IntroductionIs catching, juggling or heading a ball hard for you? If you ever tried threading a needle, did it end in frustration? Have you ever thought of blaming your eyes for this hardship? 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 in this activity!
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BackgroundHumans have two eyes, yet we only see one image. We use our eyes in synergy (cooperating 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 (also known as 3D). We can see depth and distance because our eyes are located at two different points (about three inches apart) on our head. 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 gets 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 3D films and 3D 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. Other people find it hard. Their depth perception might rely more on other clues. They might find less pleasure in 3D pictures, movies or games, and certain tasks—like threading a needle or playing ball—might be more difficult for them. Materials
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Extra: Study other parameters that might influence the shift. Do the markers shift more or less with respect to each other 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 (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 larger 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 like 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. <i>Do not</i> drive any vehicle (including a bike) with one eye covered; you will not be able to adequately estimate the distance of obstacles or oncoming traffic. Extra: If you have 3D pictures, can you find out what creates the perception of depth? Do you need to view them with a stereoscope, a virtual reality (VR) headset or red-blue glasses to get the depth perception? Why do you think this is the case? Observations and ResultsDid 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 are shifted to the right in your image. This happens because each eye looks 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 marker. If you performed some 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
CreditsSabine De Brabandere, PhD, Science Buddies
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Key Concepts
Human Body, binocular vision, stereopsis, depth perception, 3D
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