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The Eye Special Senses – Day 4
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Eye and Sense of Sight Vision system Eye Eyes Optic nerves
Vision centers in the brain Accessory structures Eye Processes light to produce images Three layers Two chambers Various Specialized parts
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Are the squares inside the blue and yellow squares all the same color?
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Bezold effect The smaller squares inside the blue and yellow squares are all the same color. They seem different (magenta and orange) because a color is perceived differently depending on its relation to adjacent colors (here blue or yellow depending on the outer square).
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Are the horizontal lines straight or crooked?
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Café Wall Illusion The horizontal lines are straight, even though they do not seem straight. In this illusion, the vertical zigzag patterns disrupt our horizontal perception.
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Does Lincoln’s face look normal?
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Some neurons in the brain seem specialized in processing faces
Some neurons in the brain seem specialized in processing faces. Faces are usually seen upright. When presented upside down, the brain no longer recognizes a picture of a face as a face but rather as an object. Neurons processing objects are different from those processing faces and not as specialized. As a consequence these neurons do not respond to face distortions as well. This explains why we miss the weird eyes when the face is inverted.
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Illusory Contour Illusory Contour: a form of visual illusion where contours are perceived without a luminance or color change across the contour
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Can you see a baby?
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How quickly can you say the color of the words below?
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Eye and Sense of Sight (cont.)
Outer – Sclera White of the eye Protects the eye Muscles attach here! Sense receptors Cornea Front of eye “Window” that allows light into eye Bends light as it enters Eye
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Eye and Sense of Sight (cont.)
Middle – Choroid Contains blood vessels Iris Colored part of eye Muscle that contracts and relaxes to open or close pupil Regulates the amount of light that enters the eye Ciliary body Muscles; controls the shape of the lens Part of the uvea: the layer of tissue that delivers oxygen and nutrients to the eye tissues Lens Posterior to iris Bends/refracts light rays Focuses light on nerve cells of retina Eye
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Eye and Sense of Sight (cont.)
Chambers of the eye Anterior chamber Front of lens Filled with aqueous humor – nourishes and bathes anterior eye Posterior chamber Behind lens Contains vitreous humor – maintains shape of eyeball and holds retina in place Eye
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Eye and Sense of Sight (cont.)
Inner – Retina Visual receptors Rods Sensitive to light Will function in dim light – “limited” night vision Do not provide sharp image or detect color Cones Function in bright light Sensitive to color and provide sharp images *Fovea Centralis contains highest amount of R/Cs Optic disc – optic nerve enters retina Eye
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Structure…
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The Art of Seeing… There are major functional differences between the rods and cones. Rods are extremely sensitive, and can be triggered by as few as 6 photons. At very low light levels, visual experience is based solely on the rod signal. This explains why colors cannot be seen at low light levels: only one type of photoreceptor cell is active Cones require significantly brighter light (i.e., a larger numbers of photons) in order to produce a signal. In humans, there are three different types of cone cells, distinguished by their pattern of response to different wavelengths of light
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Types of Cones Three types, each with different pigment:
S-cones, M-cones and L-cones Each cone is therefore sensitive to visible wavelengths of light that correspond to short-wavelength medium-wavelength long-wavelength light
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Back
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Eye and Sense of Sight: Visual Accessory Organs
Eye orbits Eye sockets Form a protective shell around the eyes Eyebrows protect eyes Eyelids Skin, muscle, and connective tissue Blinking Prevents surface from drying out Keeps foreign material out of eye
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Eye and Sense of Sight: Visual Accessory Organs (cont.)
Conjunctivas Mucus membranes Line inner surfaces of eyelids Lacrimal apparatus Lacrimal glands Lateral edge of eyeballs Produce tears Nasolacrimal ducts Medial aspect of eyeballs Drain tears into nose
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Special Senses – The Eye
Day 2
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Eye and Sense of Sight: Visual Accessory Organs (cont.)
Extrinsic eye muscles Six per eye move the eyeball Superiorly Inferiorly Laterally Medially
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Eye and Sense of Sight: Visual Pathways
Eye works like a camera Light enters the eye through the lens Refraction – cornea, lens, and fluids bend light to focus it on the retina Optic chiasm Image upside down on retina Image turned right-side up Occipital lobe of cerebrum Retina converts light to nerve impulse Optic nerve
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The Aging Eye Eyelids may droop Quality and quantity of tears decrease
Conjunctiva thins and eyes may become dryer Cornea yellows, fat deposits around it Brown spots on sclera Pupils become smaller Lens denser and more rigid Lens yellows Retinal changes – vision fuzzy Changes in ability of eye to adapt to light Impaired night vision Decreased peripheral vision; depth perception Floaters or flashes of light
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Vision Testing Professionals include
Ophthalmologist – medical doctor who is an eye specialist Optometrist – provides vision screening and diagnostic testing Opticians – fills vision prescriptions for glasses and contacts
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Vision Testing: Screening Tests
Myopia – impairment of distance vision Objects near are seen clearly, but objects farther away are blurry. Eyeball is too long Light focuses anterior to retina Snellen chart Hyperopia – impairment of near vision Distant objects clear, those close are blurry Eyeball is shorter Light focused posterior to retina Presbyopia Impairment due to aging Loss of lens elasticity
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Common Diseases and Disorders
Disorder / Disease Description Amblyopia Lazy eye; one eye is not used regularly; poor depth perception; often concurrent with strabismus Astigmatism Cornea or lens has abnormal shape; blurred images Cataracts Opaque structures in lens prevent light from passing through; vision fuzzy Conjunctivitis Pink eye; highly contagious bacterial infection Dry eye syndrome Common problem; decreased production of oil in tears
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Common Diseases and Disorders (cont.)
Disorder / Disease Description Entropion Inversion of lower eyelid Glaucoma Increase in intraocular pressure due to a buildup of aqueous humor in anterior chamber Macular degeneration Progressive disease; inadequate blood supply to retina; most common cause of vision loss; affects people over 50 years Nystagmus Rapid, involuntary eye movements Strabismus Convergent Divergent Misalignment of eyes Crossed eyes; one or both eyes turn inward Wall eye; one or both eyes turn outward
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