Laboratory Exercise in Sensory Physiology
Student Lab Manual
Introduction
Sensory organs allow us to perceive our environment by converting energy sources in the environment, like
light or sound, to nerve impulses which our brains can interpret. The process of converting environmental stimuli
into nerve impulses is called transduction. The world, which we perceive, is entirely dependent on the transduction
properties of our sense organs. These properties differ between animal species. The world to a human looks, tastes,
sounds, smells, etc. completely different from that perceived by another animal.
Each sensory organ responds to a particular kind of input, and only to a relatively small range of possible
stimuli within that modality. The visual system, for example, responds to light, which is a form of electromagnetic
radiation. We can only see a small portion of the possible wavelengths of this radiation, because our eyes can only
respond to certain wavelengths (see Table 1). Other insects, and birds, for example, can see ultraviolet light, which
is invisible to humans. Boas and pit vipers can detect infrared radiation with special, non-visual, organs. Within the
range of stimuli to which a sense organ responds, the organ will respond more readily to some stimuli than others.
In other words, sensory organs act as filters -- only allowing us to perceive those stimuli which evolution has
determined are critical for survival.
In this lab, you will use some of the methods employed by sensory physiologists to objectively quantify
human perceptual capabilities. You will see how our perception is influenced and limited by the properties of our
sense organs. You will examine three sensory modalities: (1) vision, (2) hearing, and (3) touch. In a number of the
following experiments, you will determine thresholds. A threshold is the amplitude of intensity of a stimulus at
which it is just barely detectable.
I. Vision
Vision involves the detection of light, often reflected off other objects. Light is a form of electromagnetic
radiation. It travels in waves. Different wavelengths of light produce the sensation of different colors. The range of
different wavelengths is called the spectrum (see Table 1). The visible spectrum is the familiar colors of the
rainbow. Light is detected by the eyes.
Table 1. The Electromagnetic Spectrum
Wavelength (nm)
700-1000
Description
Infrared (detectable by vipers and boas using special pit organs)
425-700
Visible spectrum (to humans)
Colors of the rainbow starting with violet at 425, ending with red at 700
300-425
Ultraviolet (visible to some birds and insects)
A schematic diagram of the human eye is shown in Figure 1. Incoming light passes through the pupil, and
is focused by the lens onto the retina. Light is actually detected by special receptor cells called rods and cones (see
Figure 2). Rods are responsible for vision under low light conditions. Cones function at higher light levels, and are
responsible for color vision (note that the first two letters of cones and color are the same). We will consider first
how rods function.
Each rod is filled with a pigment called rhodopsin, which absorbs light. When a molecule of rhodopsin
absorbs light, it breaks down into two smaller molecules. This reaction is coupled to a change in electrical potential
in the cell. This electrical potential is passed on to a cell, called a bipolar cell, which is connected directly to a cell
called a ganglion cell. Ganglion cells are nerve cells that make up the optic nerve. If the rod is sufficiently excited
(i.e. absorbs enough light), a nerve spike is caused in the ganglion cell. The greater the excitation of the rod (i.e. the
more light the pigment rhodopsin absorbs) the more nerve spikes are sent down the ganglion cell to the brain. The
ganglion cell carries the message to the brain that light has been detected and the more nerve spikes travel down the
ganglion cell, the brighter the perceived light.
Not all colors of the visible spectrum are equally good at exciting the rods. Rhodopsin is much better at
absorbing some wavelengths than others. As a result, a much smaller intensity of green light, for example, is
required to excite a rod, than of red light. Scientists have extracted rhodopsin from rods and measured its ability to
absorb different wavelengths of light. The results are shown in Figure 3. Since only rods function under dim light
conditions we can study how rods function affects our perception by analyzing our perception in dim light. As you
will observe later in the lab, under dim light conditions all the wavelengths appear to lack color. If a colored light
excites a rod to a certain degree, the brain cannot tell if the rod is being stimulated by a color to which the rod is
very sensitive, or if it is being stimulated by a very intense light of a wavelength to which it is less sensitive. A
graph of the minimum stimulus strength required to just be seen versus the wavelength of light is called a spectral
sensitivity curve. The fact that rods are differentially sensitive to different wavelengths of light does not, in itself,
allow for perception of colors. This is because there is only one type of rod. The response of rods changes in
exactly the same way if we increase brightness, or if we change to a wavelength to which the rod is more sensitive.
