You have numerous specialized sensors in your skin known as mechanoreceptors. Some respond to pressure, touch (tactile sense), pain, hot, and cold. However, these receptors are at different densities in different locations on your body. The distance between two tactile stimuli at which the two are just discerned to be separate is known as the "simultaneous spatial threshold."
1. Simultaneous Spatial Threshold:
a. Unbend a paperclip and re-bend it so the two tips are close together. Measure the distance.
b. While your partner closes his or her eyes, place the two tips of the paperclip simultaneously on the back of his/her arm. Ask your partner if he or she feels two tips or one.
c. Change the distance between the two tips. Every now and then, touch them with just one tip to be sure they aren't just guessing "two" all the time.
d. By doing this procedure repeatedly, you should be able to get a good estimate of the simultaneous spatial threshold of the back of the arm. Measure this distance with a ruler.
e. Repeat using the back of the hand and the palm of the hand and compare results. Which of the three areas is the most sensitive? Which is the least?
What distances did you measure for each area? Why do you think that these distances vary so much? What parts of the body might have the smallest spatial threshold? What parts have the largest spatial threshold?