Download Sensory Systems Lab Exercise

Some of the other senses that humans don’t have
are infrared imaging (pit vipers and some beetles),
electrosense (many fishes and aquatic amphibians and
perhaps the platypus), vomerolfaction (reptiles and
many mammals), ultrasonic detection (bats, some marine mammals), infrasound detection (many birds and
some large land mammals), magnetoreception (at least
some amphibians, reptiles and birds) and cutaneous
chemosense (fish).
Sensory Systems Lab Exercise
Bi253
PSU
Other animals may perceive sensory stimuli differently. For example, we associate chemosense in
our mouths with “taste” and chemosense in our noses
as “smell”. What quality would you put on the
chemosensors found all over the gills and fins of a fish?
May 2014
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Introduction
Sensory systems may be classified different ways.
For example, we speak of somatosensory systems as
involving skin (cutaneous) receptors, but these receptors convey a variety of sensations including temperature, pressure, pain. Thus we could instead speak of
the thermo-sense system, the pain system and so forth.
This is not quite correct either, because there are different temperature sensors inside the body anatomically
and functionally distinct from those in the skin. The
same is true for pain sensors. Thus there is no one
perfect classification system.
In this lab we will examine sensory perception in
humans, using each other as subjects. First we
will look at sensory-mediated reflex arcs, then consider chemoreception (taste and smell), cutaneous somatosense (touch, pain, and temperature), hearing,
proprioception (balance), and vision.
In the sensations lab there are many data tables to
make so you can organize your results. Some tables
are provided in the exercise, but others are not. Part
The pattern of information flow to the brain is from
of this lab exercise is to teach you to plan a simple sensory receptor cells to primary afferent neurons to
experiment and block out a table to hold the results. sensory ganglia to sensory nuclei. Afferent neurons
This encourages thinking about what you are doing!
are the first true neurons taking sensory information toward the brain. (This distinction is important because
many sensory receptor cells are derived from epithelial tissue, distinct from the embryonic tissue that gives
1.1 Some Theory...
rise to “true” neurons). Sensory ganglia are compact
collections of neuron cell bodies in the Peripheral NerYou should be aware that there are more senses than vous System (PNS), and you are a neuron of the PNS
those which humans perceive, and many senses have if you are not in the brain or spinal cord, because officially the latter comprise the Central Nervous System,
different interpretations in different animals.
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retina detect light and this information is carried
to various nuclei in the CNS before returning as
commands to muscles in and around the eye.
or CNS. The peripheral neurons communicate their information to the CNS by making connections to neurons in either the spinal cord (spinal “nuclei”) or the
brain (cranial “nuclei”). In neurospeak, a nucleus is
not part of a cell, rather it is a collection of neuron
cell bodies inside the CNS (or the CNS equivalent of a
ganglion). When neurons travel together they become
nerves. Twelve pairs of cranial nerves carry sensory
information to the brain. How many spinal nerves are
there?
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(a) Blink Reflex: In this reflex, an object approaching the eye is seen, and the eyelids
are closed automatically and very rapidly to
protect the eye. Pass a hand very rapidly in
front of the subject’s face, and observe the
response. Do not touch the eye and avoid, if
possible, stirring up air currents which may
stimulate conjunctival or corneal reflexes.
Reflex Function of the Nervous System
(b) Light Reflex: This reflex aids vision and
protects the retina. If the amount of light
striking the retina is too strong, the pupil
constricts, decreasing the amount of light
allowed to enter the eye. Conversely, if the
illumination is dim, the pupil dilates, allowing sufficient light to enter the eye for adequate vision.
Sometimes sensory information has to be processed
very quickly, and without conscious thought. Thinking takes a great deal of time, so some sensory processing is automated. The autonomic nervous system
does much of this. For example, changing the diameter of your iris in different lighting conditions or maintaining your balance are functions that should happen
while you are free to think about other matters. It is
interesting that although both are automatic reflexes,
you are aware of your state of balance but unaware of
iris changes. The fewer neurons in the loop from sensory input to motor (i.e., muscle) output, the faster the
response can be. Some of your fastest reflexes are mediated by just a few neurons in the spinal cord, and
these are mostly involved in posture and balance.
i. Measure the diameters of the pupils of
both the subject’s eyes with a millimeter scale and under ambient room light
illumination.
ii. Continue to measure both eyes while
an assistant then shines a flashlight
beam into the right eye only.
iii. Next, cover both eyes with your hands
for a minute.
