Biology 450 - Animal Physiology Lab
Fall 2007
Lab 4 – Sensory Physiology
This week’s lab exercises will cover various aspects of sensory physiology. You will
examine a vertebrate eye via dissection and investigate various sensory systems
and responses using human subjects. This will be a relatively easy lab day since
this is an exam week or Bio 449.
Be sure to answer any questions on the worksheet as you do each
exercise!
Dissection of the eye
The function of the eye relies on its physical structure as well as the functions of
its photoreceptors. In this exercise, you will dissect a cow’s eye in order to
examine this structure.
Procedure

Follow the procedure in the dissection guide provided.
Visual field
Rod and cone distribution
The highest density of cones in the retina is found at the fovea, with no rods
being found in this region. As one moves further away from the fovea, cones
become less dense and rods appear in greater numbers. The overall size of the
visual field, as well as the size of the field of for different types of
photoreceptors, can be determined through a procedure in which the subject
attempts to perceive white and colored disks using her peripheral vision.
Procedure

Locate the four colored disks (red, green, blue and white) and the
protractor with an attached string.

Have the subject sit and a directly face a well-defined object she can
focus her eyes on. With the hand opposite the eye being tested, have
subject hold the protractor in a horizontal position with its straight side
against her forehead. The 90° mark should be pointing straight ahead,
and the protractor should be held in exactly the same position in every
test.

Pull the string taut across the 180° mark (i.e., at a right angle to the
subject’s line of sight). Choose a colored disk at random and align it with
the end of the string. Slowly rotate the disk around the subject on the
end of the string. Have the subject tell you when a) she first perceives
the disk, and b) when she can first identify the disk’s color. Record the
angle in each case (it may be helpful to have a third person do this).
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
Repeat until all four colors have been used. You may wish to perform the
experiment on the other eye or on another subject to gain a second data
set.
Optic disk
The optic disk is the location on the retina behind which the optic nerve exits the
eye. Because this region lacks photoreceptors it cannot perceive images, but
higher-level visual processing normally hides this “hole”, or blind spot, in our
visual field. It is possible to demonstrate the existence of this region and
calculate where it lies relative to the fovea.
Procedure

Locate the blind spot test sheet. It shows a cross and a spot 10 cm apart.

Cover the left eye and hold the test sheet as arm’s length with the cross
in front of your right eye and the dot directly to the right of the cross.

With your eye focused on the cross, bring the sheet closer until it reaches
a point where the dot suddenly disappears. Confirm that this is your blind
spot by bringing the paper closer – the dot should reappear.

Move the paper back to your blind spot and have someone measure the
distance from your eye to the paper.

The distance between the fovea (where you focused the image of the
cross) and the optic disk (where the dot disappeared into your blind spot)
can be solved for given the distance from your eye (actually the lens) to
the paper, the distance between the dot and the cross on the paper, and
the distance from the lens to the retina using:
A a

B b
where
A = the distance between the dot and the cross on the paper (10 cm)
B = the distance from your eye and the paper
a = the distance between the fovea and the optic disk
b = the distance from the lens to the retina (assume 2 cm)
Color blindness
Several of the genes encoding photoreceptor proteins used by cone cells are
located on the X chromosome. These genes occasionally mutate, with the result
that the photoreceptors they produce may be either less sensitive than normal
or completely non-functional. These mutations tend to be recessive, but because
males carry only one X chromosome any mutation in these genes will manifest
itself phenotypically. Among Caucasians, about 1 in 10 X chromosomes carries
some type of photoreceptor mutation. As a result, about 1 in 10 men and 1 in
100 women will have deficiencies in their color vision, or the condition known as
“color-blindness”
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The most common type of color-blindness is red-green color blindness, in which
the person has difficulty distinguishing red from green. This can result from a
complete absence of functional red or green cones, or from reduced sensitivity
of either the red or green photoreceptors. The standard way to test for this
condition is to have the subject examine a series of "plates" containing colored
numbers set against colored backgrounds. People with normal color vision see
one series of numbers, while people with deficient color vision see no number at
all, or in some cases, a different number.
Procedure

Locate the Ishihara test for color blindness. Have each member of your
group examine each patch of dots and record any numbers perceived.

