February 12, 2003



Somatosenses Laboratory Exercises

Two-Point Threshold Exercise

You will work in teams of two to conduct this exercise.

Materials

Blindfold

Paper clips

Index card

Ruler

One person will be blind folded and receive tactile stimulation on the hand and forearm.

The other person will administer the tactile stimulation in the form of one or two points at various distances apart (e.g., 5, 10, 15, 20 mm) on the hand and forearm.

The blindfolded person will be asked to report whether the stimulation feels like one or two points on the skin.

Record responses for each distance on at least three different locations on the skin (e.g., fingertip, palm, upper forearm).

Distance Location # Stimulation Points Reported # of Points

5 mm fingertip _________________ ________________

palm _________________ ________________

forearm _________________ ________________

10 mm fingertip _________________ ________________

palm _________________ ________________

forearm _________________ ________________

20 mm fingertip _________________ ________________

palm _________________ ________________

forearm _________________ ________________

Stereognosis Demonstration

For this exercise, you will be asked to identify objects by tactile manipulation.

First try to identify the objects by passive touch.

An object will be placed on the dorsal surface of your hand and then in the palm of your hand. Do not manipulate the object, but try to identify it.

Then try to identify the objects by active touch (manipulation).

Summarize what you learn from this exercise.

WARM WATER PAIN

HOW DOES THAT WORK?

By soaking your hand in the cold water, the pain receptors in your skin become active and overly sensitive. When this happens, sometimes things that wouldn’t normally cause pain, like warm water, can feel like they are painful even though they are not actually hurting you (this is called allodynia).

When most people place their hand into the warm water after the cold water, they feel two distinct painful sensations. They first feel a short-lived, stinging or sharp prickling pain. This sensation is normally followed, after a brief interval, by a longer lasting dull burning pain. This unusual sequence of sensations occurs because there are two different types of pain receptors working at different speeds. The first pain that you feel is caused by you’re fast A delta pain fibers becoming active and telling your brain that something is starting to hurt you and that you should do something to stop it. The second pain comes from activation of your slower C pain fibers telling your brain that you are still being hurt.

Demonstration protocol

What you need

Two tubs/buckets/containers large enough to fit a full size hand—not too big

A cooler full of ice (you might want to put some dry ice inside to keep it colder)

A hot plate that can be regulated

A water thermometer

A water reservoir (a sink or a large jug will do)

A glass beaker to heat the water on the hot plate

Paper towel

Procedure

1. Fill the two tubs up with water: one warm (37-42 deg C) and one ice cold (you will have to continually put ice in to keep it cold)

2. Keep additional water warming on your hotplate for transfers when your warm water gets too cool

a. To “reheat” your warm water, pour it back into the beaker of hot water, swirl it around, and then pour it back into the container

3. Change all the water occasionally and always monitor your temperature (you don’t want anyone to get burned)

4. Have subjects place one hand into the cold water for 10-15 seconds (ask them to count it out SLOWLY)

5. After the 15 sec, have them place that same hand into the warm water and ask them what the feel

a. First they will still feel the cold water because its acts as a shield to the warm water until it dissipates off

b. Then they may report nothing, but generally they will feel a prickling, tingling, or numb sensation that fades quickly

c. After a brief time, they should feel the water getting warmer and then hot. Make sure they keep their hand submerged for a period of time long enough to feel each sensation (approx 1-2 minutes). People differ on how long this takes.

6. After they say the water is hot, tell them to take that hand out of the water and put their unexposed hand into the warm water

a. They should report that even though the water felt hot with their other hand, now it only feels warm with their opposite hand.

b. Now have them take out their “fresh” hand and replace the sensitized hand back into the warm water…most people will exclaim that the water is now burning hot.

c. This back-and-forth discrimination procedure may have to be repeated a few times to get that effect. Note: On occasion, some people do not report the water as being hot until they do the back-and-forth discrimination procedure.

7. The demonstration is now over, but this effect will still be evident for at least 20-25 minutes. Encourage them to come back and try it later.

8. Give them a sheet of paper towel to dry their hands.

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