Last week you measured spinal reflexes, and in today’s lab exercise we will look at eye reflexes.
iv. Quickly remove your hands, and have
your partner measure your pupillary diameters.
2.1
v. Next, cover just your right eye with one
hand.
2.1.1
Experimental Procedures
Specific Reflexes
vi. Now remove the hand and have your
partner measure the pupillary diameters of both your eyes.
1. Eye Reflexes: These reflexes are more complicated than tendon reflexes. Photoreceptors in the
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3.1
Taste and Smell
smell exhibits a high degree of adaptation, and (2) certain odors mask others. Adaptation is one of two processes where the response to a stimulus is perceived as
weaker over time even though the stimulus remains at
the same level.
Introduction
Many taste receptors are found scattered over the surface of the tongue. The circumvallate papillae, lo- 3.2 Experimental Procedures
cated on the posterior portion of the dorsal surface
of the tongue, and the fungiform papillae, located
mainly along the lateral surfaces and on the tip of the 3.2.1 Taste Receptors
tongue, however, contain large aggregations of taste
1. Location and distribution of various types of taste
buds. Four different types of taste buds have been dereceptors: To locate and study the distribution of
scribed on the basis of their responses to sweet, sour,
the taste receptors, place approximately 1 ml or
salty, and bitter substances. Although these four types
less of the 10% salt solution in a watch glass, dip
are not exclusively confined to certain areas of the
a cotton tipped applicator into the solution, and
tongue, it has been found that the lateral edges of the
apply to the surface of the tongue. Care must be
tongue are most easily stimulated by sour-tasting subexercised to be sure that all taste buds are stimustances, the tip of the tongue by salty and sweet sublated in the same manner. Avoid use of excess sostances, and the posterior portion of the tongue by bitlution on the surface of the tongue. Note the areas
ter substances. One characteristic requirement for all
on the tongue where the sensation of salt can be
four types of receptors is that the test substance must
tasted; carefully record on your data sheet those
be dissolved before stimulation can occur.
areas where the sensation is most acute. Repeat
for each of the following: 5% sucrose, 1% acetic
The receptors for the sense of smell lie in the upacid, and 0.1% quinine sulfate. Make a diagram
per part of the nasal cavity above the superior turbinate
of the upper surface of the tongue and identify
bones. The epithelium in this area contains odor recepthe regions where sensations of salty, sweet, sour,
tor cells and supporting cells. The receptor cells are
and bitter are most strongly experienced. Briefly
modified nerve cells with rod-like projections which
describe the structure of the taste buds. Place
are stimulated by odoriferous materials in the air flowsome crystals of sugar on the tongue after you
ing past the projections. Therefore, sniffing, by caushave wiped it dry with a piece of gauze. Do you
ing larger amounts of newly inspired air to flow across
taste anything? Why is it necessary for a subthe receptors, is truly an aid to odor detection.
stance to be in solution before it can be tasted? In
what part of the brain are the sensations for taste
The actual neural processing mechanisms involved
perceived?
in olfaction are not as well understood as those for
2. Threshold of taste: Using a soda straw, place one
taste, hearing, and vision, and no clear-cut classificadrop of 0.001M sucrose on the tongue, and note
tion of primary smell sensations has been established.
whether or not a sweet taste can be perceived. ReHowever, there are associated with the sense of smell
peat the test with the rest of the sucrose solutions
two rather outstanding characteristics: (1) the sense of
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stamp, an aesthesiometer (a horsehair, 3 cm long attached by means of wax to a stick); a beaker of hot
water; a beaker of ice water; glass rods; thread; dividers whose points can be separated by very small
and rather large distances; and a millimeter rule.
making sure to test them in order of increasing
concentration. The subject should rinse his/her
mouth with distilled water between tests. Note
the weakest concentration that you can taste and
record it as the threshold stimulus. Repeat the
above procedure with the various concentrations
of salt: 0.005M, 0.01M, 0.03M, 0.05M, 0.08M,
0.1M NaCl. Determine your threshold for salt.
Record your observations on a data sheet of your
design. Compare your threshold values for salt
and sugar with those of other members of the
class.
