Experiment: How Fast Your Brain Reacts To Stimuli
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Experiment: How Fast Your Brain Reacts To Stimuli

How fast do you think you are? Do you know what a reflex and a reaction are? This lesson plan tells all about the quickness of your nervous system and the muscular system, which the nervous system innervates.

Time 1 - 1.5 Hours
Difficulty Intermediate

What will you learn?

In this experiment you are going to be introduced to what a reflex and reaction are and how we go about measuring them. Do not worry we won't be throwing soccer balls at your face. . . yet!

Prerequisite Labs



Note: Backyard Brains has released a digital reaction timer that uses your body's electrical signals to measure your reaction time! If you enjoy this experiment and want to take it to the next level, check out the Backyard Brains Reaction Timer!

The speed of your reactions play a large part in your everyday life. Fast reaction times can produce big rewards, for example, like saving a blistering soccer ball from entering the goal. Slow reaction times may come with consequences.

Reaction time is a measure of the quickness an organism responds to some sort of stimulus. You also have "reflexes" too. Reflexes and reactions, while seeming similar, are quite different. Reflexes are involuntary, used to protect the body, and are faster than a reaction. Reflexes are usually a negative feedback loop and act to help return the body to its normal functioning stability, or homeostasis. The classic example of a reflex is one you have seen at your doctor's office: the patellar reflex.

This reflex is called a stretch reflex and is initiated by tapping the tendon below the patella, or kneecap. It was first independently described in 1875 by two German neurologists, Wilhelm Heinrich Erb and Carl Friedrich Otto Westphal. In their original papers Erb referred to the reflex as the "Patellarsehnenreflex" while Westphal denoted it as the "Unterschenkelphanomen". Thankfully, we now refer to it as the patellar reflex.

This reflex is also known as a "reflex arc". It is a negative feedback circuit that is comprised of three main components:

  • A sensory component or afferent neuron. These neurons take in information and translate it to an electrical signal that gets sent to the central nervous system, much like the spikes you hear when doing the cockroach leg experiments.
  • Integrating center or interneuron. These neurons act as sensory processing centers that determine the magnitude of the response to the incoming stimulus. They are located in the central nervous system (your spinal cord).
  • The efferent portion or motor neuron takes the information from the interneuron and sends it to the effectors which activate a response. The effectors are usually muscle fibers as in the patellar reflex or a gland such as the salivary gland.
  • Y = the distance you measured in centimeters
  • g0 = the acceleration due to gravity constant (980 cm/sec2 )
  • t = time in seconds

    Here is an example of the equation being used:

    It may seem tedious to convert by hand each number you recorded so instead you will be provided with a quick chart to convert your centimeter measurement to seconds. However, there are several values missing in the table. You will need to fill them out to complete the table. Use the equation above to fill out the remainder of the chart. If you are savvy you can also design a computer program to do this.

    After using the chart and converting your centimeter measurements into seconds you will have your ruler reaction time in seconds. Looking at your data you might be thinking how you compare to the human average reaction time. Here it is! The average reaction time for humans is 0.25 seconds to a visual stimulus, 0.17 for an audio stimulus, and 0.15 seconds for a touch stimulus.

    Concise Handout for the Classroom

    This handout was designed by Virginia Johnson, a graduate student who adapted our experiment here to use as a teaching tool. This handout provides great instruction for the visual, tacticle, and audible experiments performed here condensed down into one page! Thanks Virginia for Sharing it with us!

    Science Fair Project Ideas

    • Why do you think touch and audio stimuli have a faster reaction time on average? How much faster is it? Do your results match the averages mentioned above?
    • Would you expect a difference in the average reaction times between a male and female? What about a more athletic person compared to a more sedentary person?
    • Why not test the "tactile" reaction time in the choice task? How could you redesign the experimental setup to test tactile reaction times in the choice task?
    • As you know, you have a dominant vs. a non-dominant hand. With only four trials, it is too hard to see a difference. Perhaps you should repeat the experiment 10-20 times and with right and left dominant people to see if there is any difference between dominant and non-dominant hands.
    • The average conduction velocity speed is approximately 20-80 m/s. It takes approximately 1 ms for a neurotransmitter to cross the synapses.
    • Calculate the lower limit for your patella reflex vs. what you imagine for the patellar reflex of a giraffe. As a test, see if you can find a difference in reaction speed of people of differing heights.