BIOL 207: Lab 6
BIOLOGY 207 LABORATORY 6
THE NERVOUS SYSTEM
PART 2: VISION AND HEARING, THE SPECIAL SENSES.
OBJECTIVES
1. Examine the function of the eye and structure of component.
2. Describe the blind spot and fovea.
3. Demonstrate the blind spot.
4. Test for visual acuity.
5. Test your ability to accommodate.
6. Discuss the concept of refraction and the structure of lenses.
7. Examine the posterior portion of the eye.
8. Investigate the presence of afterimages. Understand both positive and negative afterimages.
9. Conduct the Rinne test.
10. Use an audiometer to establish your hearing acuity.
GENERAL STATEMENT REGARDING SUBJECT MATTER
Vision
In this section, we will be looking at some fundamental properties of the visual system. Specifically, we will
investigate properties of refraction and accommodation, take a look at your pupils and see how they respond to light,
and also examine your eye with a device designed to allow you to actually look at the posterior portion of your eye. In
addition, you will do exercises to demonstrate the blind spot and various after-images.
Refraction is the bending of light waves. Light waves entering your eye from a specific point on an object must be
focused on a specific point of your retina. If this light is not focused on a particular point, you will not see the image
clearly. Your lens and your cornea are the principle parts of your eye involved in refraction of light. Some of the
exercises you do will be to investigate this refraction and visual acuity. For instance, Myopia is the condition of
nearsightedness. This is usually due to having an eyeball that is a bit too long. Thus, the light waves focus in front of
the retina rather than right on it. You can treat this condition with lenses that are concave and pre-bend the light
waves. Hyperopia is far-sightedness. You can treat this condition with a convex lens. With hyperopia, the light waves
focus behind the retina because the eyeball is too short. The concave lens bends the light a bit more to compensate for
the short eyeball.
Accommodation is the ability of the eye to focus on objects of varied distance from your eye. You have both close-up
vision and vision for far-away objects. Your lens must change shape to alter the refraction of light arriving from
various distances in order to focus that light on your retina. With age, your lens becomes less able to change shape and
you suffer from presbyopia or the inability to focus on close objects. This presbyopia clearly indicates the importance
of your lens function in the ability to focus on close objects. Your lenses, if normal have an adjustable diopter range of
67 to about 79 (see diopters below).
Adaptation
You will also be looking at after-images and adaptation within your color vision. When you look at these images, try
to imagine the mechanism that generates this sensitivity. Think biochemistry and physiology here.
Hearing
You hear sounds because the hair cells within the Organ of Corti are vibrated by sounds. These hair cells are on the
basilar membrane and vibrations within the cochlea are the reason these hair cells move. The vibrations within the
cochlea are generated by movement at the eardrum, followed by movement of the middle ear bones, then the oval
window and the fluid within the cochlea. Those vibrations are ultimately converted into sound within your brain.
Damage to the middle ear may result in conduction deafness, an inability to transmit vibrations from your eardrum to
6.1
, BIOL 207: Lab 6
the inner ear. Damage to the inner ear results in a condition called sensory deafness. You will be testing your hearing
and doing some simple exercises to demonstrate the importance of vibrations to hearing, and to show how you
perceive the position of a tuning fork.
Audiometry
An audiometer is an instrument for measuring hearing acuity. You will listen to several tones in the normal hearing
range and establish the threshold in decibels of loudness needed to hear these tones.
READING in HUMAN PHYSIOLOGY, 14th Edition, Stuart Ira Fox
hair cells: p. 278-279 hearing and the ear: pp. 282-289 hearing
impairment: pp. 289 structure of the eyeball: p. 291 accommodation
and presbyopia: pp. 295 myopia and hyperopia: 296 astigmatism: p. 297
adaptation and vision: pp. 298-301
color vision and visual acuity: pp. 301-3
Exercises 6.1A-I: Testing the Visual System
In the following exercises, you will work in pairs to test and examine one another’s visual systems.
