Lab 6 Senses II_Vision_Hearing

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1 BIOL 226 Human Physiology Lab 6 Senses II: Vision and Hearing I. Vision Tests A. Visual Acuity “Visual acuity” means visual sharpness. Our visual acuity is our ability to clearly distinguish two closely placed objects. Normal visual acuity (emmetropia) can be affected, in part, by near- sightedness (myopia) , far-sightedness (hyperopia) , and refracted light (astigmatisms) . Recall that the job of the lens is to focus light precisely onto the retina. When light is focused in front of the retina, it is called myopia and creates a blurred image. When light is focused behind the retina, it is called hyperopia and creates blurred images. Myopia can be corrected with concave lenses and hyperopia can be corrected with convex lenses. Astigmatisms result from mis-shaped lens or cornea cause the refraction of light. A commonly used test for visual acuity is the Snellen Eye Chart. Learning Objectives: *Explain various vision tests and their physiological significance. *Measure, collect, and analyze data from various vision tests. *Explain sensory and conductive deafness and how sound is located. * Measure, collect, and analyze data from various hearing tests.
2 Note: Visual acuity is reported as 20 over some other number. The top and bottom numbers have meaning. Visual Acuity Left eye = Right eye = Both Eyes B. Near Point of Vision Near point of vision is the minimum distance an object can be placed before becoming blurry. It has to do with the amount of elasticity in the lens. As we get older, we tend to lose elasticity in the lens, leading to a condition called presbyopia (old people vision). With presbyopia, it becomes increasingly difficult for the ciliary body to change the shape of the lens because of the loss of elasticity in the lens. As we lose elastic in the lens, objects must be further away from the eye to be seen clearly. The near point of vision is approximately 10 cm in young adults, closer in young children, and further in older adults. Procedure: 1. Stand 20 feet away from the Snellen eye chart for this test. Cover one eye and read the line of letters indicated by your lab partner. 2. Continue reading progressively smaller letters. Your lab partner will your visual acuity from the smallest line of letters you can read accurately. 3. Test the visual acuity in the other eye and then in both together. 20 = when you are 20’ away, you see what 20 the average person sees from 20’ away 20 = when you are 20’ away, you see what 40 the average person sees from 20’ away 20= when you are 20’ away, you see what 10 the average person sees from 10’ away Average vision Worse than average vision Better than average vision
3 Procedure: 1. Close one eye. 2. Hold a pencil at arm’s length and slowly bring the pencil toward your eye. 3. Your lab partner will measure the distance at which the pencil first appears blurred. 4. Repeat this procedure with the other eye. C. Test of the Extrinsic Eye Muscles by Convergence There are six extrinsic eye muscles that hold the eyeball in place. These muscles also control movements to ensure light is focused onto the fovea centralis. If the tone of one muscle is weak during the test of convergence, the eye will involuntarily and slowly drift in a direction, followed by rapid movement back to correct position ( nystagmus ). Procedure: 1. Observe convergence of your lab partner’s eyes by holding a pencil about two feet in front of their face. Slowly bring the pencil toward the bridge of their nose. The eyes should move medially and converge medially on the pencil. Results _____________________________________________________________ D. Pupillary Light Reflex The pupillary light reflex is often performed clinically as a normal part of a physical exam. Under normal circumstances, bright light causes, the sphincter muscles in the iris surrounding the pupil to constrict making the pupil diameter smaller. This is caused by the parasympathetic division of the autonomic nervous system. In dim light, the sympathetic division of the autonomic nervous system is activated and causes the dilator muscles in the iris to constrict making the pupil diameter larger. Absence of normal pupillary reflexes is generally a late indication of severe trauma or deterioration of brain stem tissue. Procedure: 1. Use a room with dim light for this test. 2. After one minute in a darkened room, measure your partner’s pupil diameter using a pupi l size gauge. 3. Use a pen light to shine the light into your partner’s eye. Not the change in pupil size and measure the pupil diameter. Pupil size in dark = Pupil size in light = Near point of Vision (cm) Left eye = Right eye =
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4 E. Observation of the Retina F. Demonstration of the Blind Spot The optic disc of the retina is called the “blind spot” because is it where the axons of ganglion cell converge and become the optic nerve. When light hits the optic disc, it cannot be perceived. This blind spot is different than a blind spot in your car. You cannot wrench your neck or adjust mirrors to overcome the blind spot caused by the optic disc. Proc edure: To find your blind spot, close your left eye and look at the cross below. Start with the page about 2 feet away and slowly move it toward your face. When the disk to the right of the cross falls on your blind spot, it will disappear. This only works when one eye is closed, since the same part of your visual field cannot fall on the blind spot for both eyes. An ophthalmoscope is an instrument used clinically to view the posterior parts of the eyeball. The procedure allows practitioners to view the retina and vitreous humor and most often requires pupil dilation drops. Procedure: 1. Study the photograph of the retina as seen through an ophthalmoscope. What structures are visible? _____________________________________________________ _____________________________________________________ _____________________________________________________
5 G. Binocular Rivalry When light enters our eyes, it sends information to our brain about what is happening in the world around us. The brain collects visual data from both eyes and essentially overlaps the information to show us one combined image of the world. When both eyes receive completely different sensory information, however, the brain can get confused. At this point, rather than overlap the images from both eyes, it is possible that only one image will take dominance over the other, and that is the image that you will “ see .” This is known as binocular rivalry – literally, two eyes competing. It is a phenomenon of visual perception in which perception alternates between different images presented to each eye. Procedure: 1. Use your right hand to hold a cardboard tube (or rolled up piece of paper) against your right eye. 2. Hold your left hand, palm facing you about four inches from your left eye and touching the tube. 3. Look through both eyes and record what you see. 4. Continue to observe for an additional 30 seconds. How does the image change? H. Color Blind Tests Humans have cones that allow trichromatic color vision. These cones are blue, green, and red. Normal color vision allows humans to see approximately 125 different colors. How is this possible if we only have blue, green, and red cones? The genes for the green and red cones are located on the X chromosomes. Why are males more likely to have color blindness than females? The test used for color blindness in this lab are Ishihara’s Blindness Plates. Ishihara’s test measures the ability to tell the difference among colors. Procedure: 1. U sing Ishihara’s Plates book, observe and record the number or pattern seen in the plate with one eye covered. 2. Compare results with normal and abnormal results on the answer pages.