3.2.2
An important concept to understand about sensory
systems is that of the “receptive field”. A particular
sensory neuron may communicate with, for example,
20 sensory receptor cells of the same type, or modality. These cells are spread over a certain geographical
region. Thus the one neuron is “responsible” for informing the brain about a certain mode of sensation
(pain, temperature, etc) from a certain area. Different neurons connect to different numbers of sensory
cells, and there is a different density of sensory cells
in different regions. Skin areas where you need high
sensitivity, like fingers (so they have accurate feedback
for precise gripping) have many receptors per unit area
compared to, say, the skin surface of your lower back.
Thus a given neuron can have a small or very large
receptive field over which stimuli can affect it.
Smell Receptors
Have the subject close his eyes, and distinguish the
odors of 1) oil of cloves, 2) oil of peppermint, 3) alcohol, and 4) tincture of camphor, when vials of these
substances are held close to his/her nose. Record your
observations on the data sheet.
Occlude one nostril and have the subject smell the
tincture of camphor until the odor can no longer be de4.2 Experimental Procedures
tected. Immediately have the subject attempt to distinguish, with the adapted nostril, between oil of cloves
1. Touch Reception
and oil of peppermint. Describe the results on the data
sheet. Repeat, using alcohol instead of camphor.
(a) Distribution of touch receptors: With a
marking pen draw a grid on your skin in
an area 1) on the palm of the hand, 2) on
the back of the hand, 3) on the back of the
4 Cutaneous Sensations
arm, and 4) on the back of the neck, to study
the number and distribution of touch receptors. Pick 100 or so squares (randomly) for
4.1 Introduction
careful and detailed study. Using an aesthesiometer, stimulate each small square with
a minimal stimulus, and determine whether
The purpose of this exercise is to study several cutayour partner is capable of perceiving a senneous receptors, their distribution, and some of their
sation of touch. Record the number of touch
physiological characteristics. You will need: an ink
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a sensation following removal of a stimulus from a sense receptor; see vision section, below) - i.e. the application of a stimulus to one point followed by the application of a stimulus to a point nearby will
frequently result in the individual responding that she/he can distinguish two separate stimuli simultaneously instead of successively. Record your data.
receptors found in each of these different areas. Take care to apply the same strength of
stimulus. The subject for this experiment
should not watch the operator. Likewise,
the operator should take care not to contact
the body of the subject when the stimulus is
being given. Contrast the number of touch
receptors found in other areas of the body
such as the lobe of the ear. If time permits,
test other areas that you may desire. What
relationship is there between touch sensation and hair follicles? In what area of the
brain is touch perceived?
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(b) Tactile Discrimination: In this exercise you
will measure a person’s ability to perceive,
as two distinct stimuli, the points of a compass applied simultaneously. If two points
are close together and fall in the same receptive field, only a single neuron reports the
sensation and you can’t tell that there are actually two points touching your skin. As the
compass is widened, at some distance each
of the two points is in the receptive field of a
different neuron and so the brain learns that
there are two points of sensation.
Temperature Reception
Using the same stamped areas as in observation i),
(touch reception, above) map the distribution of cold
and warm-sensitive areas. Temperature sensitive areas
can be located using a blunt glass rod that is warmed
or cooled. First keep the glass rod in a beaker of hot
water to prevent it from cooling (It does so quickly!).
Blot the rod dry with tissue paper before use. Repeat
the experiment with a cold rod, this time keeping it in
ice water, again blotting it dry with tissue paper before
use.
In order to determine the minimal distance
for a given region of skin, adjust the spread
of the compass until two separate points can
just be distinguished. Then compare this
compass spread to that found at a different
skin region, but make sure you include: 1)
tips of the fingers, 2) the back of the arm,
and 3) the back of the neck. Measure these
distances and record your results. If time
permits, use other areas of the body.
1. Intensity of Sensation: Survey areas on the ventral and dorsal surfaces of the forearm with a cold
glass rod. Try to determine if there is any difference in intensity in the sensation of cold in different areas.
2. Thermal Adaptation: Fill the beaker with hot water. Place the index finger in the hot water for
several minutes. Then place the other index finger in the same hot water. Compare the sensations
experienced.
