inverted retina image is formed.

2. Visual Acuity- deals the ability of the retina to distinguish differences in stimulation. - 3 factors of Visual Acuitya.) sizeb.) illuminationc.) proximity

3. Visual Accomodation- process of adjusting the lenses of the eyes to varying distances within the visual field to obtain a clear vision.

4. Visual Adaptation- this is due to the difference action between the rods and cones.- Adaptation to light takes shorter than adaptation to the dark.- Adaptation to light is called photophia while adaptation to dark is called scotopia.

D.) Common Visual Deffects1. Monocular Defects ( Faulty eye structure )a.) Hyperopia or far-sightedness – this defect, the eyeballs are short and the images focused at the back of the retina. To correct this, convex lenses may be used.b.) Astigmatism – may be due to the uneven curvature of the cornea of the lenses. Cylindrical lenses are often prescribed.d.) Cross-eyedness or strabismus –here there is muscle imbalance causing the eyes to turn inward.e.) Presbyopia or old-sightedness – is a condition where the lenses harden and lose the ability to accommodate.

2. Visual Abnormalities (functional and organic)a.) Hemianopsia. There is a blindness to one side of the visual field due to lesions in the optic tract.b.) Scotoma – condition where there a blind spots in the visual field.c.) Visual paresthesia – condition where there are optic sensations.d.) Color blindness – certain color are seen as black or green. A normal person can see the light-dark called trichromat. A drichromat on the other hand lacks one system partially color blind. The monochromat is a totally color-blind person.e.) Night blindness – defect where there is the inability to see well at night due to the track of Vitamin A.

E.) Color Vision- originated by the famous scientist Sir Isaac Newton- By passing through white light through a triangular prism and getting the color of the spectrum.

Afterimages- the so-called afterimages are due to the neural action still travelling to the brain in that very small portion of a second as we transfer our gaze at a different.- two types of Afterimages: positive and negative

Contrast Effects- are maximal when the brightness of the stimulus and its surrounding is equal

Additive and Subtractive Mixtures- colored stimulus may actually becomposed of several wave lengths.- The mixture of the blue and yellow pigment reflects only green all the others are subtracted by one of the pigments.- Additive mixtures as in mixtures of light, color are projected together upon the same white surface which reflects all the wave lengths.

F.) Theories of Color vision1. Young Helmotz Theorya.) Three fiber theory- Thomas Young, a physicist combining three fundamental or primary

color red, blue, and green.- He assumed the existence of three different kinds of nervous fibers in

the retina that react specifically to these color with their corresponding perceptors in the brain center.

- This theory was further developed by Ludwig on Helmotz because of the impossibility of obtaining were raised against the theory.

b.) Theory of photo-chemical substances- According to this theory, every retinal receptor capable of arousing a

visual sensation is endowed with three different susbstances of a photo-chemical nature capable of developing any of the primary color.

2. Hering Theory (also called opponent-color theory)- Hering (physiologist) impressed by color pairs and so proposed three

processes their building up phase (anabolic) would yield the other.

3. Wundt’s Theory Wundt in 1874 proposed that every retinal excitation involves two different stimulation processes one chromatic and one achromatic. Chromatic sensations are produced by strong and weak stimuli while achromatic processes are developed only by moderate stimuli.

4. Von Kries Theory According to the theory of Von Kries, vision is due to the action of the rods and cones in the nerve endings of optic nerve.

5. Ladd-Franklin Theory This theory postulates four primaries red, green, yellow, and blue. It assumes that the primitive eye was color blind (a cone free eye) and that the yellow and blue receptors evolved from the primitive rods with further evolution producing red and green sensitive receptors, the latter having evolved from yellow processes.

Parts of the Eye

Vitereous humor – as the light reaches the retina, it crosses a clear fluid.Accommodation – lens changes its shape upon focusing in near or far image.Light rays – bent by the lens to meet a point to the retina. - The image will be out of focus if the rays meet in front of the retina (nearsighted) or behind the retina (farsighted)

Process of Accommodation

Retina – network of cells containing neurons, ganglion cells, and photoreceptors.Rods and Cones – after processing the neurons, the neural activity reaches the photoreceptor. Photoreceptor – is the origin of visual responses.Optic disk (blind spot) – the axons of the ganglion carry all visual information to the brain by first converging.Optic nerve – information that sent to the brain through a bundle of nerve fibers.Optic chasm – optic nerves leave the two eyeballs and meet at a junction referred.

Color Vision- The sun projects wavelengths of light - The fundamental color, marked by the intensity of wavelength of the light.- Neutral color (black, white, and gray) not accepted as hues because no wavelength dominates in them.

2. Audition A. Stimulus: Sound Waves - Sounds are forms of energy activated when objects vibrate.- Vibrating objects move in one direction pushing the air in front of it.Compression wave – the crowding motion that moves farther and farther out from the vibrating object.Rarefaction wave – the zone of rarefied air moving away and away from the vibrating bodies reproduces.- One compression and one rarefaction comprise one sound wave.- Sound travels faster in a warm medium than in a cool one.

Visual Sensation

Parts and Functions Light enters first by passing through the cornea, the transparent productive covering curved tissue in front of the eye. Behind the cornea, the light continues to the anterior (front) chamber which contains watery liquid (aqueous humor) to provide nutrients and keep the cornea moist. The pupil controls the amount of light by constricting (closing) on too much light dilating (opening) under dark condition. The iris is the colored muscles of the eye responsible for reducing (constricting) or enlarging (dilating) the size of the pupil. The lens is an elastic cured structure that bends light rays in order to focus a clear image to retina.

Saturation – associated with the purity color.Common Visual defects – any faulty optical mechanism will result in a certain visual defect. The most common eye defects are hyperopia, myopia, and astigmatism.Color blindness – some people have defects in their color vision. There are 2 kinds, the totally color-blind and the partially color blind.