There is an ambiguity between color and brightness. Thus, a spectral sensitivity curve, which is the minimum
intensity required to elicit vision versus the wavelength of the stimulus, is not the same as measuring the ability to
distinguish different colors.
Cones are very similar to rods but there are some important differences. First, cones are much less
sensitive than rods, and function only under bright light conditions. Second, there are three different types of cones.
Each has a different type of pigment, with a different spectral absorption function (illustrated in Fig. 4). One cone
type has peak sensitivity to red, one type to blue, and one type to green. There are two main advantages to having
three types of cones. First it expands the overall range of wavelengths of light, which are visible. Second it allows
us to perceive colors. This is because any given wavelength stimulus excites each of the three different types of
cones to a different extent. The brain determines the color of the stimulation by comparing the ratio of the neural
output from each of the tree types of cones (cones, like rods, become excited and then excite a ganglion cell). An
increase in brightness will increase the neural output from all three cone types in equal amounts and the brain will
still see the same color, because the ratio of the outputs will not change.
Another interesting aspect of the visual system is that it remains sensitive over a remarkably wide range of
light intensities. In a typical day, the brightness from midday sun exceeds the brightness of evening starlight by a
factor of 100,000,000. At any one time, each photoreceptor can only respond to a 100-fold change in brightness.
Fortunately, large changes in environmental brightness usually occur relatively slowly, and the visual system can
shift so that the intensity range of the photoreceptors is centered about the average intensity of light in the
background. The visual system has several ways of adjusting to changes in ambient light intensity, two of which we
will discuss here. First, as already mentioned, there are two types of photoreceptors, rods and cones. Rods respond
only in dim light, cones only in bright light. Second, rods change their sensitivity with time, according to the
prevailing light conditions. This phenomenon is known as dark adaptation. If you go from a lighted area to a very
dark room, you will not be able to see. After about ten minutes, however, you begin to be able to see again. This
occurs because the rhodopsin pigment, which gets broken down when it absorbs light, is continually re-synthesized
in the rods. When there is very little light, very little rhodopsin gets broken down and the amount of rhodopsin
gradually builds up, making the rod much more sensitive to dim light.
Another way in which sensitivity is improved in the rods involves the way in which they are hooked up to
the ganglion cells. As illustrated in Figure 2, a typical cone connects to one bipolar cell, which connects to a single
ganglion cell. In contrast, a number of rods pool their inputs to a single ganglion cell. By adding their inputs, each
single rod needs to be excited by only a very small amount to produce a neural spike in the ganglion cell. The
phenomenon is known as receptor convergence. There is a trade-off with this method of improving sensitivity. A
ganglion cell, which receives input from a number of rods, spread out over the retina, cannot pinpoint the source of a
light stimulus as well as a ganglion cell, which is stimulated by a single photoreceptor. In the case of a ganglion cell
receiving the added input of several rods, light hitting any one of the rods can cause a neural spike in the ganglion
cell, but the brain has no way to know which rod was excited. It is therefore harder to pinpoint the location of light
stimuli on the retina when the stimulus is detected by rods (as compared to cones). The result is that our ability to
see detail, or our resolving power, is considerably lower under dim light conditions than under bright light
conditions.
II. Hearing
Sound consists of rapid changes in air pressure. These pressure changes travel through the air as waves.
