Introduction to Psychology
Psychology S-1, 2005
LAB PROJECT – MEASURING SKIN SENSITIVITY
Background to the project
As a biological psychologist, you have been asked to determine the sensitivity to
pressure, called tactile sensitivity, of different areas of the body. With closed eyes, how well can
a subject tell whether he or she is being touched by a one-pointed or a two-pointed instrument?
How close together can you bring the two points before the subject can no longer tell whether the
instrument has one or two points? Your goal is to discover the “threshold” of discrimination,
meaning the smallest perceptible distance between two points.
You will use a crude set of calipers that you make from a bent paper clip, with the two
points initially separated by 6 cm. You will conduct 7 tests, or “trials”, of the subject’s ability to
discriminate between one point and two with closed eyes. Each time you gently touch the
subject with both points, you will ask the subject whether he or she thinks there was one point or
two. You will then increase the separation on the next two-point trial by 1 cm. if the subject
answered incorrectly and will decrease the separation if the subject answered correctly. For
example, if you touch your subject with two points separated by 6 cm. and the subject incorrectly
responds that there was only one point, you will increase the separation to 7 cm. to see whether
the subject is better at telling the difference when the separation is greater. On the other hand, if
your subject correctly responded that there were two points, then you will try reducing the
separation to 5 cm. to see whether the subject is still successful when the separation is smaller.
After your seven trials, you will have found the threshold. To see whether the threshold varies
on parts of the body, you will determine the threshold for three different body parts: the palm of
the hand; the back of the wrist; and the tip of the index finger.
In order to keep your subject from just guessing two points all the time, you will
complicate your procedure slightly by including three one-point trials in an unpredictable order
among each set of seven two-point trials. Thus there will be ten trials in all for each threshold
you determine. You will ignore the answers to the one-point trials, though, when figuring the
distance to use on the next two-point trial. For example, if the last successful discrimination in a
two-point trial was at 4 cm. of separation, the next two-point trial will be at 3 cm. of separation,
whether or not a one-point trial comes first and whether or not the subject answers a one-point
trial correctly. The schedule of one-point and two-point trials for testing each threshold is shown
on the separate data sheet. To complete all 30 trials should take only a few minutes.
Procedure
Before beginning with your subject, you will need a paper clip and a ruler. First
straighten the paper clip completely, and then bend it in half like a V, so that the two arms of the
V are approximately equal in length. Effort spent making the paper clip into a good pair of
calipers will be worthwhile. Use the ruler to set the ends of the V apart by exactly 6 cm. for the
first trial.
Show your subject the calipers and ask him or her to rest the writing hand on a table or
other surface. Then say to the subject: “Please close your eyes. We will start with the palm. I
will touch you lightly ten times. In each case, tell me whether the touch consisted of one point or
two points.” Then touch the subject’s palm on each trial with either both ends of the calipers or
just one end, following the schedule on the data sheet (0 cm. of separation = 1 point trial). The
first trial is always a two-point trial with a 6 cm. separation between the two points. Be sure to
make your hand movements the same when preparing and conducting each trial, so that the
subject can’t sense from extraneous information whether you were using one point or two. Also,
be sure to use equal pressure on all trials; for the two-point trials, be sure that you exert equal
pressure on both points.
For each trial, write down on the data sheet the amount of separation and whether the
subject was correct (C) or incorrect (I) before proceeding to the next trial. For two-point trials,
start with 6 cm. of separation. If your subject answers a two-point trial correctly, use the ruler to
reduce the separation by 1 cm. for the next two-point trial; if your subject answers incorrectly,
increase the distance by 1 cm. If there is a one-point trial before the next two-point trial, pretend
to make an adjustment both before and after it, so your subject won’t know it was one point
because something was different. However, don’t take the presence of an intervening one-point
trial into account when determining the amount of separation for the next two point trial, and
don’t take the accuracy of an intervening one-point trial into account in that process either.
Repeat the whole process, with an appropriate change of instruction to the subject, for the
back of the wrist and then for the front tip of the index finger.
Results
For each body part, you will have 7 data points, namely the two-point trials. One point
trials are not important except as they distract your subject and offer a rough measure of your
subject’s overall reliability.
Indicate at the bottom of the data sheet the threshold for each data part (the smallest
distance successfully discriminated). In addition, indicate the percentage of one-point trials that
the subject answered correctly.
Assignment Questions:
1. What is the independent variable (IV) in this experiment? Dependent variable (DV)?
2. Briefly describe your findings. What do they suggest about which body part is most sensitive
to mild pressure and which is least sensitive? Note any unexpected findings and explain.
3. What seems to be the relationship between tactile sensitivity and mobility of body parts?
Use your findings and your observations about general life in your response.
4. What would you hypothesize might be the relationship between tactile sensitivity of a body
part and size of the portion of the brain devoted to that body part?