The perception of an object as maintaining the same size despite changing images on the retina.

Key concepts
Human Body
Binocular vision
Stereopsis
Depth perception
3-D

Introduction
Is catching, juggling or heading a ball challenging for you? If you've ever tried threading a needle, did it end in frustration? Have you ever thought of blaming your eyes? Two eyes that work together help you estimate how far a ball is or where the thread is with respect to the needle. This “working together” of the eyes actually happens in the brain. The brain receives two images (one for each eye), processes them together with the other information received and returns one image, resulting in what we “see”. Are you curious about how depth perception enters the picture? “See” for yourself with this activity!

Background
Humans have two eyes, but we only see one image. We use our eyes in synergy (together) to gather information about our surroundings. Binocular (or two-eyed) vision has several advantages, one of which is the ability to see the world in three dimensions. We can see depth and distance because our eyes are located at two different points (about 7.5 centimeters apart) on our heads. Each eye looks at an item from a slightly different angle and registers a slightly different image on its retina (the back of the eye). The two images are sent to the brain where the information is processed. In a fraction of a second our brain brings one three-dimensional image to our awareness. The three-dimensional aspect of the image allows us to perceive width, length, depth and distance between objects. Scientists refer to this as binocular stereopsis.

Artists use binocular stereopsis to create 3-D films and images. They show each eye a slightly different image. The two images show the objects as seen from slightly different angles, as would be when you saw the object in real life. For some people, it is easy to fuse two slightly different images presented at each eye; others find it harder. Their depth perception might rely more on other clues. They might find less pleasure in 3-D pictures, movies or games, and certain tasks—such as threading a needle or playing ball—might be more difficult.

Materials

• Three different-colored markers or highlighters that can easily stand vertically
• Ruler
• Table
• Camera
• Flat work space, such as a tabletop

Preparation

• Place the first marker standing, vertically, 30 centimeters from the edge of the table.
• Place the next marker 30 centimeters behind it (60 centimeters from the edge) and place the last one 30 centimeters from the second marker (90 centimeters from the edge). (If your table is not long enough, you can place your three makers at 15, 30, and 45 centimeters from the edge of the table.)

Procedure

• Position yourself at the edge of the table and bend your knees so your eye level is at the level of the markers.
• Close or cover your right eye and look only with the left eye. Shift your head so all three markers are right behind the other. Is it possible to hide the second and third marker behind the first one?
• Keep the position of your head the same but now close or cover the left eye and look only with the right eye. What do you see? In your image are the second and third marker still hidden behind the first one? Why do you think this happens?
• Keeping your left eye covered, reposition yourself so the second and the third markers are hidden by the first one. Switch eyes with which you are looking again. Did it happen again? This time observe some details. In your image is the second marker to the right or the left of your first marker? What about the last marker? How far apart are the markers in your image? Do you see space between the first and the second markers? Do you see as much space between the second and the third markers (those that are farther away from you)? Are some markers still partially overlapping?
• Open or uncover your right eye and look with both eyes. What do you see? Are any markers hidden by closer markers? Try to reposition your face so, in your image, the closer marker hides the more distant markers. Is it easy? Is it even possible?
• Use a camera to study this in more detail. Position the camera so the first marker hides the second and third marker. The tops of the markers can stick out. Take a picture.
• Shift your camera about 7.5 centimeters horizontally to the side, and take a second picture. Remember whether you shifted to the right or to the left. If you shift right, the first picture represents what the left eye sees. If you shift left, the first picture represents what your right eye sees.
• Look at the pictures. These images reflect what your right and left eye register. (Human eyes are about 7.5 centimeters apart.) Are both pictures identical? In what way do they differ?
• The brain uses the different location of objects in the images received by the right and the left eyes to create depth perception. Can you find some rules the brain uses? Which marker do you think shifts most with respect to a distant point or with respect to the last marker—the closer one or the one that is farther away?
• In the first picture the three markers are lined up. In the second they are not. Measure how much the second marker is shifted with respect to the last marker. Now measure how much the first marker, which was positioned closer to the camera, is shifted with respect to the last marker. Does shift increase or decrease when objects are placed farther away from the observer?
• Look at your second picture. Is your second marker shifted to the right or the left with respect to the last marker? What about the first marker? Is this direction identical to the direction in which you shifted your camera?
• Can you imagine how the picture would look if you shifted the camera by about 7.5 centimeters to the other side? You can repeat part of the procedure where you take the pictures but now shift your camera to the other side to find out.
• Extra: Study other parameters that might influence the shift. Do the markers shift more or less with respect to one another if you (as observer) position yourself farther away from the set of markers? What happens if you gaze at a point far in the background? (That is, compare the shift with respect to a point in the background.) Pictures can help you perform a more detailed analysis. A row of equally spaced trees, light poles or other objects along a straight street can also help you perform a more elaborate investigation.
• Extra: Imagine what would happen if our eyes were separated by a longer horizontal distance. Do you think the horizontal shift would be larger or smaller? What do you think would happen if our eyes were shifted vertically instead of horizontally? Take pictures where you position the camera at slightly different locations in space to find out. Can you find some advantages and disadvantages to having eyes that are separated as they currently are in humans?
• Extra: Adequate depth perception facilitates tasks such as playing ball, threading a needle and driving. To experience how difficult playing ball and threading a needle are with monocular (or one-eyed) vision, cover one eye and perform the task. Be careful, though; this is difficult! Start by throwing a ball softly. Do not perform any dangerous tasks with one eye covered.

Observations and results
Did you see how your right eye registers the world differently from your left eye? Did you see how using both eyes created yet a different picture?

When you lined up the markers so your left eye could only see the first one, they were no longer lined up when you looked with the right eye only. Something similar happened when you lined up the markers for your right eye and you switched to a left-eye-only view. This time the markers were shifted to the right in your image. This happened because each eye looked at the row of markers from a slightly different angle.

With both eyes open it was probably very hard or impossible to position yourself so you only could see the first marker. Most people have a hard time fusing the images created by each eye in this particular setup. You might have experienced that you switched between images or had double vision.

The pictures you took with the camera allowed you to compare how much a closer marker shifted with respect to a more distant one. If you performed more tests, you might have discovered that the shift depends on the distance between the objects, the distance between you and the objects, and the point you are gazing at (also called the point of focus).

More to explore
Perception Lecture Notes: Depth, Size and Shape, from Professor David Heeger, Department of Psychology, New York University
Starry Science: Measure Astronomical Distances Using Parallax, from Scientific American.
Sight (Vision), from University of Washington

This activity brought to you in partnership with Science Buddies