ScienceMakers Toolkit Manual - The History Makers

Life Science
Safety Precautions
The materials used in two-point discrimination are normal school materials but students should always be aware
that everyday materials must be used carefully for personal safety. Points should be discarded or disinfected
before using on another person.
PROCEDURE
Exploration
1. In a pre-lab situation, demonstrate to the students a method of two-point discrimination on the skin.
Gently touch two slightly pointed objects to the skin of your forearm. It is important to apply these at the
same time and to have the subject looking away from the test site.
2. Pick up the points, move them closer together and gently touch the skin of the same area again. The
objects should be moved closer together until the test subject can no longer distinguish two separate
points.
3. Measure the distance between the points. Have students make the measurements for one side of the
body and use as much area as is feasible for your classroom setting. For example, your class may map
just face, neck, arm and hand. In other settings students may volunteer to change into shorts or swim
suits and a more extensive map can be made. Suggest that students randomly test the reliability of
subject responses by touching with only one point.
4. The students will need to take the reciprocal of the two-point discrimination measurements before
making their drawings. The areas of the skin that have many touch receptors will have a small
discrimination value. To demonstrate that the skin area would be represented by a proportionally larger
area on the cortex, the reciprocal must be taken. The reciprocal can be calculated by dividing the
measurements into 1. If the two-point discrimination on the knee is 2.0 mm, the reciprocal would be 0.5.
5. After all calculations are made, the students should use the reciprocals as estimates to predict the
relative size of the body part representations and to draw the homunculus. The smaller the distance of
the two-point discrimination, the larger the reciprocal and thus the larger the body part on the
homunculus will be. Have the students use the large squares on the graph paper as a guide for the
approximate sizes of body parts. If the calculated reciprocals for the index fi nger and the thumb are 1.0
and 2.0, the thumb should be drawn twice as large as the index fi nger. Since each hemisphere of the
cortex receives information from only one side of the body, the student drawings should be in profi le.
Concept Introduction
1. When student drawings are complete, have the students compare their maps with those of students of
different sex or ethnic background. If the maps have been made with care, they should be very similar.
2. Display the homunculi of other mammalian species on the overhead projector or with individual
hand-outs and have the students discuss the differences among the species’ representations.
Representative mammalian homunculi can be found in Kandel, 1991
3. (see bibliography).
4. Introduce the anatomical and physiological terms for the nervous system which may be appropriate for
the level of students in your class. A brain model could be used by the students to locate the
somatosensory cortex.
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