Key Concepts
Biology
Physiology
Senses
Vision
Perception

Introduction
Did you know that you have a blind spot in each of your eyes? This doesn’t mean you see a constant black spot in your field of vision. Normally you don’t notice these blind spots at all. There are, however, some ways you can make these blind spots come to light, so to speak! This activity will show you how to find them.

Background
Our eyes are complex organs that allow us to detect visual objects, colors, motion and other things happening around us. To be able to see, however, we need light. Usually what we see is light that reflects off of objects and then enters our eyes through the pupil (the pupil is the opening in the middle of the front of the eye). How much light gets into the eye is controlled by the iris, which is the colored part around the pupil that can contract and expand to open and close the pupil. Inside the eye the light lands on the retina at the back of the eye, which is a light-sensitive layer of tissue. The retina has two types of light-sensing cells: rods and cones. Rods provide black-and-white vision in dim light whereas the cones are responsible for color vision.

When light hits the rod and cone cells, nerve impulses are triggered and sent to the brain through the optic nerve. In humans and most vertebrates the optic nerve fibers pass through the retina and out of the back of the eyeball. The area where the bundled nerve fibers pass through the retina does not contain any light sensitive cells. This means we don’t see light that hits this exact spot. Although we technically cannot see this light, our brain can usually fill in the information that we are missing based on the other things around the blind spot. This is the reason why we don’t usually notice our blind spots. In this activity, however, you will see how under certain circumstances your blind spot can seem to make things disappear. Are you ready to make your blind spot visible?

Materials

  • Cardstock paper
  • Scissors
  • Ruler or measuring tape
  • Pen or pencil

Preparation

  • Carefully cut a 2-inch-high and 5-inch-long rectangle from the cardstock paper.
  • Place the paper rectangle on a surface so that it is lying long-ways (2 inches high and 5 inches wide).
  • By the left edge of the paper, halfway between the top and bottom, draw a small shape (no wider than 0.5 inches) such as a circle, heart or plus sign.
  • By the right edge, halfway between the top and bottom, draw another shape of approximately the same size.

Procedure

  • Take the rectangle in your right hand. Hold it in the middle so you can see both shapes.
  • Extend your arm with the rectangle at eye level. Focus your eyes on the left shape. With your eyes still focused on the left shape can you still see the shape on the right side of the paper?
  • Slowly move your extended arm closer to your face. While moving the paper closer keep your eyes focused on the left shape. While moving the paper can you still see both shapes clearly?
  • Cover your left eye with your left hand. Extend the right arm with the paper at eye level again. Focus your right eye on the left shape. Can you see the other shape as well?
  • With your left eye covered and your right eye focusing on the left shape, slowly move the paper closer toward your face. Keep focusing your right eye on the left shape. What happens to the shape on the right side of the paper while you move the paper closer?
  • Now cover your right eye with your right hand. Extend your left arm with the paper, and look at the right shape. Are you able to still see the left shape while focusing on the right shape with your left eye?
  • Again slowly move the paper closer to you. Keep your left eye focused on the right shape. What do you notice this time?
  • Extra: Draw a horizontal line straight across the paper from one edge to the other, running through your two shapes. Repeat the activity. How do your results change?
  • Extra: Can you measure the size of your blind spot? Cover one of your eyes, and position the paper at a distance where one of the shapes disappears from your vision. Move a pen across the paper and mark where it disappears in your blind spot from all sides. The markings allow you to measure the size of your blind spot in your field of vision.
  • Extra: Make a new paper card, but this time draw bigger shapes on each side. Does the activity still work?

Observations and Results
Did you find your blind spot? When looking at the paper rectangle with both eyes you probably always saw both shapes on each side of the paper—even when you moved your arm closer to your body. When you looked with only your right eye directly at the left shape, however, you should have noticed that at some point the right shape disappeared as you moved the paper closer to your face. At this position the shape just happened to be in your blind spot. This blind spot is there because the optic nerve fibers pass through the back of your retina inside your eye. Where the nerve passes through there are no cells receiving light. At this tiny spot, which is approximately the size of a pinhead, you are technically blind. You know that you have such a blind spot in both of your eyes because you should have seen the same thing happen when you looked at the right shape with your left eye. This time the left shape should have disappeared.

If you did the extra activity with the straight line across the card, you should have noticed that the shape still disappeared but you still saw a continuous straight line all the way to the edge of the paper. This is a great example of how your brain tries to fill in the blanks of the blind spot. Your brain noticed the straight line in the area surrounding your blind spot—and just continued the line even though it technically didn’t detect it there. The brain, however, couldn’t notice the shape because it was fully inside the blind spot. As a result the shape disappeared, but the straight line was still visible.