Oh, and make sure that the two points are
applied simultaneously. Remember that the
stimulation of a touch receptor frequently
results in an after-image (the persistence of
Now set up 3 beakers, one with hot water, one
with ice water, one at room temperature. Place
one finger in the hot water, one finger in the cold
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water and leave for several minutes. Then place sound, vibrations in the bony portions of cochlea also
both fingers in the room temperature water and stimulate the hair cells. Thus, the vibrations of a tuning fork may be transmitted through the bony struccompare the temperature sensation.
ture of the skull to stimulate the hair cells and produce
the sensation of sound without involving the tympanic
membrane or the ossicles. It is possible, therefore, to
6 Hearing
distinguish between (1) loss of hearing due to abnormalities of the cochlea and neural mechanisms (nerve
deafness), and (2) loss of hearing due to abnormalities
6.1 Introduction
of the auditory canal, tympanic membrane, or ossicles
(conduction deafness). The vibrations of a tuning fork
Sound waves entering the ear are funneled through will not be perceived as sound either when held bethe external auditory meatus to the tympanic mem- side the ear or with the butt against the skull if nerve
brane. Sound, an alternate compression and rarefac- deafness is present. However, if the deafness is due to
tion of air, causes the tympanic membrane to move conduction difficulties, the vibrations through the skull
back and forth. This movement is transmitted through will be interpreted as sound.
the mechanical linkages of the ear ossicles to the oval
window of the cochlea to set the fluid in the cochlea
into motion. The oscillating motion of fluid in the
cochlea causes displacement of the basilar membrane, 6.2 Experimental Procedures
stretched between the walls of the cochlea, and of
the tectorial membrane, parallel to the basilar membrane. Because of different attachment points for the
two membranes, they have different relative motions. 6.2.1 Webber’s Conduction-Deafness Test
Hair cell receptors ride on the basilar membrane, but
their ”hairs” or cilia are influenced by motion of the
tectorial membrane. Thus, there is different relative Place the handle of a vibrating tuning fork on the masmotion for the hair cell and its cilia. This sheer force toid process (just back of the ear) of the subject. When
bends the cilia relative to the hair cell surface mem- the subject no longer hears the sound, move the vibratbrane, opening ion channels and causing a receptor ing tuning fork in front of the ear. Can the sound again
potential. A specific region of the basilar membrane be heard? If not, what type of deafness is indicated?
moves maximally in response to sound of a particular List the various structures which might be involved in
frequency. Thus pitch, which is the perceptual equiv- air conduction deafness. Would a hearing aid help this
alent of frequency, is coded by location of active hair type of deafness? Why?
cells along the length of the cochlea. Loudness, the
perceptual equivalent of intensity or pressure, is coded
by the magnitude of the receptor potentials.
If the individual is unable to hear the tuning fork
when held either in front of the ear or on the mastoid
Since the relative motion between the hair cells process, indicate the type of deafness from which he is
and the tectorial membrane produces the sensation of probably suffering.
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6.3
Localization of Sound
6.4.1
Lets say you are playing a note at 40 decibels. What
does this exactly mean? Since decibel is a relative measurement, this 40dB reading is 40dB above
(greater than) another tone or intensity. The most
widely used standard reference pressure is called
“sound pressure level”. This is not a very good
name! The reference pressure “sound pressure level”
(or SPL) has an absolute value, 20µPa. The SPL is
used by audiologists as a reference point for all other
measurements - it is the intensity 2 term in the above
decibel formula. Therefore, if you have a 40dB SPL
noise (40 dB greater than SPL), let’s figure out what
that noise is in absolute terms.
1. Start with initial formula: x = 20 · log II20
The subject should close his/her eyes, and attempt to
locate the position of a time clock held in different
vertical and horizontal positions but at a uniform distance from the ear. In which position is the localization
of sound most accurate? Repeat with one ear closed.
What difference, if any, do you observe? Familiarize
yourself with the different theories of the mechanism
of hearing. Where is sound perceived?
6.4
Adjusting the Reference Pressure
Audiometry
2. I0 = 20µPa, andx = 40dB
An audiometer sends out amplitude- and frequency
controlled sound waves to a pair of headphones. There
(I1 )
3.
Substituting:
40
=
20
·
log
20µPa , and solve for
are two dials on the audiometer. The left dial conI
...
trols frequency; rotating it will increase the pitch of 1
the noise. The right-hand dial is used to control “deci(I1 )
4.
Divide
both
sides
by
20:
2
=
log
bels”, the level of intensity. The decibel is actually a
20µPa
ratio of two intensities:
6. Eliminate the log term: 102 =
dB = 20 · log
intensity1
intensity2
(I1 )
20µPa
7. Solve for I1 : 100 ∗ 20µPa = I1 = 2, 000µPa
This shows us that a 40dB reading means your noise
Intensity 1 is what you might measure and intensity is 100x above sound pressure level (SPL). A rule of
2 is a standard or reference pressure (a constant).
thumb: for every 20dB, you are another 10x the SPL
(60dB would be 1000x SPL, or 20µPa).