Ex. 6.1A: Diopters and Focal Length –A calculation
The strength of the lens is measured in diopters, and whether or not a lens is convex or concave is indicated
by a + or – sign, respectively. For instance, a +3 lens is convex, is used to correct for hyperopia, and has a
strength of 3 diopters. Whereas a -3 lens is concave, is used to correct for myopia and has a strength of 3
diopters. The strength in diopters is = 1 divided by the focal length of the lens in meters.
Diopters = 1 ÷ focal length (meters)
From this formula, you can see that a strong lens has a shorter focal length. Thus, a 3 diopter lens will focus
light in 1/3 of a meter, while a 5 diopter lens will focus light in 1/5 of a meter. Use the diopter formula to
calculate the focal length of a human lens on p. 6.9.
Ex. 6.1B: Visual Acuity –the Snellen Eye Chart
Your visual acuity (sharpness of vision) can be determined using the Snellen Eye Chart (the chart with the
big E and other letters). The last line, which you can read all letters correctly, determines your visual acuity.
Normal vision is 20/20 vision. The top number tells you how far you are standing from the chart and the
bottom number tells you how far someone with normal vision can stand from the chart to read that same
line. For instance, someone with 20/30 vision must stand 20 feet of the chart, whereas someone with normal
vision could stand further away at 30 feet and still read that same line. Thus, if the bottom number is greater
than 20 (e.g. 20/30 vision), then you are myopic (near-sighted). Myopia is usually due to the eyeball being
too long or the lens being too strong, such that the light is focused before it hits the retina. If the bottom
number is less than 20 (e.g. 20/10 vision), then you have better than normal vision.
1. Stand 20 feet from the Snellen Eye Chart and remove your glasses if you wear them. If you are
wearing contacts, you may leave them in. You can still check to see if your prescription has changed
or if your optometrist gave you contacts that provide you with better than normal vision.
2. While covering one eye, read the lines of letters down to the smallest letters you can actually read.
Have your lab partner stand next to the chart to determine whether you have incorrectly identified
any of the letters.
3. Record the corresponding numbers next to the last line that you can read every letter correctly.
4. Repeat for your other eye.
6.2
BIOLOGY 207 LABORATORY 6
THE NERVOUS SYSTEM
PART 2: VISION AND HEARING, THE SPECIAL SENSES.
OBJECTIVES
1. Examine the function of the eye and structure of component.
2. Describe the blind spot and fovea.
3. Demonstrate the blind spot.
4. Test for visual acuity.
5. Test your ability to accommodate.
6. Discuss the concept of refraction and the structure of lenses.
7. Examine the posterior portion of the eye.
8. Investigate the presence of afterimages. Understand both positive and negative afterimages.
9. Conduct the Rinne test.
10. Use an audiometer to establish your hearing acuity.
GENERAL STATEMENT REGARDING SUBJECT MATTER
Vision
In this section, we will be looking at some fundamental properties of the visual system. Specifically, we will
investigate properties of refraction and accommodation, take a look at your pupils and see how they respond to light,
and also examine your eye with a device designed to allow you to actually look at the posterior portion of your eye. In
addition, you will do exercises to demonstrate the blind spot and various after-images.
Refraction is the bending of light waves. Light waves entering your eye from a specific point on an object must be
focused on a specific point of your retina. If this light is not focused on a particular point, you will not see the image
clearly. Your lens and your cornea are the principle parts of your eye involved in refraction of light. Some of the
exercises you do will be to investigate this refraction and visual acuity. For instance, Myopia is the condition of
nearsightedness. This is usually due to having an eyeball that is a bit too long. Thus, the light waves focus in front of
the retina rather than right on it. You can treat this condition with lenses that are concave and pre-bend the light
waves. Hyperopia is far-sightedness. You can treat this condition with a convex lens. With hyperopia, the light waves
focus behind the retina because the eyeball is too short. The concave lens bends the light a bit more to compensate for
the short eyeball.