These waves of pressure change travel through the air and into our ears. Figure 5 shows a simplified picture of the
human ear. The sound pressure waves enter the ear and cause the eardrum, or tympanum, to vibrate back and
forth. The vibrations of the eardrum are passed through a series of three inner ear bones to an organ called the
cochlea. The cochlea is a long thin organ, which, in humans, is coiled into a snail shell shape. Two membranes run
the length of the cochlea, with tiny hairs suspended between them. One of these membranes, the basilar
membrane, vibrates at the same rate as the eardrum, the other, the tectorial membrane, does not vibrate. This
causes a shearing force in the hairs. This shearing force is converted to electrical potential, and causes nerve spikes
to be sent, via the neurons of the auditory nerve, to the brain. The amplitude of the vibration in the cochlea varies
along its length, depending on the frequency of the stimulation. Low frequency vibrations stimulate the far end of
the cochlea, while high frequencies stimulate the near end. The brain determines the frequency of a sound by noting
how far down the cochlea the nerve cell, which is sending signals, is located.
The ear is not equally sensitive to all sound frequencies. A greater amplitude is required to hear some
sound frequencies than others, while some cannot be heard at all. In this lab, you will measure the relative
sensitivity of the human ear to sounds of different frequency.
III. Touch
The sense of touch, as well as hot and cold, is mediated by a variety of sense organs located in the skin.
Sense of pain and hot/cold is mediated by free nerve ending s near the surface of the skin. There are a large number
of receptors, which consist of nerve endings wrapped around the base of hairs, which protrude from the skin and
detect light touch. There are also pressure receptors called Pascinian corpuscles, which are located below the
surface of the skin. These resemble tiny onions in shape.
The ability to precisely locate a stimulus on the skin depends on the local density of these receptors. You
will examine this effect in the lab today.
Another phenomenon that can be readily observed with the sense of touch (although it occurs in all sensory
modalities) is the phenomenon of adaptation, in which sensitivity to a stimulus declines rapidly with exposure time.
This occurs in many sense organs. It allows the brain to direct attention to important stimuli and ignore those that
are constantly present. In general, changes in stimulation are more critical for attention that is ongoing stimuli.
Thus under constant stimulation, the receptor stops sending messages to the brain. This allows us, for example, to
wear clothing without having our brain being constantly aware of the contact with our skin. If, however, someone
touches us on the back, the touch receptors detect the change and send messages to the brain that the back is being
touched.
Quantifying Sensory Stimuli
There is a variety of ways to quantify sensory perception. The most common method is to develop a graph
of threshold (the magnitude of the stimulus which is barely detectable) versus stimulus quality (e.g. frequency). In
many cases, sensory systems respond over an extremely wide range of stimulus values. Often this extreme range is
achieved because the sensory systems respond only when a stimulus value increases by a multiple of its previous
value. In order to graph such results we make use of graphs with logarithmic axes. In a logarithmic graph, each
major division on the graph represents an increase by a factor of 10. Figure 6 illustrates how one reads a
logarithmic scale. In some cases, only one axis is logarithmic. Such a graph is called a semi-log plot. When both
axes are logarithmic, the graph is called a log-log plot.
The Experiments
A set of five different experiments is described below. Each group will spend 30 minutes on each
experiment (experiments 4 and 5 will be performed in the same time period). The data from experiments 1, 2, 3, and
4 should be recorded on the data tables at the back of the lab report, and later plotted on the graphs provided.
Experiment 1:
Spectral Sensitivity
In this experiment, you will measure the minimum stimulus intensity required for a light of a given wavelength to be
visible. You will test six different wavelengths and plot a spectral sensitivity curve. You will also observe the
effects of dark and light adaptation on spectral sensitivity.
1.
Turn on the illuminator to the low setting. Figure out how to change the intensity of the beam falling on
the screen by sliding the variable density filter back and forth. The number at the arrow indicates the
percent of maximum brightness passing through the filter. Figure out how to change the color filters.
2.
Choose one person in the group to be the subject.
3.