Finally, and please pay attention to this, the audiometer we are using today is calibrated differently
for each frequency, and 0dB is not 0dB SPL for each
of the frequencies, as we would expect from a perfectly calibrated machine. This means that 0dB on the
In this part of the lab exercise, each person will find
the lowest decibel level you can hear (threshold) for a
number of frequencies. The class will make a graph
of this data showing average hearing threshold by frequency.
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Table 5: Correction Factors for Audiometer
threshold (in dB) for each frequency (in Hz). This will
give you a line graph representing at what intensity
you can hear a noise for each frequency. You will then
construct a spreadsheet with all of the class data and
derive an average threshold for each frequency. Using
the same method (constructing a line graph), you will
then make an audiogram for the class data. How does
your data differ from the average? Is there more variance at one end of the spectrum? If you notice threshold falling off at one or both ends of the frequency
range, what might explain this?
Hz
+dB
125 47.5
250 26.5
500 13.5
750
8.5
1000 7.5
1500 7.5
2000 11.0
3000 9.5
4000 10.5
6000 13.5
8000 13.0
At each frequency you are testing, look up the calibration factor and add those dB to your hearing. That
means if you are testing at 500Hz and the lowest tone
you can hear is at a dial setting of 20dB, write it down
as 20+13.5=33.5dB SPL. If you are at 750Hz with the
same threshold (20dB), it becomes 28.5dB SPL.
1. Put the headphones on. Turn the decibel level
(right knob) down to zero.
2. Set the tone to constant (or you can play around
by setting it to burst, etc.)
3. Starting at 20Hz, slowly raise the decibel level
until you can hear a tone. Slowly decrease the
intensity to verify that the noise disappears and
record the intensity (in dB) for the lowest value
(threshold) you can hear the tone.
audiometer does not equal 20µPa, as you would find
mathematically (try it for yourself!) In order to give us
a decibel value that is referenced to SPL (20µ Pa), use
the chart below as follows:
4. Write down your threshold value for that frequency in a table and add to it the proper + dB
offset from the table above.
If you want to find the absolute value (in µPa) of
your tone, first add the right decibel offset from the
chart above, then use that new dB in the mathematical
process outlined earlier, using SPL as your I0 value and
solving for I1 .
6.4.2
5. Set the intensity back to zero, increase the frequency (left knob) to the next specified value and
repeat step 3 (for the new frequency).
6. Once you have values for each frequency asked
for, put your data into a class data table. Construct an audiogram for yourself (see sample audiogram below) and compare it to the class audiogram, answering the questions above.
Constructing an Audiogram
An audiogram is a graph of frequency (in Hz, x-axis)
by the intensity (in dB, y-axis) required to just detect
the sound. You will be charting your minimum hearing
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7
7.1
Balance
Introduction
The sense of balance in mammals is mediated by inner ear hair cell organs: the saculus, utricle and semicircular canals. The saculus and utricle are otolithic
organs. That means that the hair cell stereocilia bundle is sheared when the hair cells move out from under
an inertial mass, the collection of otoliths. Otoliths
are small calcium carbonate crystals embedded in a
gelatinous cupula just above the hair bundle. When
you experience linear acceleration, for example when
you are in a car that starts to move forward, your hair
cells move along with your head, but due to inertia the
otoliths lag behind. This then creates a shear force, and
your brain understands you are accelerating forward.
Figure 1: Sample audiogram. Remember to add the
+ dB offset values appropriate to each frequency beThe semicircular canals contribute to your sense of
fore comparing your values to the above hearing loss
balance
by reporting angular, rather than linear accelranges.
eration. When you rotate your head, say from left to
right, fluid in your horizontal semi-circular canal tends
to stay at rest. As in the previous description, this
6.5 Pathology and Deafness
is due to the inertia of the fluid. The same principle
causes water to spill over the top of a cup when you
move the cup quickly. The container accelerates but
the water wants to stay put. It is then forced up the side
The above sample audiogram gives ranges for different of the container. Fluid cannot spill out of your semicirhuman hearing loss conditions. A hearing threshold cular canals, but your head can rotate out from under
above 70dB SPL in the human hearing range (20Hz- the lagging fluid. Again, this causes a shearing force
20kHz) indicates severe hearing loss, and above 90dB that moves the stereocilia bundles on hair cell organs
SPL is usually considered deaf. Often this deafness re- fixed to the inside wall of the canal. Note that the semisults from pressure-detecting hair cells in the cochlea circular canals are embedded in the “bony labyrinth”,
becoming damaged or never developing at all. Hearing a portion of your skull.