Accommodation is the ability of the eye to focus on objects of varied distance from your eye. You have both close-up
vision and vision for far-away objects. Your lens must change shape to alter the refraction of light arriving from
various distances in order to focus that light on your retina. With age, your lens becomes less able to change shape and
you suffer from presbyopia or the inability to focus on close objects. This presbyopia clearly indicates the importance
of your lens function in the ability to focus on close objects. Your lenses, if normal have an adjustable diopter range of
67 to about 79 (see diopters below).
Adaptation
You will also be looking at after-images and adaptation within your color vision. When you look at these images, try
to imagine the mechanism that generates this sensitivity. Think biochemistry and physiology here.
Hearing
You hear sounds because the hair cells within the Organ of Corti are vibrated by sounds. These hair cells are on the
basilar membrane and vibrations within the cochlea are the reason these hair cells move. The vibrations within the
cochlea are generated by movement at the eardrum, followed by movement of the middle ear bones, then the oval
window and the fluid within the cochlea. Those vibrations are ultimately converted into sound within your brain.
Damage to the middle ear may result in conduction deafness, an inability to transmit vibrations from your eardrum to
6.1
, BIOL 207: Lab 6
the inner ear. Damage to the inner ear results in a condition called sensory deafness. You will be testing your hearing
and doing some simple exercises to demonstrate the importance of vibrations to hearing, and to show how you
perceive the position of a tuning fork.
Audiometry
An audiometer is an instrument for measuring hearing acuity. You will listen to several tones in the normal hearing
range and establish the threshold in decibels of loudness needed to hear these tones.
READING in HUMAN PHYSIOLOGY, 14th Edition, Stuart Ira Fox
hair cells: p. 278-279 hearing and the ear: pp. 282-289 hearing
impairment: pp. 289 structure of the eyeball: p. 291 accommodation
and presbyopia: pp. 295 myopia and hyperopia: 296 astigmatism: p. 297
adaptation and vision: pp. 298-301
color vision and visual acuity: pp. 301-3
Exercises 6.1A-I: Testing the Visual System
In the following exercises, you will work in pairs to test and examine one another’s visual systems.
Ex. 6.1A: Diopters and Focal Length –A calculation
The strength of the lens is measured in diopters, and whether or not a lens is convex or concave is indicated
by a + or – sign, respectively. For instance, a +3 lens is convex, is used to correct for hyperopia, and has a
strength of 3 diopters. Whereas a -3 lens is concave, is used to correct for myopia and has a strength of 3
diopters. The strength in diopters is = 1 divided by the focal length of the lens in meters.
Diopters = 1 ÷ focal length (meters)
From this formula, you can see that a strong lens has a shorter focal length. Thus, a 3 diopter lens will focus
light in 1/3 of a meter, while a 5 diopter lens will focus light in 1/5 of a meter. Use the diopter formula to
calculate the focal length of a human lens on p. 6.9.
Ex. 6.1B: Visual Acuity –the Snellen Eye Chart
Your visual acuity (sharpness of vision) can be determined using the Snellen Eye Chart (the chart with the
big E and other letters). The last line, which you can read all letters correctly, determines your visual acuity.
Normal vision is 20/20 vision. The top number tells you how far you are standing from the chart and the
bottom number tells you how far someone with normal vision can stand from the chart to read that same
line. For instance, someone with 20/30 vision must stand 20 feet of the chart, whereas someone with normal
vision could stand further away at 30 feet and still read that same line. Thus, if the bottom number is greater
than 20 (e.g. 20/30 vision), then you are myopic (near-sighted). Myopia is usually due to the eyeball being
too long or the lens being too strong, such that the light is focused before it hits the retina. If the bottom
number is less than 20 (e.g. 20/10 vision), then you have better than normal vision.
1. Stand 20 feet from the Snellen Eye Chart and remove your glasses if you wear them. If you are
wearing contacts, you may leave them in. You can still check to see if your prescription has changed
or if your optometrist gave you contacts that provide you with better than normal vision.
2. While covering one eye, read the lines of letters down to the smallest letters you can actually read.
Have your lab partner stand next to the chart to determine whether you have incorrectly identified
any of the letters.
3. Record the corresponding numbers next to the last line that you can read every letter correctly.
4. Repeat for your other eye.
6.2