Have the subject face the screen. Move the variable filter to 99% transmission. Place one of the colored
filters in the holder in front of the illuminator. Now turn on the illuminator. Turn out the room lights.
4.
Gradually darken the image on the screen (by moving the variable filter) until the subject reports that
he/she cannot see the projected light on the screen.
5.
Turn on the room lights. Record the stimulus intensity at threshold (the value, in %, from the variable
filter) in Data Table 1.
6.
Return the variable filter position to 99% and choose a new filter. Repeat the steps above for all six filters.
Be sure to turn the room lights on between each test (this assures that the subject will remain light adapted).
7.
When you have recorded the threshold for each of the six filters, turn the room lights out and keep the room
totally dark for at least 15 minutes. Now repeat the entire experiment, keeping the room lights off. Use the
small flashlight covered with red cellophane to read the threshold values and change the filters.
8.
When you have time (later in the lab for example) plot the data for light and dark room on Graph 1.
Questions:
1.
Can you see a difference in the threshold levels for the dark adapted and light adapted states?
2.
Is there a difference in the wavelength of peak sensitivity for rod vision (dim light) versus cone vision
(bright light)?
3.
In World War II, fliers going out on night missions would receive their instructions in a room lit only with
red light. Based on your data, can you guess why this was done?
Experiment 2:
A. The Time Course of Dark Adaptation
When the eye remains in the dark for a period of time, the ability to see gradually improves as the quantity
of visual pigment builds up in the photoreceptors. In the following experiment you will plot the time course of this
change in sensitivity.
1.
Turn the room lights on. Choose a subject and have him/her face the screen. Turn the illuminator on and
figure out how to use the variable density filter to change the brightness of the image. Once the room
lights are turned off, only use the flashlight covered with red cellophane to record data. It is very important
that the subject see as little light as possible. For the experiment to work well, the subject should keep
his/her eyes closed except when the sensitivity is being tested.
2.
Move the variable filter to near 1 (i.e. make the image too dark to see). Start the stopwatch and turn out the
room lights. Adjust the brightness until the subject reports the spot of light is just visible. Turn the
brightness down until the subject reports it disappears. Record the position of the filter by reading the
number off the scale that the pointer points to.
3.
Repeat this measurement every 2 minutes for the next 16 minutes.
4.
Record the results in Data Table 2. Later, when time is available, plot the results on Graph 2.
Questions:
1.
Is your graph smooth all the way, or is there a break or sudden change in the shape of the plot?
2.
Can you explain the shape of your plot in terms of what you know about rods, cones, and changes in
pigment concentration?
B. Resolving Power and Ambient Illumination
Turn the room lights on. Stand at the tape and determine the smallest row of letters that you can clearly
read. Now turn the lights down (use the lamp with the dimmer switch to achieve relatively low illumination), and
observe what happens to your resolving power.
Questions:
1.
Examine Figure 2 and explain your results based on the different way rods and cones are configured in the
retina.
2.
What are the implications of these results for the relative safety of night versus daytime driving?
Experiment 3:
Audition
A. Observing Sound
A microphone is a device that converts sound (pressure waves) into electrical potentials, which can then be
observed on an oscilloscope. Observe the patterns on the oscilloscope as you talk, sing, clap, or whatever, into the
microphone. Try singing a pure tone. Change from high to low sounds. Try jangling keys into the microphone.
This will produce sounds that are too high for the human ear to detect. Many animals, such as bats, rodents, and
many insects, can hear these “ultrasounds”. Use the tuning fork to observe a pure tone. This waveform is called a
sine wave. Sounds that are not pure tones are produced by adding sine waves of different frequencies together.
B. Making an Auditory Tuning Curve
As you observed above, pure tones have the appearance of sine waves. High sounds have a high frequency
and low sounds a low frequency of repetition. The ear does not respond to all frequencies equally well. Here we
will measure the ability of one person to detect different frequency sounds. Since all sounds consist of added sine
waves, we can test the overall ability of a person to hear by stimulating with sine waves of many different
frequencies. A plot of threshold amplitude versus frequency is called a tuning curve. Tuning curves are used by
doctors to detect hearing malfunctions.
