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Chapter 4 - Sensation and Perception

Psychology in Action

Measuring Touch Sensitivity

From your study of the brain and nervous system, you know that there are receptor cells for each of our basic senses—vision, hearing, taste, smell, and touch. Of course, the receptor cells adapted for sensing light are located in the eye, those that detect motion of molecules are in the ear, and those that detect the presence of certain molecules (for example, of cinnamon or perfume) are in the nose or the taste buds on the tongue. But what about the touch receptors that are present in our skin? Have you ever wondered how these receptors are distributed within the body? Are they present in fairly uniform density all over the body, or are some areas more and less sensitive than others?

If you consider this problem, you can probably suggest some hypotheses about where the concentration of touch receptors is likely to be highest and lowest. Where are you most and least sensitive to touch? The list of body areas in the chart on page 74 will help you. You can state your hypotheses by filling in the following blanks:

HYPOTHESIS 1 I believe the highest concentration of touch receptors in the human body is (are) in the following area(s) of the human body:

HYPOTHESIS 2 I believe the lowest concentration of touch receptors in the human body is (are) in the following area(s) of the human body:

Now that you have stated your hypotheses, you are ready to conduct an experiment that will confirm or refute them.

How can you measure the density of the touch receptors at various locations of the body? One way to do so is to measure the greatest distance two stimuli can be spread apart but still be felt as a single stimulus. You might use two pencil points as stimuli and touch them to your skin. The farther apart the pencil points are, the easier it is to tell that there are two pencil points pressing against the skin. As you move the pencil points closer together, it gets increasingly difficult to tell that two pencils, not just one, are touching the skin. In any given region on the body, there will be a minimum distance below which you will be unable to tell if one or two points are touching you. If you measure this distance at various locations on your body, you have a basis for comparing the sensitivity of one location over another.

Experimental Procedure

To do this experiment, you will need two sharp pencils, a ruler, a blindfold, and a cooperative subject.

STEP 1 Ask the subject to sit in a comfortable chair in a quiet room and put on the blindfold. Explain that you will be touching various parts of his or her body with pencil points and that the subject's task is to tell if there are two pencils touching his or her skin, or just one. (You might find it more convenient if the subject is wearing shorts and a short-sleeved shirt.)

STEP 2 When the subject is comfortable, you may begin the measurement procedure. Beginning with the subject's wrist, place one pencil point on the skin. Use just enough pressure to enable the subject to feel the pencil easily, but without discomfort. Be careful to use the same amount of pressure every time you touch the pencil points to the skin. Ask, "Do you feel one or two pencil points touching your skin?" The subject should, of course, feel only one. If the subject should say "two," you should caution the subject to be very careful in making judgments and to say "two" only if he or she can clearly feel two separate points of stimulation.

STEP 3 Now put two pencil points on the subject's wrist, placing them about 2 inches apart. Ask the subject, "Do you feel one or two pencil points touching your skin?" Of course, the subject should respond "two."

STEP 4 Now you must determine the maximum distance the pencils can be apart and still be felt as a single stimulus. To do this, repeat steps 2 and 3, varying the distance at which you place the pencils in step 3. (It is important to use a single pencil sometimes in order to tell if subjects are being truthful in saying they feel two points. Obviously, they should not feel two points when only one is present.) Through repeated trials you will be able to find the greatest distance at which the two points can still be felt as a single stimulus. When you have identified this distance, measure it with the ruler and record it on the chart provided.

STEP 5 Continue this procedure for each of the body locations listed in the chart below. Notice that four blank spaces appear at the bottom of the chart. You may test some additional body locations and record them in these blanks.

Body Area Tested Distance at Which Two Stimuli Are Detected
1. Wrist
2. Back of the neck
3. Elbow
4. Fingertips
5. Back of the hand
6. Palm of the hand
7. Sole of the foot
8. Back of the leg
9. Top of the head
10. Kneecap
11. Thigh
12. Shoulder
13. Tip of the nose
14. Forehead
15. Earlobe
16. Lower lip

Analyzing the Results

Look at the distances you recorded on the chart. According to your results, which areas of the body are most sensitive to touch? Which areas are least sensitive? Can you think of any reasons why the areas that are most sensitive need to be highly responsive to touch sensations? Were your hypotheses confirmed or were they rejected? What have you learned about the distribution of touch receptors in various parts of the body?

If your findings are typical of those recorded by most subjects, you should have found than the most sensitive regions of the human body are the lips, the tongue (if you measured it), and the fingertips. Furthermore, the hands and feet are more sensitive to touch than are the upper limbs. Most students also find that their earlobes and thighs are quite insensitive to touch. Are your results consistent with these observations?

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