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The Smallest Touch: Exploring Thresholds of Sensitivity

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Author: Peggy LaCerra, Ph.D.

Grade Level: Accessible enough for 6th graders, but options provided can be challenging enough for 12th graders;  Type: Life Science (Human Perception, Tactile Sensitivity)

Objective:

  • Discover the smallest, lightest touch we can detect.
  • Learn about tactile receptors, tactile discrimination, and receptive fields
  • Optional: To learn about the peripheral and central neural mechanisms that underlie receptive fields.

Research Questions:

  • What is a "threshold of sensitivity"?
  • What is a "tactile receptor"?
  • What is a "receptive field"?
  • Why do some parts of our body have a lower threshold of sensitivity than others?
  • Optional: What are the "receptor mechanisms" that enable us to sense tactile stimuli? What is is the neural mechanism underlying a "receptive field"?

Our “tactile detection threshold” is the smallest amount of touch that we can detect. This experiment will use blindfolded subjects to test the outer and inner surface of their lower arm with increasingly thick pieces of fishing line – homemade ‘Von Frey Hairs,’ named after the researcher who first devised this testing technique.

Older, more advanced and/or more curious students are encouraged to test other parts of the body such as the cheek, compare the results with those obtained by testing the arm and discuss their results in terms of the concept of "receptive fields" and the neural mechanisms that underlie them.

Materials:

  • Popsicle sticks
  • Glue
  • Monofilament fishing line of various thicknesses
  • Blindfold or eye mask for sleeping
  • Paper and pencil

Experimental Procedure:

  1. Cut piece fishing line about one and a half inches long from each of the lines of different thicknesses.
  2. Glue each piece of line onto the end of a Popsicle stick. The stick will serve as the holder.
  3. Prepare a sheet or sheets of paper for your records using one line for each subject. Make a bold line down the center of the paper and label one side outer arm and the other inner arm. Divide each of these wide columns into columns for each thickness of fishing line you will be using and label them accordingly.
  4. Test each subject individually. Ask the subject to place the blindfold over their eyes. Then using any one of the homemade Von Frey hairs, beginning to touch the skin of either the outer or inner arm just until the hair bends. Ask the person if they felt anything and record the results.
  5. Continue to test each subject until you have recorded their data for each Von Frey hair on both the inner and outer surface of their arm.
  6. Calculate the average fishing line thickness first detected by subjects on the outside of their arms and the average first detected by them on the inside of their arms. Graph the inner arm and outer arm averages using a bar graph (be sure to label both the bars and the graph itself).
  7. Discuss your results in terms of thresholds of sensitivity and receptive fields, going into as much depth about the neural mechanisms as you find accessible to you.

Terms/Concepts: Tactile perception; Tactile discrimination; Tactile receptors (for pressure); Threshold of sensitivity; Von Frey hairs; Receptive fields

References:

  • Sensation and Perception, by Mike May (Chelsea House Publications, 2007)

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