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Lightning Fast Reflexes: A Study

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Author: Janice VanCleave

A basketball rolls across the street in front of a moving car. The driver must make split-second decisions about what to do. The driver's reaction time, how long it takes for the person to respond, involves special sensory and motor nerve cells that send and receive messages to and from the brain.

In this project, you will compare reaction times to a visual stimulus and reaction times to an auditory stimulus. You will also explore whether other factors, such as distraction, gender, and age, affect reaction time.

Getting Started

Purpose: To determine your reaction time to a visual stimulus.

Materials

  • table and chair
  • helper
  • yardstick (meterstick)

Procedure

  1. Sit with your forearm on the surface of the table with your writing hand extending over the edge.
  2. Have your helper position the yardstick (meterstick) with the zero end between, but not touching, your thumb and fingers (see Figire30.1).
  3. Instruct your helper to release the stick without warning.
  4. As soon as the stick is released, try to catch it as quickly as possible between your thumb and fingers.
  5. Record the distance the stick falls and use the following equation to determine your reaction time. See Appendix 5 for a sample calculation.

  6. Record the distance and time calculated for the one trial.

    Effect of Stimuli on Reaction Times

Results

The reaction time varies with each individual.

Why?

Your eye is the stimulus receptor that sees the stick as it starts to fall. It detects the movement of the stick and initiates a signal, a nerve impulse, in the nerve cell to which it is attached. The message is then sent along sensory neurons (special nerve cells that transmit impulses from the stimulus receptor) to the spinal cord. The spinal cord telegraphs the message to the brain, where it is processed. A message is transmitted from the brain down the spinal cord to motor neurons, which cause the muscles in your hand to contract so that your fingers clamp around the falling stick. The time for these impulses to make the complete trip from the stimulus receptor to the motor neurons is called the reaction time.

Try New Approaches

  1. Does practice affect reaction time? Repeat the experiment 10 times. Calculate your reaction time for each trial and plot the data on a graph. Study the graph to determine whether the trend of repeated trials is toward faster or slower reaction times. Explain what accounts for the results.
  2. Does using your writing hand affect reaction time? Use your other hand and repeat the procedure in the original experiment 10 times. Graph the data and compare the results for both hands.
  3. Is reaction time affected if you are distracted? Have a second helper ask you simple math problems as you repeat the procedure in the original experiment 10 times.
  4. How does using an auditory stimulus receptor affect reaction time? Repeat the procedure in the original experiment, but this time close your eyes. Have your helper say "Go" when he or she releases the stick. Try to react to the auditory stimulus as quickly as possible by catching the stick. Calculate your reaction times for 10 trials and graph them (see Figure 30.2). The time delay between the helper's releasing his or her fingers and saying "Go" is very slight, but you might want to design an experiment that eliminates this delay. One idea would be to have the stick support a lever attached to a bell. When the stick is released the bell rings, notifying you that the stick is falling.
  5. Does age affect reaction time? Repeat the procedure in the original experiment using a test group of people in good health and of the same gender but of different ages. Test each person 10 times and average the results. Plot the averages on a graph.
  6. Does gender affect reaction time? Repeat the procedure in the previous experiment using an equal number of males and females, all of whom are in good health and of the same age. Test each person ten times and average the results. Plot the averages on a graph.
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