1.
Turn on the function generator. Make sure the “sine wave” button is pressed in. Note the frequency dial
and the “frequency multiplier” buttons. These two numbers are multiplied together to get the correct
frequency.
2
Choose a subject and have her/him put the earphones on. The subject should adjust the amplitude of the
signal by himself, to avoid dangerously loud stimulation. Choose a frequency and turn the amplitude knob
until the subject can barely hear the tone. Read the threshold amplitude from the digital voltmeter. The
amplitude is given in millivolts. Fill out Data Table 3 giving the amplitude at which each of the listed
frequencies is just barely audible. Plot the data on Graph 3. Note that the axes of the graph have scales
that increase exponentially. This is called a log scale. Such scales are used when the data to be plotted
covers a very large range. It allows all of the data to be presented in a single graph. Pay careful attention
to the pattern of the intervals on the scales of the graph.
3.
Have each person in the group listen to the sine wave generator at different frequencies. Change from a
sine wave to a triangle wave, and then to a square wave. These other waves sound similar to the sine wave,
but consist of many higher frequency sine waves added to the original. Can you hear the additional high
frequencies?
If time remains, complete experiment C below.
C. Localizing Sound
The ability to localize sound depends on the detection of small differences in arrival time at the two ears.
1.
Place the ends of the acoustic pipe over the ears of the subject, who keeps his/her eyes closed. Tap lightly
on the pipe at different locations, and ask the subject to report whether the sound comes from the right or
the left. See how close to the middle of the pipe you can tap and still have the person be able to detect the
side from which it is coming. Try bending the pipe to one side, and tapping directly behind (or in front) of
the subject. Which side does the sound appear to come from? Try placing the tuning fork on the pipe.
Which is easier to localize: a tapping sound, or an extended pure tone? Many animals have warning cries
that are fairly long and pure tone in quality. This makes them difficult to locate. This allows the animal to
warn its family members without giving away its location to a potential predator.
Experiment 4:
Tactile Sensation
A. Two-Point Perception
1.
Choose a subject and have her/him place their arm on the table with the palm face up and the skin exposed.
Touch the skin with two points close together. Ask the subject if they detect one or two distinct points.
Move the points apart until they can clearly detect two points.
2.
Starting with the tip of the middle finger, measure the “two-point distance” (i.e. the distance at which the
subject can just distinguish two pints) every few centimeters along the finger and hand, and then every
10cm up the arm. Record the results in Table 4 and plot them in Graph 4. Where are the touch receptors
at the greatest density?
3.
Try the two-point distance on other locations such as the lips, the scalp, and the face.
Question:
How does the minimum two-point distance relate to density of tactile receptors in the skin?
B. Adaptation of Hot and Cold Receptors
1.
Place one hand in ice water and the other in hot water.
2.
After a few minutes, move both hands to tepid water.
Question:
Relate what you experience to the concept of adaptation.
Experiment 5:
Color Vision
The ability to distinguish different wavelengths as colors, independent of their brightness depends on the
presence of three kinds of cones. However, if you choose three different colored lights, such that each stimulates
one set of cones, you can cause the output from the different sets of cones to occur in any possible ratio, and thereby
fool the brain into seeing nearly any possible color.
1.
Turn on each of the three projectors. Note that one shines pure red light, one pure green, and one pure
blue. Each of these colors primarily stimulates one set of cones. Point the projectors so that there is a zone
of overlap of the three colors.
2.
Play with the irises in front of the projectors and create as many different colors as you can. Pick one color,
and note the positions of each of the irises. Increase the iris on each projector by the same amount. What
happens?
Conclusion:
By the end of the lab, each group should have graphs 1, 2, 3, and 4 filled out for at least one subject. We will
discuss these results together at the end of the lab period.