Activity 2: Sensory Receptors and Illusions

Many of the illusions we will focus on are optical illusions, meaning that they use the sense of sight. In this activity, we will explore just how the information goes from our external environment to our perceiving brain.

Activities and videos that introduce the sensory receptors, with a focus on the eye and optical illusions.

Activity 1.1 Sensory Receptors Bonus pdf

Click here for a pdf with more information about sensory receptors and an activity that will help you remember them.

Activity 1.2 Do Your Gustatory Receptors Need Help?

Supplies: Dry odorless foods such as sugar, salt, and plain crackers. Try to find something for each of your taste buds (sweet, salty, sour, and bitter). You might be able to find sour and bitter in a candy, but it needs to be a hard dry candy. You will also need paper towels and a glass of water.



  1. Dry the top of your tongue with a clean paper towel and keep your tongue out of your mouth.
  2. Place a small amount of one of the foods on your tongue.
  3. Keep your tongue out of your mouth and try to “taste” the flavor.
  4. Rinse your mouth out with the water and then dry your tongue again.
  5. Try the same procedure with a different food.


  1. Place a small amount of one of the foods on your tongue, keeping your tongue in your mouth.
  2. Close your mouth and try to identify the flavor.
  3. Rinse your mouth out with the water and then try to taste another flavor.

What happened? You probably could not taste the flavors as much when your tongue was kept dry. In order for your taste buds to work food must be dissolved by your saliva.

Extension: Most people’s taste buds are grouped together by flavor. You can map out your taste buds by using the following experiment.

Supplies: Solutions for each type of taste buds. Salty (dissolve salt in water), sweet (dissolve sugar in water), sour (lemon juice), and bitter (tonic water or unsweetened baking cocoa mixed in water). There is one more taste most scientists now believe we have and it is called umami (the taste of glutamate, so if by chance you have some MSG you can try this one as well). A simple drawing of your tongue, something like the one below, and 4-5 Q-tips.

Instructions: Using a Q-tip, rub the solution onto your tongue. Maker everywhere you can “taste” the flavor. Rinse your mouth out with some water and repeat with a new Q-tip and a different flavor.

Activity 1.3 Thermoreceptors

Use this simple trick to confuse your thermoreceptors quite easily.

Supplies: Two bowls approximately the same size that are large enough to fit your hand into, You will also need some ice and a timer.


  1. Fill one bowl with cold water and add ice. Fill the second bowl with lukewarm water.
  2. Place your hand in the ice-cold water for one minute.
  3. Place your hand in the lukewarm water.

Question: Is the lukewarm water hotter than it used to be? You have both hot and cold thermoreceptors. When your hand was in the cold water it was being overstimulated and some of your cold thermoreceptors started to temporarily stop working. The sensation of the warm water requires both the hot and cold thermoreceptors but your cold thermoreceptors are taking a little holiday so your brain perceives the water to be hotter than it actually is!

Activity 1.4 Olfactory Receptors:

Your olfactory receptors have more to do with what you like the taste of and your memory of how things taste than your gustatory receptors. Here is a quick illustration of the role they play in enjoying your food.

Supplies: All you need is a little snack. As teachers, we have to recommend a nice slice of fruit like an apple, but a cookie will also work.


  1. Take the first bite of your snack and eat it like you normally would.
  2. Take a second bite time as you are chewing your snack, focus on your breathing. Take a deep breath in through your nose and then exhale out your nose. Does the flavor of the snake change in intensity when you inhale and when you exhale?

What’s happening? When you are breathing out your olfactory receptors are getting a lot of information about the “smell” of the food, but when you are breathing in they are taking in less information about what is in your mouth and more about your surroundings, which are not as delicious. Enjoy the rest of your snack!

Activity 1.5 Photoreceptors and Your Nose.

Did you know that your nose is constantly in the way of your photoreceptors? Think about it, it is a large thing sticking out of your face, practically right in front of your eyes!

Take a good look at your nose. Point your face straight ahead of you and then look as far to the right as you possibly can and then close our right eye. You should be able to see your nose. It’s probably bigger than you expected. Now you can make your nose disappear by opening the right eye!

You big, beautiful, amazing brain knows you’re not “looking” at your nose so it simply erases it from your field of vision!

Activity 2.1: How the Lens works (Magnifying Glass Demonstration)

Supplies: Tape, Sheet of white paper, Magnifying glass, and a light source (from my experience windows work the best).


  1. Tape the paper on a wall that is in front of the light source.

  2. Hold the magnifying glass three to seven inches from the paper and move it back and forth between the light source and the paper, until the image comes into focus.

Things to observe. The orientation and size of the image on the paper versus the real image. Are the foreground and background clearly focused on the reflected image?

Activity 2.2: Testing your Cones and Rods

Experiment 1:

Supplies: colored sheets of paper about the size of an index card (or paint sample cards) and someone to help you.


  1. Have someone stand three to four feet to your left side, while you look straight ahead.

  2. Have the person helping you slowly hold up one of the colored papers. It is important that they move slowly because your brain will be tempted to take a quick peek if they move quickly. This is an instinctual response to movement (you might not even know you peeked).

  3. Try and guess the color of the card.

  4. Have the person helping you move towards the center where you are focusing your vision.

  5. Repeat with a different color.

    Things to observe. Did the color change as the person moved towards your focus? Did the edges of the paper become more clear as the persons moved towards your focus?

    MODIFICATION: If this experiment doesn’t work for you (you might be very sensitive to movement) or you don’t have someone to help you. You can look at your colored papers in a brightly lit room and then a dimly lit room, such as a closet or your bedroom at night with the curtains are drawn. Give your eyes time to adjust to the darkness and look at the cards. Can you still identify the colors? If so are they the same colors you saw in the well lit room?