6 II. Hearing Tests Conduction deafness results from an inability to transmit sound to an intact and functioning inner ear. This can be cause by anything that damages the outer or middle ear, including, but not limited to damage or mis-shaped auricle, infections of the middle ear or tympanic membrane, damage to one or more ossicles, and an excessive build-up of cerumen. Conduction deafness can be treated with the hearing aids. Sensory deafness results from an inability to transduce or perceive sound. This can result from the death of hair cells in the cochlea, detachment of CN VIII (Vestibulocochlear nerve), damage to the temporal lobe of the brain. In addition to congenital issues, sensory deafness can be caused by inner ear infections and exposure to prolonged excessively loud noises. Sensory deafness can be treated by cochlear implants. Procedure: A. Rinne’s Test 1. Strike the tuning fork against a book to produce vibrations. 2. Place the handle of the tuning fork against the mastoid process of the temporal bone with the prongs pointed down and behind the ear. 3. When the sound disappears, move the prongs of the tuning fork next to the same ear. In a normal ear, the sound will reappear. If there is damage to the middle or outer ear, the sound will not reappear. 4. Repeat this procedure with the other ear. 5. Perform Rinne’s test again simulating conduction deafness by plugging the ear with your finger. Ear Tuning fork against mastoid process Tuning fork next to ear Left (unplugged) Right (unplugged) Ear Tuning fork against mastoid process Tuning fork next to ear Left (plugged) Right (plugged) B. Weber’s Test 1. Place the handle of a vibrating tuning fork on the midsagittal line of the head. 2. Record the loudness in each ear. 3. Repeat this test with one ear plugged. In which ear is the sound louder? Why? Left ear and left ear unplugged Right and left ear with left ear plugged
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7 C. Sound Localization Sound localization depends on the brain’s perception of two differences. 1. The difference in the loudness of the sound reaching the two ears . The frequency of action potentials will be higher from the ear closest to the location of the sound; thus, the brain will perceive the sound as louder on one side than the other side. By comparing the difference in loudness between the right and left ears, the brain perceives the location the sound is coming from. 2. The difference in the time of arrival of the sound at each ear. The sound coming from one side will reach the ear closest to origin of the sound before the other ear. That ear’s cochlea will send action potentials to the brain before the ear furthest from the origin of the sound. By comparing the difference in the time of arrival of the action potentials from right and left ears, the brain perceives the location the sound is coming from. Procedure: 1. Your lab partner should be seated and have both eyes closed. 2. Snap your fingers at various locations around their head about one foot away. 3. After each snap, your lab partner will point to the location of the sound. 4. Record the locations where the subject was accurate and inaccurate pin- pointing the location of the sound.
8 Lab Report Questions 1. Organize your vision test results in a chart or table, excluding the blind spot demonstration and binocular rivalry. 2. What is your visual acuity with both eyes? What does this number mean? 3. Two people the same age may have different near point of vision measurements. How is this possible? 4. In the blind spot demonstration, what did you see in place of the figure that disappeared? Do not answer “nothing” because there is never a hole in our visual fields. Does “seeing” occur in the eyes or in the brain? Use the results of this demonstration to support your answer. 5. Is your visual awareness always an accurate representation of reality? Explain. 6. What abnormalities can be detected by observing the retina with an ophthalmoscope? 7. Why are vision and hearing tests given as part of a regular physical exam to young children before starting school? 8. How is sound located? Why are we often inaccurate locating sound that originates in the midsagittal plane? 9. Predict the results of Rin ne’s and Weber’s test in someone with conductive hearing loss in one ear and another person with sensory loss in one ear.

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