aids and cochlear implants can help this situation by
A semicircular canal is a toroid, or donut-shaped.
either amplifying the incoming sound pressure or electrically activating those hair cells, respectively, which The fluid can only move around in the plane of the
torus, so one canal can only report rotation in one
will then travel to the brain.
9
plane. To detect head rotation in all directions (left- 8.2 Experimental Procedures
right, up-down, and side-to-side) you need three semicircular canals, and that is just what you do have!
1. The blind spot. Evidence that a blind spot exists
As with most sensations, there are ways to trick your
in each eye is demonstrated by using the test chart
sense of balance because natural selection never had to
shown below. To test the right eye one closes the
invent a scheme to prevent an illusion that would not
left eye and stares at the plus sign with the right
normally occur in nature. For example, what would
eye as the page is moved from about 18 inches
happen if you stand at the edge of a large rotating plattoward the face. At first both the plus and dot
form that moves with constant angular velocity (no acare seen simultaneously, but at a certain distance
celeration) and close your eyes? Would you perceive
from the eye, the dot will disappear as it comes
that you are rotating?
into focus on the blind spot of the retina. Perform
this test on both of your eyes. To test the left eye
it is necessary to look at the dot instead of the
plus sign. Have your laboratory partner measure
7.2 Experimental Procedure
the distance from your eye to the test chart with a
ruler. Record the measurements.
7.2.1
Fast Pointing
The subject should whirl until dizzy. Upon stopping,
the subject should raise his/her arm to full height and
without bending his/her arm attempt to strike quickly
the outstretched finger of a partner with his/her own
forefinger. Repeat until successful contact has been
made. Which direction did the subject err in relation
to the direction of rotation? The success of this test
depends upon the rapidity with which it is conducted
as recovery progresses.
8
8.1
Vision
2. Visual Acuity is measured by determining one’s
ability to distinguish the detail of an object. The
technical way to measure acuity is to find how
many alternating light and dark bands or stripes
can be distinguished per degree of arc. Depending on the light and dark contrast difference you
can see many such bands per single degree. At
some point, though, the light and dark bands
blend together and you just see a gray image.
Clinically, a different measure is used. An eye
chart with letters on it is presented to a subject
standing 20 feet away. The line marked ”20” has
characters of a particular size, and if you can distinguish them at 20 feet you have ”20/20 vision”.
If you have instead, say, 20/50 vision then at 20
feet from the chart you can see what a 20/20 person can see at 50 feet! Of course you then could
not resolve the letters on the 20 line.
Introduction
The following exercises are simple tests that reveal
normal and abnormal characteristics of the eye.
10
(a) The teaching assistants will mount a Snellen
Visual Acuity Chart at eye level. Stand 20
feet away from the chart. Cover one eye and
read down, line by line, as far as the letters
can be clearly distinguished. Suppose that
the smallest letters that you can read are on
the line that an individual with a normal eye
can read at 40 feet. Your visual acuity is
then 20/40. The denominator expresses the
distance at which the letters on that line can
be distinguished by the normal eye. Normal
visual acuity is expressed as 20/20.
(b) Measure and record the visual acuity of
each member of the group without glasses,
and with glasses if they are worn customarily.
3. Accommodation. An individual with a normal
eye can see distant objects without effort - that
is, with the eye at rest. But if one wishes to see
near objects, muscular contraction is required to
distort the shape of the lens and bring the object
into focus on the retina. This response is known
as accommodation. The ability of the eye to accommodate has its limits however. The distance
to the nearest object that is focused on the retina
is called the near point of vision and the distance
to the point that the eye can see without accommodation is called the far point. For the normal
eye, the far point is infinity.
(a) Demonstrate the near point by proceeding
as follows: focus one eye on a pin, held upright at arm’s length. Gradually bring the
pin closer to the eye, continuing to focus on
it.
(b) Measure the distance to the point where the
pin can no longer be seen sharply. This is
the near point.
