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Tactile Discrimination: Exploring Receptive Fields

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Author: Peggy LaCerra, Ph.D.

Grade Level: Middle School; Type: Life Science Human Perception, Tactile Discrimination

Objective:

  • Learn about tactile discrimination.
  • Learn about receptive fields, and how and why their sizes vary.
  • Think about the adaptive utility of an important aspect of our physiology.

Research Questions:

  • What kind of tactile receptors are being activated in this two-point discrimination task?
  • Why would our species have evolved to have smaller receptive fields on some parts of the body than others?

What's more ticklish: your feet or your elbows? Different areas of our bodies are more sensitive than others. In this experiment, we'll discover how and why certain parts of our bodies are more sensitive than others, and we'll learn about the receptive field, the part of a sensory neuron that feels for us.

Materials: 

  • Box of toothpicks
  • Bag of corks
  • Ruler with metric markings
  • Blindfold or eye mask
  • Lined paper and pencil
  • Graph paper

Methods/Experimental Procedure:

  1. First, learn on your own about receptive fields, neurons, and tactile reporters, so that you have a basic framework in which to understand your results. Start with this Youtube video and Wikipedia's receptive field page.
  2. Create a series of two-point discrimination tool using two toothpicks pressed into one cork, placing the toothpicks the following distances apart using the metric ruler: 5 mm, 10 mm, 20 mm, 30 mm, 40 mm and 50 mm.
  3. Create your data record by taking a piece of paper and drawing two vertical lines down its length, dividing the paper into three columns. In the first column on the left write “Body Part” at the top; in the second column write “mm S-1”; and in the third column write “mm S-2 (note: ‘mm’ stands for ‘millimeters’, ‘S’ stands for ‘subject’). In the ‘Body Part’ column write one of the following body parts per line: Forehead, cheek, nose, palm, fingertip, forearm, and upper arm.
  4. Ask your subject to close his/her eyes, or put on a blindfold or eye mask. Starting with the 50 mm two-point discrimination tool, touch the apparatus to your subject’s forehead carefully and quickly, making sure that both tips touch the skin at the same time.
  5. Ask your subject if he or she felt one or two pressure points. If your subject reported two points, move to the 40 mm tool and re-test your subject.
  6. Record the minimal number of mms apart the toothpicks need to be in order for the subject to report that s/he feels only one point on his/her forehead. Then proceed to the next body part on the list and test in the same manner. [Note: If you are using two or more subjects, use fresh toothpicks for each one.]
  7. Graph Data: If you have used two subjects, average the data for each body part and use these averages to create a line graph; otherwise, just use the raw data from your single subject. Create X and Y axes on a piece of graph paper. Label the X axis "Body Parts" and the Y axis “mm distance between toothpicks." Graph your data points (the number of mm. required to perceive a single point at each body part you tested). The body parts should be labeled along the X-axis with the associated data point above it the height corresponding to the correct number of mm. Remember to label the graph itself.

Discussion:

Discuss your results in terms of receptor fields and the neural mechanisms that underlie them, as well as the adaptive utility of having receptive fields of a particular size.

Terms/Concepts: What is meant by the term ‘tactile discrimination’?; What is a receptive field?; What are the different types of tactile receptors in our skin?

References:

 

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