Charles Blair-Broeker

Blind Spot Detection

Concept:As every introductory textbook makes clear, the eye has a blind spot on the retina where the optic nerve exits. The texts usually have an illustration and instructions students can use to find their blind spot. I think it’s better for students to make their own blind spot detector. It can be done simply in a few minutes, and you can extend the activity to promote critical thinking, illustrate the Gestalt concept of closure, and make the distinction between bottom-up and top-down processing.

Materials Needed:Each student will need a pen or a pencil and a narrow strip of paper. I usually cut up scratch paper to make the narrow strips. Each strip should be 8.5 inches wide (the width of a standard piece of paper) and a couple inches long.

Procedure:You can be a little clever in terms of what “disappears” in the blind spot detection exercise. For example, you can have a dog and a disappearing bone or the Cookie Monster and a disappearing cookie. However, I like to use one that requires almost no artistic ability, so I generally have a blind spot detector where the IRS makes your money disappear.

To do this, instruct students to draw the letters IRS about a third of the way across the strip of paper, and then to make a dollar bill sign about two thirds of the way across. It’s a very forgiving activity in that the size and placement of the IRS and the $ don’t compromise the success of the demonstration as long as they are roughly close to your instructions. The slip of paper should end up looking similar to this:

Once each student has a slip ready, provide the following instructions. I tell students to work slowly and deliberately, or they will not see the effect.

  1. This is a right eye blind spot detector, so close or cover your left eye.
  2. Hold the detector at arm’s length so that it’s centered (if Pinocchio was holding it and he told a lie, his nose would pierce the slip of paper midway between IRS and $).
  3. Look directly at IRS (not $) with your right eye and very slowly bring the slip of paper closer to your face while continuing to focus on IRS and keeping the slip centered.
  4. At some point, the $ will disappear. This is the point where the image of the $ falls on the area where the optic nerve exits the retina, a spot where there are no rods or cones.

Students are usually delighted when they identify the blind spot. Give them a little time to play around with it and see how far in each direction they can move the slip of paper before the symbol comes back into view. There may be some students who struggle to get the right spot. When this happens, students can help each other or you can circulate and assist students. Usually, unsuccessful students are moving too fast or not following the rule to keep focused on IRS.

Discussion:The procedure above will provide a basis for discussing the structure of the eye and why the blind spot exists. You can easily extend this into a critical thinking activity by asking students why we aren’t aware of the blind spot unless we set up a rather elaborate activity like this one to expose it. Students will quickly identify the importance of having two eyes in this regard. An image that falls on the blind spot of one eye does not fall on the blind spot of the other eye. At this point, have students close one eye and look at an object in the room. They will not detect a blind spot even though they are using only one eye. How come? This is where you can introduce the concept of top-down processing and the Gestalt principle of closure. Your brain prefers to detect whole patterns and uses assumptions and experience to “fill in” the information lost to the blind spot.

To prove this have students add a couple lines to their blind spot detectors so they look like this:

Now have them repeat the procedure they used earlier to detect the blind spot. This time, when the $ disappears, the line will appear continuous. This is a wonderful demonstration of how top-down processing influences perception in our attempt to make sense of the world.

I do not know the original source of this activity.