Compare your perceptions to those for normal and color-blind vision,
indicated on the back of the test. You can also get a sense of what the
world looks like to a color-blind person by looking at the set of altered
Ishihara test sheets.
Equilibrium - Functions of the semicircular canals
The semicircular canals provide the central nervous system with information
about rotational movements of the head. This information is used to determine
appropriate responses by various motor systems to compensate for movements
of the body as they occur. In these exercises, you will observe this
phenomenon, as well as the results of the semicircular canals sending faulty
information about the head’s current state of motion.
One use of the sensory information provided by the semicircular canals is to
keep the eyes fixed on an object despite movements of the head and body.
Under normal conditions, the nervous system causes the eyes to rotate
appropriately to compensate for any head movements. This compensatory
rotation of the eyes is called nystagmus, and can be observed both when the
semicircular canals send accurate and inaccurate information about head
movements.
Motor responses of other parts of the body also rely on information provided by
the semicircular canals. We will observe responses to faulty information from the
semicircular canals, manifested in the phenomena known as past-pointing and
vertigo.
Procedure
Nystagmus

Seat a member of your lab group in a revolving chair.

With her head upright and eyes open, slowly revolve the subject 360°.
During this first trial, the subject is free to turn her head. Notice how the
head turns to keep the eyes fixed on one spot (head nystagmus). Note
also the direction of head turning in relation to the direction of revolution

Proceed as above, but have the subject keep her head immobile. Notice
the eye movements (rotatory ocular nystagmus) – slow drift to keep the
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eyes fixed on one spot (slow phase) followed by a rapid shift back to
“center” (fast phase). Note the direction of each phase with relation to
the direction of revolution.

Have the subject close her eyes and revolve her rapidly 20 times. Stop
the subject and have her open her eyes. Observe the direction of slow
and fast phase nystagmus in relation to the direction of the revolutions
(postrotatory ocular nystagmus).
Past-pointing

Repeat the last procedure, but after stopping the subject have her keep
her eyes closed. Record the time. Now have the subject extend her arm
and point straight ahead with the index finger. Note the direction of
finger “drift” relative to the direction of revolution. Now have the subject
retract the arm and ask her to re-extend it in the same direction, again
observing drift. Repeat until no further drift occurs, and record the time
again.
Vertigo

Have the subject report the direction that she feels she is turning relative
to the direction of revolution.
Compensatory movements

Again revolve the subject quickly 20 times with her eyes closed, then
have her stand up and (if possible) attempt to walk in a straight line. Do
not let the subject fall! Note the direction of any turning or balancing
movements relative to the direction of revolution. Have the subject
describe her sensations while standing or walking.
Effects of the orientation of revolution
It is possible to change the effective orientation of the revolutions by having the
subject hold her head completely forward or tilted strongly to one side. This will
influence which semicircular canal is most strongly affected by the rotational
movements.
Procedure

Repeat a number of the above exercises with the subject’s head in
different orientations.
Tactile sensations - Cutaneous receptors
Tactile distribution
Receptors sensitive to touch or pressure are found in varying densities across
the body’s surface. Some areas, such as the fingers and lips, have many touch
receptors per cm2, while other areas, such as the back and legs, have a much
lower density of receptors. One way to measure the density of these receptors is
to determine the ability of a subject to perceive two point stimuli as distinct as
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the distance between the stimuli varies. The distance at which this occurs is
known as the two-point threshold.
Procedure

Have the subject sit with eyes closed.

Use a caliper to apply tactile stimuli to the subject’s skin. Start with the
points close together and slowly increase the distance between them.
Touch the subject with one or two points randomly to differentiate
between the true two-point threshold and the subject’s imagination.
When using two points, be sure to apply both points simultaneously.
Measure the distance (in mm) between stimuli required for the subject to
accurately differentiate between the two points.