Experiment 2:

Measuring the location of cones and rods in your field of vision.

Supplies: Thick paper (or cardboard, glue/tape, and regular paper), colored markers/pencils, scissors, and something that is cylindrical and about the diameter of your face (a coffee can will work well).

  1. Trace the circle from the cylindrical object onto a sheet of paper.

  2. Cut out the circle.

  3. Fold the circle in half and cut on the folded line creating two half circles. Glue or tape the two half circles together. (If you don’t have thick paper you can glue or tape one of the half circles to the cardboard and then cut it out.

  4. Create a protractor by using your finger to mark equal distance segments from the center point of the curve and down each edge. Number each segment starting at one on each end.

  5. Cut a sheet of paper that is about 2inches by ¼ inch for each of your colored markers/ pencils. (If you don’t have thick paper cut your paper twice the width and then fold it in half and glue or tape it together.)

  6. On the top of each paper draw a shape. You can repeat shapes but make sure and use more than one shape.

  7. Hold the protractor horizontally to your face so that the center is between your eyes. Look straight forward and randomly pick up one of the papers with the shape on it (don’t look at it). Hold the paper vertically at the left or right end of the protractor and slowly move it towards the center point.

  8. Write down when you first noticed the movement, shape, and color of the marks on the paper. If you have someone who can help write down your answers for you it is easier, if not focus on one thing at a time.

    Things to observe. On average, at what point did you detect the movement, the shape, and the color? Was the average for the direction of color the same for all colors or do you detect different colors quicker?

    Extension: Do you think age plays a role in how well we see color? Build off of one of the experiments above by testing people in your home. Do you get the same results?

Activity 3.1 Bird in a Cage Negative After Image

Click here to create the Bird in a Cage negative after image from the San Francisco Exploratorium

Extension: Design and create your own negative after image. Using bright colored markers or cutout brightly colored sheets of paper and glue them on to a white or black sheet of craft paper. Stare at the image you created and then look at a sheet of white paper to see if it works. If it worked, conduct an experiment to see how long you need to stare at the image to get the best negative after image. Send a picture of the negative afterimage you created and how long someone needs to stare at it to create the illusions to and we will add it to our website. If you would like credit, include your first name and school you attend.

Activity 3.2: Hermin Grid

It’s not just your cones that can create negative after images but also your rods. Look at the illusion below.

Do you see gray dots appear and then disappear when you try to look directly at them?

Experiment: Investigate the effect of the color on the illusion. You can investigate digitally here or you can use markers or a computer to create your own Hermin Grid.

How does it work? You can learn more about the theories behind how the mind creates the illusion of the grey dots here.

Activity 3.3: Waterfall Illusion

Illusionists can also create motion afterimages by overstimulation of the receptors that detect motion. Click here to experience the waterfall illusion and learn more about the theory of how it works.

Experiment: Does the amount of time you stare at the moving lines affect how well you see the illusion or how long the illusion lasts? What about age, does it have an effect? Test several different people in your house or vary the amount of time you spend looking at the moving lines and time how long the illusion lasts to find out.

Activity 4.1 Find Your Blind Spot

Click here to go to The Brain from Top to Bottom website had a good experiment you can try if you’d like to find your blind spot.


You can try to follow these instructions to find your blind spot:

Draw a small circle on the left side of a sheet of white paper and then put an X about 6 inches to the right of the small circle. Hold the paper out at arms’ length in front of you, with the middle of the paper lined up with your nose. Close your right eye and focus your left eye on the X. You should still be able to see the circle in your peripheral (side) vision. Slowly bring the paper to your face while keeping your left eye focused on the X. Before you get to your face the circle should disappear. We are going to add these instructions and some links to some other websites that will help you “see” your blind spot.

If you want to see how good your brain is at filling in the gaps, draw a straight line from about an inch to the right of the dot all the way to the X. This time when you find your blind spot and the dot disappears, you will still see an entirely straight line! You never actually “saw” your blind spot; you actually stopped receiving information about the dot.

Activity 5.1: Binocular Vision

Place sticky notes numbered 1-10, 6 inches apart from each other on a wall in your house. If you don’t have sticky notes ask if it is okay to tape little squares of paper to the wall.

Stand in the middle of the room and hold up your finger directly in front of your nose at arm’s length. Alternate opening and closing each eye without moving the finger. Record the sticky the finger in front of when the left eye is open and when the right eye is open. Now go to the back of the room and do the same task.

How many sticky notes did your finger jump when you were in the middle of the room compared to the back of the room?

Activity 5.2: Little Sausage Finger

Instructions: Hold your hands about ten inches from your face in front of your nose with the tips of your index fingers pointed towards each other. Look past your finger and focus on an object in the background. Slowly move your fingers towards each other and a little sausage finger should appear between the two fingers. This new finger will disappear if you focus on it so it is important to keep your focus on the object in the background.

Activity 5.3: Hole in the Hand.

Supplies: You will need a paper towel tube or a sheet of white paper rolled lengthwise to create approximately a 1-inch diameter hole.

Instructions: Hold the tube up to your right eye and look straight through it, focusing on an image in the background. Hold your left hand so that the fingers are pointing up and place it so that the middle of the right side of your palm is touching the tube (you should be able to see the entire palm of your hand with your left eye. Keep your left eye focused on your left hand and your right eye focused on the object in the background (still looking through the tube). Move the left hand to and from your face (sliding along the tube) until a hole appears in your left hand allowing you to see the object in the background.

If you want to learn more and do some fun activities with vision and visual illusions. We recommend you check out the neuroscience for kids vision page by clicking here.


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