TWO-POINT DISCRIMINATION
NAME
Human skin contains a variety of receptors for sensation, each triggered by a
specific stimulus including touch, light pressure, deep pressure, pain, hot and cold (see
attached diagram). These receptors are not distributed evenly at different areas of the
body. The distribution of touch receptors can be assessed by a simple experiment.
MATERIALS
per pair
Compass with a pencil
Toothpicks
Tape
Ruler
Two-person teams
TO DO AND NOTICE
Tape toothpicks to the compass point and the pencil so that they protrude at least
2 cm from the ends of the compass point and pencil. (We are using toothpicks instead of
compass points/pencil points to avoid injury). One person will be the subject, the
other the experimenter. Roles will be reversed at the end of the activity.
The experimenter sets the distance between the toothpicks. Do not reveal this distance
to the subject.
1) The subject closes their eyes. The experimenter will gently but firmly touch the
toothpick tips to the subject's skin, being careful that both toothpick points touch the
skin simultaneously. Try this test on as many areas of the body as you can.
2) The subject will say whether they feel one distinct point of contact or two.
3) Record your results in a data table.
4) Alter the distance separating the compass points and repeat. You may wish to alter
the distance until you find the difference that you can discriminate one from two points
at various body parts. Which part of your body can two points be detected with the smallest
tip separation? This is the most sensitive part of your body.
This activity can be repeated on multiple body surfaces to map the differences in
distribution of sense receptors across various body surfaces.
WHAT'S GOING ON?
The sense of touch is triggered by the stimulation of specific touch receptors. To
discriminate two distinct points of touch, at least one touch receptor must remain
unstimulated between two adjacent stimulated receptors. For example, if there are three
touch receptors in a row and all three are being stimulated, the touch would be
perceived as only one point. If the middle receptor was not stimulated and the lateral
two were stimulated, the two lateral receptors would send two distinct signals and the
sensation of touch would be perceived as two distinct spots being touched. Since touch
receptors are distributed sparsely on some body areas, such as backs, knees and elbows,
and densely on other areas such as the palms of the hands and lips, the distance needed
to discriminate between two tiny point sources of touch may be large on some body
areas and small on other parts.
SO WHAT?
This test is one actually used by neurologists in assessment of nervous system function.
Receptors in the skin. All of the tactile receptors are actually modified dendrites.
Palm surface of the fingertips: 60 pain, 100 touch receptors
Back of finger: 100 pain, 9 touch receptors
TWO POINT DISCRIMINATION DISCOVERIES
Create a data table showing the areas of the body that were tested and the distances
and the amount of points the test subject was able to detect at those distances.
1. Which area(s) were most sensitive? What does this say about the number of receptors in this/these
area(s)?
2. Which area(s) were least sensitive? What does this say about the number of receptors in this/these
area(s)?
3. Why do you think that our bodies have different densities of receptors in different areas? How does
this help humans survive/interact with their environment?
Below, you need to draw on the human body outlines to show how sensitive the areas are to the sense
of touch. Create a key below the outlines showing the amount of distance needed to feel two pressure
points. The key should have a color or pattern next to each distance (ex. 2mm
).
Front
Back
Site
Threshold Distance
Fingers
2-3 mm
Upper Lip
5 mm
Cheek
6 mm
Nose
7 mm
Palm
10 mm
Forehead
15 mm
Foot
20 mm
Belly
30 mm
Forearm
35 mm
Upper arm
39 mm
Back
39 mm
Shoulder
41 mm
Thigh
42 mm
Calf
45 mm