Carry out this procedure on the following areas:
o
o
o
o
o
o
o
Back of the neck
Fingertip
Forearm (inner surface)
Tip of nose
Palm
Lips
Upper arm (if possible)
Tactile localization
As receptors become less dense, the field of perception for each sensory neuron
also becomes larger, and hence less precisely focused. It therefore becomes
more difficult to determine the exact location of a stimulus. This can be
demonstrated by asking a test subject to locate the exact point of a stimulus
given by another person. The difference between the actual stimulus and the
perceived location is the error of location.
Procedure

Have the subject sit with eyes closed.

Touch the skin with a pen, or with a pointed pencil so that a mark or
indentation is left. Have the subject try to touch the same point with
another pencil, and measure the error of location (in mm). Repeat the
procedure using the same stimulation point, and note whether localization
ability improves.

Carry out this procedure on the following areas:
o
o
o
o
Palm
Fingertip
Forearm (inner surface)
Lips
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Adaptation of sensory neurons
Sensory receptors often show “adaptation” to a continuous stimulus; that is, the
response to the stimulus changes with time. One possible result is that the
stimulus is no longer perceived by the central nervous system. Alternatively, a
continuing level of simulation may become the new “reference value,” with
future changes in intensity being perceived relative to that value. We will
observe the former phenomenon using cutaneous pressure receptors and hairfollicle mechanoreceptors, and the latter using cutaneous thermoreceptors and
the eye’s photoreceptors.
Mechanoreceptors
Procedure

Have the subject sit with eyes closed.

Place a coin on the inside of the subject’s forearm. Determine how long
the sensation of pressure produced by the coin persists by having the
subject announce when she can no longer perceive the coin’s presence.

Repeat as above in a new location, but after sensory adaptation takes
place, add two more coins on top of the first one. Note whether the
pressure sensation returns, and if so for how long.
Another test of mechanoreceptor adaptation:

Have the subject sit with eyes closed.

Choose a hair on the subject’s hand or arm and, using a pin or pen or
pencil point, slowly fold the hair backwards (from its natural orientation),
then let it spring away again. Note when the subject’s sensation of the
hair’s movement is greatest.
Photoreceptors
Photoreceptors become adapted to ambient light levels within about 30
seconds. In bright light, this means that photoreceptors are no longer very
sensitive to lower light levels. An important point is that photoreceptors
adapt individually. Thus particular rods and cones exposed to intense light
become less sensitive to dim light, while other photoreceptors continuously
exposed to low light levels remain more sensitive. In the case of cones, the
color of a bright light can also influence which cones become light adapted
and which do not. A bright red light, for example, will not stimulate bluesensitive cones.
Procedure

Stare at a bright but confined light source (such as a flame, light bulb or
distant window) for about 30 seconds, then shift your gaze quickly to a
white surface.

You should be able to perceive the effects of adaptation by the
photoreceptors exposed to the bright light. Blinking a number of times
may help enhance the effect.
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
Now stare at the strangely colored American flag graphic under good light
for 30 seconds (you can use the red dot as a focal point), then shift your
gaze to a white surface.

Again, try to perceive the effects of adaptation by the different cone types
in your retina.
Weber’s law
Weber’s law states that the change in a stimulus that will be just noticeable is a
constant ratio of the original stimulus. For example, if an initial pressure of 50
mmHg is applied to the skin and an increase of 10 mmHg (to 60 mmHg total) is
required before the person perceives a change, then an initial pressure of 100
mmHg should require a 20 mmHg increase (to 120 mmHg) before the change is
perceived. The veracity of Weber’s law can be tested using weights of various
sizes supported in the upturned hand
Procedure

Have the subject place his hand, palm up, on the table and close his eyes.

Place a weighing boat containing a 10 g weight on the ends of the index
and middle fingers and have the subject note the resulting sensation.

Remove the boat and increase the total weight by 1 to 2 g and replace
the boat on the subject fingers. Repeat until the subject detects a weight
difference and record the final weight. Note – You may want to
occasionally replace the boat without changing the weight to keep your
subject honest.

Repeat the experiment using starting weight of 20, 40 and 80 g. (You
may want to increase the weight in larger increments as the total weight
increases.)

For each starting weight, calculate Weber’s fraction as W/W, where W is
the initial weight and W is the Just Noticeable Difference (the weight
increase required before a difference is perceived).
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