How Your Skin Senses
There is a map of your body surface on your brain’s cortex.
But, some parts use more space than others. Sensitive parts like your face and fingers use more space in
your brain than less sensitive parts like your legs or back, they use less brain space.
You can test the sensitivity of body parts using two-point discrimination. This means, when two nearby
objects touch your skin, it can feel like just one object. The more sensitive the body part is, the easier it
is to feel both points!
By testing what you feel on different body parts, you can make a cortisol representation of the body,
called a cortical homunculus. In a cortical homunculus, more sensitive body parts are larger, and less
sensitive body parts are smaller. You might asking, “What is a homunculus?” Well, homunculus is, Latin
for "little man".
Doctors use a cortical representation of the body to determine the degree of sensory loss following
disease or trauma that affects the nervous system.
In this experiment, you will take sensitivity measurements and enter them into the web page to make
your own cortical homunculus!
Materials:


Data sheet
2 measured index cards
Take Measurements
Three roles are needed to perform the experiment: the test subject, the experimenter, and the
data collector. If needed, one person can be both the experimenter and the data collector.
The test subject allows the experimenter to gently touch different areas of their body with the
toothpicks and tells the experimenter whether two or one point was detected.
The experimenter uses the toothpick cards to test the two point discrimination of the test subject.
The data collector enters data collected by the experimenter.
EVERYONE WILL GET A CHANCE AT EACH ROLE.
Instructions for each role:
Test subject:

Sit down and close your eyes. When the experimenter touches you with toothpicks report
whether you feel one point or two. Don't open your eyes.
Data collector:
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
Fill in the homunculus data table as you go.
When the experimenter calls out a measurement for a body part, write the measurement
for that body part in the correct spot on the table.
Experimenter:




Test the head then the torso, arms, hands, and legs, shown in the pictures.
To test a body part, begin with the 60mm pair. Gently touch the tips of the tooth picks to
the test subject. The two tips should touch the skin at the same time and pressure on each
side should be equal. This is very important for the experiment.
Ask the subject whether they felt one point or two. If they feel one tell the data collector
"60 millimeters for the arm". If they feel two, test the body part using the 30mm pair.
Don't tell the subject if they are right or wrong.
Keep testing smaller and smaller pairs (15mm, 7.5mm, 3.75mm) until the subject reports
that they only feel one. Do not always start with the 60 mm side; you do not want the test
subject to know what measurement you are using.
Name:_________________________________
Section _____________
Title:_______________________________________
Purpose/Problem (what do we want find out)________________________________________________
_____________________________________________________________________________________
_____________________________________________________________________________________
Hypothesis: ___________________________________________________________________________
_____________________________________________________________________________________
Trial
Head
Torso
Arms
Hands
Legs
1
2
It is time to see what your Homunculus would like as a person! The Max Planck Institute has designed a
great website to do just that. Follow the instruction below, and you will see what your somatosensory
system looks like according to your nervous system.
You will be using YOUR results in this part of the experiment.
1. Go to the website http://www.maxplanckflorida.org/fitzpatricklab/homunculus/
and scroll down until you see pictures.
2. Choose the picture you want your Homunculus to look like and click on it.
3.
Click on the drop
down arrow and
select measurement
form your table
Continue to do
this for each body
part
For Feet enter 60mm
About your results
While individual results vary, most studies show that sensitivity is high for the hands and face.
Sensitivity is usually low for the torso and legs. This is because there is a higher density of touch
receptors in your fingers and your face. This also means that there are more neurons in your
cerebral cortex to process the information from your fingers and face.
If your results are non-typical, it could mean that you are unique! But, it may also be a result of
not testing enough times.
Scientists run the same test many times in order to make an accurate measurement. If you only
test once you can record the wrong value due to chance (maybe you pressed too hard or too
lightly). If you do the same test many times and take the average result you will get a more
accurate recording.
On a separate sheet of paper answer the following questions.
1. Since more sensitive body parts are usually smaller and less sensitive body parts are usually
larger; make a table to demonstrate which of your parts are more and less sensitive.
2. According to the data given, do your results match known scientific results? (If not, what might
be the reason?)
3. Why are some parts of the body more sensitive than others?
4. Where are the neurons located, that process the sensory information?
5. Why isn’t your skin’s sensitivity in proportion to the size of the body part? Is it reversed?
6. What would happen to the Homunculus if the skin sensitivity in your hands stopped working?
7. Was your hypothesis correct? (use data to prove your answer)
8. What could be done to improve this experiment?