Test Your Sense of Touch

Is the Glass of Water Hot or Cold?

With this experiment, test your skin's ability to perceive whether an object is hot or cold.

Materials:

• Three tall glasses of water, one filled with very warm or hot water (not burning), one filled with room-temperature water, and one filled with ice water
• A clock to time yourself

Procedure:

1. Grab the glass of hot water with one hand, making sure that your palm is touching the glass. Grab the glass of ice water with your other hand, holding the glass in a similar fashion.
2. Hold the glasses for at least 60 seconds.
3. After holding the hot and cold glasses for 60 seconds, grab the room-temperature glass with both hands, palms touching the glass.
4. Does the glass of room-temperature water feel hot or cold?

What's Happening?

Your brain just received confusing messages from your hands about what the temperature of the third glass was. The hand originally holding the hot glass told you the third glass was cold, whereas the hand originally holding the cold glass told you the third glass was hot. But they were both touching the same glass. How can this be?

You received these confusing messages because our skin does not perceive the exact temperature of an object. Instead, your skin can sense the difference in temperature of a new object in comparison to the temperature of an object the skin was already used to ("relative temperature"). This is why entering a body of water, such as a pool or lake, seems really cold at first (your body was used to the warmer air) but then gradually "warms up" after being in the water for a while (your body adjusts to the temperature of the water).

Two-Point Discrimination

Is your skin equally sensitive all over your body? Try this experiment to find out more about how well your skin perceives touch.

Materials:

• Ruler that measures in millimeters
• Two toothpicks
• Partner
• Blindfold (optional)

Procedure:

1. Prepare for this activity by setting up a chart like the one listed below. You may need to go beyond 10 mm in this activity, and you may want to test more areas of the body than what is listed below. Some suggestions are: back of finger, back of hand, wrist, neck, stomach, top of foot, sole of foot, calf, thigh, forehead, nose, lip, and ear.

	1 mm	2 mm	3 mm	4 mm	5 mm	10 mm
Tip of Finger
Palm of Hand
Upper Arm
Back
Cheek

2. Explain to your partner that you are going to lightly poke her with either one or two toothpicks on various places on her skin. Her job is to tell you whether or not she feels one poke or two pokes. To make sure she is not cheating, she needs to either wear a blindfold or keep her eyes closed.
3. Without telling your partner this, hold the two toothpicks so that the points measure 1 mm apart and lightly poke her on the palm of her hand. Ask her if she felt one or two points on her skin. If she says one point, separate the two points of the toothpicks so that they measure 2 mm apart and lightly poke her in the palm again. Keep pulling the points apart until she says that she feels two points. Record the measurement at which she felt points on the palm of her hand.
4. Repeat step 3 with other parts of the body, such as the fingertips, the upper arm, the back, the stomach, the face, the legs, and feet. Make sure to record the smallest distance at which each area of the body felt two distinct points when poked with the toothpicks.

What's Happening?

The ability to distinguish between one point or two points of sensation depends on how dense mechanoreceptors are in the area of the skin being touched. You most likely found that certain areas of your body are much more sensitive to touch than other areas. Highly sensitive areas such as the fingertips and tongue can have as many as 100 pressure receptors in one cubic centimeter. Less sensitive areas, such as your back, can have as few as 10 pressure receptors in one cubic centimeter. Because of this, areas such as your back are much less responsive to touch and can gather less information about what is touching it than your fingertips can.

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