4. Eye Dominance
(a) While looking with both eyes at a small object 10 or more feet away from you, bring
11
the index finger into line with the object (as
in pointing it out). Hold the finger in this
position, and close the left eye. What is the
result?
(b) Now look at it with the left eye only (close
the right eye). What is the result? Most
right-handed persons are also right-eyed,
and many left-handed persons are also lefteyed. Try to confirm this.
5. Astigmatism. When the lens in an eye has different curvatures in different axes, astigmatism
results. Thus, when the lens has part of an object in focus in one axis, the object is blurred in
other axes. To determine the presence of astigmatism, one simply has to look at the center of the
diagram in the next figure and see if all radiating lines are in focus and have the same intensity
of blackness. If all lines are sharp and equally
black, no astigmatism exists. (The presence of
other refractive abnormalities can make this test
impractical.)
Look at the wheel-like chart (Figure 3) and note
whether the more vertical lines are sharply in focus than the horizontal ones, or vice-versa.
6. Myopia and Hypermetropia are the two most
common aberrations of vision. These conditions
may be due to defects of the cornea or lens or to
the shape of the eyeball.
Myopia, or nearsightedness, is a condition in
which the rays of light converge before striking
the retina, causing blurred vision. In this case,
the lens may be too convex or the eyeball is too
long. The condition can be corrected with concave (negative) lenses (lenses that are thicker at
the edges than in the middle). Such a lens moves
the plane of focus farther back.
Hypermetropia, or farsightedness, exists when
the light rays passing through the lens come to
Consider the following experiment where you fix
your gaze on a bright source, a light bulb. The
bulb is small enough (or you are sufficiently far
away) so that a central portion of each retina
gets the bright light bulb image but a surrounding region gets mostly the ambient room light. In
other words, some “central” photoreceptors are
very stimulated and other “surrounding” ones are
much less stimulated.
Figure 3: Astigmatism Chart
Are the lines sharp no matter which direction? If, for
example, your cornea curvature is correct vertically
but not correct horizontally, then the 12 o’clock - 6
o’clock lines will be sharp but the 9 o’clock - 6 o’clock
lines will be fuzzy.
focus at a point behind the retina. A too-short
eyeball, too-flat a lens, or corneal imperfection
may cause this condition. Correction is achieved
with a convex (positive) lens (lenses thicker in the
center than at the edges). Such a lens brings the
plane of focus further forward.
7. Afterimages are a visual illusion caused by the
relatively slow biochemistry of the phototransduction process in the retina. Light interacts with
rhodopsin in photoreceptor cells. Rhodopsin is
composed of a protein and a chromophore, a
chemical that can absorb light of certain wavelengths. When exposed to light, the bond structure of the rhodopsin molecule changes and the
protein and chromophore dissociate. Cells in the
retina enzymatically regenerate the rhodopsin,
but in strong light the rhodopsin can be used
faster than it can be regenerated.
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After fixing your gaze on the bright light bulb,
and then turning away for a brief period you will
still perceive an image of the bulb. This is because the photoreceptors that used to receive light
from the bulb are still “in high gear” and the biochemical reactions producing a high response are
still proceeding. This is a positive after-image.
After a short while you will now perceive the inverse effect, a dark bulb. This is because both
the “central” photoreceptors and the surrounding photoreceptors are now receiving exactly the
same light BUT, the central ones are fatigued and
report less signal than is actually present.
Procedure:
(a) Gaze at a light bulb for 10 seconds. Quickly
close your eyes and describe the image you
“see”. Now look at a white piece of paper
and observe the color of the perception.
(b) Fix your gaze for approximately one minute
on a small white square (1 cm 2) of paper on
a black background. Now look at just the
blank white portion of the paper. Describe
your perception. Is this a positive or negative after-image?
(c) Focus your eyes steadily on a colored object
for about one minute. Then look at a white
piece of paper and report the color you see.
Is this a positive or negative after-image?
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Discussion Questions
1. What is olfactory adaptation?
2. What is the adaptive value of the patellar reflex?
3. What is the difference between your ability to
adapt to thermal stimuli and your ability to adapt
to pain?
4. In the auditory system what is the difference between nerve deafness and conduction deafness.
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Table 1: Light Reflex Data
Table 2: Taste Modalities Data
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Table 3: Tongue Location Data
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Table 4: Olfactory Adaptation Data
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Figure 2: Blind Spot Tester
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